14th International Symposium on Particle Image Velocimetry

Tracking particles in Poiseuille flow for several pipe diameters in three dimensions

2021-09-01T14:29:58+00:00

A setup is devised to track suspended particles in a pipe in three-dimensional space using the ShadowgraphyPTV technique. This system consists of a single camera and a mirror, and is used to track particles for over 20 pipe diameters at three downstream locations. Pipe to particle diameter ratios (D/d) of 18, 9, and 6 are investigated. The bulk Reynolds number is varied between Reb = 300-1250. As expected, particles are observed to migrate radially to a location corresponding to the Segre-Silberberg annulus. In addition, ´we observe particles also moving in the azimuthal direction (clockwise or counter-clockwise), with some particles moving as much as 180◦ during their passage through the field of view. This helical motion persists throughout the pipe (600D long) and the azimuthal velocity increases with the Reynolds number (Reb). The effect of particle size and the Reynolds number on this previously undocumented, three-dimensional motion is studied.

Tomographic PIV measurement of a re-entrant jet in a cavitating venturi

2021-09-01T19:04:06+00:00

Partial cavitation occurs when low-pressure regions caused by separated shear layers are filled with vapours. Partial cavitation is inherently unsteady and leads to periodic cloud shedding. The periodically generated re-entrant jet travelling beneath the vapour cavity is considered as one of the mechanisms responsible for the periodic cloud shedding (Callenaere et al. (2001)). However, the exact physical mechanism that drives the shedding remains unclear. The re-entrant flow exists as a thin liquid film wedged between the wall and the vapour cavity. The flow in this thin film is generally assumed to move with the same order of magnitude as the bulk flow, yet in the opposite direction. There have been several attempts to measure the velocity of the re-entrant flow to get insight into the physics of re-entrant flow and its contribution to cloud shedding. However, the flow topology of the re-entrant jet poses a major challenge to experimentally study it. The unsteady nature of the flow and the opacity of the cavitation cloud adds to the further complexity. In this work, we show that tomographic PIV (Elsinga et al. (2006)) can be extended to exploit the flow topology to accurately measure the velocity and thickness of the re-entrant flow. This in turn provides better insight into the role of re-entrant flow in periodic cloud shedding.

An Experimental Study of Unsteady Heat Transfer and Phase Changing Process Upon Impacting of Ice Crystals onto Heated Surfaces Pertinent to Aero-Engine Icing Phenomena

2021-09-01T20:06:10+00:00

Ice accretion on exposed surfaces of aero-engine components has been widely recognized as a significant hazard to aviation safety in cold weathers. Icing process due to the impingement of the supercooled water droplets suspended in the cloud onto the cold surfaces of inlet components of aeroengines have been studied extensively for decades. Since ice particles were believed to simply bounce off from the exposed surfaces of aero-engine components, ice crystals in the clouds were initially considered not to pose a threat to aviation safety. Therefore, the ice accretion process due to the impacting of ice crystals onto the surfaces of hot engine componentes has not been studied until recently. It has been found recently that, tiny ice particles in the cloud may be partial/full melting upon impacting onto the hot surfaces of aero-engine components, such as heated Inlet Guide Vanes (IGV) and various probes. The partially/fully melted ice crystals were found to stick onto the hot surfaces and form thin water film, which would intercept more oncoming ice particles and lead to significant ice accretion over the surfaces of the hot engine components. The ice crystal induced ice accumulation on the critical aero-engine components has been found to cause significant engine performance loss and erroneous data being read from the probes.

In the present study, a series of experimental investigations were conducted to elucidate the underlying physics of the dynamic ice accretion process pertinent to ice crystal icing phenomena. A novel ice crystal icing test rig with the capacity of generating controllable amount of ice crystals and flying speed up to 100 m/s was developed in a temperature-controllable environment chamber for the ice crystal icing studies. By using a high-speed imaging system, a digital particle image velocimetry(PIV), and an Infrared (IR) thermal imaging system, a comprehensive experimental campaign was performed to characterize the transient impacting process of ice crystals, dynamic ice accretion and unsteady heat transfer process associated with the impacting of ice crystals onto heated surfaces, in comparison to those due to the impingement of supercooled water droplets. By using an ultra-sensitive force sensor and a high-speed image system, a comparative study is conducted to examine the differences in the transient impinging dynamics of single water droplets, supercooled water droplets, and ice crystals onto solid surfaces with different wettability and stiffness. By upgrading the unique Icing Research Tunnel of Iowa State University (i.e., ISU-IRT) with additional ice crystal icing capability, a set of explorative studies are also conducted to examine the characteristics of the dynamic ice accretion processes over the heated surfaces of an aero-engine Inlet Guide Vane (IGV) model under both ice crystal icing and supercooled droplet icing conditions. The anti-/de-icing performance of a novel hybrid strategy by integrating icephobic coatings and minimized surface heating are also evaluated under both supercooled water droplet icing and ice crystal icing conditions. The new findings derived from the present studies are very helpful to gain further insights into the ice crystal icing phenomena for the development of more effective and robust anti-/de-icing strategies to ensure safer and more efficient aircraft/aero-engine operations in cold weathers.

Evaluation of velocity fields in horizontal gas-liquid intermittent flows using Stereoscopic-PIV and instantaneous masking procedure

2021-09-02T18:39:25+00:00

The main goal of this work was to obtain well-converged liquid velocity profiles for intermittent gas-liquid flows in a horizontal pipe. To this end, air and water with superficial velocities of J_G = 0.5 m/s and J_L = 0.3, 0.4 and 0.5 m/s, respectively, were driven into a 18-m acrylic test section with an inner diameter of 40 mm. All three-components of the velocity vectors were measured in a pipe cross-section using a highfrequency stereoscopic PIV system, together with the laser induced fluorescence technique. Photogates were used to measure the unit cell translational velocity, as well as to trigger data acquisition, allowing the calculation of ensemble-averaged velocity fields at specific positions, referenced to the gas-bubble nose tip position. An instantaneous image masking procedure was implemented, allowing the determination of non-dimensional ensemble-averaged velocity profile in the liquid film, referenced to gas-bubble boundary. The high-frequency system employed allowed the determination of the influence of the faster-moving gas bubble on the liquid velocity field in the plug region. The data presented are relevant to the validation and improvement of one-dimensional two-phase numerical models, as well as to better understand this complex flow.

Micro-PIV Measurements of Multiphase Flow in Deforming Porous Media Subject to Mineral Dissolution

2021-09-15T20:06:58+00:00

Mineral dissolution is studied in novel calcite-based porous micromodels under single- and multiphase conditions, with a focus on the interactions of mineral dissolution with pore flow. Microscopic particle image velocimetry (PIV) was utilized to simultaneously characterize the local velocity field and the instantaneous shapes of the dissolving grains. The preliminary results provide a unique view of the coupled dynamics between pore flow and mineral dissolution.

PIV investigation of cavitating flows around circular cylinders with hydrophobic coatings

2021-09-15T21:05:40+00:00

The treatment of the hydrophobic properties of solid surfaces is considered as a passive method to reduce the drag in water flows (Rothstein, 2010) and to potentially affect the flow separation and vortex shedding (Sooraj et al., 2020). The manufacturing of surfaces with micro- and nano-scale roughness allows to extend the hydrophobicity towards superhydrophobicity with the contact angle close to 180°. In such conditions the solid surface is not wetted completely and the air-water interphase partially remains on the surface texture. This results in so-called flow slip effect. Therefore, a local phase transition during the flow cavitation or gas effervescence in near-wall low-pressure regions may additionally affect the slip effect for hydrophobic surfaces. The present work is focused on the comparison between cavitating and noncavitating flows around circular cylinders with lateral sectors with hydrophobic and non-hydrophobic coatings. The experiments are performed in a water tunnel, which consists of a water outgassing and cooling/heating section, honeycomb, contraction section, test section and diffuser. The water flow is driven by an electric pump, providing a bulk velocity up to 10 m/s in the transparent test section with 1 m length and 80×150 mm²rectangular cross-section. The facility is equipped with an ultrasonic flowmeter, temperature and pressure sensors. Besides, the static pressure inside the water tunnel can be varied by using a special shaft section. The measurements are performed by using high-repetition and low-repetition PIV systems. The former is used for the analysis of large-scale flow dynamics in the wake region, whereas the latter one is used for high-resolution measurements in near-wall regions by using a long-distance microscope. The Reynolds number based on the bulk velocity of the flow, diameter of the cylinders (D = 26 mm) and kinematic viscosity of the water is varied up to 2×10⁵.
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Phase separation and flow measurement of dilute bubbly jet with 2-D PIV and LIF

2021-09-21T15:10:52+00:00

A measurement technique with a combination of PIV and LIF is suggested to measure gas-phase and liquid phase separately to resolve flow structures of a bubbly jet. In the bubbly jet, distribution of the bubble population shows a Gaussian-like function but translated outward. Spreading nature of each phase does not correspond to each other due to lack of the number of bubbles to be redistributed.

Time Resolved PIV of the Flow Field Underneath an Accelerating Meniscus

2021-09-21T19:34:09+00:00

We present an experimental analysis of the flow field near an accelerating contact line using time-resolved Particle Image Velocimetry (TR-PIV). Both advancing and receding contact lines are investigated. The analyzed configuration consists of a liquid column that moves along a vertical 2D channel, open to the atmosphere and driven by a controlled pressure head. Large counter-rotating vortices were observed and analyzed in terms of the maximum intensity of the Q-field. To compute smooth spatial derivatives and improve the measurement resolution in the post-processing stage, we propose a combination of Proper Orthogonal Decomposition (POD) and Radial Basis Functions (RBF). The RBFs are used to regress the spatial and temporal structures of the leading POD modes, so that “high-resolution” modes are obtained. These can then be combined to reconstruct high-resolution fields that are smooth and robust against measurement noise and amenable to analytic differentiation. The results show significant differences in the flow topology between the advancing and the receding cases despite velocity and acceleration of contact lines are comparable in absolute values. This suggests that the flow dynamics are tightly linked to the shape of the interface, which significantly differs in the two cases.

Simultaneous two-phase flow measurements in a high-speed particle-laden under-expanded jet

2021-09-27T16:29:40+00:00

The objective of this study is to understand the dynamics of a high-speed particle-laden under-expanded jet, motivated by landings on extraterrestrial bodies. In this setup, inertial particles are entrained and accelerated by an under-expanded jet. But, due to their inertia, the particle velocity is significantly lower than that of the surrounding gas close to the nozzle, so the two phases are coupled through aerodynamic drag. Sub-micron oil droplets are dispensed upstream to serve as tracers, whose velocity is determined through a PIV system; inertial particles, after image segmenting is performed to separate them from PIV data, will be tracked over time with a PTV system. This was accomplished with a single laser pulse and the camera straddle time to produce image pairs and shorten the pulse width. The results will help to understand particle-laden flow in a new regime where the background flow is compressible and the Mach number based on the slip velocity is not negligible, which may help to pave a foundation for future studies in compressible multiphase flows.

Intermittency effects in single- and multi-phase flow and anomalous transport in heterogeneous porous media

2021-09-27T16:38:36+00:00

Multi-phase flow and transport in porous media is prevalent in a wide range of challenging fluid mechanics problems related to sustainability, energy, and the environment. Accurate prediction of the displacement and interaction of such flows is vital in addressing these problems. In particular it is critical to understand the small- or pore-scale flow and its spatial and temporal evolution, which can impact behaviors at system scales in a nontrivial manner. Intermittency is a phenomenon currently observed in numerical and experimental studies of single-phase flow (Anna et al., n.d.; Morales et al., n.d.), but the case of multi-phase flow has yet to receive much study due to challenges faced in both simulations and experiments. The underlying physics of spreading, mixing, and interfacial processes must be understood for accurate predictions of transport in multi-phase flow systems. Therefore, a comprehensive understanding of multi-phase flow at these very small scales is necessary in the development of accurate system-scale prediction models. We present results from a coordinated numerical and experimental study of intermittency effects over a range of viscous and inertial flow regimes in single- and multi-phase flows in 2D heterogeneous micromodels to quantify Lagrangian flow statistics to better inform pore-scale models. The applicability of different modeling frameworks such as the correlated-continuous time random walk is tested by studying statistics of particle trajectories obtained by particle tracking velocimetry (PTV) measurements and Lattice Boltzmann simulations from single- and multi-phase flows. The results make particular note of the influence of the pore Reynolds number (Re) and inertial effects on intermittency, and compare these effects in the two flow regimes.

Velocity measurements of dilute suspensions over and through various porous media models

2021-09-28T16:14:36+00:00

This study is focused on the motion of a dilute suspension containing rigid, spherical, non-Brownian, noncolloidal particles flowing over and through porous media models. The flow is confined to very low Reynolds numbers. To examine the velocity distribution particle image velocimetry (PIV) was applied in conjunction with refractive index matching (RIM) techniques. This study is the first of its kind analyzing the interaction between two common engineering systems: suspension fluid and porous media.

Particle pair statistics of inertial particles at small separation using stereoscopic particle tracking

2021-09-01T15:37:02+00:00

Particle pair statistics of inertial particles having average Stokes numbers of 2.1 and 14 are measured in isotropic turbulence at a Reynolds number of Re_λ = 240. The radial distribution function (RDF) and mean relative approach velocity are obtained at small separation distances using 2-frame stereoscopic particle tracking velocimetry (stereo-PTV). At small separation distance, the RDF varies by an order of magnitude in the range of Stokes numbers investigated. However, the mean relative approach velocity is found to have a weak dependence on Stokes number. The results are shown to have high accuracy when compared to analogous mono-PTV datasets, and can be used to provide a more reliable estimate of the inter-particle collision rate. The main limitation of the measurement is observed at separation distances less than the laser sheet thickness, where the technique tended to underestimate the mean relative approach velocity.

Ghost particle reduction in particle streak velocimetry

2021-09-01T15:53:44+00:00

Ghost particles are ambiguities in the process of the 3D-reconstruction of seeding particles detected in short-exposure imaging for volumetric flow velocimetry. 3D Particle Streak Velocimetry (3D-PSV) relies on long-exposure images, where the pathlines of the seeding particles are imaged as streaks. In this work, we demonstrate the inherent suitability of 3D-PSV for ghost particle rejection by calculating the probability of ghost streak generation in different scenarios and comparing our results to simulations.

Fluid structure interactions of a pitching wing at high angle of attack

2021-09-02T16:00:50+00:00

This paper deals with the study of the flow around a pitching wing at high angle of attack. Different pitching amplitudes and frequencies are studied using DIC and LPT measurements. The Fluid Structure Interactions are shown and exhibit that the vortex shedding could be reduced by actuating the wing at specific frequencies.

Visualization of Three-Dimensional Acoustic Streaming Flow Patterns around an Inclined Triangular Obstruction using Digital In-Line Holographic Micro-Particle Tracking Velocimetry

2021-09-02T20:50:03+00:00

Acoustic Streaming is a flow phenomenon with many applications in the field of microfluidics, such as micro mixing[1, 2] and particle manipulation[3]. With the manufacturing techniques evolves, more complicated geometries can be designed for microfluidic device and 3-D acoustic streaming patterns may occurs. In this study, 3-D trajectories of particle induced by acoustic streaming around an inclined triangular obstruction in a microchannel were visualized by a volumetric tracking method using Digital Inline Holographic Microscopy (DIHM)[4-6]. The triangular obstruction has a tip angle of 20° and an inclined angle of 30°. The acoustic streaming is created under 12 kHz oscillation of a piezo plate driven by 20V voltage. Illuminated by a 450nm continuous laser, the magnified hologram of the motion of 1.79μm tracer particles was recorded by a low-cost 10X industrial microscope with a machine vision camera of 10 fps (frames per second). Using RayleighSommerfeld back-propagation method[7], particle locations was reconstructed frame by frame and 3-D tracking of individual particles was performed afterwards. The trajectories of each particle were reconstructed to reveal the vortical structure of the acoustic streaming flow. For the current system setup, the measurable range was estimated to be 550×685×840 μm³. The 3-D location reconstruction accuracy was verified with a calibration target and the location sensitivity was found to be linear throughout the measurable range. Reconstruction at different depth locations show that the dick-shaped calibration dots and the spherical polystyrene particles have different intensity profiles. The calibration dots show local minimum of intensity at the correct depth location, while polystyrene particles show local maximum of intensity instead. Resolved particle trajectories show that the acoustic streaming flows cause particles to move with 3-D spiral shaped motions near the side of the triangular obstruction, while particles away from the obstruction shows planar motions.

Shake-The-Box particle tracking with variable time-steps in flows with high velocity range (VT-STB)

2021-09-07T19:45:41+00:00

We present a novel evaluation mode for Lagrangian Particle Tracking methods in general, applied to the Shake-The-Box method specifically. The aim is to attain high levels of accuracy and a removal of false (‘ghost’) tracks in flow situations, where significant amounts of particles show small relative movement with respect to each other in consecutive time-steps.

An iterative approach using variable time separations is employed, which starts by tracking particles at high timeseparations, followed by an iterative reduction of time separation, while feeding the particle tracked within the previous iterations. The process allows for applying tracking parameters fine-tuned to the different flow regimes tracked within each iteration.

Experimental validation was performed using a dataset on impinging jet flow, created in collaboration with the School of Mechanical Engineering of Pusan National University. Evaluation of this flow with high velocity range shows distinct advantages in reduction of ghost tracks and in tracking accuracy.

Highly accurate optical flow method based on volumetric segmentation for 3D PIV

2021-09-07T19:52:20+00:00

In this study, we present a new three-dimensional optical flow method based on volumetric segmentation for the velocity estimation of fluid flow. The proposed method uses a segmented smoothness term that is designed on the assumption that the particle velocity varies continuously in each segmented volume and discontinuously on the surfaces of the segmented volumes. Subsequently, the data term is proposed on the basis of the segmented volumes and the fluid mass conservation equation, which is derived from the Reynolds transport equation. In addition, the robust local level-set method is applied to segment the particle volume according to the velocity distribution of fluid flow. The proposed method is evaluated quantitatively on synthetic data and qualitatively on experimental data, and the velocity results are compared to the advanced 3D velocity estimation methods. The results indicate that the proposed method can obtain velocity fields with greater measurement accuracy for Tomo-PIV.

3D Particle Tracking Velocimetry applied to droplets generated by breaking waves

2021-09-07T20:50:45+00:00

One of the environmental difficulties of exploring the polar regions is marine icing. The understanding of this phenomenon is important for the safety of installations, ships and people that operates in these environments. One of the main sources of marine icing is wave breaking. Therefore, experimental and field work has been conducted to understand the break-up of waves in different situations and some explanation have been proposed to the instabilities that create the spray formation. In this work, two different situations of wave breaking were studied: 1. Solitary waves were created and steepened by the use of a beach. The waves impacted on a vertical wall with different wall heights. 2. Violent plunging breakers were created by a focusing wave train and a sloping beach. The main objective of these experiments was to quantify the production of droplets from the impact by using Particle Tracking Velocimetry in 3 dimensions. It was found that the initial distribution of droplet sizes is similar in both experiments. These distributions are compared with previous studies, where the distribution of droplet sizes in different experimental cases were approximated by lognormal, Weibull or G-distributions respectively.

Reynolds stress tensor and pressure-related turbulence transport terms measured by time-resolved tomographic-PIV

2021-09-15T20:47:30+00:00

Turbulence is inherently a three-dimensional and time dependent flow phenomenon (Pope, 2001). Because of the ubiquitous existence of turbulent flows in nature, accurate characterization of turbulent flows, either through experimental measurements or through direct numerical simulations, is of paramount importance for modeling turbulence (Liu and Katz, 2018). Since its inception in 1984 (Adrian, 1984), Particle Image Velocimetry (PIV), among several other conventional techniques used for turbulence measurements, has been a valuable tool for providing reliable experimental data for turbulence research. Several advancements in hardware such as high-speed cameras, together with innovative algorithms and procedures, have extended the scope of PIV to a variety of applications. Westerweel et al. (2013) point out in a recent review article that one of the main advantages of the PIV measurement is its unique ability in measuring quantitatively spatial derivatives of the flow field. With the development of Tomographic PIV introduced by Elsinga et al. (2006), it is now possible to measure simultaneously the distributions of three velocity components in a three-dimensional flow field, thus enabling us to measure all the velocity derivatives of a turbulent flow. However, for a thorough characterization of a turbulent flow, in addition to the velocity gradients, the instantaneous pressure distribution in the 3D flow field also needs to be measured.

Development and Uncertainty Characterization of Rotating 3D Velocimetry using a Single Plenoptic Camera

2021-09-16T14:04:48+00:00

Rotating 3D velocimetry (R3DV) is a single-camera PIV technique designed to track the evolution of flow over a rotor in the rotating reference frame. A high-speed (stationary) plenoptic camera capable of 3D imaging captures the motion of particles within the volume of interest through a revolving mirror from the central hub of a hydrodynamic rotor facility, a by-product being an undesired image rotation. R3DV employs a calibration method adapted for rotation such that during MART reconstruction, voxels are mapped to pixel coordinates based on the mirror’s instantaneous azimuthal position. Interpolation of calibration polynomial coefficients using a fitted Fourier series is performed to bypass the need to physically calibrate volumes corresponding to each fine azimuth angle. Reprojection error associated with calibration is calculated on average to be less than 0.6 of a pixel. Experimental uncertainty of cross-correlated 3D/3C vector fields is quantified by comparing vectors obtained from imaging quiescent flow via a rotating mirror to an idealized model based purely on rotational kinematics. The uncertainty shows no dependency on azimuth angle while amounting to approximately less than 0.21 voxels per timestep in the in-plane directions and correspondingly 1.7 voxels in the radial direction, both comparable to previously established uncertainty estimations for single-camera plenoptic PIV.

Three-dimensional density measurements of a heated jet using laser-speckle tomographic background-oriented schlieren

2021-09-17T14:18:37+00:00

Three-dimensional density field measurement techniques can be used to understand the complex heat transfer and mixing processes that occur in turbulent flows. Tomographic background-oriented schlieren (BOS) is an optical technique that can be used to measure the instantaneous three-dimensional density field in turbulent flows. Light rays propagating through the flow are deflected from their ambient path due to variations in refractive index related to the spatial density gradients. In BOS, a camera is placed looking through the flow at a reference image, which captures path-integrated information on the refractive index gradients in the form of apparent image displacements Richard and Raffel (2001). The displacements recorded simultaneously from many cameras placed around the flow form the basis of a tomographic reconstruction of the three-dimensional refractive index gradients Goldhahn and Seume (2007), from which the density field is obtained through integration of the gradients and application of the Gladstone-Dale relation.

Volumetric Lagrangian Particle Tracking, Air water interface, insect locomotion, surface tension

2021-09-17T14:30:29+00:00

Over a thousand animal species are capable of walking on the interface between air and water. These speciesinclude water striders, a family of insects from the order Hemiptera that has an almost unique ability to walk on the surface of the water without penetrating it. They achieve this outstanding feat by making use of the surface tension and their long hydrophobic legs. Experiments have revealed that water striders transfer some momentum to the underlying fluid through capillary waves and hemi-spherical vortices and that both waves and vortices contribute to the mechanism of propulsion. However, the exact momentum and energy carried by waves and vortices have never been quantified together, and only the energy of the surface waves has been quantified to date. An analysis of the complete energy balance between the interface and the body of the water requires measurement of the free surface topography, together with the three-dimensional (3D) flow field in the water under the surface. The ultimate aim of this work was to develop a method capable of doing this.

Examining the Effects of Fluid Velocity Gradients on 4D Digital Holographic PIV/PTV Measurements

2021-09-17T14:43:43+00:00

4D digital holographic PIV/PTV (4D-DHPIV/PTV) methods have demonstrated theoretical viability due to their relative ease of setup and high spatial resolution (Soria (2018)). This study investigates how velocity gradients related to different flow regimes and their magnitudes affect 3-component–3-dimensional (3C-3D) digital holographic PIV measurement uncertainty.

The error introduced by velocity gradients within the interrogation volume is studied by simulating particles in a velocity field, with a given constant velocity gradient superimposed on a uniform flow from which a time-series of hologram pairs are generated and the 3C-3D velocity fields and their errors are determined using 4D-DHPIV/PTV Sun et al. (2020). Hologram pairs are simulated by modelling the propagation and particlediffraction of coherent laser light using the angular spectrum method (Goodman (1996)). The hologram reconstruction then involves direct reconstruction, followed by deconvolution, a particle position refinement and a hologram subtraction step (Sun et al. (2020)). The particle positions obtained from 4D-DHPIV/PTV are then used to resolve particle displacement measurements using 3D cross-correlation digital analysis with a 3D Gaussian fit to sub-pixel resolution (Soria (2006)).

The effects of velocity gradients on the displacement uncertainty and bias error have been investigated by undertaking Monte Carlo simulations under a range of velocity gradient environments. Specifically, 5 common velocity gradients have been studied, which included pure strain, pure vorticity and x, y and z-directional shear.

The results indicate that the novel 4D-DHPIV/PTV has poorer accuracy and precision in the z-propagation axis, resulting in larger minimum uncertainties and bias errors. The errors in the z axis are also significantly less affected by velocity gradients in the z direction when compared to the effects of x and y directional velocity gradients on x and y errors respectively. Furthermore, the rate of cross-correlation maximum and SNR decrease are approximately 1.36 times slower due to velocity gradients in the z axis than other axes.

First challenge on Lagrangian Particle Tracking and Data Assimilation: datasets description and planned evolution to an open online benchmark

2021-09-21T14:29:56+00:00

In the last decade, Lagrangian Particle Tracking (LPT) has emerged as one of the leading measurement techniques for the quantitative determination of fluid flows in three-dimensional domains (see e.g. Schanz et al., 2016), due to its accuracy in reconstructing particles velocities and material accelerations. Due to the scattered nature of the obtained result, at the particles positions only, significant research efforts have also been placed in the development of dedicated Data Assimilation (DA) techniques, aiming at finally reconstructing full 3D velocity and pressure fields on regular Cartesian grids (see, e.g., Schneiders et al. 2016).

Mounting and support for pseudo biaxial Scheimpflug focusing for unity-magnification, high-speed particle velocimetry

2021-09-21T21:37:18+00:00

A camera mount that can support both heavy cameras and heavy optics allowing a total of seven degrees of freedom shared between them has been designed. This allows for Scheimpflug focusing along one or two axes. A paper proposing a solution to two- axes Scheimpflug focusing has been examined and a new nomer is proposed for two-axes Scheimpflug focusing. The newly designed mounts allow for a broader range of solutions for combinations of positioning and alignment than traditional Scheimpflug mounts.

A novel laboratory pushing the limits for optics-based basic turbulence investigations

2021-09-22T14:39:03+00:00

Developments in theoretical investigations and experimental techniques are reaching a level of maturity for which it is finally becoming possible to answer some of the most pressing questions in turbulence. The prevailing classical theories all have their strengths and drawbacks based on their respective principal assumptions. To better understand the implications of these assumptions, we have developed a theoryintensive experimental strategy. For these purposes, a laboratory has been established at the Department of Mechanical Engineering, Technical University of Denmark. The objective being to provide the data necessary to test the (bounds of) validity of the existing theories; Most prominently the classical Richardson-Kolmogorov-Batchelor paradigm, but also other generally adopted views such as Rapid Distortion Theory and Equilibrium Similarity. The measurements will be analyzed within a novel theoretical framework that enables not only quantification of the degree to which the small and intermediate scale turbulence behaves according to the existing theories (and their central assumptions), but also unveiling the underlying processes that create the respective state of turbulent flow. The present work will describe the current state of the developments of building up the laboratory.

Development of an Optical set-up for 3D PIV with a Large Volume

2021-09-22T15:01:44+00:00

Measurements of 3D volumetric velocity fields are of great theoretical interest with numerous practical applications. These measurements are essential for studying volumetric flows that do not exhibit inherent flow symmetry, such as turbulence or vortex breakdown. In the past decade, several technological innovations facilitated the emergence of 3D-PTV techniques for measuring velocity fields at kHz rate with volumes of interest up to 104 cm³ that contain 300 µm helium-filled soap bubbles. However, when a commercial laser beam with millijoule pulse-energy is expanded and shaped to fill volumes above 102 cm³for 3D-PTV experiments with 15 µm air filled soap bubbles, one finds that the power density of the laser source is insufficient to generate a signal image. This is because the power density of the laser beam falls inversely with respect to its cross-section area and due to the quadratic dependence of Mie-scattering on the particle diameter. Here, we report of the analysis and development of two optical techniques for extending the volume of measurement in volumetric PTV. In particular, when a volume about 103 cm³ is seeded with 15 µm air-filled soap bubbles and a laser with a pulse energy of few single mJ illuminates it. The first technique uses multi reflections between two opposing parallel mirrors. The second technique is a development of laser scanning PIV for volumetric scanning: The potential to increase the scanned volume is examined by experimenting with an acousto-optic modulator for fast scanning. Furthermore, by employing an off-axis parabolic mirror, we obtain parallel beam scanning, which increases the efficiency and quality of the scanning.

Color contamination matrix property assessment for improvement of colored smoke PIV

2021-09-22T17:11:11+00:00

A single-camera color PIV system that can acquire PIV data of three separated layers has been redesigned, purposing improvement of wind tunnel applicability. We target smoke image that has particle-per-pixel values higher than unity. The system constitutes of a high-power color-coding illuminator and a digital color high-speed video camera. RGB values in recorded image involves severe color contaminations due to five optical and digital sequences (Fig. 1). To quantify this, a snapshot calibration is proposed to describe the contamination matrix equation (Eq. (1)). Taking the inverse matrix (Eq. (2)) allows in-plane PIV in each color layer to be accurately implemented. We also derive mathematical limits to operate the colored smoke PIV, which is explained by the matrix property (Eq (3)). Feasibility of the proposed method has been demonstrated by application to a turbulent wake behind a Delta wing (Fig. 2) and also to a boundary layer flow along heated chocolate.

Using differential phase for 3D localization of tracer particles in digital inline holographic microscopy PIV/PTV (DIHM-PIV/PTV)

2021-09-22T17:56:52+00:00

Digital inline holographic microscopy PIV/PTV (DIHM-PIV/PTV) has the ability to provide 4-dimensional (4D), i.e. time-resolved, 3-component 3-dimensional (3C-3D) flow measurement with high spatial and temporal resolution, compact optical setup and minimal calibration Sun et al. (2020) compared to most other volumetric techniques such as tomo-PIV, defocusing PIV, etc. Despite all these advantages DIHMPIV/PTV has not yet developed into a standard laboratory tool due to some major limitations such as the extended depth-of-focus (DOF) problem and the virtual image effect which cause artefacts in the standard reconstruction volume limiting the seeding concentration and thus the achievable velocity spatial resolution. In order to mitigate the above-mentioned limitations we present a novel particle localization and extraction methodology which allows the minimization of these artefacts from the standard reconstruction and perform PIV/PTV analysis on the particle volume fields only. The proposed algorithm is based on the differential phase, which is the axial phase shift of the object wave compared to the reference plane wave propagation.

LaPIV using multigrid warping and proxy regularization

2021-09-22T18:08:01+00:00

The Lagrangian Particle Image Velocimetry (LAPIV) method was firstly proposed in Yang et al. (2019) as a prototype approach to achieve the goal of accurate and efficient reconstruction of 3D Eulerian velocity field of fluid flow from multi-view particle images. After validating against synthetic datasets, the prototype has already shown significant advantages in revealing more small scale flow structures than other stateof-the-art Eulerian velocity estimation methods, such as TomoPIV (Scarano, 2013) and VIC# (Jeon et al., 2019). However, at this early stage, LAPIV can not be easily applied to other datasets. In the current work, we focus on extending LAPIV to operational search by incorporating several essential and wellestablished paradigms: multi-resolution, warping, and proxy regularization. Recent approaches, Lasinger et al. (2019) and Cornic et al. (2020), function in the same vein as LAPIV, aiming at reconstructs the dense Eulerian volumetric flow directly from multi-view particle-seeded images Another pipeline consists of firstly reconstructing the Lagrangian flow using the Lagrangian Particle Tracking (LPT), then optimally interpolating the Lagrangian flow to Eulerian grids, taking into account the Eulerian dynamics constraints as in Flowfit (Gesemann, 2020) and VIC# (Jeon et al., 2019). If Eulerian flow is required, LAPIV is the preferred approach due to its simplicity and ability to utilize the original rich image features.

Three-Dimensional Particle Tracking Velocimetry using a Single Time-of-Flight Camera

2021-09-22T19:35:27+00:00

Time of Flight (ToF) cameras are a type of range-imaging camera that provides three-dimensional scene information from a single camera. This paper assesses the ability of ToF technology to be used for threedimensional particle tracking velocimetry (3D-PTV). Using a commercially available ToF camera various aspects of 3D-PTV are considered, including: minimum resolvable particle size, environmental factors (reflections and refractive index changes) and time resolution. Although it is found that an off-the-shelf ToF camera is not a viable alternative to traditional 3D-PTV measurement systems, basic 3D-PTV measurements are shown with large (6mm) particles in both air and water to demonstrate future potential use as this technology develops. A summary of necessary technological advances is also discussed.

Lagrangian Particle Tracking: a link between localization error and fraction of missed particles

2021-09-27T21:07:12+00:00

This paper aims at analysing the behaviour of particle localisation error in 3D Lagrangian Particle Tracking (LPT) techniques, with a particular emphasis on general properties, independent of a specific algorithm. Based on the hypothesis that in LPT algorithms, errors on the image formation models are solely due to random noise, we show/prove the existence of a best achievable root mean square error (RMSE) on particle localisation, that, for a setup at a given seeding density, depends only on the noise level. We provide a procedure to estimate this lower bound, and show that it can only be reached if there are no missed detections; further on, we establish a link between localisation error and fraction of missed particles. We illustrate the consistency of this model on the results of the recent First Challenge on LPT (see ISPIV21 papers by Leclaire et al. and Sciacchitano et al.)

A novel volumetric velocity measurement method for small seeding tracers in large volumes

2021-09-28T16:21:10+00:00

This paper presents the volumetric velocity measurement method of small seeding tracer with diameter 5µm ∼ 100µm for volumes of ≥ 500cm³. The size of seeding tracer is between helium-filled soap bubbles (HFSB) and di-ethyl-hexyl-sebacic acid ester(DEHS) droplets. The targeted measurement volume dimension is equivalent to the volume of HFSB, which will give a higher resolution of turbulence study. The relations between particle size, imaging and light intensity are formulated. The estimation of the imaging results is computed for the setup design. Finally, the methodology is demonstrated for turbulence velocity measurements in the jet flow, in which the velocities of averaged diameter 15µm air filled soap bubbles are measured in a volume of 7200cm³.

Peregrine falcon wakes examined using Volumetric PIV

2021-09-28T21:22:40+00:00

This study presents time-resolved volumetric measurements in the wake of a peregrine falcon model. The experiments were performed in a water flume with a freestream velocity of 10 cm/s and at an angle of 3.25°. The TSI volumetric PIV system, using Insight V3V-4G software, was used to capture the time-resolved volumetric flow field. The results compare well with previous Stereo PIV measurements; however, the present results also provide true 3-dimensional flow field information which helps decode the reason for the superior maneuverability. This is attributable to the vortex dominated flow field promoted by its morphology.

Main results of the first Lagrangian Particle Tracking Challenge

2021-09-29T14:08:29+00:00

This work presents the main results of the first Lagrangian Particle Tracking challenge, conducted within the framework of the European Union’s Horizon 2020 project HOMER (Holistic Optical Metrology for Aero-Elastic Research), grant agreement number 769237. The challenge, jointly organised by the research groups of DLR, ONERA and TU Delft, considered a synthetic experiment reproducing the wall-bounded flow in the wake of a cylinder which was simulated by LES. The participants received the calibration images and sets of particle images acquired by four virtual cameras, and were asked to produce as output the particles positions, velocities and accelerations (when possible) at a specific time instant. Four different image acquisition strategies were addressed, namely two-pulse (TP), four-pulse (FP) and time-resolved (TR) acquisitions, each with varying tracer particle concentrations (or number of particles per pixel, ppp). The participants’ outputs were analysed in terms of percentages of correctly reconstructed particles, missed particles, ghost particles, correct tracks and wrong tracks, as well as in terms of position, velocity and acceleration errors, along with their distributions. The analysis of the results showed that the best-performing algorithms allow for a correct reconstruction of more than 99% of the tracer particles with positional errors below 0.1 pixels even at ppp values exceeding 0.15, whereas other algorithms are more prone to the presence of ghost particles already for ppp < 0.1. While the velocity errors remained contained within a small percentage of the bulk velocity, acceleration errors as large as 50% of the actual acceleration magnitude were retrieved.

An experimental investigation on stall flutter over a vertically mounted rigid finite wing

2021-09-29T14:32:34+00:00

As stall flutter has relevant engineering implications, such as in blades of wind turbine and HALE (highaltitude long-endurance aircraft). This work presents the experimental investigation of rigid wing setup in a closed-circuit wind tunnel having 2.1 m × 1.5 m test section. The experimental campaign reached stable and symmetrical LCO within the freestream range from 9 m/s up to 14 m/s (1.69 × 10⁵ < Re < 2.63 × 10⁵ ). Two techniques were used for position tracking: one mechatronic and one image-based. The latter used ‘shakethe-box’ method applied to a body, which has proven a successful approach as a non-intrusive tool.

Aerodynamics of a cycling wheel in crosswind by coaxial volumetric velocimetry

2021-09-29T14:54:01+00:00

The aerodynamic characteristics of a modern road cycling wheel in crosswind are studied through force measurements and 3D velocimetry in TU Delft’s Open Jet Facility. The performance of the 62 mm deep rim is evaluated for two tire profiles, and yaw angles up to 20^◦ . All measurements are executed at 12.5 m/s (45 km/h) freestream- and wheel-rotational velocity. The wheel’s rim-tire section in crosswind is found to behave similar to an airfoil at incidence, ultimately resulting in a reduction of the wheel’s aerodynamic resistance with increasing yaw angle magnitude. This trend, also referred to as the sail-effect, is limited by the stall angle of the tire-rim profile. The stall angle is found to be dependent on the tire surface texture and varies between 14^◦ and 20^◦.

Tracer-based 3D surface reconstruction

2021-09-29T15:03:42+00:00

The continuous advancements of particle imaging techniques for flow field measurements have led to imaging systems and processing approaches matching the demands for 3D velocimetry at large scale (Schanz et al., 2016; Discetti and Coletti, 2018). Often, the flow past an object immersed in the fluid is of key interest, and in some cases the experimentalist exploits the velocimetry data for analysis of the near-surface flow properties such as pressure. It follows that knowledge of the object shape and position is essential.

For 2D studies, the issue of identifying the fluid-solid interface often reduces to detection of the intensity gradient resulting from the light sheet striking the object. The latter task is well explored, with a variety of methods providing the object interface in the measurement plane (Canny, 1986; Malik et al., 2001; Gui et al., 2003, among others). These approaches, however, are not applicable in volumetric studies where the illumination is diffuse. A frequently applied alternative in fluid-structure-interaction studies is a dual-measurement approach, where a second measurement system tracks the object shape (e.g., Acher et al., 2019; Zhang et al., 2019) The complexity of operating two measurement systems may not be affordable however, motivating the development of 3D interface detection methods that rely solely on the flow imaging system.

Particle imaging based interface detection approaches in 3D have been addressed from various perspectives, containing the detection of fluid-fluid interfaces. Examples utilizing tomographic PIV measurements include the studies of Adhikari and Longmire (2012), Im et al. (2014), and Ebi and Clemens (2016). The two latter examples identify the fluid-solid interface by discriminating a seeded phase (the fluid) from a void phase (the solid). The present work is inspired by this principle, but it assumes a discrete 3D particle distribution as obtained from a generic particle tracking algorithm such as IPR by Wieneke (2012), or “Shake-The-Box” by Schanz et al. (2016), as a foundation. Summarizing, this work aims to detect the surface of a solid object immersed in a seeded flow, solely based on the spatial distribution of flow tracers as recorded by a generic 3D PTV measurement.

Large-scale 3D flow investigations around a cyclically breathing thermal manikin in a 12 m³ room using HFSB and STB

2021-09-29T16:30:38+00:00

Exhalation of small aerosol droplets and their transport, dispersion and (local) accumulation in closed rooms have been identified as the main pathway for indirect or airborne respiratory virus transmission from person to person, e.g. for SARS-CoV 2 or measles (Morawska and Cao 2020). Understanding airborne transport mechanisms of viruses via small bio-aerosol particles inside closed populated rooms is an important key factor for optimizing various mitigation strategies (Morawska et al. 2020), which can play an important role for damping the infection dynamics of any future and the ongoing present pandemic scenario, which unfortunately, is still threatening due to the spreading of several SARS-CoV2 variants of concern, e.g. delta (Kupferschmidt and Wadman 2021). Therefore, a large-scale 3D Lagrangian Particle Tracking experiment using up to 3 million long lived and nearly neutrally buoyant helium-filled soap bubbles (HFSB) with a mean diameter of ~ 370 µm as passive tracers in a 12 m³ generic test room has been performed, which allows to fully resolve the Lagrangian transport properties and flow field inside the whole room around a cyclically breathing thermal manikin (Lange et al. 2012) with and without mouth-nose-masks and shields applied. Six high-resolution CMOS streaming cameras, a large array of powerful pulsed LEDs have been used and the Shake-The-Box (STB) (Schanz et al. 2016) Lagrangian particle tracking algorithm has been applied in this experimental study of internal flows in order to gain insight into the complex transient and turbulent aerosol particle transport and dispersion processes around seated breathing persons.

Robust approach to monitoring Lagrangian transport in very large volume

2021-09-29T16:43:45+00:00

State-of-the-art flow measurements utilize four or more high-speed cameras to perform highly-accurate Lagrangian particle tracking (LPT) in small to medium-sized measurement volumes (Schanz et al., 2016). Hou et al. (2021) suggested a novel approach to allow measurements in significantly larger measurement volumes (O(10m³)) while reducing the experimental effort. A single camera is used to track centimeter-sized soap bubbles in three dimensions by not only evaluating the bubble-center location but also the bubbleimage size. Possible applications of the suggested approach include - but are not limited to - measurements in industrial wind tunnels (Hou et al., 2021), full-scale measurements in the atmospheric boundary layer (Rosi et al., 2014; Toloui et al., 2014), and the characterization of airflow in indoor spaces, such as offices or classrooms (Kahler et al., 2020). In the context of the recent pandemic, the latter application could ¨help to reduce infection risk by designing appropriate air circulation. Hereby, frequent air exchange is recommended, while direct airflow from individual to individual should be avoided (WHO, 2020). The present study strives to optimize and simplify the experimental set-up as well as to characterize the accuracy of the novel single-camera approach. Figure 1(a) shows the set-up used to characterize the novel approach.

Spatially and temporally resolved measurements of turbulent Rayleigh-Bénard convection by Lagrangian particle tracking of long-lived helium-filled soap bubbles

2021-09-29T16:48:35+00:00

We present spatially and temporally resolved velocity and acceleration measurements of turbulent RayleighBénard convection spanning the whole volume (~ 1 m³) of a cylindrical sample with aspect ratio one. With the "Shake-The-Box" (STB) Lagrangian particle tracking (LPT) algorithm, we were able to instantaneously track up to 560,000 particles, corresponding to mean inter-particle distances down to 6 - 8 Kolmogorov lengths. We used the data assimilation scheme ‘FlowFit’, which involves continuity and Navier-Stokesconstraints, to map the scattered velocity and acceleration data on cubic grids, herewith recovering the smallest flow scales. Lagrangian and Eulerian visualizations reveal the dynamics of the large-scale circulation and its interplay with small scale structures, such as thermal plumes and turbulent background fluctuations. As a result, the complex time-dependent behavior of the LSC comprising azimuthal rotations, torsional oscillation and sloshing can be extracted from the data. Further, we found more seldom dynamic events, such as spontaneous reorientations of the LSC in the data from long-term measurements.

Particle position prediction based on Lagrangian coherency for flow over a cylinder in 4D-PTV

2021-09-29T17:14:57+00:00

Recent developments in time-resolved Particle Tracking Velocimetry (4D-PTV) consistently improved tracking accuracy and robustness. We propose a novel technique named ”Lagrangian coherent predictor” to estimate particle positions within the 4D-PTV algorithm. We add spatial and temporal coherency information of neighbour particles to predict a single trajectory using Lagrangian Coherent Structures (LCS). We found that even a weak signal from coherent neighbour motions improves particle prediction accuracy in complex flow regions. We applied Finite Time Lyapunov Exponent (FTLE) to quantify local boundaries (i.e. ridges) of coherent motions. Synthetic analysis of the wake behind a smooth cylinder at Reynolds number equal to 3900 showed enhanced estimation compared with the recent predictor functions employed in 4D-PTV. Results of the experimental study of the same flow configuration are reported. We compared predicted positions with the optimised final positions of Shake The Box (STB). It was found that the Lagrangian coherent predictor succeeded in estimating particle positions with minimum deviation to the optimised positions.

Large-scale structures of scalar and velocity in a turbulent jet flow

2021-09-01T15:43:52+00:00

We study the relation between large-scale structures in the concentration field with those in the velocity field in a dye-seeded turbulent jet. The scalar concentration in a plane is measured using laser-induced fluorescence. Uniform concentration zones of an advected scalar are identified using cluster analysis. We simultaneously measure the two-dimensional velocity field using particle image velocimetry. The structures in the velocity field are characterized by finite-time Lyapunov exponents. The measurement of the scalarand velocity fields moves with the mean flow. In this moving frame, turbulent structures remain in focus long enough to observe well-defined ridges of the finite-time Lyapunov field. This field gauges the rate of point separation along Lagrangian trajectories; it was measured both for future and past times since the instant of observation. The edges of uniform concentration zones are correlated with the ridges of the past-time Lyapunov field, but not with those of the future-time Lyapunov field.

An Experimental Investigation on the Wind-driven Runback Motion of Water Droplets over Solid Surfaces with Different Wettabilities

2021-09-01T20:39:47+00:00

Aircraft icing is widely recognized as one of the most serious weather hazards to flight safety. Specially designed hydro-/ice-phobic coatings are currently undergoing development for aircraft icing mitigation. It was found that hydro-/icephobic coatings would delay the ice accretion iover airframe surfaces so that the impacted supercooled water droplets could be blown away by the airflow from the airframe surface before being frozen into ice. It is of fundamental importance to understand the wind-driven runback behavior of water droplets over surfaces treated with different coatings, since the corresponding knowledge would be very helpful and essential to develop more efficient anti-/de-icing systems for aircraft icing protection.

With the rapid development of surface engineering, a series of specially designed surface coatings succeed in icing mitigation using airflow to remove the remained water. While various hydro-/ice-phobic coatings/surfaces have been developed in recent years, the “state-of-the-art” icephobic coatings/surfaces can be generally divided into three categories, i.e., 1). Lotus-leaf-inspired superhydrophobic surfaces (SHS) with micro-/nano-scale surface textures to achieve very high contact angles (typically > 150°); 2). Pitcherplant-inspired slippery liquid infused porous surfaces (SLIPS) with a layer of liquid lubricant (which is immiscible with water) being sandwiched between ice and solid substrate materials; and 3). Icephobic elastic materials/surfaces with deformable structures/surfaces. SHS has a water droplet contact angle (CA) larger than 150° and a sliding angle (SA) less than 10° . SHS always has a hierarchical structure which is similar to the lotus leaf, and water droplets on SHS appear as water beads which can easily roll off the surface by wind or gravity before frozen. Another strategy to reduce ice adhesion strength to a solid surface is to use a layer of liquid lubricant, which is immiscible with water, between ice and the solid surface. The use of such lubricated surfaces was investigated as early as 1960s, and has gained increasing attentions again recently with the introduction of a concept called Slippery Liquid-Infused Porous Surfaces (SLIPS). SLIPS concept is inspired by the Nepenthes pitcher plants, which have evolved highly slippery, liquidinfused micro-textured rim to capture insects. SLIPS surfaces were not only found to be able to suppress ice/frost accretion by effectively removing condensed moisture even in high humidity conditions, but also exhibit at least an order of magnitude lower ice adhesion than most SHS coatings. More recently, elastic materials/surfaces, such as Polydimethylsiloxane or PDMS in short, which would be structurally deformed/altered dynamically upon applying extra mechanical stress, have also been suggested for icing mitigation. Elastic materials display ultra-low adhesion to ice due to their low work of adhesion and liquidlike deformability, while maintaining good mechanical durability due to their solid-like rigidity. It is found that water droplets would not only be more readily rebounding away from the surface after impingement, but also be able to roll away before frozen due to the hydrophobicity of PDMS. Considering the differences in wettabilities and mechanisms of water repellency, it is necessary to have a systematic understanding of how efficient the surfaces are when the aerodynamic force is applied to remove the adhered water droplets. In the present study, a comprehensive experimental campaign was conducted to characterize the transient runback behaviors of wind-driven water droplets over the surfaces of test plates coated with different hydro-/icephobic coatings (i.e., SHS, SLIPS and PDMS). A high-resolution Particle Image Velocimetry (PIV) system was used to achieve quantitative measurements of the velocity field of the airflow around the wind-driven water droplets on the test surfaces with different wettabilities. With the detailed PIV measurements of the airflow field around the runback water droplets and the droplet profiles, the aerodynamic forces and the adhesion forces acting on the water droplets were estimated. While Fig. 1 shows the experimental setup used in the present study, Fig. 2 to Fig. 3 given some of the typical measurement results. More measurement results and comprehensive analysis and discussions will be provided in the full version of this research paper.

Measurement of Lagrangian Trajectories in a 3 L Stirred Tank Reactor using 4D Particle Tracking Velocimetry with Shake-the-Box

2021-09-08T19:27:32+00:00

Stirred tank reactors are widely used in the chemical industry and bioprocess engineering and, consequently, a large number of scientific publications deal with the characterization of those apparatuses. However, there is very little information about the flow conditions. This is mostly due to the fact that these apparatuses are generally made of stainless steel, which restricts optical access. Furthermore, three-dimensional flow field measurements are still not trivial and involve costly equipment, therefore, investigations often reduce to two-dimensional PIV measurements. Nevertheless, recent works (Rosseburg et al., 2018; Taghavi and Moghaddas, 2020; Kuschel et al., 2021) impressively show the formation of compartments which hinder and delay mixing. However, these measurements are based either on instantaneous concentration profiles by means of pLIF measurements or on a two-dimensional projection of the system and thus do not allow conclusions about the development of the three dimensional compartments and the exchange rates between the compartments. In this work, for the first time, instantaneous flow field measurements with high spatial and temporal resolution are performed in the entire volume of a 3L stirred tank reactor based on 4D particle tracking velocimetry. The highly resolved particle trajectories further allow detailed Lagrangian analysis of the mixing dynamics inside the reactor, data that was previously inaccessible.

Simultaneous PIV and DIC Measurements in a Towing Tank Environment with a Flexible Hydrofoil

2021-09-01T18:47:42+00:00

Due to their good mechanical properties composite materials are increasingly applied for the construction of lifting surfaces in the maritime industry. However, besides improving the strength to weight ratio of a structure, the anisotropic material properties can also exhibit bend-twist coupling, when exposed to higher loads. In order to experimentally measure the fluid structure interaction, the object of investigation needs to exposed to the same fluid loadings, as it would experience during operation. To investigate the possibility to obtain simultaneous deformation and flow field measurements in a large hydrodynamic testing facility simultaneous PIV and DIC measurements are performed to obtain the deformation of a flexible NACA 0008 hydrofoil and to measure the flow field in the wing tip region. For the assessment of the performance of the methods two scenarios are presented including tests in stationary conditions with constant angles of attack and forced plunging oscillations. The calibration of both measurement systems is done independently and the wing tip, visible in the PIV images, is used for triangulation to find the position of the wing within the PIV coordinate system. The combination of both measurement techniques allows for an accurate determination of tip vortex center positions with respect to the deformed wing and their evolution downstream of the wing. During forced plunging motions, the phase lag of the wing tip and the influence on the wing tip vortex is observed.

Large Scale PIV for confined fires

2021-09-02T20:09:27+00:00

Fire safety engineering, including knowledge of fire dynamics and fire-related hazards is crucial for securing people as well as rescue teams during interventions. One of the main critical aspects remains in determining the smoke dynamics at openings where fresh air and hot fumes mix. This particular phenomenon, encountered in many enclosures fires can reveal either well ventilated or under-ventilated fires. The response techniques of rescue teams are different depending on the ventilation status. Merci et al. (2016), Bengtsson et al. (2001) and Pretrel et al. (2012) have studied fire in enclosures that occur in oxygen-limited conditions. Generally, smoke dynamics are studied by using different devices or techniques. These include, among others, Pitot probes and bidirectional probes or McCaffrey probes, McCaffrey and Heskestad (1976). However, these probes are intrusive and potentially affecting the smoke dynamics. Moreover, only one-point data are evaluated. To overcome this difficulty, laser techniques such as PIV can be set up, see Tieszen et al. (2002) , Hou et al. (1996) or Koched et al. (2012). PIV technique has already been used in case of well-ventilated and under-ventilated fires conditions. A natural extension of this technique remains in applying the PIV technique close to the outlet of the container in order to highlight exchanges between hot exhaust fumes and fresh incoming air.

The objectives of the paper remain threefold:
1. First, we propose a specific design of enclosure fire to ensure large scale PIV measurements inside the enclosure.
2. Second, the transition from ventilated to under ventilated fire conditions is evaluated

Effects of Particle Properties on Visualizing Flows in a Two- Stage Electrostatic Precipitator Using Particle Image Velocimetry

2021-09-02T20:34:23+00:00

The amount of particulate matter (PM) in the environment has been confirmed to be health risks on human bodies[1, 2], and therefore removing suspended particles has become the research goal of many studies. Electrostatic precipitator (ESP) is one of the high-efficiency particle collection technologies[3-7]. Particle Image Velocimetry (PIV) has been an effective tool for visualizing the flow patterns in experimental fluid mechanics, and many studies adopted this technique to study flows in ESP[8-10]. However, particles charged by the electric field can cause deviation in measurement results since it does not follow the ionized air flow which can be charged differently from the tracer particles. In this study, the observation of the effects of different particle properties on flow field in a two-stage ESP is the objectives of this study. A two-stage ESP was built and four different seeding particles, aluminum oxide (Al₂O₃) particle, oil droplet particle, sodium chloride (NaCl) particle, and titanium dioxide (TiO₂) particle, are tested in the current study. In this study, the streamwise velocity of the flows ranges from 2.36 m/s to 4.18 m/s, the voltage of the corona electrode varies from 8 kV to 12 kV with a positive polarity, and the voltage of the collector electrode is fixed at 16 kV. To investigate the 3-D flow patterns inside the channel, data at different planes were taken for comparison. The results show that by increasing charge voltage from 8 kV to 12 kV with a streamwise flow velocity the 2.36 m/s, the y-component velocity for Al₂O₃ particle, oil droplet particle, NaCl particle and TiO2 particle increased by 50.6%, 76.0%, 33.5% and 51.9%, respectively. Moreover, for the case of the 4.18 m/s primary flow, the y-component velocity for Al₂O₃ particle, oil droplet particle, NaCl particle and TiO₂ particle increase by 52.7%, 59.2%, 59.4% and 65.9% after the voltages increase from 8 kV to 12 kV. PIV results for oil droplet particle shows slower y-component velocities, which can be due to the lower Archimedes number of 3.12E-06 and the mobility number that is larger than 3. On the contrary, in most of results from TiO₂ particles show high y-component velocity, which is due to the highest Archimedes number of 1.15E-03 of the seeding particles tested in this study. This result shows that the particle is less affected by buoyancy effect. The PIV results of the middle plane also shows that the ycomponent of velocity from -2.6 m/s to -0.5 m/s, in contrast to -1.0 m/s to 1.0 m/s from the near wall observation plane. These results are consistent to simulation results of the electric field distribution, whichshows unequal electric field strengths between the middle and near wall regions of the test section. Only half of the cage shape distribution of the electric field can be observed, and primary flow influences the ionic wind to move to the downstream area. Based on the results, the oil droplet and TiO₂ particles are more suitable for the role of tracer particles compared to aluminum oxide and sodium chloride particles.

High Speed PIV Measurements in Water Hammer

2021-09-08T20:38:55+00:00

An experimental study addressing the challenge to measure relaxation coefficient of very fast phenomena such as water hammers is presented. An acrylic projectile containing water is accelerated and impacts a metal wall creating a water hammer. State of the art laser measurements techniques will be deployed in order to achieve such goal. A compressed air custom built cannon is used to accelerate the projectile and create the impact leading to the water hammer. First experimental results for Shadowgraphy and PIV measurements are presented and discussed with focus on the future development for the presented facility.

The Stereo-PIV investigation of the unsteady flow in the draft tube of a model hydro turbine

2021-09-15T21:23:51+00:00

Varying the generator load of a hydro turbine results in short-term changes in the rotation frequency of the runner, leading inevitably to flow instability and strong flow swirling behind the turbine. This may lead to the formation of unsteady flow regimes featured by vortex instability of the swirling flow behind the runner, known as the precessing vortex core (PVC) Dorfler et al. (2012). This effect causes dangerous periodic pressure pulsations that propagate throughout the water column in the draft tube. The present study reports on stereo PIV measurements of the air flow field inside a transparent draft tube of a model hydro turbine for a wide range of operation conditions. The research is focused on the time-averaged flow properties (mean velocity field and the second-order moments of velocity fluctuations), pressure pulsations and coherent flow structures in the velocity field.

Simultaneous Particle Tracking and Temperature Measurements during Additive Manufacturing using a High-speed Spectral Plenoptic Camera

2021-09-16T14:27:15+00:00

In this work, a high-speed spectral plenoptic camera was used for three-dimensional (3D) simultaneous particle tracking and pyrometry measurements of hot spatter particles ejected during the metal additive manufacturing process. Additive manufacturing (AM) has an increasing role in the aerospace, energy, medical and automotive industry (DebRoy et al., 2018). While this new technology enables the production of highly advanced parts, research on the fundamental mechanisms governing the laser-matter interactions are an ongoing challenge because of the spatial and temporal resolution inherent to the AM process. One challenge is the characterization of spatter particles ejected from the melt pool, as these particles can be incorporated into the final part affecting the mechanical properties (Deng et al., 2020). One potential solution for simultaneously measuring velocity and temperature of the spatter particles is the spectral plenoptic camera.

Energy spectra of Sub-Surface Velocity Fields Beneath Faraday Waves

2021-09-20T19:58:19+00:00

Faraday waves form on the surface of a fluid which is subject to vertical forcing, and are researched in a large range of applications. Some examples are the formation of ordered wave patterns and the controlled walking or orbiting of droplets (Couder et al. (2005); Saylor and Kinard (2005)). Moreover, recent studies discovered the existence of a horizontal velocity field at the fluid surface, called Faraday flow, which was shown to exhibit an inverse energy cascade and thus properties of two-dimensional turbulence (von Kameke et al., 2011, 2013; Francois et al., 2013). Additionally, three-dimensionality effects have been part of recent investigations in quasi-2D flows (both electromagnetically-driven (Kelley and Ouellette, 2011; Martell et al., 2019) or produced by parametrically-excited waves (Francois et al., 2014; Xia and Francois, 2017)). Furthermore, the occurrence of an inverse cascade in thick layers is also subject of current studies on the coexistence of 2D and 3D turbulence (Biferale et al., 2012; Kokot et al., 2017; Biferale et al., 2017). By performing 2D PIV measurements at horizontal planes beneath the Faraday waves, we recently showed that pronounced three dimensional flows occur in the bulk, with much larger spatial and temporal scales than those on the surface (Colombi et al., 2021), when the system is not shallow in comparison to typical length scales of the surface flow (fluid thickness exceeding half the Faraday wavelength λ_F). This in turn reveals that an inverse energy cascade and aspects of a confined 2D turbulence can coexist with a three dimensional bulk flow. In this work, 2D PIV measurements of the velocity fields are carried out at a vertical cross-section xz-plane and at four distinct horizontal xy-planes at different depths in Faraday waves. The results reveal that small and fast vertical jets penetrate from the surface into the bulk with fast accelerating bursts and strong momentum transport in the z−direction. Furthermore, the fraction of flow kinetic energy in the vertical direction is found to peak inside a layer of approximately 10 mm (one Faraday wavelength) below the fluid surface.

Time-resolved flow field investigation in an industrial centrifugal compressor application involving TR-PIV synchronized with unsteady pressure measurements

2021-09-21T14:24:44+00:00

We report on combined velocity and unsteady pressure measurements obtained on an radial compressor with vaneless diffuser and asymmetric volute. Time-resolved PIV recordings were acquired at 26 kHz both upstream of the impeller as well as within the vaneless diffusor at several rotation speeds at clean conditions and prior to the onset of instabilities within the rotor. The velocity data was acquired with a high-repetition rate, double-pulse laser system consisting of two combined DPSS lasers and a high-speed CMOS camera that was synchronized with multi-point unsteady pressure measurements. Details on the facility, the utilized instrumentation and data processing are provided with particular focus on the spectral and coherence analysis. Power spectra obtained from time records of the inlet velocity and unsteady pressure reveal an increase of low-frequency fluctuations below the blade passing frequency and the occurrence of a mode-locked behaviour indicating the presence of rotating instabilities. High levels of correlation between velocity and unsteady pressure signals not only confirm the temporal coherence of the acquired data but also reveal a direct coupling between flow field and pressure signature that is more prominent upstream of the rotor rather than in the diffusor.

Experimental Study of Darrieus Turbines in Confined Free-Surface Flows

2021-09-21T16:52:49+00:00

Both scientific and industrial communities have a growing interest for marine renewable energies. There is a wide variety of technologies in this domain, with different degrees of maturity. Our work focuses on two models of an H-type vertical axis Darrieus (1931) tidal turbine with the objective of studying the effect of fluid-structure interactions on their performances. The
experimental studies were carried out on the 15m long and 1m² square section free surface channel of the Environmental Hydrodynamics Platform at PPRIME Institute (Figure 1). The maximum flow discharge through the channel is Q = 500 l/s.

Applications of Particle Tracking Velocimetry to severe nuclear accident experimentation

2021-09-23T13:56:42+00:00

Experimental research into severe nuclear accidents may entail the discharge of a very high-temperature lava-like molten fuel mixture, corium, either into a pool of less-dense, more-volatile coolant or onto a solid substrate where the corium will spread and cool. In both instances, remote, high-speed video imaging is usually required to interpret these transient interactions and PTV represents a powerful tool for the characterisation of the dynamic properties of discrete melt fragments or distinctive features in the surface of the melt during spreading. Nuclear fuel-coolant interactions present particular challenges for PTV analysis as a molten jet and its fragments can exhibit high rates of inter-frame deformation and undergo fragmentation with a relatively high frequency. A PTV algorithm, adapted to these challenges, is presented whereby a user-defined tolerance in the evolution of certain particle properties is used to refine the potential candidate particles prior to particle matching. This candidate refinement step is used to distinguish between acceptable levels of deformation between successive sightings of a given particle, and more substantial changes consistent with fragmentation or coalescence, requiring the tracking of a new particle. Implementation of the PTV algorithm is presented for (1) an X-ray video from the FCINA-30-1 experiment between a jet of molten stainless steel and liquid sodium, conducted at the JAEA’s MELT facility, and (2) video imaging of the VE-U9-ceramic experiment of a molten corium-thermite mixture spreading on a zirconium substrate, conducted at the CEA’s VULCANO facility. The latter case-study enabled the characterization of > 70,000 local velocity vectors at locations corresponding to distinctive temperature heterogeneities in the surface of the spreading melt, providing extensive insight into the spreading dynamics for the validation of corium spreading models.

Dense flow field interpolations from PTV data in the presence of generic solid boundaries

2021-09-01T18:59:21+00:00

Three-dimensional flow measurements by Particle Tracking Velocimetry (PTV) provide scattered flow information, that often needs to be interpolated onto a regular grid. Therefore, the use of experimental data assimilation approaches such as VIC+ (Schneiders and Scarano, 2016) were proposed to enhance the instantaneously available spatial resolution limits beyond that of the PTV measurements. Nevertheless, there exists no prior attempt to perform the data assimilation when the flow is in direct contact with physical objects. Thus, in order to handle generic solid body intrusions within the flow fields of VIC+ application, the utilization of Arbitrary Lagrangian-Eulerian and immersed boundary treatment approaches of the computational fluid-structure interaction (FSI) frameworks are proposed. The introduced variants over the standard VIC+ are assessed with a high fidelity numerical test case of flow over periodic hills. The accuracy superiority of the flow field reconstructions with the proposed approaches are denoted especially in close proximity of the interaction surface. An experimental application of the introduced methods is demonstrated to compute the pressure distribution over an unsteadily moving elastic membrane surface, revealing the time-resolved interaction between the flow structures and the membrane deformations.

Time-resolved Velocity Estimation from Inflow Pressure Measurements in a Subsonic Jet Using Machine-Learning Methods

2021-09-08T14:31:49+00:00

The goal of this study is to estimate aspects of the time-resolved (TR) velocity field that is associated with pressure fluctuations measured in a subsonic jet using machine learning (ML) approaches. The experiments were conducted in the Anechoic Jet Test Facility at the University of Florida using a round converging nozzle operated at at a Mach number of 0.3 and Re_D = 3.8 × 10⁵. Planar PIV was utilized to record nonTR, 2D velocity snapshots on the streamwise plane. A B&K 4138 1/8” microphone and a GRAS 46DD 1/8” microphone were employed to measure inflow pressure fluctuations synchronously with the PIV. Both microphones were equipped with aerodynamically-shaped nosecones and were placed on the upper and lower jet liplines. The nosecone tips were streamwisely aligned and were placed just downstream of the PIV window (see Figure 1(a)). Pressure signals were recorded synchronously with PIV, but at different sampling rates, 80 kHz and 12 Hz, respectively. A total of 8000 PIV snapshots were acquired in the experiment.

Approximate Bayesian framework for 3D reconstruction in a volumetric PIV/PTV measurement

2021-09-08T20:24:47+00:00

Non-invasive flow velocity measurement techniques like volumetric Particle Image Velocimetry (PIV) (Elsinga et al., 2006; Adrian and Westerweel, 2011) and Particle Tracking Velocimetry (PTV) (Maas, Gruen and Papantoniou, 1993) use multi-camera projections of tracer particle motion to resolve three-dimensional flow structures. A key step in the measurement chain involves reconstructing the 3D intensity field (PIV) or particle positions (PTV) given the projected images and known camera correspondence. Due to limited number of camera-views the projected particle images are non-unique making the inverse problem of volumetric reconstruction underdetermined. Moreover, higher particle concentration (>0.05 ppp) increases erroneous reconstructions or “ghost” particles and decreases reconstruction accuracy. Current reconstruction methods either use voxel-based representation for intensity reconstruction (e.g. MART (Elsinga et al., 2006)) or a particle-based approach (e.g. IPR (Wieneke, 2013)) for 3D position estimation. The former method is computationally intensive and has a lesser positional accuracy due to stretched shape of the reconstructed particle along the line of sight. The latter compromises triangulation accuracy (Maas, Gruen and Papantoniou, 1993) due to overlapping particle images for higher particle concentrations. Thus, each method has its own challenges and the error in 3D reconstruction significantly affects the accuracy of the velocity measurement. Though, other methods like maximum-a-posteriori (MAP) estimation have been previously developed (Levitan and Herman, 1987; Bouman and Sauer, 1996) for computed Tomography data, it has not been explored for PIV/ PTV 3D reconstruction. Here, we use a MAP estimation framework to model and solve the inverse problem. The cost function is optimized using a stochastic gradient ascent (SGA) algorithm. Such an optimization can converge to a better local maximum and also use smaller image patches for efficient iterations.

Unsupervised Recurrent All-Pairs Field Transforms for Particle Image Velocimetry

2021-09-21T14:41:17+00:00

Convolutional neural networks have been successfully used in a variety of tasks and recently have been adapted to improve processing steps in Particle-Image Velocimetry (PIV). Recurrent All-Pairs Fields Transforms (RAFT) as an optical flow estimation backbone achieve a new state-of-the-art accuracy on public synthetic PIV datasets, generalize well to unknown real-world experimental data, and allow a significantly higher spatial resolution compared to state-of-the-art PIV algorithms based on cross-correlation methods. However, the huge diversity in dynamic flows and varying particle image conditions require PIV processing schemes to have high generalization capabilities to unseen flow and lighting conditions. If these conditions vary strongly compared to the synthetic training data, the performance of fully supervised learning based PIV tools might degrade. To tackle these issues, our training procedure is augmented by an unsupervised learning paradigm which remedy the need of a general synthetic dataset and theoretically boosts the inference capability of a deep learning model in a way being more relevant to challenging real-world experimental data. Therefore, we propose URAFT-PIV, an unsupervised deep neural network architecture for optical flow estimation in PIV applications and show that our combination of state-of-the-art deep learning pipelines and unsupervised learning achieves a new state-of-the-art accuracy for unsupervised PIV networks while performing similar to supervisedly trained LiteFlowNet based competitors. Furthermore, we show that URAFT-PIV also performs well under more challenging flow field and image conditions such as low particle density and changing light conditions and demonstrate its generalization capability based on an outof-the-box application to real-world experimental data. Our tests also suggest that current state-of-the-art loss functions might be a limiting factor for the performance of unsupervised optical flow estimation.

Enhanced data assimilation of 4D LPT with physics informed neural networks

2021-09-21T15:15:35+00:00

According to recent trend of explosive growth of computation power and accumulated data, demand for the deep learning application in various research fields is increasing. As following this trend, remarkable achievements are presented in the experimental fluid mechanics field. One of the most outstanding research is Physics Informed Neural Networks (PINN) Raissi et al. (2020). Physical knowledge, which has been accumulated by humans, is imposed on the neural networks. PINN was used the automatic differentiation for implementing the governing equations as a physical constraint. By utilizing this concept, physical constraints make neural networks finding physical meaning of phenomena instead of simply fitting to the label data.

A network-based perspective on coherent structure detection from very-sparse Lagrangian data

2021-09-21T20:15:52+00:00

Coherent structure detection (CSD) is a long-lasting issue in fluid mechanics research as the presence of spatio-temporal coherent motion enables simpler ways to characterize the flow dynamics. Such reducedorder representation, in fact, has significant implications for the understanding of the dynamics of flows, as well as their modeling and control (Hussain, 1986). While the Eulerian framework has been extensively adopted for CSD, Lagrangian coherent structures have recently received increasing attention, mainly driven by advancements in Lagrangian flow measurement techniques (Haller, 2015; Hadjighasem et al., 2017). Lagrangian particle tracking (LPT), in particular, is widely used nowadays due to its ability to quantity fluid-parcel trajectories in three-dimensional volumes (Schanz et al., 2016).

Investigating Optimal Training and Uncertainty Quantification for CNN-based Optical Flow

2021-09-21T20:29:41+00:00

Optical Flow (OF) techniques provide “dense estimation” flow maps (i.e. pixel-level resolution) of timecorrelated images and thus are appealing to applications requiring high spatial resolutions. OF methods revolve around mathematical descriptions of the image as a collection of features, in which the pixel-level light intensity is the primary variable (Horn and Schunck, 1981). Feature tracking often involves the notion of scale invariance. Traditional OF approaches, merely based on mathematical formulations, have suffered from many challenges, especially when directly applied to images of fluid flows textured with tracer particles (hereafter PIV-like images). Due to the limited number of computationally manageable features and suboptimal regularization methods, successful implementation of past approaches has been limited to highly textured images and small displacement dynamic ranges.

On the determination of 3D position and orientation of spheroidal particles using defocusing and deep learning

2021-09-21T21:29:18+00:00

Tracking the 3D position of tracer particles or small objects like cells or unicellular organisms in miniaturized lab-on-a-chip or biomedical devices is complicated since it is often not possible in these setups to use multi-camera approaches. Most successful single-camera approaches for these applications are based on holography or defocusing. Holographic methods have been used to track complex objects such has bacteria (Bianchi et al. (2019)) and even to estimate their orientation (Wang et al. (2016)). However, these methods require a complex and expensive experimental setup which is not always available in research laboratories. On the other hand, defocusing methods work with conventional microscopic optics, are easy to implement, and have shown excellent results in 3D PTV experiments (Qiu et al. (2019)). One main drawback is that they normally work only with spherical and mono-dispersed tracer particles. A defocusing method that has potential to measure non-spherical particles is the General Defocusing Particle Tracking (Barnkob and Rossi (2020)) which is based on pattern recognition and can be conceptually extended to more complex tasks by extending the reference library of particle images, including not only spherical particles at different depth positions, but also non-spherical particles at different orientations. However, whether this approach could work in practice is still unknown. First, is the information contained in simple defocused images sufficient to reconstruct depth and orientation of non-spherical particles, and eventually under which circumstances? Second, how to practically collect the labelled reference images?

Eulerian time-marching in Vortex-In-Cell (VIC) method: reconstruction of multiple time-steps from a single vorticity volume and time-resolved boundary condition

2021-09-22T21:41:50+00:00

A data assimilation approach is proposed to enhance the dynamic range of the Vortex-In-Cell (VIC) method by simulating future- and past- instances. The VIC method mainly considers a vorticity field from which velocity and acceleration fields are calculated through Poisson equations, respectively bounded by prescribed conditions. In addition, a vorticity time derivative is also available by the vorticity transport equation. The proposed approach focuses on such already available data, i.e., the vorticity and its time derivative fields, for simulating additional instances and getting feedbacks from the corresponding measurement instances, e.g., particle image velocimetry (PTV). However, the self-simulated flow field can be depleted due to a lack of incoming information, which is out of the reconstruction domain at the source instance. To supply that kind of information and thus sustain the simulation, boundary conditions of the simulated instances are required and considered. As a result, the proposed approach can gather corrections from multiple PTV instances while optimizing a single vorticity volume and time-resolved boundary conditions. Since the boundary grid points are much smaller in number than that of the whole volume, one can expect an increased dynamic range. A former work, VIC# (Jeon et al. 2018), which supplements additional constraints and coarse-grid approximation to VIC+ (Schneiders and Scarano 2016), is selected as a 3D method to which the proposed 4D approach is applied. Two explicit Eulerian time-marching methods are tested as a simulation scheme: the forward Euler and the Runge-Kutta methods. A numerical assessment is conducted using the synthetic PTV data, whose ground truth is known, and returns reconstruction qualities based on the velocity and the identified vortical structures. Other practical features regarding convergence and computation complexity are also reported. To visually verify an improvement by the proposed approach, two kinds of time-resolved Shake-the-Box (STB) measurements, which were acquired in high-speed systems, are processed and discussed.

GANs-based PIV resolution enhancement without the need of high-resolution input

2021-09-23T14:03:52+00:00

A data-driven approach to reconstruct high-resolution flow fields is presented. The method is based on exploiting the recent advances of SRGANs (Super-Resolution Generative Adversarial Networks) to enhance the resolution of Particle Image Velocimetry (PIV). The proposed approach exploits the availability of incomplete projections on high-resolution fields using the same set of images processed by standard PIV. Such incomplete projection is made available by sparse particle-based measurements such as super-resolution particle tracking velocimetry. Consequently, in contrast to other works, the method does not need a dual set of low/high-resolution images, and can be applied directly on a single set of raw images for training and estimation. This data-enhanced particle approach is assessed employing two datasets generated from direct numerical simulations: a fluidic pinball and a turbulent channel flow. The results prove that this data-driven method is able to enhance the resolution of PIV measurements even in complex flows without the need of a separate high-resolution experiment for training.

Data reconstruction of homogeneous turbulence using Lagrangian Particle Tracking with Shake-The-Box and machine learning

2021-09-27T13:50:49+00:00

This paper proposes a data reconstruction of homogeneous turbulent flow combined machine learning (ML) approach using experimental Lagrangian Particle Tracking (LPT) data with Shake-The-Box (STB). The LPT with STB was adopted to measure a von Kármán flow with a homogeneous turbulent region in the center [1]. The STB results have been stored and a temporal filter using 3rd order B-splines has been applied with optimal weighting coefficients to be used as input for FlowFit data assimilation method [2]. FlowFit data was used as ground truth to train ML algorithm. The low-resolved data of the velocity and acceleration field was reconstructed using an Adaptive Neuro-Fuzzy Inference System (ANFIS) with the downsampled LPT data as an input to predict homogeneous turbulent flow [3]. The training process can be mathematically regarded as an optimization problem to determine the weighting factor.

Particle Detection by means of Machine Learning in Defocusing PTV

2021-09-27T20:24:12+00:00

The accurate measurement of a fluid flow inside a measurement volume (MV) with limited optical access poses a challenge since the view on the MV is often partially obstructed for all but one viewing angle. Defocusing particle tracking velocimetry (DPTV) can be used to determine the instantaneous threedimensional velocity field of the flow with a standard PIV setup, requiring only a single optical axis. Current detection algorithms reach an out-of-plane accuracy in an order of magnitude lower than the planar accuracy, on top of a low rate of detected particles in comparison to other PTV approaches. These drawbacks originate from the low image quality due to noise, fluctuations in illumination, reflections and overlapping particle images. It has been shown that Machine Learning (ML) based detection is more robust against these adverse effects, due to the ability to leverage a higher amount of optical features for detection than conventional algorithms (Lecun et al. (1998)). Therefore, the present work addresses the applicability of ML algorithms in the post-processing of DPTV experiments, which will be evaluated on the ground of the DPTV experiments conducted by Leister and Kriegseis (2019). The setup of these experiments can be seen in Figure 1(a) and a section of a raw image recorded during the experiments in Figure 1(b).

Main results of the first Data Assimilation Challenge

2021-09-29T14:15:50+00:00

This work presents the main results of the first Data Assimilation (DA) challenge, conducted within the framework of the European Union’s Horizon 2020 project HOMER (Holistic Optical Metrology for Aero-Elastic Research), grant agreement number 769237. The challenge was jointly organised by the research groups of DLR, ONERA and TU Delft. The same synthetic test case as in the Lagrangian Particle Tracking (LPT) challenge (also presented in this symposium) was considered, reproducing the flow in the wake of a cylinder in proximity of a flat wall. The participants were provided with three datasets containing the measured particles locations and their trajectories identification numbers, at increasing tracers concentrations from 0.04 to 1.4 particles/mm³ . The requested outputs were the three components of the velocity, the nine components of the velocity gradient and the static pressure, defined on a Cartesian grid at one specific time instant. The results were analysed in terms of errors of the output quantities and their distributions. Additionally, the performances of the different DA algorithms were compared with that of a standard linear interpolation approach. Although the velocity errors were found to be in the same range as those of the linear interpolation algorithm, typically between 3% and 12% of the bulk velocity, the use of the DA algorithms enabled an increase of the measurement spatial resolution by a factor between 3 and 4. The errors of the velocity gradient were of the order of 10-15% of the peak vorticity magnitude. Accurate pressure reconstruction was achieved in the flow field, whereas the evaluation of the surface pressure revealed more challenging.

Error propagation dynamics of PIV-based pressure field calculation (3): What is the minimum resolvable pressure in a reconstructed field?

2021-09-01T19:09:02+00:00

An analytical framework for the propagation of velocity errors into PIV-based pressure calculation is established. Based on this framework, the optimal spatial resolution and the corresponding minimum field-wide error level in the calculated pressure field are estimated. This minimum error is viewed as the smallest resolvable pressure. We find that the optimal spatial resolution is a function of the flow features, geometry of the flow domain, and the type of the boundary conditions, in addition to the error in the PIV experiments, making a general statement about pressure sensitivity is difficult. The minimum resolvable pressure is affected by competing effects from the experimental error due to PIV and the truncation error from the numerical solver. This means that PIV experiments motivated by pressure measurements must be carefully designed so that the optimal resolution (or close to the optimal resolution) is used. Flows (Re=1.27 × 10⁴ and 5×10⁴) with exact solutions are used as examples to validate the theoretical predictions of the optimal spatial resolutions and pressure sensitivity. The numerical experimental results agree well with the analytical predictions.

Effect of the boundary conditions, temporal, and spatial resolution on the pressure from PIV for an oscillating flow

2021-09-01T20:35:59+00:00

The pressure field of an impinging synthetic jet has been computed from time-resolved, three-dimensional, three-component (3D-3C) particle image velocimetry (PIV) velocity field data using a Poisson equationbased pressure solver. The pressure solver used in this work can take advantage of the temporal derivative of the pressure to enhance the temporal coherence of the calculated pressure field for time-resolved velocity data. The reconstructed pressure field shows sensitivity to the implementation of the boundary conditions, as well as to the spatial and temporal resolution of the PIV data. The pressure from a 3D Poisson solver that does not consider the temporal derivative of the pressure shows high random error. Invoking the temporal derivative of the pressure eliminates this high-frequency noise, however, the calculated pressure exhibits an unphysical temporal drift. This temporal drift is affected by both the temporal resolution of the PIV data and the spatial resolution of the PIV vector field, which was systematically evaluated by downsampling the instantaneous data and increasing the interrogation window size. It was observed that decreasing the temporal resolution increased the drift, while decreasing the spatial resolution decreased the drift.

A Grid-free least-squares method for pressure evaluation from LPT data

2021-09-08T14:07:14+00:00

Lagrangian particle tracking Shake-the-box (STB) method (Schanz et al., 2016) acquires the 3D positions of tracer particles from the temporal sequences of their 2D projection images even for rather high seeding densities. Approximation of tracks by analytical functions (Gesemann, 2015) provides an accurate evaluation of tracers’ local velocity and acceleration. This data, which is obtained on non-regular grid, can be used to estimate local pressure fluctuations based on the Navier–Stokes equation. The present paper describes a grid-free least-squares method for the gradients and pressure evaluation based on irregularly scattered LPT data with random noise minimization.

Gridless Determination of Aerodynamic Loads Using Lagrangian Particle Tracks

2021-09-08T18:58:05+00:00

The aerodynamic loads on a flexible wing in terms of the surface pressure distribution and the lift force along the span are determined experimentally based on non-intrusive Lagrangian particle tracking (LPT) measurements. As the flexible wing deforms under the aerodynamic loads, its deformed shape is first reconstructed based on structural LPT measurements conducted together with the flow measurements in an integrated approach. Based on the reconstructed wing shape, flow tracers data are collected along surface normals to evaluate the surface pressure, as well as along elliptic paths around the wing to determine the circulation. The lift force is calculated from the surface pressure by integrating the pressure difference along the chord, as well as from the circulation using the Kutta-Joukowski theorem. The circulation-based lift results are in very good agreement with reference measurements from a force balance, with differences in the total lift force on the wing of less than 5%. The lift estimation based on the extrapolated surface pressure is consistently lower than the circulation-based lift, by about 10%, due to the limited accuracy of the pressure extrapolation near the leading edge region, where a considerable fraction of the lift is generated.

Pressure calculation for flows with moving surface boundaries from particle tracking velocimetry (PTV)

2021-09-08T19:03:20+00:00

The examples of flow conditions, where an object of a fixed or deformable body moves in a fluid, or the interface between the flow phases instantaneously changes its topology, are numerous in industry and natural sciences. The advent of particle image velocimetry (PIV) [1] and particle tracking velocimetry (PTV) [2] enabled the measurement of the instantaneous velocity fields in these types of complicated flow fields. As a next step, several methodologies have been developed in the past decade to calculate the pressure fields from PIV or PTV data [3,4]. These methods were developed based on the assumption of a stationary flow domain, with surface boundaries that are fixed and independent of time. This makes the current pressure calculation methods inapplicable to a flow domain with deformable moving surface boundaries. Also, for most of the two-phase flows, the capillary forces are significant and the pressure drop over the two-phase interface must be considered. Therefore, the current pressure calculators require an improvement in the formulation of the algorithms to account for the deformable volume conditions and the effect of the surface tension force. For the calculation of pressure from sparse PTV velocity data, firstly, a tessellation method is required to interconnect the irregularly spaced vectors in the flow field using a highquality mesh grid. The mesh must be dynamic and adjust itself to the moving boundaries. This tessellation method has already been developed by the current authors [5]. As the next step, equations of motion for a deformable C.V. need to be coupled with the tessellation method to calculate the instantaneous pressures in a two-phase flow field, with a moving interface, which will be the ultimate goal of the current study.

3D pressure field reconstruction from time-resolved stereoscopic PIV measurements by relaxation of Taylor's hypothesis

2021-09-08T20:15:44+00:00

This work presents reconstructions of 3D pressure fields starting from 2D3C stereoscopic-PIV (SPIV) measurements. In Fratantonio et al. (2021), we presented a new reconstruction algorithm, the “Instantaneous convection” method, capable of producing 3D velocity fields from time-resolved SPIV measurements. For reconstructions in flows with strong shear layers and high turbulence intensity, this method is able to provide time-resolved 3D velocity volumes that are more accurate than those that can be obtained from the more frequently employed reconstruction method based on the Taylor’s hypothesis and on the use of a mean convective field. Here we investigate the possibility of reconstructing the 3D pressure field from the timeresolved series of reconstructed 3D velocity data. A pseudo-tracking method is employed for computing the velocity material derivative, and the pressure field is then reconstructed by solving the 3D Poisson equation. The velocity and pressure reconstructions are validated on the Direct Numerical Simulation data of the turbulent channel flow taken from the John Hopkins Turbulence Database (JHTDB), and an application to experimental SPIV measurements of an air jet flow in coflow carried out at the Turbulent Mixing Tunnel (TMT) facility at Los Alamos National Laboratory is presented.

An Intensity-based and a lifetime-based PSP imaging method with enhanced sensitivity

2021-09-21T21:21:29+00:00

A pressure-/temperature-sensitive paint (PSP/TSP) has been developed and used as a measurement tool for the two-dimensional distribution of pressure and temperature on aerodynamic surfaces. In recent years, although the concern with measuring a pressure difference of several Pa, such as countermeasures against the noise of small fans, has been growing, the resolution of current PSP measurements is limited to several 10 Pa, even with carefully conducted measurements. For highly accurate measurements, researches on the advanced coating films in PSP/TSP have eagerly been conducted to date. However, measurement resolution and accuracy deteriorate when quantum efficiency or lifetime decrease under high pressure or high temperature conditions.

Extended-POD based acoustic analysis of separated aerofoils via simultaneous time-resolved PIV and force measurements

2021-09-22T16:46:43+00:00

We present a cross-correlation based analysis of the acoustic field generated in the wake of NACA 0012 and NACA 65410 aerofoils at a chord-based Reynolds number Re_c = 75000 as obtained from pressure fields reconstructed from a series of planar time-resolved Particle Image Velocimetry (PIV) experiments. The experiments are performed in a water channel facility located at the University of Southampton with an overhead carriage system to allow precise control of the angle of attack α of the aerofoils as well as to mount them to a six-axis force/torque transducer to allow for simultaneous load measurements. Angles of attack in the range 4^◦ < α < 17^◦are explored corresponding to a range of conditions from zero flow separation to fully stalled.

Towards the closure of Collar’s triangle by optical diagnostics

2021-09-22T19:42:16+00:00

An experimental methodology is proposed for the study of aeroelastic systems. The approach locally evaluates the forces involved in Collar’s triangle, namely aerodynamic, elastic, and inertial forces. The position of flow tracers as well as of markers on the object surface is monitored by a volumetric PIV system. From the recorded images, the flow tracers and surfare markers are separated based on their optical characteristics. The resulting images are then analysed by Lagrangian particle tracking. The inertial and elastic forces are obtained solely analysing the motion and the deformation of the solid object, whereas the aerodynamic force distribution is obtained via the pressure-from-PIV technique. Experiments are conducted on a benchmark problem of fluid-structure interaction, featuring a flexible panel installed at the trailing edge of a cylinder. A polynomial fit of the markers’ positions is carried out to determine the panel’s instantaneous shape, from which the inertial and elastic forces are evaluated. The pressure loads on the panel are determined via solution of the Poisson equation for pressure, imposing adaptive boundary conditions that comply with the panel. The simultaneous measurement of the three forces allows to assess the equilibrium of forces, and in turn to close Collar’s triangle.

Experimentally obtained velocity and pressure fields of an open channel flow around a cylinder using RIM-SPIV

2021-09-22T21:25:47+00:00

The quantification of velocity and pressure fields over streambeds is important for predicting sediment mobility and water exchange between stream and sediment interstitial spaces (Schmeeckle and Nelson, 2003; Tonina and Buffington, 2009). The latter is typically referred as hyporheic flow, which consists of surface water that flows through the streambed sediment pores (Tonina and Buffington, 2009). These fluxes are mainly driven by pressure gradients at the water sediment interface. In this paper, we report an experimental investigation of the time-averaged velocity and pressure field, quantified in a set of laboratory experiments using stereo PIV (Particle Image Velocimetry) with a non-toxic index-matched fluid, for an open channel flow around a barely submerged vertical cylinder as a model for plant stalk over a plane bed of coarse granular sediment, mimicking a stream gravel bed. This is the first time that such a velocity and pressure field is characterized experimentally for a free surface flow with irregular floor contour.

Pressure from data-driven-estimated velocity fields using snapshot PIV and fast probes

2021-09-22T21:33:05+00:00

This work explores the use of data-driven techniques to retrieve time-resolved information from snapshot PIV by exploiting the information from synchronized high-repetition rate sensors measuring flow quantities in few points, and to compute from it the instantaneous pressure field leveraging the Navier-Stokes momentum equation of the flow. This work focus on a technique rooted in the Extended Proper Orthogonal Decomposition, which already proven good performances in estimating time-resolved velocity fields from a finite number of probes synchronized with field measurements. The performances of the technique and its robustness to noise are tested on 2 synthetic dataset, a laminar one and a turbulent one, and compared to the most commonly applied technique to retrieve time-resolved information from snapshot PIV which exploits Taylor’s hypothesis.

Pressure reconstruction of a planar turbulent flow field within a multiply-connected domain with arbitrary boundary shapes

2021-09-27T19:46:02+00:00

Over the past two decades, it has been demonstrated that the instantaneous spatial pressure distribution in a turbulent flow field can be reconstructed from the pressure gradient field non-intrusively measured by Particle Image Velocimetry (PIV). Representative pressure reconstruction methods include the omnidirectional integration (Liu and Katz, 2006; Liu et al., 2016; Liu and Moreto, 2020), the Poisson equation approach (Violato et al., 2011; De Kat and Van Oudheusden, 2012), the least-square method (Jeon et al., 2015), and most recently, the adjoint-based sequential data assimilation method, which also essentially utilizes the Poisson equation to reconstruct the pressure(He et al., 2020). Most of these previous pressure reconstruction examples, however, were applied to simply-connected domains (Gluzman et al., 2017) only. None of these previous studies have discussed how to apply the pressure reconstruction procedures to a multiply-connected domain (Gluzman et al., 2017). To fill in this gap, this paper presents a detailed report for the first time documenting the implementation procedures and validation results for pressure reconstruction of a planar turbulent flow field within a multiply-connected domain that has arbitrary inner and outer boundary shapes. The pressure reconstruction algorithm used in the current study is the rotating parallelray omni-directional integration algorithm, which, as demonstrated in reference (Liu and Moreto, 2020) based on simply-connected flow domains, offers high-level of accuracy in the reconstructed pressure. While preserving the nature and advantage of the parallel ray omni-directional pressure reconstruction at places with flow data, the new implementation of the algorithm is capable of processing an arbitrary number of inner void areas with arbitrary boundary shapes. Validation of the multiply-connected domain pressure reconstruction code is conducted using the DNS (Direct Numerical Simulation) isotropic turbulence field available at the Johns Hopkins Turbulence Databases, with 1000 statistically independent pressure gradient field realizations embedded with random noise used to gauge the code performance. For further validation, the code is also applied for pressure reconstruction from the DNS pressure gradient in the ambient flow field of a shock-induced non-spherical bubble collapse in water (Johnsen and Colonius, 2009). The successful implementation of the parallel ray pressure reconstruction method to multiply-connected domains paves the way for a variety of important applications including, for example, experimental characterization of pressure field changes during the process of cavitation bubble inception, growth and collapse, non-intrusive unsteady aerodynamic force assessment for an arbitrary body shape immersed in flows, and multi-phase flow investigations, etc. In particular, as an immediate follow-up effort, the parallel ray pressure code will be used for the instantaneous pressure distribution reconstruction of the turbulent flow surrounding cavitation inception bubbles occurring on top of a cavity trailing corner based on high-speed PIV measurements.

Error propagation dynamics of velocimetry-based pressure field calculations (2): on the error profile

2021-09-28T14:33:50+00:00

This work investigates the propagation of error in a Velocimetry-based Pressure field reconstruction (VPressure) problem to determine and explain the effects of error profile of the data on the error propagation. The results discussed are an extension to those found in Pan et al. (2016). We first show how to determine the upper bound of the error in the pressure field, and that this worst scenario for error in the data field is unique and depends on the characteristics of the domain. We then show that the error propagation for a V-Pressure problem is analogous to elastic deformation in, for example, a Euler-Bernoulli beam or Kirchhoff-Love plate for one- and two-dimensional problems, respectively. Finally, we discuss the difference in error propagation near Dirichlet and Neumann boundary conditions, and explain the behavior using Green’s function and the solid mechanics analogy. The methods discussed in this paper will benefit the community in two ways: i) to give experimentalists intuitive and quantitative insights to design tests that minimize error propagation for a V-pressure problem, and ii) to create tests with significant error propagation for the benchmarking of V-Pressure solvers or algorithms. This paper is intended as a summary of recent research conducted by the authors, whereas the full work has been recently published (Faiella et al., 2021).

Investigation on computed pressures from PIV, a study of how boundary definitions affect pressure accuracy along objects on the example of a cylinder flow.

2021-09-29T14:45:46+00:00

This work discusses an approach to compute pressure fields from planar PIV measurement using standard CFD tools. In particular, we propose a combination of interpolation and mesh adaptation to import the PIV measurements on a grid that is morphed around objects, and is fine enough to solve the Poisson equation accurately. The whole process of meshing, interpolation and pressure computation is carried out using the popular open-source solver OpenFoam®. The method is tested and validated on a classic benchmark test case, namely, the unsteady flow past a cylinder. A 3D multiphase flow simulation is used to generate the reference data and analyze the impact of both, the PIV interrogation and the interpolation on the morphed grid. The simulation uses an Euler-Lagrangian one-way coupling approach to simulate the flow field and the dynamics of seeding particles. The analysis compares the pressure field from the 3D CFD simulation with the solution of a 2D Poisson equation based on the 2D velocity field obtained by either down-sampling the CFD data or by PIV interrogation of synthetic images built from the CFD data. Finally, we challenge the proposed method with the pressure reconstruction in a TR-PIV experiment in similar conditions.

Experimental investigations of the turbulent/non-turbulent interface over surface with spanwise heterogeneity

2021-09-01T19:18:55+00:00

The sharp but irregular interface that separates the instantaneous turbulent and irrotational flows is termed as the turbulent/non-turbulent interface (TNTI). TNTI can be widely observed in various types of flow, such as turbulent boundary layers, jets and combustion flame fronts. Due to its importance on the intermittency and entrainment process, TNTI has been widely explored in its geometry and dynamic properties (da Silva et al., 2014). Most of the studies focus on the TNTIs in smooth plane boundary layer, while few investigate the effects of wall shapes. However, the wall conditions in many engineering applications are complex and heterogeneous, which will induce large-scale heterogeneity (Barros and Christensen, 2014) and require further investigations. To shed new light on the intermittency and entrainment above complex surfaces, the TNTI over spanwise heterogeneity are investigated here with time-resolved stereoscopic PIV (TR-SPIV).

The model and TR-SPIV experimental set-up are shown in Fig. 1. The experiments are conducted in the low-speed water channel at Beijing University of Aeronautics and Astronautics. The spanwise distance S between two adjacent ridges is S/(δ) = 1.35, where (δ) is the spanwise-averaged boundary layer thickness. This spanwise distance is selected to induced strong secondary vortices (Vanderwel and Ganapathisubramani, 2015; Wangsawijaya et al., 2020). The Reynolds number based on the streamwise location x is Re_x = 7.2×10⁵. The field of view is around 2S×1.8S, and is captured by two CMOS cameras (2048×2048 pixel) with sampling rate as 500Hz. The averaged resolution is about 8 pixels per Kolmogorov scale (calculated at y/(δ) = 0.6), which is high enough for TNTI-related research (Borrell and Jimenez, 2016). The ´TNTI is detected by the magnitude of local enstrophy ω²/2, and the threshold is selected to be the value where changing the threshold has the smallest influence on the TNTI-mean-height (Watanabe et al., 2018).

The time-mean velocity and TNTI location are present in Fig.2(a). A pair of counter-rotating largescale secondary vortices (SVs) are induced over the ridge-type roughness. At the position where SVs induce upwash flow, a low-momentum pathway (LMP) can be observed, while the time-mean height of TNTI (y_I) is brought higher. As a contrast, where downwash flow induces high-momentum pathway (HMP), (y_I) is lower.

TNTI properties are further discussed from two aspect. The geometry properties are firstly investigated. The fractal dimension of the TNTI keeps as 2.3 along the spanwise direction. This value is consistent with the result over smooth plate (Borrell and Jimenez, 2016; Wu et al., 2020) and riblets plates(Cui et al., 2019),´ which indicates that the wall shapes do not influence the multiscale properties of the TNTI. The streamwise wavelength of the TNTI (λ_I) is further obtained by calculating the streamwise pre-multiplied spectrum of the TNTI. It is found that at each spanwise location, λ_I is identical to the wavelength of streamwise velocity fluctuation at the TNTI mean height. This shows that the large-scale fluctuation of TNTI is controlled by the large-scale streamwise velocity fluctuation structures. Secondly, the p.d.f. of TNTI instantaneous height is investigated, as shown in Fig. 2(b). It can be observed that the p.d.f. of TNTI height above LMP shows a negative skewness, while the p.d.f. above HMP skews positively. A closer look at instantaneous structures shows that the skewness is attributed to the different probability of Q2/Q4 events in LMP and HMP.

Time-resolved sphere and fluid motions in turbulent boundary layers

2021-09-01T19:59:28+00:00

This paper extends the study by Tee et al. (2020) to investigate the effect of large coherent structures on motion of spheres with specific gravities of 1.006 (P1) and 1.152 (P3) at Re_τ = 670 and 1300 (d⁺ = 56 and 116). The sphere and fluid motions are tracked simultaneously via 3D particle tracking and stereoscopic particle image velocimetry over the streamwise-spanwise plane, respectively. With sufficient mean shear, sphere P1 lifts off of the wall upon release before descending back towards the wall at both Re_τ. It typically accelerates strongly over a streamwise distance of less than one boundary layer thickness before approaching an approximate terminal velocity. By contrast, the denser sphere P3 does not lift off upon release but mainly slides along the wall. At lower Re_τ where wall friction is stronger, this sphere translates with unsteady velocity, significantly lagging the local fluid. The streamwise velocities of both spheres correlate strongly with the fast- and slow-moving zones that approach and move over them. In most runs, both spheres lag the local coherent structures and travel with either fast- or slow-moving zones throughout the observed trajectories. Vortex shedding, which is most prevalent for sphere P3 at Re_τ = 670, is also important. The sphere spanwise motion is prompted by wall friction, spanwise fluid motion, and/or meandering of the coherent structures, and spheres do not appear to migrate preferentially into slow-moving zones.

Investigation of turbulent boundary layer flows with adverse pressure gradient by means of 3D Lagrangian particle tracking with Shake-The-Box

2021-09-07T20:41:43+00:00

A large-scale 3D Lagrangian particle tracking (LPT) investigation of a turbulent boundary layer (TBL) flow developing across different pressure gradient regions is presented in this study. Three high-speed multi-camera imaging systems, LED illumination and helium-filled soap bubbles (HFSB) tracers have been adopted to produce time-resolved sequences of particle images over a large volume encompassing approximately 3 m in the streamwise direction, 0:8 m in the spanwise direction and 0:25 m in the wall-normal direction. Individual tracers have been reconstructed and tracked within the imaged volume by means of the Shake-The-Box algorithm (STB, Schanz et al. (2016)); the FlowFit data assimilation algorithm (Gesemann et al. (2016)) has been used to evaluate the spatial velocity gradients and to interpolate the scattered LPT results onto a regular grid. Thanks to the large size of the investigated volume and to the time-resolved nature of the recorded images, the entire spatial extent of the large-scale coherent motions within the logarithmic region of the TBL (i.e. superstructures) could be captured and their dynamics investigated during their development over several boundary layer thickness in the streamwise direction, from the zero pressure gradient region (ZPG) to the adverse pressure gradient region (APG). Two free-stream velocities were investigated, namely 7 and 14m=s, corresponding to Re_t ~ 3,000 and 5,000 respectively.

The results confirm the location and scale of the elongated high- and low-momentum structures in the logarithmic region, as well as their meandering in the spanwise direction. Two-point correlation statistics show that the width and spacing of the superstructures are not affected by the transition from the ZPG to the APG region. The analysis of the instantaneous flow realizations from both a Lagrangian and Eulerian perspective indicates the presence of significant fluid particle elements exchange across the interfaces of the large-scale structures.

Multi-resolution, time-resolved PIV measurements of a Decelerating Turbulent Boundary Layer near Separation

2021-09-08T16:15:50+00:00

We report on measurements of the time-evolving velocity profile of a turbulent boundary layer subjected to a strong adverse pressure gradient (APG) at Reynolds numbers up to Re_θ ≈ 55 000 with an upstream friction Reynolds number exceeding Re_τ ≈ 10 000. Near the point of flow separation high-resolution imaging at high camera frame rates captured the time evolving velocity profile using the so-called “profile-PIV” technique in a nested imaging configuration of two cameras operating at different image magnifications. One camera used an image magnification better than unity to resolve the viscous scales directly at the wall while the remainder of the roughly 200 mm thick boundary layer is simultaneous captured by the second camera. In the APG the variance of the stream-wise velocity exhibits no “inner peak” commonly found in turbulent boundary layers without pressure gradient influence. Spectral analysis further shows that the peak energy within the boundary layer shifts away from the wall toward lower frequencies. The overlap between the simultaneously imaged areas allows to assess and, to first order, correct for the effect of spatial smoothing on statistical quantities, spectra and related quantities. A multi-frame cross-correlation algorithm was used to process the extensive data base. In addition, a newly developed 2D-2C “Shake-The-Box” algorithm (STB) provided highly resolved particle tracking data beyond the reach of conventional PIV processing.

Cross-plane stereo-PIV measurements in a refractive-index-matched environment of flow associated with barchan dunes immersed in a turbulent boundary layer

2021-09-08T16:29:37+00:00

Barchan dunes are crescent-shaped bedforms that form in aeolian (i.e., wind-driven) environments (including both Earth and other planets, such as Mars) as well as subaqueous environments. Under the forcing of the aloft turbulent boundary layer, they migrate downstream at a rate inversely proportional to their size, which results in complex interactions between neighboring dunes of disparate scales. In particular, it has been observed that dunes will interact at a distance, causing changes in morphology without contacting each other, which is thought to be driven by the way dunes modify the local flow field Bristow et al. (2018); Assis and Franklin (2020).

In this study, the coherent structures formed in the wakes of barchan dunes are investigated using measurements of the flow over fixed-bed (i.e., solid) barchan models, both in the wake of an isolated barchan and the interdune region between interacting barchans (Fig. 1(a)). Furthermore, the interactions between the flow structures shed by the dunes and the structures in the incoming boundary layer are analyzed.

PIV measurements in a turbulent boundary layer overlying a spanwise heterogeneous roughness

2021-09-08T16:42:37+00:00

In nature and engineering applications, wall-bounded flow often encounter a heterogeneous surface condition, such as the atmosphere boundary layer at the urban boundary and flow over riveted aircraft surfaces. In a particular scenario, when the surface heterogeneity is predominantly in the spanwise direction of the flow, this roughness heterogeneity can generate secondary flow in cross flow plane which is very different from smooth-wall or homogeneous rough-wall boundary layers.

Effect of Gust Wind on Flow over a Wall-Mounted Fence

2021-09-15T20:54:54+00:00

In this work, the characteristics of incoming and wake flows downstream of wall-mounted fences under wind gust were explored with wind tunnel experiments. A time-resolved particle image velocimetry was used to capture the flow dynamics across two different fence heights. The results show that during the gust period, the wake presents distinct meandering and strong flow mixing. The Probability Density Function distribution of flow velocities indicates that the mixing effect increases with the streamwise distances. Specifically, for locations above the fence top tip, the growth of streamwise distance decreases the footprint of wind gust. However, for locations lower than the fence top tip, the local wind flows exhibit stronger variations before and after wind gust with the growth of downstream distance. Overall, at the same relative streamwise and spanwise locations downstream of fences within the wake region, the higher fence better suppresses the influence of gust wind.

PIV measurement of buffer and logarithmic layers with detached eddies which mimics the neutral atmospheric surface layer

2021-09-22T16:56:31+00:00

Accurate comprehension of turbulence characteristics in the atmospheric surface layer (ASL) under near neutral conditions, which is a lower part of the atmospheric boundary layer and a very high-Re number flow, is critically required in view of the increasing and broadening use of numerical weather prediction models. The models need to estimate turbulence fluxes of momentum, heat and moisture in the ASL as boundary conditions. On the other hand, observations (Högström 1990, Drobinski et al. 2007) have revealed that the fluxes under near-neutral conditions are often inconsistent with Monin-Obukhof theory, which has been widely used in models. The observations were conducted over flat surfaces with homogeneous roughness, and thus the violation from the theory might not be due to the underlying surface conditions. Thus, aiming to investigate an origin of the violation from the theory, we have carried out a wind tunnel experiment on the logarithmic layer along a smooth flat wall with a larger-scale disturbance, which mimics the near-neutral atmospheric surface layer (Hattori et al. 2010). In the present study, we especially examine a PIV measurement with a long-distance microscope lens to discuss the interaction of turbulences structures between buffer and logarithmic layers, which must give a clue on Reynolds number effects

Investigation of Large Scale Motions in Zero and Adverse Pressure Gradient Turbulent Boundary Layers Using High-Spatial-Resolution PIV

2021-09-27T16:26:16+00:00

Particle image velocimetry (PIV) has been used to capture the high-spatial-resolution (HSR) two-component, two-dimensional (2C-2D) velocity fields of a zero-pressure-gradient (ZPG) turbulent boundary layer (TBL) and of an adverse-pressure-gradient (APG) TBL. Proper Orthogonal Decomposition (POD) is performed on the measured velocity fields to characterize the velocity fields as large or small scale motions (LSMs or SSMs), with further characterisation of the LSMs into high and low momentum events. This paper reports the findings of the PIV experiment and the subsequent analysis of the high Reynolds number ZPG and APG TBLs

Analysis of the Contribution of Large Scale Motions to the Skin Friction of a Zero-Pressure-Gradient Turbulent Boundary Layer Using the Renard-Deck Decomposition

2021-09-27T16:46:50+00:00

Coherent flow structures in turbulent boundary layers have been an active field of research for many decades, as they might be the key to reveal the mechanics of turbulence production and transport in turbulent shear flows. Renard and Deck (2016) proposed a theoretical decomposition for the mean skin-friction coefficient based on the mean kinetic energy budget in the streamwise direction. This decomposition, referred to as the Renard-Deck (RD) decomposition, decomposes the mean skin friction generation into three physical mechanisms in an absolute reference frame, namely, direct viscous dissipation, turbulent kinetic energy production, and spatial growth. In this study, the large scale motions (LSMs) are extracted using a proper orthogonal decomposition (POD) of the velocity field based on high-spatial-resolution two-dimensional – two-component particle image velocimetry (HSR 2C-2D PIV) of a zero-pressure-gradient turbulent boundary layer (ZPG-TBL), and their effect on the skin friction via RD decomposition.

Flow field of impinging sweeping jets

2021-09-01T19:33:06+00:00

Sweeping jets are oscillating jets generated by fluidic oscillators, i.e., devices designed to produce an oscillation of the flow without the use of any moving parts (Raghu, 2013). A typical configuration of such devices consists of an expansion chamber connected to a high-pressure supply via a converging nozzle and provided with feedback channels. The oscillating motion in the expansion chamber is triggered by an inherent flow instability and sustained by the flow rate across the feedback channels. Recently, sweeping jets have been studied in flow control applications for noise reduction, separation and circulation control over airfoils, control of resonant cavity oscillations and deflection of jets. The advantageous features of fluidic actuators, among which are the wide range of operating frequencies (up to kHz with meso-scale) and the distributed momentum addition, have also stimulated an increasing interest in their application to electronics cooling. Several recent studies on the convective heat transfer from impinging sweeping jets (e.g., Hossain et al., 2018; Park et al., 2018) have shown that, compared to conventional round jets, they offer higher cooling rates with better uniformity at least for small jet-to-plate spacings.

An Experimental Study to Quantify the Rotor-to-Rotor Interaction Characteristics of a Small Unmanned-Aerial-Vehicle

2021-09-01T20:19:32+00:00

The flow interactions between laterally aligned rotors were investigated experimentally to study the rotor-to-rotor interactions on the aerodynamic and aeroacoustic performance of small unmanned aerial vehicles (UAVs). Two identical rotors, similar to the dimensions of Phantom 3 (DJI), were mounted separately on different stages in a wide-open space. High-accuracy force and sound measurements were conducted to document the thrust and noise at various separation distances. The detailed flow structures and corresponding vortex evolutions behind the rotors were resolved clearly by using high-resolution PIV measurements. As the rotor separation distance decreased, intensified flow interactions were noted within the rotors. More specifically, the twinrotor with separation distance of L= 0.05D exhibited a significantly enhanced thrust fluctuation (i.e., ~ 240% higher) and augmented noise level (i.e., ~ 3dB) in comparison with that of baseline case. Measured PIV results indicated that a strong recirculation region existed near the top-right of the twin-rotor case, which is believed to be the reason for the increased thrust fluctuations and aeroacoustic noise level.

Wake behind circular cylinder excited by spanwise non-uniform disturbances

2021-09-01T20:51:04+00:00

We studied a modification of wake behind a circular cylinder using a plasma actuator. The plasma actuators were arranged in the spanwise direction of the cylinder to give temporal periodic disturbances having Strouhal number St = 0.18-2.3 with a burst ratio BR = 20 and 40%. The Reynolds number was set in a rage of Re = 4200 to 8400. Two types of plasma actuator were prepared; one is a single strip of the actuator placed at each side of the cylinder to give a spanwise uniform disturbance, and another is an array of small piece of actuators placed at the same location to create a spanwise non-uniform disturbance with temporal phase difference, φ = 0 or π, between adjacent electrodes. A conventional two-component PIV and stereo PIV was used to measure the flow field. Figure 1 shows the instantaneous spanwise component of vorticity at Re = 4200 evaluated by two-component PIV. Under no disturbance condition, the laminar shear layer extends straight to around x / d = 1.5 and then forms a wake vortex, as shown in Fig.1(a). In the case of spanwise non-uniform forcing with St = 1.09 and φ =π, rapid roll up of the initial shear layer leads to arrangement of wake vortices closer to the cylinder., as shown in Fig.1(b). With higher Strouhal number case with St = 1.09 and φ = 0, shown in Fig.1(c), a series of fine scale vortices are generated behind both side of the cylinder without forming regular Karman vortices. The spanwise non-uniform forcing was effective to suppress the formation of large scale vortices just behind the cylinder. Figure 2 shows surface of constant vorticity magnitude and vortex lines under St =1.09 and φ = π case. These were computed from a phase-averaged threecomponents velocity field evaluated by stereo PIV. The value of the surface was selected to display the boundary layer formed on the cylinder, and the vortex lines are selected to visualize the vortex structure formed in the following shear layer. A bundle of vortex lines are shaped in a wavy pattern along spanwise direction with 180 degrees out of phase to the adjacent bundle upstream of downstream. This structure, so called ‘chain-line fence structure’ was already found in planar free shear layer [Nygaard, K.J. and Glezer, A., 1990, Phys. Fluids A, 2, 461] and planar jet [Sakakibara, J., Anzai, T., 2001, Phys. Fluids, 13, 1541], but it became evident to create it in the wake of circular cylinder in this study.

PIV Measurements of the Wake Formation from a Rough Flat Plate

2021-09-08T19:20:33+00:00

Wake flows are prevalent in a wide range of engineering applications and their behaviour can significantly impact engineering design and performance. A considerable body of work exists on smooth body wake structures and flows over rough bodies, however, there is a lack of fundamental physical understanding of the amalgamation of the two fields. Two-component two-dimension particle image velocimetry (2C-2D PIV) is used to investigate the effect of surface roughness on the formation of large scale structures in the near wake of a thin flat plate. Both high-speed and low-speed, high-resolution PIV setups have been used to investigate the effect of surface roughness on the boundary layer and the near wake of the plate to gain insight into the underlying physical connection between these regions.

Application of SPOD analysis to PIV data obtained in the wake of a circular cylinder undergoing vortex induced vibrations

2021-09-08T19:44:54+00:00

Vortex induced vibrations (VIV) of a circular cylinder have been investigated experimentally using a cyberphysical apparatus with m^∗ = 8, ζ = 0.005, and Re = 4000. This study considers the application of proper orthogonal decomposition (POD) and spectral POD (SPOD) analysis to the wake dynamics of the low-mass-ratio VIV of a circular cylinder in the lower branch at U^∗ = 7.5. SPOD has been previously shown to better separate frequency-centered modal dynamics, compared to POD. Coherent POD and SPOD modes were compared and the newly separated third SPOD mode pair was found to have a periodicity characteristic of vortex shedding and a peak in the temporal coefficient spectra at St = f D/U_∞ = 0.2248. The literature has identified that the wake dynamics within the lower branch are synchronized to the cylinder motion; however the present study suggests that some hidden dynamics persist at the Strouhal frequency. Low order models based on the first eight POD and SPOD modes were compared, and it was found that the filtering operation in SPOD removes the uncorrelated stochastic energy component of the POD modes while producing a comparable representation of the coherent deterministic part of the wake dynamics. Using SPOD to separate the distinct frequency-centered dynamics into unique, interpretable mode pairs will simplify future efforts to develop sparse dynamical models of the flow.

Combining high-speed planar PIV and motion tracking of a flexible cylinder in cross-flow

2021-09-16T13:54:47+00:00

Most modeling studies investigating the flow dynamics in vegetation canopies are limited to rigid models as proxies for vegetation elements. However, most canopies embody some degree of structural flexibility, resulting in aeroelastic mechanisms coupling the motion of the vegetation with the surrounding flow. Studies addressing flexible canopies typically quantify either the flow or the plant motion independently, thus missing the instantaneous coupling between turbulent stresses and structural deformations. Few experiments have been devoted to measuring both quantities simultaneously. Okamoto and Nezu (2009) utilized a combined PIV-PTV technique to capture both flow and canopy motion. However, only the motion of the stem tips was captured, as opposed to the deformation of the entire stem. Py et al. (2006) employed digital image correlation (DIC) to quantify the motion of crop canopies using in-field images. However, the wind itself was not measured across the domain.

The present work presents an experimental technique that can be utilized to study the flow–structure interaction in flexible canopies, and that could be extended to other flexible and/or moving objects. High-speed PIV data of the flow surrounding an idealized canopy element, consisting of a flexible cylinder, together with the corresponding displacement field throughout the cylinder were simultaneously obtained combining fluorescent imaging and refractive index matching (RIM).

PIV measurement of a jet from a gasper in an aircraft cabin mockup

2021-09-16T14:17:11+00:00

The air conditioning system of most commercial aircrafts consists of a main system, which operates on the principle of mix ventilation, and a personalized system called gasper. Field studies show that passengers prefer to keep gasper parcially open or redirect it away from the head due to the discomfort. Therefore, there is a demand to characterize the flow of this device for future improvements. In this way, the present work aims to experimentally study the gasper jet inside a real cabin mockup using PIV. The results indicate that passengers are subjected to a high speed jet and the air in their breathing zone is mostly supplied by the mixed ventilation system due to the large entrainment ratio.

Experimental investigation on flow structures of steadily translating low-aspect-ratio wings in low Reynolds number flow

2021-09-17T14:23:52+00:00

In recent decades, Micro Air Vehicles (MAVs) have been a hot topic for their promising future. But the promotions of MAVs are hindered by their short endurances. To solve this problem, inspirations are brought from migratory butterflies who utilize the ‘flapping-gliding’ skill during long-distance migration to improve the flight efficiency. The butterfly’s gliding flights, which can be simplified by considering the steadily translating fixed wings, have drawn high attentions. Previous studies mainly focus on the aerodynamics of the low-aspect-ratio fixed wings at Re ≈ 10⁵ via force measurements. However, few experimental studies have measured the 3D flow fields. Consequently, the underlying high lift-to-drag ratio mechanisms in the steadily translating butterfly-shaped wings are still not clear. To shed new light on this problem, the 3D flow structures around butterfly-shaped wings were captured and investigated in detail.

Detecting vortical structures in time-resolved volumetric flow fields

2021-09-20T16:34:01+00:00

The detection of three-dimensional coherent vortical structures that get advected as well as deformed with time is a challenge. However, it is critical for the statistical analysis of these vortices, for example, the quasi-streamwise vortices (QSVs) in the near field of a turbulent shear layer, where cavitation inception typically occurs. These structures exhibit underlying correlations among different properties that can be derived from the velocity gradients. Exploiting these correlations, a pseudo-Lagrangian vortex detection method is proposed that uses k-means clustering based on vorticity magnitude and direction, values of λ₂, strain rate structure, axial stretching, and location. The method facilitates the finding that QSVs have pressure minima that are lower than those in the surrounding flow, including the primary spanwise vortices. These minima typically appear after a period of axial stretching and before contraction events.

Plunging jets from orifices of different geometry

2021-09-21T15:21:38+00:00

Plunging jets are used in many industrial and civil applications, as for example in sewage and water treatment plants, in order to enhance aeration and mass transfer of volatile gases. They are also observed in natural processes as rivers self-purification, waterfalls and weirs.

Many investigations dealt with the plunging jets in different configurations, but the dependence on Reynolds number and jet geometry were still not sufficiently addressed. For example, Mishra et al. (2020) studied an oblique submerged water impinging jet at different nozzle-to-plate distances and impingement angles, but only at a rather small Reynolds numbers (2600). On the other hand, different jet geometries have been extensively considered, but not for the plunging jet configuration (Mi, 2000; Hashiehbaf &Romano, 2013).

In this work, plunging water jets issuing in air from orifices of different shape are considered. The aim of the work is to detail and compare jet behaviors in terms of velocity fields generated after impacting the air-water interface, as a function of Reynolds number and orifice geometry. However, air bubbles entrainment is mainly avoided in order to study the jet characteristics in a simpler case and use it as a reference starting point for future works.

Influence of span-wise coherence on the acoustic radiation in a cylinder wake

2021-09-21T16:39:37+00:00

The aim of this study is to detect and visualise the influence of span-wise coherence on propagating sound waves emanating from a flow around circular cylinders with span-wise variations of the local radius. Synchronous particle image velocimetry (PIV) and microphone measurements are performed in a circular wind tunnel with a nozzle size of 0.4 m×0.4 m at a maximum flow speed of U∞ = 43m s⁻¹ . The test section is surrounded by a full anechoic chamber of approximately 9 m×9 m×5 m.

Super-large-scale flow visualization using natural snowfall for the study of utility-scale wind turbine flows

2021-09-21T21:14:44+00:00

With the rapid growth of wind turbine installation in recent decades, fundamental physical understanding of the flow around wind turbines and farms is becoming increasingly critical for further efficiency increases. However, the effort to develop this understanding is hindered by the significant challenges involved in modelling such a complex dynamic system with a wide range of relevant scales (blade boundary layer thickness at ∼ 1 mm to atmospheric scales at ∼ 1 km). Additionally, conventional methods used to measure air flow around wind turbines in the field (e.g., lidar) are limited by low spatio-temporal resolutions.

TR-PIV in highly pulsatile flow: pulsation frequency and wake dynamics case study

2021-09-22T18:27:36+00:00

Experimental study of highly pulsatile flows presents a number of challenges, primarily the inherently large dynamic range of velocities. Herein, we use time-resolved particle image velocimetry processed with a technique known as pyramid sum-of-correlation to study highly pulsatile flow around a surface-mounted hemisphere. The frequency of pulsation is varied from low- frequency, quasi-steady pulsation to high frequency pulsation. We present a conceptual overview of the wake regimes observed and compare the flow physics of the high-frequency case to that of a vortex ring produced by a single impulse of fluid.

Volumetric Lagrangian particle tracking measurements of jet impingement on convex cylinder

2021-09-27T13:45:05+00:00

Impinging jets are widely used for heat and mass transfer because they are applicable to any type of body and can be easily implemented. They are also used in various industrial fields and design techniques. Previous research investigated e.g. a jet impinging onto a flat surface. However, since most of the mechanical parts have curvature on their surface, it is necessary to study more detailed properties of the jet impinging on a curved surface using advanced measurements. Therefore, in this study, three-dimensional flow structures of a round jet impinging on a convex cylinder surface were measured using volumetric Lagrangian particle tracking (LPT).

On the unsteady dynamics of synthetic leaves: Laboratory experiments using synchronized PIV and DIC

2021-09-27T14:22:44+00:00

The unsteady 3D dynamics of various synthetic leaves and the induced turbulence are systematically studied experimentally for representative Cauchy numbers in a wind tunnel under nearly uniform incoming flows. Synchronized digital image correlation (DIC) and high-frame-rate particle image velocimetry (PIV) are employed to track the structure dynamics simultaneously and the surrounding flow field to uncover the fluid-solid interaction. A high-resolution six-axis load cell is also used to quantify the synthetic leaves' induced force and torque under various flows. The shapes of synthetic leaves inspected are representative of selected environments (e.g., calm to windy weather; tropical to temperate climate). The Cauchy number is set to resemble those observed in natural conditions. This presentation will discuss insights from synchronized PIV-DIC techniques on the synthetic leaves' distinct behavior and wake flow response. Particular emphasis is placed on characterizing flow instability and the leave shape's role in the motions and force. For this purpose, we inspected the instantaneous force and torque as well as their structure. We will also discuss the relationship between leave shapes with force and torque fluctuations linking them with the leaf motion obtained from DIC measurements. In particular, the results show that selected leaf shapes experience significantly larger and distinct force and torque fluctuations and larger pitch magnitude, as shown in Fig. 5. A shared monotonically decreasing trend of the nondimensional frequency (Strouhal number, St = fL/U) is evidenced for standard environmental conditions.

Flow characteristics analysis for flow past the porous spheres: wake structures and drag force coefficients

2021-09-27T14:45:40+00:00

Flow past a permeable sphere is different from that of a solid sphere due to the penetration of the fluid within porous structures, which can arise a change of flow fields. In this work, flow past porous spheres with Darcy numbers (Da) ranging from [0,10⁻³ ] were measured using planar Particle Image Velocimetry (PIV). The whole flow fields, including both leading edge and trailing edge, were captured at six different Reynolds numbers (Re) varying from 400 to 1400. Time-average flow fields were calculated based on instantaneous flow fields within fully-developed stages. Local minimum method was used to search for stagnation point positions. The results show positions of stagnation points are nearly proportional to the logarithm of Re. For most porous spheres, positions of stagnation points are extended to farther downstream positions than that of a solid sphere. However, at some certain Darcy numbers, ranging from 5 ∗ 10⁻⁶ to 2 ∗ 10⁻⁵, positions of stagnation points are closer to the sphere centers than that of an impermeable one.

A representative driven system to interrogate passive dynamics of an airfoil in the wake of a cylinder

2021-09-27T15:57:21+00:00

Passive motion of an airfoil in the wake of a circular cylinder is compared with driven motion of an airfoil in the same configuration, through simultaneous measurement of both the airfoil dynamics and the surrounding flow field. The passive mounting allows the airfoil to move in the transverse (heaving) direction in response to oncoming forcing, while introducing significant parasitic effects to the dynamics including friction. The driven motion of the airfoil reproduces important characteristics of the imperfect passive motion, validating idealized sinusoidal motion as a model for dynamics of the passive airfoil operating in a more realistic engineering context. Particle Image Velocimetry (PIV) of the driven case is then used to illuminate flow structures contributing to observed power and thrust production in both cases.

PIV measurement of complex flow characteristics in open-cell metal foam replica

2021-09-27T18:23:20+00:00

Time-resolved 2-D particle image velocimetry was used to study on turbulent flow characteristics inside an open-cell metal foam under the laminar and turbulent inlet conditions. A study on the effect of Reynolds number was conducted with different three channel Reynolds numbers, 1000, 5000 and 10000. Uniform upstream flow is divided by the pore network of metal foam and it is found that there are flow disturbances induced by metal foam structure even at a laminar inlet condition. It is confirmed that there is a similarity of the preferred flow path flows take regardless of Reynolds number.

Investigation of the shear-layer instabilities in supersonic impinging jets using dual-time velocity measurements

2021-09-27T19:35:01+00:00

Motivated by applications in the propulsion industry, the fundamental study of phase-locked shear-layer instabilities in supersonic impinging jets has been of research interest for long time. While such flows have been experimentally investigated in various research studies using time-unresolved particle image velocimetry (PIV) techniques, the understanding of the shear-layer dynamics is limited, due to the absence of temporal information. Time-resolved PIV measurements for high-speed flows require a large bandwidth, which is challenging to achieve with the current state of technology. An alternate approach using time-unresolved double-PIV measurements is presented in the current study, which provides multiple samples of dual-time data, depicted in figure 1. Such data can be obtained using two co-visual PIV systems, triggered at a user-selectable time-offset, ∆t. As shown by Sikroria et al. (2020), the application of techniques like dynamic mode decomposition (DMD) on time-unresolved dual-time data provides valuable information about the flow structures governing the shear-layer instabilities. The experimental setup for such measurements in supersonic impinging jets, followed by the determination of the relevant dynamical flow structures from the data, will be presented in the conference.

Stereoscopic Micro PIV Investigation of Velocity Boundary Layer Near Piston Top of a Tumble Enhanced SI IC Engine

2021-09-01T21:10:58+00:00

To develop higher efficiency and lower emission gasoline engines, ultra-lean burning under high Reynolds number conditions is desired. It is believed that enhancement of tumble flow in the engine cylinder is effective for increase of turbulent intensity, resulting in improvement of characteristics of flame propagation and ignition under a strong discharge, while the enhancement of tumble flow might cause heat loss from the wall. Investigations of characteristics of turbulence and distributions of wall and gas temperature in engine cylinders are still challenging due to transient and high pressure, and due to cycle-to-cycle variations. In the previous study (Jainski et al., 2013), a micro particle image velocimetry (micro PIV) measurements were conducted in an engine cylinder at up to 1100 rpm and the characteristics of velocity boundary layer around a cylinder head were investigated. The study has shown that the log-law does not properly present the measured velocity distributions near the wall. In our previous study (Shimura et al., 2018), a micro PIV was conducted in a motored engine cylinder to investigate velocity boundary layer characteristics near piston top before the top dead center (TDC) at a constant engine speed of 2000 rpm to deepen understanding characteristics of velocity boundary layer in engine cylinder with tumble flow. The velocity boundary layer was well fitted to the Blasius theory at 30 CAD before TDC and deviated from the theory at 15 CAD before TDC. However, the obtained data was two components of velocity in the measurement plane, which means that effects of magnitude of velocity were not clear in the previous measurement. In this study, stereoscopic micro PIV was conducted to elucidate the effects of magnitude and direction of velocity on the characteristics of velocity boundary layer near the piston top in the tumble enhanced SI IC optical engine. The tumble enhanced SI IC optical engine used in the previous study (Shimura et al., 2018; Matsuda et al., 2019) was used also in this study. The bore is 75 mm and the stroke is 112.5 mm. Length of the connecting rod is 250 mm. The engine has two intake valves of the diameter 29 mm and two exhaust valves of the diameter 25 mm. The compression ratio is 13.0. The optical access is achieved through the quartz glass cylinder. A tumble enhancing intake port is used for the sake of improvement of ignition and flame propagation. The engine speed can be set up to 2000 rpm at the maximum. The overall flow fields taken by a preliminary PIV experiment can be seen in the literatures (Shimura et al., 2018; Matsuda et al., 2019). The laser beams for PIV are from two Nd:YAG lasers (LOTIS, LS-2131, 150 mJ/pulse, 532 nm) are led to the same optical axis by a mirror and a polarizing beam splitter. The laser beam is formed into laser sheet of 180 µm thickness by laser sheet forming optics and led into the engine cylinder. The scattering light was collected by long distance microscope lenses (Quester, SZM100) and imaged onto CCD cameras (Princeton Technology, ES4020) in the stereoscopic alignment with 18 degrees. To compensate for the difference in the focal length caused by the quartz engine liner, a cylindrical lens of 1000 mm focal length was placed in front of each long distance microscope lens. SiO2 of 1 µm mean diameter was used for tracer particles. The micro PIV was operated at about 6.6 Hz to be synchronized with engine speed. The time separation of the successive particle images was 1.5 µs.

The field of view of the micro PIV was 3.5 mm × 3.5 mm on the piston top including central axis of the cylinder. Here, x and y coordinates are set to the direction from the exhaust to the intake valves and the direction from the piston to the pent roof, respectively. z coordinates is perpendicular to x and y axes, and the orizin of the coordinates is set at the center of the piston top. The spatial resolution of PIV, which is defined by the size of interrogation region, is 108.8 µm × 54.4 µm. Vector spacing is 54.4 µm × 27.2 µm. The first vector position is about 27.2 µm away from the wall. The measurements were conducted at 345 CAD. The engine was motored at 2000 rpm and operated for three intake valve open timings of -30 CAD.

The operation condition of the engine tested contain strong cycle-to-cycle variations, which results in the large root-mean-square values of velocity fluctuation near the center of the piston top (Shimura et al., 2018). To evaluate flow characteristics in the cycle-to-cycle variations, conditional averaging based on magnitude of fluid velocity is used in this study. Figure 1(a) shows a histogram of the magnitude of combined velocity of u and w. The magnitude of velocity can be considered as momentum of fluid because few fluctuation of density is considered and temperature boundary layer is enough thin compared to the velocity boundary layer. The large variations in the momentum can be observed in Fig. 1(a). The large variations are considered to be caused by the variations of tumble core locations. The fraction of the large momentum Here, the momentum are classified into C1 to C4 based on fractions (C1: 54.4%, C2: 19.5%, C3: 19.5%, C4: 6.6%). Figure 1(b) and (c) shows mean velocity distribution classified into C1 and C4 in Fig. 1(a). The distribution is fitted to the log-law velocity profile of developed wall turbulence. The mean velocity profile for C1 shows large discrepancy from that of general turbulent boundary layers, while that for C4 show relatively close to that of general turbulent boundary layer. C2 and C3, which are not shown here, have trend between the C1 and C4 profiles. These results show that the velocity profiles which can be assumed to be the developed turbulent boudary layer in the targeted condition is less than half of cycles, which means partial applicability of conventional CFD models for prediction of boundary layer of the engine condition.

Rayleigh-Bénard convection in air: out-of-plane vorticity from stereoscopic PIV measurements

2021-09-01T21:54:13+00:00

We present results from stereoscopic PIV measurements in a Rayleigh-Benard convection (RBC) cell filled with (compressed) air at Rayleigh numbers: Ra = 1.5×10⁴, 2×10⁴, 1×10⁵, 2×10⁵, 5×10⁵, and Prandtl number Pr ' 0.7. The three largest Rayleigh numbers are obtained pressurising the whole set-up including cameras and objective lenses, up to 4.5 bars. The main goal of this study is to reproduce DNS data that are acquired at the same Rayleigh numbers to study far-tail events of the out-of-plane vorticity component (ω_z). The measurements are performed in a RBC cell with aspect ratio Γ = W/H = 10, where W is the width and H = 3 cm is the height of the domain. The cell is equipped with a transparent bottom plate heated by a thin oxide layer (for details see Kastner et al. (2018)), which allows us to measure 3C2D velocity fields on ¨a horizontal plane at mid height of the cell. The RBC cell set-up is inserted in the SCALEX facility of TU Ilmenau, a pressure vessel with several optical accesses that can be pressurised up to 10 bars

The experiments aim firstly at improving the quality of previous measurements performed in the sameset-up [Kastner et al. (2018), Cierpka et al. (2019)], regarding the accuracy of the out-of-plane velocity ¨component. This has been realised by positioning the cameras at a larger stereo angle (about 25^◦), which is possible by placing them inside the pressure vessel. Major challenges of the current measurements are caused by optical distortions due to the temperature gradients that are typical for thermal convection (see Valori (2018), Valori et al. (2019)).

Probability Density Functions (PDFs) of ω_z from stereo PIV experiments and from DNS data are shown respectively in figure 1(a) and 1(b) for all Rayleigh numbers studied. We can observe that for both kind of data the tails of the PDF becomes wider while increasing the Rayleigh number, which may be connected to intermittency. This crossover from Gaussian to intermittent statistic was recently studied in Valori and Schumacher (2021) from DNS. Figure2(a) shows the temporal evolution of ωz at the position of its largest (extreme) value at Ra = 2.5 × 10⁵, while figure2(b) shows the spatial distribution of ω_z at the time of its extreme event in the experiments.

The experimental results are able to reproduce well the statistics of DNS data of the same flow, and allow the study of extreme events of ω_z.

On the challenges of precise velocity measurements in vertical convective wall jets

2021-09-07T20:12:54+00:00

For the transition of our energy supply towards a higher share of renewables, thermal energy storage (TES) systems are, besides electric batteries and chemical energy storage systems, one promising solution to overcome the volatile nature of renewable energy sources. For the most efficient operation, the liquid storage material in the tank should be stratified by its temperature-dependent density. As a result, the cold fluid remains at the bottom, and the heated fluid rises to the top (Alva et al. (2018)). Typically steel tanks are used for TES, and thus, the wall material has a thermal diffusivity that is one to two orders of magnitude
higher than that of the storage fluid. Consequently, the tank’s sidewalls work as a thermal bridge between the stratified layers. In recent studies, the authors have shown that the resulting heat flux induces two counterdirected, convective wall jets near the sidewalls of the tank, which increase mixing of the stratification and thus lowers the exergy content and the storage efficiency (Otto et al. (2019, 2020)). Using a model experiment of a TES, the entire vertical extent of the detected wall jets is investigated. Hence, the typical flow structures of vertical, natural convection under the influence of non-zero temperature gradients in the ambient fluid can be analyzed, which can help to improve storage tanks in the future.

The velocity in the region of the wall jets is measured via 2d particle-image velocimetry (PIV) in a rectangular model experiment of 750mm height on a base area of 375mm×375mm made from polycarbonate. The jets evolve on the surface of an aluminum plate simulating the storage tank’s sidewall. The measuring system consists of four cameras with a resolution of 2160×2560 pixels combined with objective lenses with 100mm focal length capturing the raw images in a plane perpendicular to the aluminum wall. A Nd:YAG laser with a wavelength of 532nm illuminates the measuring plane. Simultaneously using up to four cameras adjacent to each other and stitching their resulting vector fields, the vertical extent of the field of view increases from 38mm up to 140mm. Despite this, the field of view is still much smaller than the vertical extent of the model experiment, so that seven consecutive runs are performed to cover the entire height. Disturbing reflections of the laser light sheet on the aluminum wall are eliminated using optical filters for the cameras that are opaque for the green laser light in combination with fluorescently (Rhodamine B) dyed PMMA tracer particles with a diameter between 1–20μm. The particles emit light at a wavelength of 610nm (orange light) and can therefore be detected through the cameras’ filters. During four separate measuring periods, where each lasts for two minutes, double frame images are captured with a time difference of 19.981 ms (maximum possible value) at a measuring frequency of 7 Hz. Figure 1 shows a schematic of the camera setup next to the model experiment and the measurement and evaluation procedure to finally receive one time-averaged velocity field per measuring period of the full height of the experiment.

The raw data evaluation process starts with calculating the vector fields of all cameras used at a certain measuring position and stitching them to one flow field of this position. Since the wall jets’ horizontal extents are with 2–7mm relatively small and they show high velocity gradients, the raw images are evaluated in both single-frame and double-frame mode. With a velocity threshold that corresponds to a pixel displacement of 1/4 of the interrogation window size and the time difference of the single-frames, the resulting vector fields are masked and merged into one final vector field. This vector field consists of high velocities evaluated in double-frame mode and low velocities evaluated in single-frame mode (see Figure 2) thus minimizing the relative error. The algorithm used in this work is similar to the multi-frame PIV approach introduced by Hain and K¨ahler (2007). Figure 3 shows the time-averaged results of the first measuring period for each of the seven measuring positions in height.

Application of simultaneous time-resolved 3-D PTV and Two Colour LIF in Studying Rayleigh-Benard Convection

2021-09-27T13:55:31+00:00

To study the flow topology and temperature distribution of Rayleigh-Benard convection in a highly slender cell, measurement of the simultaneous velocity and temperature in the 3-D domain is required. For this aim, implementing a simultaneous time-resolved 3-D PTV and two-colour PLIF is planned. As a part of this development, for both PTV and two-colour PLIF techniques, the experimental setup has been implemented separately to measure time-resolved 2-D velocity and temperature and is presented in this paper. For PTV, a scanning system is also utilized to scan the flow field to capture the planar velocity in different depths of the flow domain. Progress on calculation of the out-of-plane velocity component including the theory is discussed. Finally, results of the time-resolved 2-D PTV and PLIF systems are presented.

PIV measurements of entrainment process of directly injected media in internal combustion engines

2021-09-27T20:36:27+00:00

PIV measurements have been successfully applied to various flow fields relating internal combustion engines such as in-cylinder air motion, air flow in an intake port, and even a discharging passage of an ignition plug. Measurements of induced air motion around diesel sprays can be said to be a significant example of the PIV applications because the air motion is reflected in an unsteady complicated flow structure. Instead of the apparent entrainment exaggerated by spray droplet dispersing, substantial air entrainment through momentum exchange between liquid and gas was finally obtained by combining PIV and spray profile observation.

PIV measurements of this kind were extensionally applied to other direct fluid injection by the authors. The second object was a high-pressure gas jet directly injected under gas pressure as high as 30 MPa. It was found the gas jet has strong air entrainment through momentum exchange in a single gaseous phase between fuel gas and ambient air. The third directly injected medium in internal combustion engines should be torch flame ejected from nozzle holes of a pre-combustion chamber (PCC) to a main combustion chamber (MCC) of a so-called DF (dual-fuel) engine.

In this study, mixture entrainment process of torch flames is discussed on the PIV results for the first time. However, chamber configurations of a real DF engine are hard to simulate since it
requires several auxiliary PCC devices such as an ignition plug, a sub gas injector, and so on. All of them should be actuated synchronously with an engine crank angle. In the case of a constant volume vessel (CVV), the synchronization is not necessary, but the mixture control in the PCC becomes problematic because of the lack of compression and expansion strokes that assures PCC
gas exchange. For overcoming the situation, rupture of a membrane was introduced in this study. The membrane turns the upper part of the PCC into an air pressure reservoir and low-pressure air jets eject from the nozzle holes after a solenoid-driven needle pierces the membrane for rupturing. The differential pressure between the upper chamber and the lower one was chosen as a main parameter of the experiment.

Since the measurements and analysis of the entrainment of the low-pressure air jets are yet to finalize, the outlook of the CVV, the PIV specifications, and prime results of the air entrainment are attached herewith. After all, the PIV measurements revealed essential difference among air entrainment processes of the above three directly injected media in internal combustion engines.

Flow Field Study of a Top Heated Immiscible Liquid Layer Adjacent to Ice

2021-09-01T14:29:53+00:00

A series of experiments were conducted to investigate the flow field of a top-heated liquid fuel adjacent to an ice block. The experimental setup consisted of a borosilicate container containing an ice wall and a layer of n-heptane heated from above. Particle Image Velocimetry (PIV) and Background Oriented Schlieren (BOS) measurements were conducted on the liquid -phase. PIV measurements showed a surface flow toward the ice caused by surface -tension forces, which is driven by the horizontal temperature gradients on the liquid surface. A recirculation zone was observed under the free surface and near the ice. The combination of the two flow patterns caused lateral intrusion in the ice, instead of a uniform melting across ice surface. BOS measurements indicated presence of density gradients below the free surface of n-heptane and in regions near the ice block. These density gradients were created by local small-scale temperature gradients. The current experiments were conducted to explore the processes that influence the ice melting by immiscible liquid layers.

Ultrasound PIV Uncertainty Quantification

2021-09-02T13:59:55+00:00

Ultrasound Particle image velocimetry (UPIV) is a non-invasive flow measurement technique where acousticopaque flow tracers are injected into a working fluid and ensonified to create ultrasound images. These images are processed using PIV cross-correlation based algorithms to measure the velocity field (Kim et al., 2004). UPIV is useful for opaque flows, primarily where complex flows exist, accordingly, it is used in many industrial and clinical research applications such as studying intracardiac flow (Crase et al., 2007). Furthermore, the measurement provides suitable temporal and spatial resolutions for improved diagnostic metrics. Mentioned applications and the sensitive diagnostic industrial and clinical decisions made based on these measurements intensifies the importance of characterizing the UPIV measurement accuracy and associated uncertainty. However, quantifying UPIV measurement uncertainty is non-trivial due to the complexity of possible uncertainty sources, their combination, and propagation through the measurement chain.

The formation of a particle image by ultrasound significantly differs from optical imaging, introducing unique aspects to image quality that must be considered. Particle images are formed across several ultrasound scan lines, yielding an elliptical particle image shape. Furthermore, the particle’s reflected pressure wave is converted to a digital signal that undergoes signal modulation, and this process forms a non-Gaussian point spread function (PSF) along the scan line direction. Additionally, clusters of tracers produce a single, bright image intensity and speckle image pattern. Compared to conventional PIV images, UPIV incurs significantly higher image noise due to lack of filtration for the ultrasound reflection of the non-tracer obstacles.

Visualization and Quantification of the Cerebral Microcirculation using Contrast-enhanced Ultrasound Particle Tracking Velocimetry

2021-09-15T19:50:13+00:00

Noninvasive measurements of the regional microvascular perfusion might lead to sensitive biomarkers for the changes in intracranial hemodynamics that could guide timely surgical interventions for neonatal brain injuries. The current work utilizes a clinically available contrast enhanced ultrasound (CEUS) system and particle tracking velocimetry to perform ultrasound localization microscopy for measuring the microcirculation in piglets. A new deep learning method based on U-net is proposed for enhancing noisy raw CEUS images and detecting the microbubbles. Subsequently, the bubbles are tracked using a Kalman filter based method, which incorporates conditions of spatio-temporal consistency in flow direction and globally optimizes the assignment of bubbles to trajectories. Based on analysis of synthetic data, the U-net results demonstrate significant improvement in the processing speed and localization accuracy over a conventional blind deconvolution method. Visualization of the microvasculature is performed by superposing the bubble trajectories, enabling depiction of a complex micro-vessel network, where neighboring vessels separated by 40 µm can be distinguished. The corresponding perfusion map shows the velocity distribution in these vessels. Based on the current frame rate (44 fps), speeds in the 0.1 to 12 cm/s range can be well captured. These methods show promise as potential clinical tools for bedside measurement of cerebral microcirculation.

Development of Echo-LPT for the study of particle-wall interactions in dense suspensions

2021-09-29T16:52:32+00:00

Examining the behaviour of dense suspensions has proven to be difficult, both experimentally and numerically. Using super water–absorbent polymer, PIV measurement was successfully conducted in a hydrogel suspension with a volume fraction (VF) of Φ =20% (see Zhang and Rival, 2018). However, due to the slightly refractive index mismatch, the image quality will degrade significantly as the particle loading of the hydrogel is increased. In order to achieve flow measurements in suspensions with high volume fractions, non-optical based techniques such as ultrasound imaging velocimetry (UIV) should be implemented. UIV has been developed for fluid dynamics applications and embraced by many researchers to study fluid flows (Gurung and Poelma, 2016; Jeronimo et al., 2019). Although, UIV provides useful information about the flow physics, it is unable to provide Lagrangian quantities such as particle trajectories, which is a key parameter to study entrainment and particle-wall interactions.

Measurement of energy spectrum by using 100 eyes Tomographic PIV

2021-09-02T14:13:34+00:00

PIV is one of the methods to measure velocity in a flow field, but its dynamic velocity range is narrower than other flow velocimeter. This disadvantage is particularly apparent in measurements of spectrum in turbulent boundary layers, where the higher wave number side of the spectrum cannot be measured with high accuracy.

In this study, we captured images of the same particle in the flow field from many different direction simultaneously, and reduced the measurement error of the particle displacement by averaging the acquired particle positions, so called ‘Multiple Eye PIV’ [Maekawa, A., Sakakibara, J., 2018, Meas. Sci. Tech., 29, 064011]. We applied this method to obtain the energy spectrum in a turbulent pipe flow aiming for resolving higher wave number. Particle images were captured by a single high-speed CMOS camera (Fastcam Nova S6, 6000 fps, Photron) through a mirror array consists of 110 flat mirrors arranged in the shape of an axisymmetric ellipsoid (Fig.1), as shown in Fig.2. The images were evaluated by Tomographic PIV method to resolve three-dimensional velocity field.

Fig.3 shows energy spectrum in a pipe measured by Tomographic-PIV with number of mirrors, N, up to 100 in addition to the 2D2C-PIV with a single mirror. Although the spectrum curve for the result of Tomographic-PIV begins to depart from the reference curve at wavenumber beyond 10^-1 , such wavenumber grows as N increases, and consequently the plateau of the curve appeared at lower energy. Such a downward shift of the plateau is expected due to the improvement of the dynamic velocity
range, which is approximately one order in energy, i.e. three times in velocity, found between N=4 and 100. Note that the cases of N=4 and 40 loses the dynamic range against the 2C2D-PIV case. From the above, we can summarize that the advantage of Multiple Eye PIV over the 2C2D-PIV is effective when the number of mirrors is more than 40.

In this experiment, the issue is that particles images flickered. In order to resolve this issue, we tried to use fluorescent particles, and obtained a clear particle images in the following experiment. We are now analyzing whether the energy spectrum can be measured with higher accuracy due to improved resolution of the particles.

Validation of Multi-Frame PIV Image Interrogation Algorithms in the Spectral Domain

2021-09-07T19:30:46+00:00

Multi-frame correlation algorithms for time-resolved PIV have been shown in previous studies to reduce noise and error levels in comparison with conventional two-frame correlations. However, none of these prior efforts tested the accuracy of the algorithms in spectral space. Even should a multi-frame algorithm reduce the error of vector computations summed over an entire data set, this does not imply that these improvements are observed at all frequencies. The present study examines the accuracy of velocity spectra in comparison with simultaneous hot-wire data. Results indicate that the high-frequency content of the spectrum is very sensitive to choice of the interrogation algorithm and may not return an accurate response. A top-hat-weighted sliding sum-of-correlation is contaminated by high-frequency ringing whereas Gaussian weighting is indistinguishable from a low-pass filtering effect. Some evidence suggests the pyramid correlation modestly increases bandwidth of the measurement at high frequencies. The apparent benefits of multi-frame interrogation algorithms may be limited in their ability to reveal additional spectral content of the flow.

Extending the Frequency Limits of "Postage-Stamp PIV" to MHz Rates

2021-09-07T19:37:06+00:00

The effective frequency limits of postage-stamp PIV, in which a pulse-burst laser and very small fields of view combine to achieve high repetition rates, have been extended by increasing the PIV acquisition rate to very nearly MHz rates (990 kHz) by using a faster camera. Charge leaked through the camera shift register at these framing rates but this was shown not to bias the measurements. The increased framing rate provided oversampled data and enabled use of multi-frame correlation algorithms for a lower noise floor, increasing the effective frequency response to 240 kHz where the interrogation window size begins to spatially filter the data. The velocity spectra suggest turbulence power-law scaling in the inertial subrange steeper than the theoretical -5/3 scaling, attributed to an absence of isotropy.

smartPIV – an app for flow visualization by cross-correlation and optical flow using smartphones

2021-09-08T16:23:42+00:00

In recent years smartphones considerably changed our communication and are used on a daily (or even every minute) basis especially by students without any difficulties. Fluid flows also belong to our daily experiences. However, the education of the basic principles of fluid mechanics is sometimes cumbersome due to its non-linear nature. This problem may be tackled in practical sessions applying flow visualization techniques in wind or water tunnels and directly learn from own observations. Nowadays, often optical methods like particle imaging velocimetry (PIV) or particle tracking velocimetry (PTV) are used for these purposes. A typical PIV/PTV setup consists of a (double)pulse laser, a scientific camera and a synchronization device. The costs for this equipment can easily add up to more than 100,000 euros and the installations and set up of the systems requires experiences and is complex. For these reasons Universities often only offer practical courses for a small amount of students and the students may not be allowed to use and set up the systems by their own as the equipment is also needed for scientific research. Due to the COVID-19 pandemic it is also often not allowed to share equipment or even to work in larger groups during practical sessions.

Calibration correction of arbitrary optical distortions by non-parametric 3D disparity field for planar and volumetric PIV/LPT

2021-09-20T20:11:49+00:00

In this study, a new image calibration approach is presented that corrects arbitrary optical distortions by utilizing non-parametric, 3D disparity fields. A calibration plate with a high spatial resolution (i.e., high density of calibration marks) was used to identify optical distortions that remain after the initial calibration, which were then used to create a correction field for the pinhole or polynomial mapping functions. Results from a pipe flow experiment with four cameras using volume self-calibration (VSC) and Shake-the-Box Lagrangian particle tracking (STB LPT) are presented and the impact of the improved calibration is discussed. Using the calibration marks with the correction field, distortions of initially more than 20 pixels are reduced below 1 pixel. Using VSC with the correction field allows further reduction of average calibration disparities below 0.02 pixels (maximum 0.5 pixels), whereas without a correction field the remaining average disparity is much higher at 1 pixel (maximum 5 pixels). STB analysis of the data shows a considerable higher spatial resolution at the pipe wall and a consistent spatial distribution of the number of detected particles in the measurement volume.

Adjustable window for 2D PIV estimation based on local Lagrangian coherency

2021-09-22T14:28:05+00:00

We present a novel approach to adjust shapes of the interrogation windows (IW) in Particle Image Velocimetry (PIV) measurements as a function of temporal and spatial local coherent motions. Lagrangian Coherent Structures (LCS) has been widely utilized to determine local flow boundaries. We propose using Finite-Time Lyapunov Exponent (FTLE) to quantify LCS separatrix boundaries (i.e. ridges) and adjust the interrogation window. We integrated the proposed method with a local optical flow PIV algorithm. The evaluation was performed using synthetic particle images of 2D homogeneous isotropic turbulence obtained from Direct Numerical Simulation (DNS). The results showed significant improvements in regions with complex flow behaviours, particularly shear, vortex and hyperbolic motions. We studied improvements of the velocity estimation in a real experiment of the wake flow behind a cylinder at Reynolds number equal to 3900. It was found that optical flow featured by coherency based interrogation window (coherent optical flow) reveals detailed vector field estimations in regions with complex behaviours inside the wake flow.

A stereoscopic PIV system for the Princeton Superpipe

2021-09-22T18:57:56+00:00

Herein, we describe the design and testing of a stereoscopic PIV system uniquely adapted for the high pressure environment of the Princeton Superpipe. The Superpipe is a recirculating pipe facility that utilizes compressed air as the working fluid to attain very high Reynolds numbers. Commercial piping is used as the pressure vessel to hold pressure up to 220 bars, and a test pipe is enclosed inside with a development length of 200 diameters that ensures a fully-developed condition at the test section. The highest achievable Reynolds number (based on the bulk velocity and the pipe diameter) is 35×10⁶, corresponding to a maximum friction Reynolds number of 5×10⁵.

The unprecedented range of Reynolds number has enabled a number of new insights in the behavior of high Reynolds number wall-bounded turbulence (Zagarola and Smits, 1998; Hultmark et al., 2013). However, past measurements in the Superpipe have been primarily restricted to single-component, one- or two-point statistics of fully-developed pipe flows. The present work aims to expand the capability of the Superpipe to study turbulent coherent structures and multi-point statistics by means of a new stereoscopic PIV system. The high pressure environment and the confined space inside the pressure vessel pose challenges to both imaging and seeding, the solutions to which will be discussed.

Investigation of Best Practices in Volumetric Reconstruction for Plenoptic PIV

2021-09-22T19:23:23+00:00

This paper investigates the effect of smoothing operation in 3D reconstruction using a plenoptic camera. A plenoptic camera - also known as light field camera - features a commercial off the shelf camera with added microlens array (MLA) behind the imaging lens, directly in front of the sensor. The main lens focuses the light to the MLA plane, where each microlens then re-directs the light to small regions of pixels behind, each pixel corresponding to different angle of incident (T. Fahringer (2015)) (Adelson and Wang (1992)). Thus, MLA encodes angular information of incident light rays into the recorded image that assist to acquire 4D information (u,v,s,t) of light-field including both position and angular information of light rays captured by the camera (Ng et al. (2005)) (Adelson and Wang (1992)).

Super-resolution PIV using multi-frame displacement information

2021-09-23T14:14:24+00:00

High-resolution (HR) fluid-flow velocity information is important to reliably analyze fluid measurements in particle image velocimetry (PIV), such as the boundary layer and turbulent flow. Efforts in PIV to enhance the resolution of flow fields are mainly based on single-frame information, which follows the velocity field estimation and may influence the final reconstruction accuracy. In this study, we propose a novel super-resolution (SR) reconstruction technology from another perspective, which consists of two parts: a multi-frame imaging system and a Bayesian-based multi-frame SR reconstruction algorithm. First, a splitbased imaging system is developed to obtain particle image pairs with fixed displacements. Subsequently, we present a Bayesian-based multi-frame SR (BMFSR) reconstruction algorithm to obtain an SR particle image. Multi-frame particle images collected by the developed system are used as the input low-resolution images for the following novel SR reconstruction algorithm. Synthetic and experimental particle images have been tested to verify the performance of the proposed technology, and the results are compared with the traditional and advanced reconstruction methods in PIV. The results and comparisons show that the proposed technology successfully achieves good performance in obtaining finer particle images and a more accurate velocity field.

Multi-spectral imaging for Thermochromic Liquid Crystal based Particle Image Thermometry: A proof of concept

2021-09-27T16:20:12+00:00

In this contribution, a novel imaging approach for Thermochromic Liquid Crystal (TLC) based Particle Image Thermometry (PIT) is demonstrated. In contrast to state of the art approaches, a multi-spectral camera was used to record the color response of the Thermochromic Liquid Crystals seeding particles. An experiment with a transparent, water-filled, cylindrical cell as the central element was set up to investigate the novel approach. The temperature in the cell can be controlled by adjusting the temperature of the bottom and top plate. Calibration images at eleven different temperatures ranging from 18 ^◦C to 21.6 ^◦C, as well as images of a stable thermal stratification, were recorded. 90 percent of the calibration data was used to train a neural network (NN) to predict the temperature. The remaining 10 percent of the calibration data and the data of the stable thermal stratification were used to test the NN. The tests show that the deviation between predicted and ground truth temperature is mostly below 0.1 K and that the linear profile of the stable thermal stratification can be predicted with a maximum deviation of ≈ 0.15 K. This shows that multi-spectral imaging with neural networks for data processing is feasible and a promising concept.

A novel method to accurately align the laser sheet for planar and stereoscopic PIV

2021-09-27T18:02:58+00:00

Several techniques including two-dimensional (2D) and three-dimensional (3D) calibration are used for the calibration of two-component two-dimensional (2C-2D) particle image velocimetry (PIV) and three-component two-dimensional (3C-2D) stereoscopic PIV (SPIV) systems. A major requirement of these techniques is to keep the calibration target exactly at the position of the laser sheet within the field of view (FOV), which is very difficult to achieve (Raffel et al., 2018). In 3C-2D SPIV, several methods offer different correction schemes based on the disparity between the FOV of two stereo cameras produced due to misalignment, to account for the misalignment error. These techniques adjust the calibration or the measured displacement field in different ways to reduce the error which may introduce an unintended error in the measurement position and/or velocity such as a bias in the measured three-component 3C displacements. This paper introduces a novel method to align the laser sheet with the calibration target so that the uncertainty in displacement measurements is minimal. Ideally, it should be of the order of the uncertainty associated with PIV measurement so that no ad hoc post-correction scheme is required.

A novel streak velocimetry technique based on 2D fits of decaying phosphor particle images

2021-09-27T18:07:14+00:00

A new 2D velocimetry technique based on streaks formed by individual phosphor particles, which are moving during their luminescence decay following pulsed excitation is proposed in this study. Tin-doped phosphor particles (Sr,Mg)₃(PO4)₂:Sn²⁺ are dispersed into flows and excited by a pulsed UV light sheet. During the phosphor decay time (~27 µs), the emission streaks due to particle motion are recorded. A 2D fitting is then applied on each particle streak against the analytical expression of intensity distribution, to obtain the velocity information for each particle. Unlike Particle Tracking Velocimetry (PTV) this technique does not rely on any particle image searching procedure.

TrackAER: Real-Time Event-Based Particle Tracking

2021-09-27T18:14:12+00:00

Event-based cameras (Lichtsteiner et al., 2008; Posch et al., 2010; Gallego et al., 2020) operate fundamentally different from frame-based cameras: Each pixel of the sensor array reacts asynchronously to relative brightness changes creating a sequential stream of events in address-event representation (AER). Each event is defined by a microsecond-accurate time stamp, the pixel position and a binary polarity indicating a relative increase or decrease of light intensity. Thus, event-based cameras only sense changes in a scenery while effectively suppressing static, redundant information. This renders the camera technology promising also for flow diagnostics. In established approaches like PIV or PTV vast amounts of data are generated, only for a large part of redundant information to be eliminated in data post-processing. In contrast, eventbased cameras effectively compress the data stream already at the source. To make full use of this potential, new data processing algorithms are needed since event-based cameras do not generate conventional framebased data. This work utilizes an event-based camera to identify and track flow tracers such as helium-filled soap bubbles (HFSBs) with real-time visual feedback in measurement volumes of the order of several cubic meters.

Utilization of direct forcing immersed boundary methods for the optimization of inertial focusing microfluidics

2021-09-02T14:31:37+00:00

Inertial focusing microfluidics have gained significant momentum in the last decade for their ability to separate and filter mixtures of particles and cells based on size [1-3]. However, the most important feature is that the separation is passive, without the need for external forces. At the heart of inertial focusing is the balance between counteracting lift forces: shear and wallinduced lift. Shear-induced lift is a product of the curvature of the fluid flow and the rotation of the particle in the flow, while wall-induced lift is generated by the disturbance of the fluid by the particle near a wall. This phenomenon was first observed by Segre and Silberberg for the focusing of particles in a pipe, and was later extended to the focusing of cells and particle in rectangular channels [4]. Taking advantage of inertial focusing we explore particle capture utilizing an expanded channel microfluidics chip design. By expanding a small region of the straight channel microvortices form in the well, which allows for size selective trapping of particles.

Characterization of interactive force acting on colloidal particles near an electrode in presence of a high-frequency (>10 kHz) AC electric field using particle diffusometry

2021-09-02T18:48:38+00:00

Colloidal particles like polystyrene beads and metallic micro and nanoparticles are known to assemble in crystal-like structures near an electrode surface under both DC and AC electric fields. Various studies have shown that this self-assembly is governed by a balance between an attractive electrohydrodynamic (EHD) force and an induced dipole-dipole repulsion (Trau et al., 1997). The EHD force originates from electrolyte flow caused by interaction between the electric field and the polarized double layers of both the particles and the electrode surface. The particles are found to either aggregate or repel from each other on application of electric field depending on the mobility of the ions in the electrolyte (Woehl et al., 2014). The particle motion in the electrode plane is studied well under various conditions however, not as many references are available in the literature that discuss the effects of the AC electric field on their out-of-plane motion, especially at high frequencies (>10 kHz). Haughey and Earnshaw (1998), and Fagan et al. (2005) have studied the particle motion perpendicular to the electrode plane and their average height from the electrode mostly in presence of DC or low frequency AC (<1 kHz) electric field. However, these studies do not provide enough insight towards the effects of high frequency (>10 kHz) electric field on the particles’ motion perpendicular to the electrode plane.

Determination of the Near Wall Flow of a Multi-Stage Tesla Diode Using PIV and PTV

2021-09-27T16:21:16+00:00

Particle image velocimetry (PIV) and particle tracking velocimetry (PTV) are two popular methods to measure the velocity in complex geometries such as the Tesla valve. This paper provides an investigation on the application of a tessellation meshing method for interpolating non-uniform velocity vectors calculated using PTV. The procedure to apply this method containing mask generation and mesh study is described. The results are compared to the PIV results particularly where the near wall results are important. The result of the flow field calculated by the application of the tessellation method on the PTV results are presented for a two-stage Tesla valve operated in the range of Re = 100 to 600 both in forward and reverse configuration.

DPTV-based analysis of the flow-structure/wall-shear interplay in open wet clutches

2021-09-27T19:30:13+00:00

The trend to lower energy consumption in the automotive industry still offers potential in various fields of application. One powerful saving strategy is described by the idling behavior of wet clutches, where the speed difference between drive and output, and the cooling oil in combination with a sub-millimeter spacing leads to significant amounts of wall shear stress (WSS) and accordingly drag torque. Minimization of this adverse effect has been found to be possible by means of grooved clutch-disk geometries, which have been demonstrated to correlate with the drag torque (see e.g. Neupert et al., 2018). The main interplay between torque and fluid flow in open wet clutches has been analyzed by Leister et al. (2020) in a dimensionless way. Today, a detailed investigation of a clutch flow, however, is missing for a larger variety of groove patterns and the cause-effect relations remain yet to be fully understood. Especially, the clear identification of the so-called foot print of a particular groove geometry in the flow field and corresponding WSS – thus drag-torque predictions – still requires further research efforts.

Quantification of Brownian motion under presence of flow using particle diffusometry

2021-09-27T21:13:58+00:00

Particle diffusometry (PD), a quantification method for the Brownian motion, is performed by recording temporally sequential images and using correlation analysis to obtain an ensemble diffusion coefficient for all particles captured in the imaging region (Clayton et al., 2017). PD is proven to be successful in the detection of the waterborne pathogen V. cholerae in environmental samples using different imaging techniques, including an inverted fluorescence microscope as well as a handheld hardware device operated with a smartphone (Clayton et al., 2019; Moehling et al., 2020). Although we intend to use PD to calculate diffusion coefficients in quiescent fluid, oftentimes unintentional fluid flows occur, creating measurement error when calculating the diffusion coefficient. In previous work, recordings under the presence of flow were discarded to avoid incorrect measurements of the sample.

Measurement of the acoustic streaming pattern in a standing surface acoustic wave field

2021-08-31T19:00:52+00:00

The application of standing surface acoustic waves (sSAW) has enabled the development of many flexible and easily scalable concepts for the fractionation of particle solutions in the field of microfluidic lab-ona-chip devices. In this context, the acoustic radiation force (ARF) is often employed for the targeted manipulation of particle trajectories, whereas acoustically induced flows complicate efficient fractionation in many systems [Sehgal and Kirby (2017)]. Therefore, a characterization of the superimposed fluid motion is essential for the design of such devices. The present work focuses on a structural analysis of the acousticallyexcited flow, both in the center and in the outer regions of the standing wave field. For this, experimental flow measurements were conducted using astigmatism particle tracking velocimetry (APTV) [Cierpka et al. (2010)]. Through multiple approaches, we address the specific challenges for reliable velocity measurements in sSAW due to limited optical access, the influence of the ARF on particle motion, and regions of particle depletion caused by multiple pressure nodes along the channel width and height. Variations in frequency, channel geometry, and electrical power allow for conclusions to be drawn on the formation of a complex, three-dimensional vortex structure at the beginning and end of the sSAW.

Volumetric Flow Rate Measurement using Surface Imaging Techniques

2021-09-02T15:51:31+00:00

The purpose of our research is to validate an experimental method developed by Johnson and Cowen (2016) aimed at measuring volumetric discharge in an open channel using Surface particle image velocimetry (SPIV) combined with turbulent boundary layer analysis to infer the bathymetry and calculate volumetric flow rate, ultimately extending this work to natural systems (Hendrickson, 2020).

An Infrared Quantitative Imaging Technique (IR-QIV) for Remote Sensing of River Flows

2021-09-02T16:13:31+00:00

Stage and discharge are some of the oldest measurements in environmental fluid mechanics and are vital in forecasting water supply and flood safety. These measurements are traditionally manpower intensive, hence expensive, and dangerous under high flow conditions. Considering climate change and the planet’s increasing population there is a critical need for better, more accurate, and frequent, in space and time, data for model and forecast guidance. This need spans monitoring small-scale turbulent processes to calibrating and nudging continental scale river dynamics models. Driven by applications from river gaging networks to fish behavior modeling to flood and erosion forecasting, and more generally, the near-shore environment of lakes, estuaries and the coasts, remote sensing with quantitative imaging tools is a rapidly expanding field. Such tools can be deployed from fixed platforms, drones, planes and satellites with valuable information contained within the visible to infrared spectral bands.

Large Scale Infrared-Based Remote Sensing of Turbulence Metrics in Surface Waters: Going Beyond Mean Flow

2021-09-02T16:31:06+00:00

In recent years field-scale applications of image-based velocimetry methods, often referred to as large scale particle image velocimetry (LSPIV), have been increasingly deployed. These velocimetry measurements have several advantages—they allow high resolution, non-contact measurement of surface velocity over a large two dimensional area, from which the bulk flow can be inferred. However, visiblelight LSPIV methods can have significant limitations. The water surface often lacks natural features that can be tracked in the visible and generally requires seeding with tracer particles, which creates concerns regarding the fidelity with which tracer particles track the flow, and introduces challenges in achieving sufficient and uniform seeding density, in particular in regions with appreciable velocity accelerations such as turbulence. In LSPIV, image collection is generally limited to daylight hours, and can suffer from non-uniformity of illumination across the camera’s field of view. Due to these issues LSPIV often requires spatio-temporal averaging, and as a result is generally able to extracting the mean, but not the instantaneous, velocity field, and hence is often not a suitable tool for calculating turbulence metrics of the flow.

Characterization of Non-linear Internal Waves Using PIV/PLIF Techniques

2021-09-08T19:08:41+00:00

Internal waves are ubiquitous in the ocean. They often form in regions of high temperature or salinity variability as the pycnocline oscillates to form the wave (Phillips, 1966). They can be generated either from the interaction of tidal currents with submarine bathymetry (Garrett and Kunze, 2007) or by wind stress at the ocean surface (Munk and Wunsch, 1998). The current study addresses non-linear internal waves due to their importance in the mixing and dynamics of both atmospheric and oceanographic flows.

Due to the significance of this phenomena, numerous investigations have been conducted to obtain satisfactory theoretical solutions for internal waves in several types of fluid systems. The verification of these models requires precise and accurate experimental data. It should be noted that such models generally assume simple two-layer stratified system separated by a sharp interface. In reality, there is a gradient of density at the interface of the two layers, which can make both experimental and theoretical analysis more challenging. To date, most experimental studies for several types of internal waves have been performed using ultrasonic probes, conductivity probes, resistance-type wave gauges, or salinity-sensor-type wave gauges, as given by Davis and Acrivos (1967),Koop and Butler (1981),Michallet and Barthelemy (1998) and Umeyama (2002). There is one recent study that used the particle image velocimetry (PIV) technique to determine the Eulerian velocity field of internal waves, but it lacks the detailed density measurements necessary to fully understand the flow (Umeyama and Matsuki, 2011).

The current work aims to fully understand the dynamics of internal waves by measuring the density and velocity fields, and then comparing the experimental results with the theoretical non-linear wave solution. A laboratory-scale apparatus was created to replicate the flow characteristics of internal waves in a twolayer stratified system. An experimental configuration is presented with a density jump of 1.1 and 1.5 σ_t separately. Experiments are conducted in the tank (2.438 m × 50 cm × 50 cm), which was constructed from clear acrylic sheets with thickness of 1.905 cm. The schematic of the internal wavemaker apparatus is shown in Fig. 1(a) (Mohaghar et al., 2020). A line diffuser (PVC) was installed along the middle of the tank floor to be used to fill the tank. A half-cylinder plunger-type wavemaker was used to create a perturbation at the pycnocline represented by the interface between the density layers. On each revolution of the drive mechanism, the switch sent a voltage signal to the external trigger port of a pulse generator. By precisely controlling the delay following the external trigger signal, the pulse generator sent a signal to the Nd:YAG laser and the camera to capture an image at a targeted phase of the wave cycle. Images are recorded with a high resolution 29 MP CCD camera, (14-bits, 6600 × 4400 pixels).

PIV was used to measure the velocity field, and the fluids in both layers were seeded with neutrallybuoyant particles. The seeded particles were illuminated using a dual-cavity New Wave Research Gemini PIV laser at wavelength of 532 nm, which is diverged into a sheet. Light entering the PIV camera passed through a 532 nm bandpass filter. The image pairs were processed with Insight 4G^TM software using a 32 × 32 pixel final spot size with 50% overlap. A sample of PIV vector field for the case of ∆ρ = 1.5σ_t is shown in Fig. 1(b). In order to measure the density fields, the flow is visualized using the planar laser-induced fluorescence (PLIF) method for scalar visualization. A laser-fluorescing dye, Rhodamine 6G, is mixed into the heavier layer and the light sheet is used to fluoresce the dye. Following the procedures outlined by Mohaghar (2019), the dye concentration is then inferred from the digital images. In order to capture only fluorescence emitted by Rhodamine 6G, the camera is equipped with a notch filter blocking the 532 nm wavelength of light. A sample of a final processed PLIF image for the case of ∆ρ = 1.5σ_t is shown in Fig. 1(c).

The interface location, density gradient, wave amplitude and period, velocity and vorticity fields, kinetic energy and shear strain rate are quantified by several phases in one wave cycle and subsequently compared with the corresponding predictions based on third-order Stokes internal-wave theory.

Measurements of intracrater flow dynamics utilizing a mound-bearing crater in a refractive index matched environment

2021-09-17T14:13:10+00:00

The processes controlling crater mound formation are the subject of ongoing research (Bennett and Bell III, 2016). Several theories exist on the formation of a central mound, with those pointing to wind processes as the predominant driving mechanisms being among the most compelling (Kite et al., 2013; Day et al., 2016; Anderson and Day, 2017). Few experimental studies have been conducted to uncover impact crater driven flow dynamics. As such, direct and experimental flow measurements that could be used to validate previously developed fluid-topography interaction theories are not yet available. The objective of this study is to elucidate the intra- and extra-crater circulation induced by unidirectional winds using experimentation on scaled models coupled with high spatial and temporal resolution flow measurements.

An experimental approach to analyze aerosol and splatter formation in a dental practice

2021-09-27T21:19:15+00:00

The flow velocity, trajectories, and size distribution of droplets produced during a dental scaling procedure using a Cavitron ultrasonic scalar (CUS) has been investigated by optical flow tracking velocimetry and shadowgraphy measurements. The droplet sizes are found to vary from 5 -500 µm; these correspond to droplet nuclei that could carry viruses. The droplet velocities also vary between 0.7 m/s and 1.3 m/s. These observations confirm the critical role of aerosols in the transmission of disease during dental procedures, providing invaluable knowledge for developing protocols and procedures to ensure the safety of both dentists and patients especially during COVID-19 pandemic.

Three-dimensional temperature and velocity measurements in fluids using thermographic phosphor tracer particles

2021-09-29T16:37:43+00:00

Many flows of technical and scientific interest are intrinsically three-dimensional. Extracting slices using planar measurement techniques allows only a limited view into the flow physics and can introduce ambiguities while investigating the extent of 3D regions. Nowadays, thanks to tremendous progress in the field of volumetric velocimetry, full 3D-3C velocity information can be gathered using tomographic PIV or PTV hence eliminating many of these ambiguities (Discetti and Coletti, 2018; Westerweel et al., 2013). However, for scalar quantities like temperature, 3D measurements remain challenging. Previous approaches for coupled 3D thermometry and velocimetry combined astigmatism PTV with encapsulated europium chelates particles (Massing et al., 2018) or tomographic PIV with thermochromic liquid crystals particles (Schiepel et al., 2021). Here we present a new technique based on solid thermographic phosphor tracer particles, which have been extensively used for planar fluid temperature and velocity measurements (Abram et al., 2018) and are applicable in a wide range of temperatures. The particles are seeded into a gas flow where their 3D positions are retrieved by triangulation from multiple views and their temperatures are derived from two-colour luminescence ratio imaging. In the following, the experimental setup and key processing steps are described before a demonstration of the concept in a turbulent heated jet is shown.

Fluorescent PIV using Atomized Liquid Particles

2021-09-07T15:30:16+00:00

It is shown that aerosolized fluorescent particles generated using a Venturi-type atomizer, from a solution of fluorescent Kiton Red 620 dye in a water/glycol fluid, provide effective flow seeding for fluorescent PIV. The atomized liquid particles were found to be of acceptable size for PIV purposes, with 92% of detected particles by number concentration measuring < 1 μm in diameter. A PIV application was conducted in a wind tunnel (freestream velocity U_∞ = 27 m/s), using the particles for measurement of the boundary layer flow approaching a forward-facing step (approach boundary layer momentum thickness Reynolds number of Re_θ = 5930), to identify potential benefits in near-wall regions normally affected by unwanted laser reflections from tunnel surfaces. Particles were generated from solutions with dye molar concentrations of 2.5 × 10⁻³and 1.0 × 10⁻² mol/L, and PIV images were obtained for both elastic Mie scattering and filtered, Stokes-shifted fluorescent light. Raw images indicate that the fluorescence yield of the 1.0 × 10⁻² mol/L solution provides PIV images with high contrast, even in the near-surface regions where Mie scattering images are highly affected by surface reflections. Boundary layer profiles are processed in the adverse pressure gradient region leading up to the forward-facing step, where the fluorescent PIV performed comparably to the most optimized Mie scattering PIV; both obtained data as near to the wall as 30 μm, or 2 viscous wall units in our flow of interest. These results indicate that the new seeding method holds excellent promise for near-surface measurement applications with more complicated three-dimensional geometries, where it is impossible to arrange PIV cameras to reject surface-scattered light.

Validation of model-based correction for non-Stokesian tracers

2021-09-22T20:10:07+00:00

In-situ flow-tracking measurements at scales on the order of 10 m³and larger remain a challenge. The large size of the tracers required for optical visibility results in an inertial lag and inherently low seeding density. For instance, natural snowfall, fake snow and soap bubbles on the order of 2 cm have been used as tracers for field measurements and extracted statistical quantities (Nemes et al., 2017; Wei et al., 2021; Rosi et al., 2014). There is also growing interest in networks of sensors for remote- measurement where optical access is impossible (Bolt et al. 2020; Villa et al. 2016). Onboard inertial measurement units (IMU) are a promising tool for high-resolution measurements over large spatial domains without optical access. However, due to the intrinsic lag, a dynamic-model-based correction is required for the tracking of transient phenomena, sketched in figure 1. In the present study, the tracer-velocity correction is evaluated by quantifying the residual error in measured flow velocity after the method of Galler et al. (2021) is applied.

Soap bubbles seeding for quantitative time resolved velocity measurements of a turbulent wake flow behind a cylinder

2021-09-22T20:27:40+00:00

The wake flow behind a cylinder of 100mm diameter is investigated using time resolved 2D PIV technique applied to an air flow generated in a closed loop open test section wind tunnel. The flow is seeded using a micro soap bubble generator (BG-1000, TSI Inc.). The bubbles in the air flow were illuminated with a CW laser source and imaged using a high-speed camera. The main purpose of this study is to show features and advantages of using soap bubbles as seeding for a relatively large-scale PIV investigation under low power illumination conditions.

Development of micro soap bubble generator for PIV tracer using home stereolithography 3D printer

2021-09-27T20:32:29+00:00

A micro soap bubble generator for tracers for PIV measurement was developed using a home stereolithography 3D printer. The nozzle has a coaxial triple pipe structure, and an orifice cap is attached to the nozzle head. The inner diameter of the central pipe is 0.7 mm, and the wall thickness of the central pipe is 0.7 mm. From the comparison of the smoke wire visualization result of the flow around the cylinder placed under the mainstream flow velocity of 3 m/s and the PIV measurement result, it was confirmed that the generated micro soap bubbles have good followability to the flow. Generated bubbles’ particle size was estimated to be Φ0.2 mm at the minimum and Φ6.3 mm at the maximum. The most common was Φ0.9 mm ± 0.1 mm, accounting for more than 50% of the total.

PIV measurement of turbulence over a streamwise preferential porous medium

2021-09-07T20:27:29+00:00

This study examines the possibility of orthotropic porous medium whose streamwise permeability is larger than the wall-normal permeability to reduce turbulent friction inspired by recent numerical studies of Rosti et al. (2018); G´omez-de Segura and Garc´ıa-Mayoral (2019). Because G´omez-de Segura and Garc´ıa-Mayoral (2019) used Brinkman equation to approximate the flow in the porous media, it is uncertain that such porous media really reduce the friction. We make a layered porous medium, which satisfies the drag reducing condition suggested by G´omez-de Segura and Garc´ıa-Mayoral (2019), and carry out particle image velocimetry measurements of turbulent square duct flows over it and examine the drag reduction probability. From the analyses of the obtained data, it is found that the friction on the porous-wall is nearly the same as that of the smooth-wall at Re_b < 10000 and tends to increase at Re_b > 10000.

Moving Surface Actuation, Effects of Frequency-based Shear Layer Excitation on the Response of a Bluff Body Wake

2021-09-08T20:01:44+00:00

The effect of actuation frequency, using moving surface actuation, is investigated for a square cylinder bluff body wake. Pressure sensor data are used to optimize actuation characteristics through the implementation of an NSGA-II evolutionary algorithm. Velocity field data are obtained using Particle Image Velocimetry (PIV) for baseline and optimized actuation cases. A Proper Orthogonal Decomposition (POD) analysis shows that the vortex shedding frequency shifts between frequencies associated with the actuation, moving between regions of lock-on and quasi-periodicity. Additionally, the POD shows that the energy contained in the coherent shedding motion is reduced through actuation, while the total energy in the velocity field stays relatively constant. A reconstruction of the first 10 POD modes indicates that the coherent contribution to the Reynolds stresses significantly decreases compared to the non-actuated case. The mechanism for drag reduction is investigated using the shed circulation flux and Kochin’s drag formulation model. The drag obtained using PIV measurements and Kochin’s formulation is consistent with trends observed for the base pressure as a function of actuation frequency.

Stereo PIV measurements of oscillatory plasma forcing in the cross-plane of a channel flow

2021-09-20T20:51:06+00:00

The present work describes an experimental investigation that applies stereo particle image velocimetry in a cross-plane of a turbulent channel flow that is additionally perturbed by spanwise oscillatory body forces, induced by a plasma actuator and designed to mimic the effect of spanwise wall oscillations. The experiment is aimed at retrieving the forcing-correlated scales and the turbulent flow stochastic fluctuations for the measured cross-plane. The first are macroscopic scales and require a larger investigation domain while the latter benefit of a higher resolution. Furthermore, the extended flow-field dynamic range posed a challenge on the experiment design, finally leading to an optimal tradeoff. The results of the unactuated flow compare well to the direct numerical simulations of Hoyas and Jimenez ́ (2008), while the actuated case demonstrates strong near-wall momentum addition and spanwise modulation of the streamwise flow component.

Design of experiments: a statistical tool for PIV uncertainty quantification

2021-09-07T20:34:04+00:00

A statistical tool called Design of Experiments (DOE) is introduced for uncertainty quantification in particle image velocimetry (PIV). DOE allows to quantify the total uncertainty as well as the systematic uncertainties arising from various experimental factors. The approach is based on measuring a quantity (e.g. time-averaged velocity from PIV) several times by varying the levels of the experimental factors which are known to affect the value of the measured quantity. In this way, using Analysis of Variances (ANOVA), the total variance in the measured quantity can be computed and hence the total uncertainty. Moreover, the analysis provides the individual variances for each of the experimental factors leading to the estimation of the systematic uncertainties from each factor and their contribution to the total uncertainty. The methodology is assessed for an experimental test case of the flow at the outlet of a ducted Boundary Layer Ingesting (BLI) propulsor to quantify the total uncertainty in time-averaged velocity from stereoscopic PIV measurements as well as the constituent systematic uncertainties due to the experimental factors, namely, camera aperture, inter-frame time separation, interrogation window size and stereoscopic camera angle.

On the uncertainty of defocus methods for 3D particle tracking velocimetry

2021-09-08T16:35:38+00:00

Defocus methods have become more and more popular for the estimation of the 3D position of particles in flows (Cierpka and Kahler, 2011; Rossi and K ¨ ahler, 2014). Typically the depth positions of particles are ¨ determined by the defocused particle images using image processing algorithms. As these methods allow the determination of all components of the velocity vector in a volume using only a single optical access and a single camera, they are often used in, but not limited to microfluidics. Since almost no additional equipment is necessary they are low-cost methods that are meanwhile widely applied in different fields. To overcome the ambiguity of perfect optical systems, often a cylindrical lens is introduced in the optical system which enhances the differences of the obtained particle images for different depth positions. However, various methods are emerging and it is difficult for non-experienced users to judge what method might be best suited for a given experimental setup. Therefore, the aim of the presentation is a thorough evaluation of the performance of general advanced methods, including also recently presented neural networks (Franchini and Krevor, 2020; Konig et al., 2020) based on typical images.

TrackFit: Uncertainty Quantification, Optimal Filtering and Interpolation of Tracks for Time-Resolved Lagrangian Particle Tracking

2021-09-08T20:47:57+00:00

Advanced Lagrangian Particle Tracking methods (such as the STB algorithm (Schanz et al. 2016)) are a very useful tool for uncovering properties of flow. As a measurement technique, the results of such methods are perturbed by different sources of errors and noise. This work addresses the problem of optimal filtering of particle tracks as well as estimating uncertainties of derived quantities such as location, velocity and acceleration of observed particles. The behavior and performance of this new filtering method (“TrackFit”), first introduced at Gesemann et al. (2016) is analyzed and compared to the Savitzky–Golay filter (Savitzky and Golay (1964)) which is commonly used for these purposes. The optimal choice of parameters of this filtering method as well as the uncertainty quantification of the reconstructed tracks can be extracted from a spectral analysis of the recorded raw particle tracking data. This is in contrast to a Savitzky–Golay filter where the choice of parameters might often be driven by experience and gut feeling. Estimating the power spectral density (PSD) of the particle trajectory signals for the purpose of optimal filtering parameter selection represents a challenge due to possibly short trajectory signals. In the following work we will present a method for PSD estimation that is applicable in this scenario. In addition, we show that regardless of the choice of Savitzky–Golay filter parameters, the resulting filter will not approximate the ideal noise reduction filter well unlike the “TrackFit” described in this work.

A Kalman filtering approach to particle track filtering and track uncertainty quantification for 3D PTV measurement

2021-09-15T20:18:00+00:00

Three-dimensional Particle Tracking Velocimetry (3D-PTV) is a non-invasive flow measurement technique that computes the velocity field by reconstructing 3D particle positions of individual tracer particles and by subsequently tracking those positions. The particle velocity measurement accuracy depends on the faithful reconstruction of 3D particle positions. The complex measurement chain in 3D-PTV involves several steps, from calibration to 3D position reconstruction and particle position tracking, each having its own source of error. Additionally, higher seeding density increases the uncertainty in particle reconstruction and tracking, which in turn, increases the noise in the estimated tracks. A noisy track decreases the measurement accuracy and amplifies any noise in the PTV-derived quantities of interest, which includes acceleration, pressure and vorticity. Thus, track filtering techniques are critical in a 3D-PTV measurement. Track fitting using polynomial functions, filtering methods adopted from signal processing and object tracking are among the well-established techniques used to achieve smooth position, velocity estimates from reconstructed particle trajectories. The Kalman filter is one such filtering technique that is widely used in various applications. The strength of the Kalman filter lies in its ability to perform noise reduction that is informed by existing physical models and the uncertainty estimates of recorded measurements. However, the measurement uncertainty input to the Kalman filter needs to be known at priori, which in many cases may not be available or could be difficult to estimate. In the literature on Kalman filters and their variants applied to 2D-PIV/PTV, the position uncertainty data fed to the filter is either user-defined or estimated based on global noise levels in the PTV measurements. But instantaneous position and velocity uncertainty quantification for individual particle positions/tracks has been challenging in the 3D PTV community. Recent work by Bhattacharya and Vlachos (2020) provides an estimate of the uncertainty in the reconstructed particle positions for a 3D PTV measurement. This position uncertainty estimate dynamically updates the filter gain for each track and enables the evaluation of the performance of the Kalman filter in 3D PTV track filtering.

Uncertainty Quantification for PTV/LPT data and Adaptive Track Filtering

2021-09-21T15:27:39+00:00

Particle Tracking Velocimetry (PTV) or Lagrangian Particle Tracking (LPT) picked up a lot of interest over the last years due to their ability to acquire global flow fields at high spatial and temporal resolution. The most recent research focused mainly on algorithmic advancements in order to increase the obtainable data density and on its application to new flow cases. Only a small amount of studies tried to quantify the measurement uncertainties linked to these volumetric measurement approaches. Within this contribution we want to present how to acquire measurement uncertainties for the position, velocity and acceleration for each data point along a trajectory by means of linear regression analysis tools. Based on these uncertainties, an adaptive filtering approach is introduced, which eliminates the user’s choice of the filter kernel length and which automatically determines its optimal value.

Uncertainty Estimation for Ensemble Particle Image Velocimetry

2021-09-22T17:24:06+00:00

We present a novel approach to estimate the uncertainty in ensemble particle image velocimetry (PIV) measurements. Ensemble PIV is widely used when the cross-correlation signal-to-noise ratio (SNR) is insufficient to perform a reliable instantaneous velocity measurement. Despite the utility of ensemble PIV, uncertainty quantification for this type of measurement has not been studied. The existing uncertainty quantification algorithms for PIV are developed and used only for instantaneous PIV measurement and do not account for the improved SNR in ensemble PIV. Existing instantaneous uncertainty quantification methods can be divided into direct and indirect categories. Indirect methods require calibration based on the effect of various image parameters (such as noise, particle size, density, velocity gradient, etc.) on the correlation SNR. Indirect methods have not been calibrated for error sources relevant in an Ensemble PIV measurement. Also, they have lower sensitivity to the error sources compared to direct approaches. Direct methods, such as the moment of correlation (MC) and Image Matching (IM), find the uncertainty based on the images and correlation planes without any calibration and are more reliable (Bhattacharya et al., 2018; Sciacchitano et al., 2013). Ensemble PIV is based on ensemble correlations; therefore, MC, which uses the generalized cross-correlation (GCC) plane as a measure of uncertainty, is the most suitable method to be modified to be applicable for the ensemble PIV. The GCC plane is the inverse Fourier transform of the phase correlation and represents the probability density function (PDF) of particles’ displacements (Bhattacharya et al., 2018; Eckstein and Vlachos, 2009). We replaced instantaneous GCC with ensemble GCC and modified MC’s normalization factor to account for the number of ensembles. The MC’s primary limitation is that it assumes a Gaussian shape for the PDF of displacements and estimate the standard deviation of the underlying PDF using a fitted Gaussian. However, the PDF deviates from Gaussian distribution due to velocity gradient or non-Gaussian random displacements. Therefore, MC’s reliability and applicability are reduced for flow fields with non-Gaussian PDFs. Also, our analysis shows that ensemble MC consistently underestimates the uncertainty. So, a generalized and reliable method for uncertainty quantification for ensemble PIV is needed.

Uncertainty of PIV/PTV based pressure, using velocity uncertainty

2021-09-22T18:38:23+00:00

Pressure reconstruction from velocity measurements using particle image velocimetry (PIV) and particle tracking velocimetry (PTV) has drawn significant attention as it can provide instantaneous pressure fields without altering the flow. Previous studies have found that the accuracy of the calcualted pressure field depends on several factors including the accuarcy of the velocity measurement, the spatiotemporal resolutions, the method for calculating pressure-gradient, the algorithm for pressure-gradient integration, the pressure boundary condition, etc. Therefore, it is critical and challenging to quantify the uncertainty of the reconstructed pressure field. The recent development of the uncertainty quantification algorithms for PIV and PTV allows for the local and instantaneous uncertainty estimation of velocity measurement, which can be used to infer the pressure uncertainty. In this study, we introduce a framework that propagates the standard velocity uncertainty defined as the standard deviation of the velocity error distribution through the pressure reconstruction process to obtain the uncertainty of the pressure field. The uncertainty propagations through the calculation of the pressure-gradient and the pressure-gradient integration were modeled as linear transformations, which can reproduce the effects of the spatiotemporal resolutions, the numerical schemes, the integration algorithms, and the pressure boundary condition on the accuracy of the resulting pressure fields. The proposed uncertainty estimation approach also considers the effect of the spatiotemporal and componentwise correlation of the velocity errors in common PIV/PTV measurements on the pressure uncertainty.

Uncertainty propagation and truncation errors in LPT kinematics

2021-09-22T18:48:14+00:00

Lagrangian Particle Tracking (LPT) has become a near-standard approach for performing accurate 3D flow measurements, thanks notably to the technical breakthroughs brought by the Iterative Particle Reconstruction (IPR: Wieneke, 2013) and Shake-the-Box (STB: Schanz et.al, 2016) procedures. These decisive progresses have triggered a number of studies relative to the eduction of flow kinematics and dynamics based on particle trajectory analyses. Novara & Scarano (2013), and others, focused on polynomial approximations of the trajectories, which analytically provide the material derivatives used to estimate pressure gradients. In particular, approximations based on second order polynomials fits of a small number of particle positions are used in commercially available softwares and among research teams as a straightforward solution to obtain the first and second order derivatives with a limited effect of the measurement noise. Additionally the analyses conducted during the 2020 LPT challenge (Leclaire, 2020 ; Sciacchitano, 2020) have addressed the performance of methodologies used by different groups with respect to second order trajectory fits for both multi-pulse and four-pulse (Novara et. al, 2016) LPT cases. On more advanced theoretical grounds, Geseman et. al (2016) have proposed the trackfit approach using penalized B-splines with considerations on the time-varying acceleration rate (i.e. jolt or jerk) and spectral content of noisy particle tracks

Noise canceling to compute second order statistic from SPIV

2021-09-27T20:09:03+00:00

Stereoscopic PIV (SPIV) is performed near the wall in a turbulent boundary layer at high Reynolds numbers (Re_τ = 2272, 3840) in the streamwise-wallnormal plane. A novel method for denoising the statistics is proposed by the use to two independent SPIV systems to capture the same field of view, with the objective being that the random noise associated with the velocity fluctuations are not correlated between two independent PIV systems. The derivatives in the third (spanwise) direction are obtained by axisymmetry assumptions [George and Hussein (1991)] and continuity equation. The statistics are in agreement with that of DNS at comparable Reynolds number, from y ⁺ = 25 for lower Re_τ experiment and from y ⁺ = 40 for higher Re_τ experiment.

Numerical Uncertainty in Density Estimation for Background Oriented Schlieren

2021-09-27T20:50:38+00:00

Background Oriented Schlieren (BOS) is an image-based density measurement technique. BOS estimates the density gradient from the apparent distortion of a target pattern viewed through a medium with varying density using cross-correlation, tracking, or optical flow algorithms. The density gradient can then be numerically integrated to yield a spatially resolved estimate of the density [1]. A method was recently proposed to estimate the a-posteriori instantaneous and spatially resolved density uncertainty for BOS [2] and showed good agreement between the propagated uncertainties and the random error. However, the density uncertainty quantification method could not account for the systematic uncertainty in the density field due to the discretization errors introduced during the numerical integration, which could be much larger than the displacement random errors [2]. In this work, we propose a method to estimate the numerical uncertainty introduced by the density integration in BOS measurements, using a Richardson extrapolation framework. A procedure is also introduced to combine this systematic uncertainty with the random uncertainty from the previous work to provide an instantaneous, spatially-resolved total uncertainty on the density estimates. The method will be tested with synthetic fields and synthetic BOS images.

Towards capturing a database of respiratory exhalations from flow visualisations

2021-09-08T14:42:45+00:00

One of the most common modes of infection transmission is through pathogen laden droplets expelled during natural human respiratory exhalations such as speaking, coughing, and sneezing. Infection control guidelines for the prevention of respiratory infection make assumptions about two key parameters: the safe distance between an infected and healthy individual and the size of large and small droplets (Bahl et al., 2020). Studies in the past have utilised flow visualisation techniques to understand the dynamics of respiratory flows but most of them provide only qualitative data on respiratory droplets and do not provide sufficient detail to estimate accurate flow velocities (Bourouiba et al., 2014; Vansciver et al., 2011; Scharfman et al., 2016). One of the reasons this remains a demanding application is the vast range of droplet sizes that are expelled at various velocities. Here, we present an experimental framework using particle tracking to understand the flow dynamics of the expelled droplets. Three different illumination techniques were used to capture high-speed frames of different exhalations (see figure 1). The high density of droplets in case of sneezing lead to overlap of droplet trajectories with volume illumination approach, which was resolved using tailored optics to illuminate only a slice of sneeze flow. Thereafter, the image processing techniques required for precise PTV were refined to examine droplet dynamics of various exhalations (see figure 2). The techniques were applied to multiple cases of respiratory exhalations to understand subject to subject variability.

The results for sneezing revealed a mean droplet velocity of 2 m/s to 5.4 m/s across different subjects. Additionally, less than 1% of droplets were expelled at velocities greater than 10 m/s and almost 80% of were expelled at velocities less than 5 m/s. These values were substantially lower than the values usually assumed in studies modelling or replicating sneezes (Xie et al., 2007; Atkinson and Wein, 2008). The results also revealed a high variation in the droplet dynamics, even among the sneezes from the same subject. Flow direction, spread angle, and head movement were also quantified, and the results reveal substantial variation between the subjects. In the case of coughing, maximum droplet velocities observed were in the range of 10−15 m/s however, these high velocities were detected only during the initial 0.05 s.

This work addresses the critical gaps in the understanding of the respiratory transmission of infection by providing valuable data on the droplet dynamics of various exhalations, on which the experimental data was very limited in the existing literature. Furthermore, this data will aid in numerical modelling of respiratory flows, particularly for sneezes, as studies to date rely only on airflow data of the exhalations.

Measurement of kinetic constant of protein binding using microfluidics and particle diffusometry

2021-09-08T14:56:00+00:00

Protein-protein interaction is widely used in biological science and biomedical engineering research Moreira et al. (2007). Accurate measurement of binding kinetics is essential for understanding protein-protein interactions. Current gold standard assays, such as surface plasmon resonance (SPR), bio-layer interferometry (BLI) and quartz crystal microbalance (QCM), can generate precise and real-time kinetics data. However, these methods usually require expensive instruments housed in core facilities and high-level expertise, which is not convenient for most labs to implement. We developed a new method based on microfluidics and particle diffusometry (PD) to measure protein binding kinetics, which only needs very general lab equipment including a fluorescent microscope to take photos, a syringe pump to inject solutions, capillary tubing, a simple chip made on a glass slide and a computer to process images. To measure the binding rate of a protein pair, both proteins are conjugated with beads of different sizes, respectively. The bead solutions are diluted to appropriate concentrations and injected into a Y-junction channel by a syringe pump. In the microchannel, the two kinds of beads will meet at the interface and bind due to surface protein interactions. Therefore, the size of the beads in solution gradually increases and the Brownian motion will be less and less drastic until the reaction is saturated. Taking photos recording this dynamic process, the apparent change in size of the beads can be measured by particle diffusometry and used for extracting binding kinetics. Particle diffusometry is a correlation-based and non-intrusive optical detection method to analyze properties of fluid and particle such as viscosity, temperature and particle diameter Chamarthy et al. (2009); Clayton et al. (2016, 2017b,a); Hohreiter et al. (2002). It was initially developed to determine errors caused by thermal noise in particle image velocimetry (PIV). PD always analyzes image pairs. A single image of a particle laden flow is first used to do auto-correlation, correlating with itself, which will generate a high and sharp peak. Then it is cross-correlated with a successive image with a known time interval ∆t. Because particles slightly deviate away from the initial positions after time ∆t, due to Brownian motion, the correlation peak is lower and broader than that of auto-correlation. Auto- and cross-correlation peaks are fit into Gaussian function to find peak widths, by which the particle size can be computed as long as the viscosity and temperature do not change. Processing the image sets of the protein-conjugated particles’ binding process, we acquire the relation of particle size and time, which can be used to solve protein binding kinetics. An equation of protein interaction and particle volume is derived to work out association rate from particle diameter data acquired by PD. In this study, we measured streptavidin-biotin binding rate. Streptavidin is conjugated with 20nm beads and biotin is immobilized onto 200nm beads. Proteins on the two kinds of beads bind rapidly after mixing in the main channel. It is necessary to choose a narrow area at the interface of particle streams that diffusion does not limit the reaction. Since the liquid is flowing, there is both Brownian motion and advection in particle images. We used EDPIV, a software package developed by Prof. Steven Wereley’s lab, to measure advection velocity. When doing PD analysis, images are shifted following the PIV data to catch up with the flow. The photos are taken at the center layer in the middle of the channel, where there is no velocity gradient. Measuring a series of photo sets along the main channel at several points with known distances to each other, the relation of complex bead size and time can be acquired. Solving for the association constant, the measured value is 1.74 × 10⁷M⁻¹s⁻¹, which is close to that of current gold standard assays. This novel PD-based method is accurate and requires only general lab facilities, making protein binding kinetics measurements accessible and practical for biological and biomedical labs.

Bubble PIV technique to measure the velocity field of a free-swimming California sea lion

2021-09-22T17:37:17+00:00

Fish et al. (2014) adapted laboratory PIV for safe use on larger animals. As opposed to seeding the entire flow with reflective particles and illuminating a plane of the flow with a laser, they produced a sheet of small bubbles and used sunlight for global illumination. Underwater cameras imaged the flow in a method similar to traditional PIV. This technique was used to measure the flow around a swimming dolphin and estimate the thrust produced during a tail stand maneuver (Fish et al. (2014, 2018)). In the current work, we will extend the modification of PIV of Fish et al. to measure the flow produced by a swimming sea lion also using bubbles as seeding particles and sunlight as illumination. This is the first time that the flowfield of a swimming sea lion has been directly measured. We will present an extensive extension to the image processing required to measure flow under field conditions. Finally, we will present the flow generated by propulsive strokes of an adult female (Cali) sea lion freely swimming through a pool of stationary water.

Particle Image Velocimetry Measurements of a Dry Powder Inhaler Flow

2021-09-29T14:22:18+00:00

Inhalation therapy for respiratory disorders is being increasingly delivered via dry powder inhalers (DPIs), which are breath-actuated devices that deliver pharmaceutical drug particles to the lungs. The motion of inhalation air, produced when a patient inhales through this device, supplies all energy for the entrainment, de-agglomeration, and dispersion of powder drug agglomerates into a fine drug particle aerosol. The aerosol performance is directly related to the fluid-mechanics of a given DPI device. These flow mechanisms are complex as they depend on the device design, inhalation flow rate, and the properties of the dry powder formulation used. Among these, the role of device design is crucial as it significantly affects not only the generation and properties of delivered aerosol, but also the capability of targeted regional drug deposition.

Characterisation of vortex shedding on a hydrofoil using PIV measurements

2021-09-08T15:12:25+00:00

An experimental study of vortex shedding on a hydrofoil Eppler 817 was conducted using two-dimensional two components Particle Image Velocimetry. This foil section’s characteristics are adapted for naval applications but sparsely documented. The characterization of the flow modes was realized based on statistical data such as the mean velocity field and the standard deviation of the vertical velocities. The data were acquired at very low Reynolds number which are not often covered for such hydrofoil and at four angles of attack ranging from 2^◦to 30^◦. A map of different characteristic flow modes was made for this space of parameters and was used to identify flow configurations exhibiting particular dynamics.

Separation Control of NACA0015 Airfoil using Plasma Actuators

2021-09-08T21:07:43+00:00

Separation control of NACA0015 airfoil using plasma actuators was investigated. Plasma actuators in spanwise array, which consists of 21 electrodes, were located at the leading edge of the airfoil to give temporal periodic disturbances with phase variations into its boundary layer. The cord length of the airfoil was c = 100mm and corresponding Reynolds number was fixed at Re = 63,000. Non-dimensional frequency of the disturbance was chosen at F⁺ = 0.5 or 6. The gap between adjacent electrode was set as 1mm, and phase difference of the temporal periodic disturbances between adjacent electrode was set at φ = 0 or π. Velocity field was measured by conventional two-component PIV using a CCD camera (Imperx, B1922, 1920 x 1460pixels) and Nd-YAG laser (Quantel, Evergreen, 140mJ/pulse). Both large field of view (FOV) images capturing whole wing with surrounding flow and smaller FOV images focused on the separation bubble near leading edge were evaluated. Surface pressure was monitored by pressure transducers through pressure taps on the upper surface of airfoil. Lift and drag against the airfoil were measured using a two-component force balance.

Flow Field around the Badminton Shuttlecock during Flipping Motion

2021-09-08T21:29:44+00:00

Badminton is one of the most popular sports in the world. The shuttlecock is used in badminton game has the unique shape. The shuttlecock is truncated cone-shaped and consists of a cork, gaps and a skirt portion. The shuttlecock has aerodynamic properties which differ from the ball used in other racquet sports. As an example of unique aerodynamic property, the shuttlecock shows high deceleration. It is known that the initial velocity immediately after smashing may reach up to 137m/s (493 km/h) at maximum. The velocities of the shuttlecock are reduced from the initial velocity of 67 m/s to the terminal velocity of approximately 7 m/s for approximately 0.6 s (Hubbard et al. 1997). In addition, turnover refers to the flipping experienced by a shuttlecock when undergoing heading change from nose pointing against the flight path at the moment of impact and a shuttlecock indicates the aerodynamically stable feature for the flip movement just after impact (Cohen et al. 2015). The turnover stability of a series of feather and synthetic shuttlecocks was measured to compare the performance of synthetic shuttlecocks to that of feather shuttlecocks (Calvin et al. 2013). The turnover stability of the shuttlecock is investigated through experiment and simulation, and the angular response of the shuttlecock in turnover was modelled and studied (Calvin et al. 2015). Furthermore, it was reported that the aerodynamic stability of the shuttlecock during flip movement was affected by gaps of the shuttlecock skirt in a previous study (Nakagawa et al. 2017). However, the mechanism of turnover stability of the shuttlecock has not been fully understood. The purpose of this study is to investigate the unsteady flow field around the shuttlecock during flip movements. In the present, we simulated the flipping motion by wind tunnel experiments and visualized the flow field around the shuttlecock by a PIV technique.

PIV and deformation measurements on the rotor blade of a rotating, scaled model wind turbine with flexible blades under tailored inflow conditions

2021-09-29T14:00:04+00:00

Wind turbines face harsh inflow conditions when operating in the atmospheric boundary layer or in the wake of other wind turbines. The incoming velocity field can change within seconds due to the turbulent structures it contains, resulting in a rapid change of several degrees in the angle of attack for the rotating blade. Aerodynamics are hence rapidly altered, leading to changes of the occurring forces on the rotor. Such dynamic forces cause the blades to twist and bend, accelerating fatigue and reducing the lifetime of a turbine Spinato et al. (2009).

Hilbert transform revisited – Proper orthogonal decomposition applied to analytical signals of flow fields

2021-09-20T16:06:46+00:00

The modes delivered by proper orthogonal decomposition (POD) are uncorrelated as per definition; but interestingly, they are not necessarily independent in terms of spatio-temporal flow-pattern dynamics. For instance, periodic structures that travel as waves through a series of snapshots often consist of pairs of modes with harmonic functions shifted 90 degree in phase and/or a spatial offset by a quarter of the spatial wave length of the convective flow pattern. Identification of such pairs, however, largely builds upon experience, visual inspection and/or the analysis of the reconstructed coefficients in cyclograms (Lissajous figures). This effort becomes even more challenging if measurement noise or other spurious information contaminates the raw data under consideration. One possibility to automatically pair corresponding patterns with common POD algorithms is the immediate application of the POD method to complex data (see Pfeffer et al., 1990). As outlined by Horel (1984), the Hilbert transform is a well-known and straight forward means to obtain the required extension of the original signal with an appropriate 90 degrees phase shift, which is independent of the fundamental frequencies. The complex extension of the original (real) signal X_i and its (discrete) Hilbert transform HT{X_i} as the imaginary part X_i +iHT{X_i} with the imaginary unit i is commonly known as the so-called analytical signal.

Enhanced functional binning for one- and two-point statistics using a posteriori Uncertainty Quantification of LPT data

2021-09-21T16:29:48+00:00

In the evaluation of Lagrangian particle tracking (LPT) measurement data the use of spatially binned flow statistics in the form of one, two or multi-point statistics is often an essential step towards better understanding of the measured flow fields. Increasingly there is a focus towards uncertainty quantification of the measurement system however these evaluations are seldom used to directly improve the statistics by directly involving them into the calculation. We present our Functional Binning approach which makes use of such uncertainty information as a core component for the calculation of improved statistics. The improvements towards prior approaches are shown utilizing synthetic data as well as data from a real-world subsonic jet experiment. Beyond the initial formulation for one-point statistics, we show that this approach is readily extended towards two-point statistics and explore more advanced utilizations of uncertainty information for the optimal selection of particle pairs. Furthermore, the benefits of more individualized particle error estimations are investigated and some strategies for archiving such information are investigated.

Lagrangian Strain- and Rotation-Rate Tensor Evaluation Based on Multi-pulse Particle Tracking Velocimetry (MPTV) and Radial Basis Functions(RBFs)

2021-09-28T15:26:28+00:00

Physical conservation laws are inherently Lagrangian. However, analyses in fluid mechanics using the Lagrangian framework are often forgone in favor of those using the Eulerian framework. This is perhaps due to a lack of experimental techniques with high temporal and spatial resolution that track the movement of fluid tracers in a flow domain. The development of time-resolved Particle Tracking Velocimetry/Accelerometry (TR-PTV/A) that measures flows with high seeding density has made the use of the Lagrangian framework more accessible. A challenge facing PTV/A is the need for robust mesh-free numerical schemes that handle random particle locations. Such a scheme can be created with high-order accuracy using Radial Basis Functions (RBFs). RBFs allow direct evaluation of derivatives of vector and scalar fields at random locations with infinite-order smoothness. The current work uses RBF-based differential schemes to develop a post-processing tool for PTV/A data, which can accurately evaluate spatial derivatives directly from Lagrangian particle tracks. This RBF-based strain/rotation-rate tensor evaluation tool is validated with two and three-dimensional flows from analytical solutions and is then tested with experimental data measured by a multi-pulse PTV/A system.

Physics-based universal outlier detector for flow statistics

2021-09-28T21:14:45+00:00

Outlier detection for PIV velocity fields is still nowadays an active field of research. In the last decades, several image pre-processing and processing algorithms have been developed aiming at increasing the dynamic velocity range of PIV measurements and reducing the measurement uncertainty. Nevertheless, PIV velocity fields are still often characterised by the presence of outliers, which potentially hamper the correct interpretation of the flow physics and negatively affect the evaluation of the flow statistics. The outlier detection strategies presented in literature are mainly based on the statistical analysis of the velocity vector with its immediate neighbour. Most of these algorithms have been demonstrated to be effective for instantaneous flow fields, where the errors associated with the outliers are order of magnitude larger than the expected measurement uncertainty. However, these approaches are not as effective for the flow statistics, where the outliers yield errors of the same order of the measurement uncertainty. To overcome this limitation, this paper proposed an outlier detection approach based on the agreement of the flow statistics to the constitutive equations, more specifically to the turbulent kinetic energy (TKE) transport equation. The focus is posed on the ratio between the local advection terms of TKE and a robust estimation of the TKE production along the local streamline. It is demonstrated that, in presence of outliers, the proposed principle yields a clear separation between the correct and the erroneous vectors. In order to assess the performance of the proposed principle, three different test cases are considered. For all of them, the results are compared with a reference outlier detection methodology, namely the universal outlier detection method proposed by Westerweel and Scarano (2005). The proposed turbulence transport-based approach exhibits higher performance in terms of percentage of outliers correctly identified in the flow statistics.