Particle tracking research publications

technique, algorithm, application, physics

If you are looking for a starting point of your literature survey in the field of particle tracking, here is the answer. We share a list of recently published articles in Particle Tracking Velocimetry (PTV) / Lagrangian Particle Tracking (LPT) every month. You can also check the list of related books.

We index four major journals, including Experiments in Fluids (Exp. Fluids), Physics of Fluids (PoF), Measurement Science and Technology (MST), and the Journal of Fluid Mechanics (JFM).

Experiments in Fluids
Journal of Fluid Mechanics
Measurement Science and Technology
Physics of Fluids

November 2021 release

Volume 01, Issue 04

Special issue on uncertainty quantification in particle image velocimetry and Lagrangian particle tracking

Andrea Sciacchitano, and Stefano Discetti

Measurement Science and Technology.

Abstract: The determination of a confidence interval is a key requirement for any measurement. In particle image velocimetry (PIV), the quantification of the measurement uncertainty strengthens the suitability of the technique for the discovery of new physics of fluid flows, as well as for the validation of numerical simulations by computational fluid dynamics. The present special issue on uncertainty quantification (UQ) in PIV and Lagrangian particle tracking (LPT) follows the successful one published in 2015 [1], which focused on the a-posteriori UQ of instantaneous two-component PIV measurements. Since then, the PIV technique has greatly advanced, further extending its range of applicability from micrometric scales to metre-scale three-dimensional flow measurements. In this special issue, the uncertainty associated with different aspects of the PIV and LPT techniques is addressed, including seeding particles, system calibration and imaging, the evaluation of derived flow properties, and novel processing algorithms based on neural networks.

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DOI: https://doi.org/10.1088/1361-6501/ac2c49

On the lifespan of recirculating suspensions with pulsatile flow

Mark D. Jeronimo and David E. Rival

Journal of Fluid Mechanics.

Abstract: A Lagrangian analysis is performed to measure the rate at which recirculating fluid is replaced (depleted) in pulsatile flows. Based on this approach, we then investigate how depletion is affected in dense suspensions. Experiments are conducted for pure liquid as well as suspensions with volume fractions of 𝛷=5% , 10 % and 20 %. Using Lagrangian tracking and pathline extension techniques, the depletion of the recirculation region is quantified via the trajectories of individual fluid parcels exiting the domain. Pulsatile flows with varying concentrations of hydrogel beads, up to a volume fraction of 20 %, are compared at mean Reynolds numbers of 𝑅𝑒=4800 , 9600 and 14 400, while the Strouhal number ( 𝑆𝑡=0.04 , 0.08 and 0.15) and amplitude ratio ( 𝜆=0.25 , 0.50 and 0.95) are systematically varied. A so-called ‘depletion efficiency’ is calculated for each test case, which is shown to increase with increasing Strouhal number and amplitude ratio. For most pulsatile cases, periodic vortex formation significantly increases depletion efficiency through enhanced entrainment of recirculating fluid. Conversely, low-amplitude pulsatile flows are dominated by Kelvin–Helmholtz instabilities, which do not penetrate into the recirculation region, and thus their depletion efficiency is markedly lower as a result. The efficiency trends and depletion mechanisms remain virtually unchanged between the pure liquid and each of the suspension concentrations under almost all flow conditions, which forms an unexpected conclusion. The only exception is for low-amplitude and steady flows, where increasing the suspension volume fraction is shown to suppress fluid transport across the shear layer, which in turn slows depletion and decreases the overall depletion efficiency.

Keywords: suspensions, vortex interactions, separated flows

DOI: https://doi.org/10.1017/jfm.2021.752

Evidence of preferential sweeping during snow settling in atmospheric turbulence

Jiaqi Li, Aliza Abraham, Michele Guala and Jiarong Hong

Journal of Fluid Mechanics.

Abstract: We present a field study of snow settling dynamics based on simultaneous measurements of the atmospheric flow field and snow particle trajectories. Specifically, a super-large-scale particle image velocimetry (SLPIV) system using natural snow particles as tracers is deployed to quantify the velocity field and identify vortex structures in a 22 m × 39 m field of view centred 18 m above the ground. Simultaneously, we track individual snow particles in a 3 m × 5 m sample area within the SLPIV using particle tracking velocimetry. The results reveal the direct linkage among vortex structures in atmospheric turbulence, the spatial distribution of snow particle concentration and their settling dynamics. In particular, with snow turbulence interaction at near-critical Stokes number, the settling velocity enhancement of snow particles is multifold, and larger than what has been observed in previous field studies. Super-large-scale particle image velocimetry measurements show a higher concentration of snow particles preferentially located on the downward side of the vortices identified in the atmospheric flow field. Particle tracking velocimetry, performed on high resolution images around the reconstructed vortices, confirms the latter trend and provides statistical evidence of the acceleration of snow particles, as they move toward the downward side of vortices. Overall, the simultaneous multi-scale particle imaging presented here enables us to directly quantify the salient features of preferential sweeping, supporting it as an underlying mechanism of snow settling enhancement in the atmospheric surface layer.

Keywords: -

DOI: https://doi.org/10.1017/jfm.2021.816

Flow dynamics of droplets expelled during sneezing

Prateek Bahl, Charitha de Silva, C. Raina MacIntyre, Shovon Bhattacharjee, Abrar Ahmad Chughtai, and Con Doolan

Physics of Fluids.

Abstract: Respiratory infections transmit through droplets and aerosols generated by the infected individual during respiratory emissions. It is essential to study the flow dynamics of these emissions to develop strategies for mitigating the risk of infection. In particular, the dynamics of droplets expelled during violent exhalations such as sneezing is crucial, but has received little attention to date. Here, for the first time, we present the results of droplet dynamics of 35 sneezes, obtained from four volunteers, using particle tracking velocimetry experiments. Our results reveal a mean droplet velocity of 2–5.4 m/s across the different subjects. These values are significantly lower than what is usually assumed in the studies simulating or replicating sneezes. Furthermore, the large variation in droplet speeds, flow direction, spread angle, and head movement is also quantified. These findings will enable the refinement of models and simulations of sneezes toward improving infection control guidelines.

Keywords: Image processing, Diseases and condition, Viscosity Flow dynamics, Aerosols Optical imaging, Flow visualization, Pathogens Velocimetry

DOI: https://doi.org/10.1063/5.0067609

Sensitivity analysis and measurement uncertainties of a two-camera depth from defocus imaging system

Wu Zhou, Yukun Zhang, Benting Chen, Cameron Tropea, Rixin Xu & Xiaoshu Cai

Experiments in Fluids.

Abstract: A two-camera Depth from Defocus imaging system has been previously proposed for the measurement of size and position of particles or drops. It makes use of two images with different degrees of defocus blur captured at different imaging planes. In the present study, the influence of system and particle parameters on the calibration functions and measurement uncertainties of such a system have been comprehensively investigated and experimentally verified. The parameters varied included the distance between the imaging planes, the image bit resolution, pixel resolution and the random image noise. Furthermore, the intensity and position of the backlighting and the type of particle and its relative refractive index have been varied. Experimental verification was conducted using calibration dot targets, polystyrene latex spheres, and pulverized coal powders. A sensitivity analysis was performed using relative errors or differences for the three quantities - calibration function, particle size and particle position. The study concludes with explicit recommendations on which quantities are most influential in determining measurement accuracy and therefore, must be carefully controlled.

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DOI: https://doi.org/10.1007/s00348-021-03316-2

Bespoke flow experiments to capture the dynamics of coughs and sneezes

Charitha M de Silva, Prateek Bahl, Con Doolan and C Raina MacIntyre

Measurement Science and Technology.

Abstract: Here, we detail and analyse a set of tailored experiments to capture the flow dynamics of coughs and sneezes. Specifically, conventional particle tracking-based flow experiments are tailored to capture the wide range of spatial and temporal scales in the flow through the use of high-speed and high-resolution imaging systems. In doing so, we captured droplet velocities over a wide range of particle sizes with a large field-of-view in the order of a metre to capture the full flow field, which is challenging to achieve through conventional methods. A simultaneous direct measure of droplet sizing and velocity is also obtained through back-illuminated imaging experiments, albeit over a smaller spatial extent. A statistical assessment of the droplets' flow-field reveals good agreement to prior works, now extending this over a much larger spatial extent. Through a statistical analysis of representative sneeze and cough samples, we highlight the potential of the presented experimental techniques to provide detailed flow dynamics. In particular, we observe that droplets are expelled over a significantly shorter period than the airflow generated in a cough; in contrast, droplets appear to be continuously expelled during the span of a sneeze. Furthermore, due to the lower momentum in coughs, droplets exhibit aerosolised behaviour at an earlier stage than sneezes, where the stronger flow-field expelled dictates the droplet dynamics through the bulk of the sneeze duration. These findings reveal crucial differences between coughs and sneezes, which are essential when modelling such flow accurately or designing pressure-driven simulators.

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DOI: https://doi.org/10.1088/1361-6501/ac2010

October 2021 release

Volume 01, Issue 03

Extending the reach of Lagrangian analysis in turbulence

Nicholas T. Ouellette

Journal of Fluid Mechanics.

Abstract: The hypothesis in the classical Kolmogorov picture of turbulence with perhaps the most far-reaching consequences is that of universality, the notion that small-scale turbulence dynamics are independent of the way the turbulence was generated. The assumption of universality can be evaluated by comparing measurements taken in many kinds of flows. However, up to now the range of flows that can be used to study universality from a Lagrangian viewpoint has been highly constrained, because large-scale Eulerian inhomogeneity manifests as Lagrangian non-stationarity. The recent work of Viggiano et al. (J. Fluid Mech., vol. 918, 2021, A25) significantly extends this range by showing how the dynamics along Lagrangian trajectories can be continuously renormalised using local Eulerian scales, at least in flows whose development is self-similar. They demonstrate their results on a turbulent jet, a classical flow that is well studied from the Eulerian perspective, though not in a Lagrangian sense. Their work provides an exciting roadmap for expanding the scope of Lagrangian analysis of turbulent flows.

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DOI: https://doi.org/10.1017/jfm.2021.493

Perspective on the development and application of light-field cameras in flow diagnostics

Zu Puayen Tan and Brian S Thurow

Measurement Science and Technology.

Abstract: Multi-camera flow diagnostics have made large gains in recent years in the field of three-dimensional and multi-physics measurements. However, cost, complexity and optical access pose challenges that place multi-camera techniques out of reach for many labs. In that context, light-field (LF) imaging represents an alternative approach that can potentially alleviate some of these challenges. LF flow diagnostics is a branch of measurement techniques introduced within the last decade that are based on a plenoptic camera's unique ability to capture three-dimensional and multi-spectral data via a single objective lens and image sensor-albeit at reduced lateral resolutions and depth-to-lateral uncertainty ratios of 3–10 due to limited parallax angle. Thus far, LF flow diagnostics have successfully achieved significant camera-reduction alongside other performance improvements in 3D flow velocimetry, 3D particle tracking, 3D scalar-field tomography, micro-fluidic velocimetry and multi-spectral imaging, as well as early demonstrations of single-camera multi-physics measurements for applications such as 3D fluid-structure interactions. Here, we discuss the state of development in LF flow diagnostics, highlight on-going challenges, and project potential advancements in the near future.

Keywords: light-field, plenoptic, 3D flow diagnostics, particle image velocimetry, scalar-field tomography, 3D background-oriented schlieren, multi-spectral imaging

DOI: https://doi.org/10.1088/1361-6501/ac026e

A GPU-accelerated particle-detection algorithm for real-time volumetric particle-tracking velocimetry under non-uniform illumination

Yu Zhao, Xiaojun Ma, Chengbin Zhang, Jiujiu Chen and Yuanhui Zhang

Measurement Science and Technology.

Abstract: Real-time volumetric particle tracking velocimetry (VPTV) equipped with field-programmable gate array (FPGA) cameras has been used for open-space, low particle density, and large-scale airflow measurements with long measurement periods. However, the particle detection accuracy of FPGA cameras is inevitably hindered by non-uniform illumination, resulting in a reduction in the particle detection ratio and positional accuracy. In this article, we propose to use both synchronized FPGA and grayscale cameras in a VPTV system, where grayscale cameras utilize a new algorithm based on two-frame centroid and corner extraction (TFCCE) under non-uniform white-light illumination. To keep the frame rate of the FPGA cameras the same, the TFCCE algorithm was accelerated by a graphics processing unit (GPU). The simulation results showed that the 2D particle detection ratio of TFCCE was enhanced to approximately 80% with a positional accuracy of 0.57 pixels, compared to 30% and 0.94 pixels for the single-frame centroid extraction used in the FPGA. The GPU version of TFCCE was 15.09 times faster than the CPU version, resulting in a calculation time of 4.55 ms per image, compared to 68.70 ms when using the CPU. This system was also validated by the measurement of a turbulent jet flow in real-time at 120 fps. The experimental results correspond well with data published in the literature. Therefore, this new algorithm can improve VPTV systems in terms of particle detection ratio and positional accuracy in real time under conditions of non-uniform illumination.

Keywords: volumetric particle tracking velocimetry (VPTV), graphics processing unit (GPU), real-time, corner detection

DOI: https://doi.org/10.1088/1361-6501/ac000a

Experimental investigation of immersed granular collapse in viscous and inertial regimes

Yunhui Sun (孙云辉), Wentao Zhang (张炆涛), Yi An (安翼), Qingquan Liu (刘青泉), and Xiaoliang Wang (王晓亮)

Physics of Fluids

Abstract: This paper presents an experimental investigation of immersed granular collapse with an initially dense packing, mainly focusing on the collapse characteristics of different flow regimes and the influence of the initial aspect ratio. A novel experimental setup and imaging method are introduced to simultaneously observe the motion of the particles and the fluid. The collapse dynamics, including the collapse acceleration, steady propagation velocity, and collapse duration, are analyzed based on the front propagation. It is found that the collapse procedures in the inertial and viscous regimes differ significantly, with the transitional regime possessing some unique characteristics of both. The inertial regime exhibits a faster collapse process, sharper final deposition, and a depression near the right wall in the case of high columns. The viscous regime collapses from the upper-left corner, from where particles drop to the bottom and form the flow front in advance of the particles initially at the bottom, and exhibits a triangular final deposition. The inertial regime exhibits swirling fluid motion, which helps the granular transport, whereas the fluid flow in the viscous regime mainly follows the granular flow. The collapse regime characteristics are more pronounced in higher columns.

Keywords: -

DOI: https://doi.org/10.1063/5.0067485

September 2021 release

Volume 01, Issue 02

Lagrangian coherent track initialization

Ali Rahimi Khojasteh, Yin Yang, Dominique Heitz, and Sylvain Laizet,

Physics of Fluids.

Abstract: Advances in time-resolved three-dimensional Particle Tracking Velocimetry (4D-PTV) techniques have consistently revealed more accurate Lagrangian particle motions. A novel track initialization technique as a complementary part of 4D-PTV, based on local temporal and spatial coherency of neighbor trajectories, is proposed. The proposed Lagrangian Coherent Track Initialization (LCTI) applies physics-based FiniteTime Lyapunov Exponent (FTLE) to build four frame coherent tracks. We locally determine Lagrangian coherent structures among neighbor trajectories by using the FTLE boundaries (i.e., ridges) to distinguish the clusters of coherent motions. To evaluate the proposed technique, we created an open-access synthetic Lagrangian and Eulerian dataset of the wake downstream of a smooth cylinder at a Reynolds number equal to 3900 obtained from three-dimensional direct numerical simulation. Performance of the proposed method based on three characteristic parameters, temporal scale, particle concentration (i.e., density), and noise ratio, showed robust behavior in finding true tracks comparedto the recent initialization algorithms. Sensitivity of LCTI to the number of untracked and wrong tracks is also discussed. We address the capability of using the proposed method as a function of a 4D-PTV scheme in the Lagrangian particle tracking challenge. We showed that LCTI prevents 4D-PTV divergence in flows with high particle concentrations. Finally, the LCTI behavior was demonstrated in a jet impingement experiment. LCTI was found to be a reliable tracking tool in complex flow motions, with a strength revealed for flows with high velocity and acceleration gradients.

Keywords: Particle tracking velocimetry, Lagrangian coherent structure, coherent motion

DOI: https://doi.org/10.1063/5.0060644

Topological colouring of fluid particles unravels finite-time coherent sets

Charó, G., Artana, G., & Sciamarella, D,

Journal of Fluid Mechanics.

Abstract: This work describes the application of a technique that extracts branched manifolds from time series to study numerically generated fluid particle behaviour in the wake past a cylinder performing a rotary oscillation at low Reynolds numbers, and compares it with the results obtained for a paradigmatic analytical model of Lagrangian motion: the driven double gyre. The approach does not require prior knowledge of the underlying equations defining the dataset. The time series taken as input corresponds to the evolution of a position coordinate of an individual fluid particle. A delay embedding is used to reconstruct the dynamics in phase space, and a cell complex is built to characterize the topology of the embedding. Fluid particles are said to belong to the same topological class when the Betti numbers, orientability chains and weak boundaries of the associated cell complexes coincide. Topological colouring consists of labelling or ‘colouring’ advected particles with the topological class obtained in their finite-time analyses. The results suggest that topological colouring can be used to distinguish between regions of the flow where trajectories exhibit different finite-time dynamics.

Keywords: nonlinear dynamical systems, general fluid mechanics, chaotic advection

DOI: https://doi.org/10.1017/jfm.2021.561

On the PIV/PTV uncertainty related to calibration of camera systems with refractive surfaces

Gerardo Paolillo, and Tommaso Astarita,

Measurement Science and Technology.

Abstract: This paper investigates the calibration and measurement uncertainty related to the use of different camera models in optical systems that include refractive surfaces. A refractive surface is an interface between media with different optical properties which introduces distortions in the imaging process due to the refraction of the lines-of-sight. This is an issue common to all the investigations of fluids flowing around or inside transparent solid geometries and is of relevance for a strong curvature of the solid/fluid interface. Appropriate modelling of the refractive effects is possible by integrating the pinhole camera model with a ray-tracing method, as demonstrated in a previous work (Paolillo and Astarita 2020 IEEE Trans. Pattern Anal. Mach. Intell.). On the other side, analytical camera models with a pure mathematical foundation, like those based on polynomials or rational functions, are classically used in the PIV/PTV community. Due to the non-linear nature of the involved distortions, the accuracy of these models in representing the imaging process in presence of refractive geometries depends strongly on the polynomial order and noise of the data used for the calibration. The current work provides a numerical estimate of the uncertainty inherent to the analytical camera models by using data generated via a reference refractive camera model. The present results show that high accuracy requires high orders, which implies a large number of calibration parameters and high demand for computational resources. In particular, the rational mapping functions exhibit superior performance compared to the polynomials, although their calibration is found to be sensitive to image noise and they might yield large extrapolation errors. An experimental verification is also reported, which shows that for the estimation of the velocity statistics a 7th order polynomial model offers results comparable to those of a refractive camera model.

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DOI: https://doi.org/10.1088/1361-6501/abf3fc

DNS of a turbulent boundary layer using inflow conditions derived from 4D-PTV data

Jason Appelbaum, Duncan Ohno, Ulrich Rist, Christoph Wenzel,

Experiments in Fluids.

Abstract: Unsteady, 3D particle tracking velocimetry (PTV) data are applied as an inlet boundary condition in a direct numerical simulation (DNS). The considered flow case is a zero pressure gradient (ZPG) turbulent boundary layer (TBL) flow over a flat plate. The study investigates the agreement between the experimentally measured flow field and its simulated counterpart with a hybrid 3D inlet region. The DNS field inherits a diminishing contribution from the experimental field within the 3D inlet region, after which it is free to spatially evolve. Since the measurement does not necessarily provide a spectrally complete description of the turbulent field, the spectral recovery of the flow field is analyzed as the TBL evolves. The study summarizes the pre-processing methodology used to bring the experimental data into a form usable by the DNS as well as the numerical method used for simulation. Spectral and mean flow analysis of the DNS results show that turbulent structures with a characteristic length on the order of one average tracer particle nearest neighbor radius rNN or greater are well reproduced and stay correlated to the experimental field downstream of the hybrid inlet. For turbulent scales smaller than rNN , where experimental data are sparse, a relatively quick redevelopment of previously unresolved turbulent energy is seen. The results of the study indicate applicability of the approach to future DNS studies in which specific upstream or far field boundary conditions (BCs) are required and may provide the utility of decreasing high initialization costs associated with conventional inlet BCs.

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DOI: https://doi.org/10.1007/s00348-021-03287-4

August 2021 release

Volume 01, Issue 01

Advanced iterative particle reconstruction for Lagrangian particle tracking

Tobias Jahn, Daniel Schanz & Andreas Schröder,

Experiments in Fluids.

Abstract: The method of iterative particle reconstruction (IPR), introduced by Wieneke (Meas Sci Technol 24:024008, 2013), constitutes a major step toward Lagrangian particle tracking in densely seeded flows (Schanz et al. in Exp Fluids 57:1–27, 2016). Here we present novel approaches in several key aspects of the algorithm, which, in combination, triple the working range of IPR in terms of particle image densities. The updated method is proven to be fast, accurate and robust against image noise and other imaging artifacts. Most of the proposed changes to the original processing are easy to implement and come at low computational cost. Furthermore, a bundle adjustment scheme that simultaneously updates the 3D locations of all particles and the camera calibrations is introduced. While the particle position optimization proved to be more effective using localized ‘shake’ schemes, this so-called global shake scheme constitutes an effective measure to correct for decalibrations and vibrations, acting as an in-situ single-image volume-self-calibration. Further optimization strategies using such approaches are conceivable.

Keywords: IPR, Shake The Box

DOI: https://doi.org/10.1007/s00348-021-03276-7

On the determination of vortex ring vorticity using Lagrangian particles

Outrata, O., Pavelka, M., Hron, J., La Mantia, M., Polanco, J., & Krstulovic, G,

Journal of Fluid Mechanics

Abstract: Particles are a widespread tool for obtaining information from fluid flows. When Eulerian data are unavailable, they may be employed to estimate flow fields or to identify coherent flow structures. Here we numerically examine the possibility of using particles to capture the dynamics of isolated vortex rings propagating in a quiescent fluid. The analysis is performed starting from numerical simulations of the Navier–Stokes and the Hall–Vinen– Bekarevich–Khalatnikov equations, respectively describing the dynamics of a Newtonian fluid and a finite-temperature superfluid. The flow-induced positions and velocities of particles suspended in the fluid are specifically used to compute the Lagrangian pseudovorticity field, a proxy for the Eulerian vorticity field recently employed in the context of superfluid 4He experiments. We show that, when calculated from ideal Lagrangian tracers or from particles with low inertia, the pseudovorticity field can be accurately used to estimate the propagation velocity and the growth of isolated vortex rings, although the quantitative reconstruction of the corresponding vorticity fields remains challenging. On the other hand, particles with high inertia tend to preferentially sample specific flow regions, resulting in biased pseudovorticity fields that pollute the estimation of the vortex ring properties. Overall, this work neatly demonstrates that the Lagrangian pseudovorticity is a valuable tool for estimating the strength of macroscopic vortical structures in the absence of Eulerian data, which is, for example, the case of superfluid 4He experiments.

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DOI: https://doi.org/10.1017/jfm.2021.662

Pressure evaluation from Lagrangian particle tracking data using a grid-free least-squares method

Maxim Bobrov, Mikhail Hrebtov, Vladislav Ivashchenk, Rustam Mullyadzhanov, Alexander Seredkin, Mikhail Tokarev, Dinar Zaripov, Vladimir Dulin and Dmitriy Markovich

Measurement Science and Technology.

Abstract: The Lagrangian particle tracking shake-the-box (STB) method provides accurate evaluation of the velocity and acceleration of particles from time-resolved projection images for high seeding densities, giving an opportunity to recover the stress tensor. In particular, their gradients are required to estimate local pressure fluctuations from the Navier–Stokes equations. The present paper describes a grid-free least-squares method for gradient and pressure evaluation based on irregularly scattered Lagrangian particle tracking data with minimization of the random noise. The performance of the method is assessed on the basis of synthetic images of virtual particles in a wall-bound turbulent flow. The tracks are obtained from direct numerical simulation (DNS) of an initially laminar boundary layer flow around a hemisphere mounted on a flat wall. The Reynolds number based on the sphere diameter and free stream velocity is 7000, corresponding to a fully turbulent wake. The accuracy, based on the exact tracks and STB algorithm, is evaluated by a straightforward comparison with the DNS data for different values of particle concentration up to 0.2 particles per pixel. Whereas the fraction of particles resolved by the STB algorithm decreases with the seeding density, limiting its spatial resolution, the exact particle positions demonstrate the efficiency of the least-squares method. The method is also useful for extraction of large-scale vortex structures from the velocity data on non-regular girds.

Keywords: uncertainty quantification, Lagrangian particle tracking, pressure evaluation, STB algorithm

DOI: https://doi.org/10.1088/1361-6501/abf95c

Particle radial distribution function and relative velocity measurement in turbulence at small particle-pair separations

Hammond, A., & Meng, H,

Journal of Fluid Mechanics.

Abstract: Particle in turbulent flow are critical to particle agglomeration and droplet coalescence. The collision kernel can be evaluated by radial distribution function (RDF) and radial relative velocity (RV) between particles at small separations r. Previously, the smallest r was limited to roughly the Kolmogorov length η due to particle position uncertainty and image overlap. We report a new approach to measuring RDF and RV near contact (r/a ≈ 2.07, where a is particle radius). Three-dimensional particle tracking velocimetry using the four-pulse shake-the-box algorithm recorded short tracks with the interpolated midpoints registered as particle positions, avoiding image overlap and track mismatch. We measured RDF and RV of inertial particles in a one metre diameter isotropic air turbulence chamber with Taylor Reynolds number Reλ = 324, a = 12–16 μm (≈0.12η) and Stokes number ≈0.7. At large r the measured RV agrees with the literature, but when r < 20η the first moment of negative RV starts to increase, reaching 10 times higher values than direct numerical simulations of non-interacting particles. Likewise, RDF scales as r−0.39 when r > η, reflecting the well-known scaling for polydisperse particles, but when r η, RDF scales as r−6, yielding 1000 times higher near-contact RDF than simulations. Such RV enhancement and extreme clustering at small r can be attributed to particle–particle interactions including hydrodynamic interactions, which are not well-understood. Uncertainty analysis substantiates the observed trends. This first-ever simultaneous RDF and RV measurement at small separations provides a clear glimpse into the clustering and relative velocities of particles in turbulence near-contact.

Keywords: particle/fluid flow, isotropic turbulence

DOI: https://doi.org/10.1017/jfm.2021.486