Particle Tracking Velocimetry Research Articles

Experimental study on the motion characteristics of non-spherical biomass particulate systems in a fluidization tube

This study experimentally investigated the transport characteristics of flaky, fibrous and streaky biomass particles in a fluidization tube. The movement and distribution of these non-spherical biomass particles in different sections of the fluidization tube were visualized and analyzed by using a Particle Tracking Velocimetry measuring approach. The method for calculating the solidity rate distribution of particles in the fluidization tube was also developed. Furthermore, the distribution patterns of non-spherical biomass particles with three different morphologies in the near-wall region of the fluidization tube were significantly analyzed. It could be observed that the area of the non-spherical biomass particles in the near-wall region exhibited an ’M’ shape. Three empirical formulas for predicting the maximum area of non-spherical biomass particle clusters in the fluidization tube were firstly proposed. Among the three prediction formulas, the correlation coefficients are 0.7142, 0.8797, and 0.9567, respectively.

Statin-treated RBC dynamics in a microfluidic porous-like network

The impact of therapeutic interventions on red blood cell (RBC) deformability and microscale transport is investigated, using statins as an exemplar. Human RBCs were treated in vitro with two commonly prescribed statins, atorvastatin and rosuvastatin, at clinically relevant concentrations. Changes in RBC deformability were quantified using a microfluidic-based ektacytometer and expressed in terms of the elongation index. Dilute suspensions of the statin-treated RBCs were then perfused through a microfluidic pillar array, at a constant flow rate and negligible inertia, and imaged. Particle Tracking Velocimetry (PTV) was applied to track RBCs, identify preferential paths and estimate their velocities, whereas image processing was used to estimate cell dynamics, perfusion metrics and distributions. The findings were compared against those of healthy, untreated cells. Statins enhanced RBC deformability in agreement with literature. The extent of enhancement was found to be statin-dependent. The softer statin-treated cells were found to flow in straight, less tortuous paths, spend more time inside the pillar array and exhibit lower velocities compared to healthy RBCs, attributed to their enhanced deformation and longer shape recovery time upon impact with the array posts. The in vitro microfluidic approach demonstrated here may serve as a monitoring tool to personalise and maximise the outcome of a therapeutic treatment.

Making Mobile Nanotechnology Accessible: Is the Explicit Preparation of Janus Nanoparticle Necessary to Achieve Mobility?

This study compares the mobility behaviour, in a H2O2 environment, of three different geometries of hybrid particle made of silica core functionalized by gold (nanoparticles or layer). It is known that the decomposition of H2O2 on gold surfaces drives mobility; however, the link between mobility orientation and the organization of gold on silica surfaces is still questionable. While conventional wisdom posits that asymmetric designs are crucial for generating phoretic forces or localized bubble propulsion, recent research suggests that symmetrical particles may also exhibit motility. To address this debate, we developed a robust workflow for synthesizing gold grafted silica nanoparticles with precise control over size and shape, enabling the direct comparison of their motile behaviour by dynamic light scattering and particle tracking velocimetry. Our results indicate, first, that a combination of techniques is necessary to overcome their intrinsic limitation and, second, that the inherent asymmetry generated by isotropic gold nanoparticle deposition onto silica surfaces may enable particle motility.

A Multichamber Pulsating-Flow Device With Optimized Spatial Shear Stress and Pressure for Endothelial Cell Testing.

Design and analysis are presented for a new device to test the response of endothelial cells to the simultaneous action of cyclic shear stresses and pressure fluctuations. The design consists of four pulsatile-flow chambers connected in series, where shear stress is identical in all four chambers and pressure amplitude decreases in successive chambers. Each flow chamber is bounded above and below by two parallel plates separated by a small gap. The design of the chamber planform must ensure that cells within the testing region experience spatially uniform time-periodic shear stress. For conditions typically encountered in applications, the viscous unsteady flow exhibits order-unity values of the associated Womersley number. The corresponding solution to the unsteady lubrication problem, with general nonsinusoidal flowrate, is formulated in terms of a stream function satisfying Laplace's equation, which can be integrated numerically to determine the spatial distribution of shear stresses for chambers of general planform. The results are used to optimize the design of a device with a hexagonal planform. Accompanying experiments using particle tracking velocimetry (PTV) in a fabricated chamber were conducted to validate theoretical predictions. Pressure readings indicate that intrachamber pressure variations associated with viscous pressure losses and acoustic fluctuations are relatively small, so that all cells in a given testing region experience nearly equal pressure forces.

3D-3C measurements of flow reversal in small sessile drops in shear flow

Sessile drops play an important role in nature and the operation of many technical and biological systems as for instance fuel cells, cleaning processes, lab-on-a-chip devices and single-cell analysis. Nonetheless, their dynamic behavior in a shear flow is still not fully understood (e. g. detachment mechanism). Challenges exist regarding the precise simulation of two-phase flows as well as difficulties in conducting flow measurements with sufficient temporal resolution of more than 1 kHz. In this article, we present the first three-dimensional flow measurements in strongly oscillating drops stemming from a shear flow by using a monocular 3D localization microscope based on a Double-Helix Point Spread Function combined with Particle Tracking Velocimetry. The high temporal resolution and the large measurement volume - in terms of microscopy - make it possible to measure the time- and phase-averaged flow in small drops with Bond numbers (Bo) smaller than 1. Water drops were placed in an air flow channel and measurements were conducted for Reynolds numbers (Red) from about 750 to 1700. Our measurements show that the results of previous investigations for drops with Bo>1 concerning vortex pattern and flow reversal inside the drop apply for smaller drops as well. In addition, we are able to reveal the three-dimensional flow structure and multiple vortex-pattern in time and space. We discover a periodic 3D vortical flow pattern that corresponds to the first and second eigenfrequency of the drop. Moreover, we demonstrate the potential of adaptive optics to correct measurement errors stemming from time-varying light refraction when conducting measurements through the fluctuating drop surface, in particular for opaque substrates. The results may help in understanding the coupling of inner and outer flow for sessile drops in shear flow which allows for an analysis of the onset motion of these drops, i.e. drop removal. Removal of sessile drops plays a crucial role in many applications, which includes among other things the water management of fuel cells where small drops with Bo<1 predominantly occur.

Whole-channel acoustic energy and acoustophoretic efficiency frequency spectrum by the in-flow focusing method

The in-flow focusing method, introduced herein, enables rapid and straightforward measurements of the whole-channel acoustic energy of acoustic flow-through devices. The method is applied to assess the whole-channel acoustic energy and acoustophoretic efficiency frequency spectra of a bottom-actuated silicon-glass and a side-actuated glass chip. The input variables are the time-independent geometrical shape of the particle trajectories along the manipulation chamber, the channel cross-section geometry, the physical properties of the reference particles and fluid, and the volumetric flowrate. The outputs are the total acoustic energy, the acoustophoretic efficiency, as well as the acoustic energy density distribution along the channel, which provide a comprehensive overview of the performance of acoustically driven lab-on-a-chip setups. The acoustophoretic efficiency is an important parameter that relates the energy dissipation, and therefore the heat development in the device, to the useful acoustic energy in the channel. Unlike the current state-of-the-art methods such as particle image velocimetry, particle tracking velocimetry, and optical trapping, which are typically limited to small channel areas and are time-consuming, the in-flow focusing method can be applied to long channels and enables rapid measurements of the whole-channel acoustic energy. Published by the American Physical Society 2024

Drag reduction utilizing a wall-attached ferrofluid film in turbulent channel flow

This study explores the application of a wall-attached ferrofluid film to decrease skin-friction drag in turbulent channel flow. We conduct experiments using water as a working fluid in a turbulent channel flow set-up, where one wall is coated with a ferrofluid layer held in place by external permanent magnets. Depending on the flow conditions, the interface between the two fluids is observed to form unstable travelling waves. While ferrofluid coating has been previously employed in laminar and moderately turbulent flows (Reynolds number $Re&lt;4000$ ) to reduce drag by creating a slip condition at the fluid interface, its effectiveness in fully developed turbulent conditions, particularly when the interface exhibits instability, remains uncertain. Our primary objective is to assess the effectiveness of ferrofluid coating in reducing turbulent drag with particular focus on scenarios when the ferrofluid layer forms unstable waves. To achieve this, we measure flow velocity using two-dimensional particle tracking velocimetry (2D-PTV), and the interface contour between the fluids is determined using an interface tracking algorithm. Our results reveal the significant potential of ferrofluid coating for drag reduction, even in scenarios where the interface between the surrounding fluid and ferrofluid exhibits instability, with observed drag reduction rates up to 95 %. In particular, waves with an amplitude significantly smaller than a viscous length scale positively contribute to drag reduction, while larger waves are detrimental, because of induced turbulent fluctuations. However, for the latter case, slip outcompetes the extra turbulence so that drag is still reduced.

Granular Cooling of Uni‐Sized Inelastic Particles in an Obstructed Chute

AbstractThis research aims to experimentally characterize cooling, clogging and jamming of a dry granular flow in a chute partially obstructed by a stopping wall with two slits adjacent to the side walls. We ensemble‐average velocities, determined with Particle Tracking Velocimetry, and its fluctuations, to compute mean flow and granular temperature fields. Full chute‐wide jamming is triggered by the formation of stable arch‐like clogging structures in front of the slits. The statistical distribution of the clogging instant is not heavy‐tailed, which indicates that clogging occurs only when the flow through the slits is liquid‐like. An upstream‐progressing jamming wave eventually forms, similar to that observed in fully obstructed chutes. There is no triple point anywhere, since the flow cools down to a granular liquid before jamming. We identified three main stages of jamming wave propagation. The initial buildup is characterized by high values of the upstream Froude number, slow progression, and transformation of kinetic into potential energy. This occurs with significant granular head losses, as particles attempt to flow over the jam. In the second stage, accretion becomes dominant, characterized by smaller head losses and, consequently, higher values of the jamming wave celerity. In the third stage, the jamming wave propagates against a cooler but faster flow that pushes against the jam, slightly increasing the wave strength. Accretion is the main mechanism of jammed mass increase, justifying a further increase of wave acceleration. The macroscopic aspects of the jamming wave dynamics can be described, as a first approximation, by shallow‐water theory.

Vortex Trapping of Suspended Sand Grains Over Ripples

AbstractCoastal hydrodynamics and morphodynamics integrate the effects of small‐scale fluid‐sediment interactions; yet, these small‐scale processes are not well understood. To investigate sediment trapping by turbulent coherent structures or vortices, the transport of coarse sand over ripples was analyzed in a small‐oscillatory flow tunnel with phase‐separated Particle Image and Tracking Velocimetry. Results from one of the first direct measurements of vortex‐trapped sand grains under oscillatory flows are presented. The vortices mobilized sand grains along the ripple slopes just prior to flow reversal and transported the suspended sediment grains. During several flow cycles, some sand grains were temporarily trapped in the vortex, prescribing semi‐circular trajectories off‐center from the vortex core in quadrants of the vortex that were closest to the ripple slope, as illustrated by Nielsen (1992, https://doi.org/10.1142/1269). Comparisons of the horizontal sediment grain velocity with the horizontal fluid velocity yielded a linear relationship with a slope of 0.87. The vertical grain velocities also varied linearly with the vertical fluid velocity with a slope of approximately 1 and an offset of −0.08 m . The offset is close to the still water settling velocity for coarse sand grains, as hypothesized during vortex trapping. Additionally, estimates of the off‐center distance, between the centers of the semi‐circular sediment paths and vortex cores, compared well with the ratio of the settling velocity to the radian frequency of the vortex yielding a linear regression slope of 0.99. Improved understanding of vortex trapping effects on sediment dynamics may decrease uncertainty in model predictions of large‐scale coastal hydrodynamics and sediment transport.

Characterizing firebrands and their kinematics during lofting

Spot fires pose a major risk and add to the already complex physics, which makes fire spread so hard to predict, especially in the wildland urban interface. Firebrands can not only cross fuel breaks and thwart other suppression efforts but also directly damage infrastructure and block evacuation routes. Transport models and computational fluid dynamics tools often make simplifications when predicting spot fire risk, but there is a relative lack of experimental data to validate such parameterizations. To this end, we present a field experiment performed at the University of California Berkeley Blodgett Research Forest in California where we recorded the flame and firebrands emanating from a nighttime hand-drawn pile fire using high-frequency imaging. We used image-processing to characterize the fire intensity and turbulence as well as particle tracking velocimetry to measure ejected firebrand kinematics as they are lofted by the plume. We further collected embers that settled around the fire at varying distances and measured their size, shape, density, and settling distributions. We also examine existing physics-based time-averaged models of firebrand lofting and note discrepancies between such models, often used due to their speed and simplicity, and our experimental observations. Finally, we discuss some implications our observations could have on future modeling efforts by considering the time-dependent fire dynamics, intermittency in the plume turbulence, and in the firebrand generation rate. To the best of our knowledge, these are the first in situ observations of firebrand generation and lofting from representative fuels, addressing a major source of data gap and uncertainty in the wildland fire literature.

Improvement in the Number of Velocity Vector Acquisitions Using an In-Picture Tracking Method for 3D3C Rainbow Particle Tracking Velocimetry

Particle image velocimetry and particle tracking velocimetry (PTV) have developed from two-dimensional two-component (2D2C) velocity vector measurements to 3D3C measurements. Rainbow particle tracking velocimetry is a low-cost 3D3C measurement technique adopting a single color camera. However, the vector acquisition rate is not so high. To increase the number of acquired vectors, this paper proposes a high probability and long-term tracking method. First, particles are tracked in a raw picture instead of in three-dimensional space. The tracking is aided by the color information. Second, a particle that temporarily cannot be tracked due to particle overlap is compensated for using the positional information at times before and after. The proposed method is demonstrated for flow under a rotating disk with different particle densities and velocities. The use of the proposed method improves the tracking rate, number of continuous tracking steps, and number of acquired velocity vectors. The method can be applied under the difficult conditions of high particle density (0.004 particles per pixel) and large particle movement (maximum of 60 pix).

Investigating supercritical flow characteristics and movement of sediment particles in a narrow channel bend using PTV and video footage

This experimental study investigates the cause of nonuniform invert abrasion observed at sediment bypass tunnel (SBT) bends by examining the variations in velocity distributions, turbulence properties, bed shear stress, and bulk sediment movements under three supercritical bend flow conditions, detailed investigation of such flow is scarce. Using a laboratory-scaled model (1:22) of the downstream bend at Solis SBT, Switzerland, the research utilized particle tracking velocimetry and high-speed cameras with spherical sandstones and glass spheres representing sediments. The results indicate that as the secondary currents develop in the flow direction, the flow properties and sediments redistribute across the channel: the high-momentum fluids are directed toward the outer wall, the bed shear stress increases toward the outer wall, and the sediments are pushed toward the inner wall, which then follow this path downstream, even in straight sections, despite lower bed shear stress. This distribution of sediments, driven by secondary currents, leads to deeper invert abrasions toward the inner wall at SBT bends and downstream sections. Thus, these abrasions are primarily influenced by sediment movement rather than the bed shear stress alone. The study's findings are also valuable for validating future numerical simulations.

Particle motion under turbulent eddies: Inspiration for fine minerals flotation

Turbulent flow ubiquitous in flotation machines and contain multi-scale eddies. Compared to static environments, turbulence can increase particle kinetic energy and promote particle-bubble collision in mineral flotation. However, there have been few quantitative studies on turbulent eddies that affect particle motion, which limits the precise flow control of flotation processes. In this study, the motion characteristics of particles in isotropic turbulence were measured by particle tracking velocimetry, and the relationship between turbulent eddy and particle motion was quantitatively analyzed by 7-scale wavelet transform and fast Fourier transform. The measurement results show that the particle moves upward following a rotating turbulent eddy, and turbulent eddies with sizes of 35–119 µm and 127–288 µm drives the motion of dp = 74 µm and dp = 200 µm particles respectively. The turbulent acceleration of particles within a turbulent eddy can be calculated as aλ=Cε2/3λ-1/3. This study provides an eddy control basis for mineral flotation performance improvement.

Resonant oscillation of droplets under an alternating electric field to enhance solute diffusion

This study investigates a novel microfluidic mixing technique that uses the resonant oscillation of coalescent droplets. During the vertical contact-separation process, solutes are initially separated as a result of the combined effects of diffusion and gravity. We show that the application of alternating current (AC) voltage to microelectrodes below the droplets causes a resonant oscillation, which enhances the even distribution of the solute. The difference in concentration between the top and bottom droplets exhibits frequency dependence and indicates the existence of a particular AC frequency that results in a homogeneous concentration. This frequency corresponds to the resonance frequency of the droplet oscillation that is determined using particle tracking velocimetry. To understand the mixing process, a phenomenological model based on the equilibrium between surface tension, viscosity, and electrostatic force was developed. This model accurately predicted the resonance frequency of droplet flow and was consistent with the experimental results. These results suggest that the resonant oscillation of droplets driven by AC voltage significantly enhances the diffusion of solutes, which is an effective approach to microfluid mixing.

One-shot omnidirectional pressure integration through matrix inversion

Abstract In this work, we present a method to perform 2D and 3D omnidirectional pressure integration from velocity measurements with a single-iteration matrix inversion approach. This work builds upon our previous work, where the rotating parallel ray approach was extended to the limit of infinite rays by taking continuous projection integrals of the ray paths and recasting the problem as an iterative matrix inversion problem. This iterative matrix equation is now ``fast-forwarded'' to the ``infinity'' iteration, leading to a different matrix equation that can be solved in a single {\color{blue}step}, thereby presenting the same computational complexity as the Poisson equation. We observe computational speedups of $\sim10^6$ when compared to brute-force omnidirectional integration methods, enabling the treatment of grids of $\sim 10^9$ points and potentially even larger in a desktop setup at the time of publication. Further examination of the boundary conditions of our one-shot method shows that omnidirectional pressure integration implements {\color{blue} a boundary condition where the boundary points are treated as interior points to the extent that information is available}. Finally, we show how the method can be extended from the regular grids typical of particle image velocimetry to the unstructured meshes characteristic of particle tracking velocimetry data.

Investigation of models to estimate flight performance of gliding birds from wakes.

Mathematical models based on inviscid flow theory are effective at predicting the aerodynamic forces on large-scale aircraft. Avian flight, however, is characterized by smaller sizes, slower speeds, and increased influence of viscous effects associated with lower Reynolds numbers. Therefore, inviscid mathematical models of avian flight should be used with caution. The assumptions used in such models, such as thin wings and streamlined bodies, may be violated by birds, potentially introducing additional error. To investigate the applicability of the existing models to calculate the aerodynamic performance of bird flight, we compared predictions using simulated wakes with those calculated directly from forces on the bird surface, both derived from computational fluid dynamics of a high-fidelity barn owl geometry in free gliding flight. Two lift models and two drag models are assessed. We show that the generalized Kutta-Joukowski model, corrected by the streamwise velocity, can predict not only the lift but also span loading well. Drag was predicted best by a drag model based on the conservation of fluid momentum in a control volume. Finally, we estimated force production for three raptor species across nine gliding flights by applying the best lift model to wake flow fields measured with particle tracking velocimetry.

KF-PEV: a causal Kalman filter-based particle event velocimetry

Event-based pixel sensors asynchronously report changes in log-intensity in microsecond-order resolution. Its exceptional speed, cost effectiveness, and sparse event stream make it an attractive imaging modality for particle tracking velocimetry. In this work, we propose a causal Kalman filter-based particle event velocimetry (KF-PEV). Using the Kalman filter model to track the events generated by the particles seeded in the flow medium, KF-PEV yields the linear least squares estimate of the particle track velocities corresponding to the flow vector field. KF-PEV processes events in a computationally efficient and streaming manner (i.e., causal and iteratively updating). Our simulation-based benchmarking study with synthetic particle event data confirms that the proposed KF-PEV outperforms the conventional frame-based particle image/tracking velocimetry as well as the state-of-the-art event-based particle velocimetry methods. In a real-world water tunnel event-based sensor data experiment performed on what we believe to be the widest field view ever reported, KF-PEV accurately predicted the expected flow field of the SD7003 wing, including details such as the lower velocity in the wake and the flow separation around the underside of an angled wing.

Settling velocity and effective density analysis for aquaculture floc particles: An approach through bivariate parametric copula

The study of flocs' characteristics in aquaculture tanks remains challenging due to their complex composition. Nevertheless, the application of experimental methods, such as particle tracking velocimetry (PTV), has made it possible to measure particle diameters and settling velocities. With the experimental data, bivariate distribution functions through copula modelling were employed to provide a more accurate estimation of the effective density of flocs through the empirical model proposed by Lau and Krishnappan. It was observed that the primary particle density constituting the flocs varied between 1051 kg/m³ and 1426 kg/m³. Moreover, when considering a variable primary particle density, effective floc density values ranging from 980 kg/m³ to 1500 kg/m³ were obtained for floc diameters ranging from 0.068 mm to 1.9 mm, respectively. This variation confirms that the b and c parameters of the Lau-Krishnappan model change for each floc diameter range following the quantiles associated with the conditional probability of diameter and settling velocity. Thus, employing copula approximation, a more accurate fit of the Lau-Krishnappan model was achieved, considering a wide range of particle diameters at the tails. This approach offers better estimates of floc effective density and settling velocity, essential for enhancing the selection and design of aquaculture tanks and settlers to ensure efficient solids removal.

Study on the microscopic characteristics of pentanol/highly active fuel spray based on high-speed droplet tracking velocimetry technology

Pentanol blending with highly reactive fuels for internal combustion engines holds considerable promise. The atomization of sprays closely influences the efficiency and combustion characteristics of the system. Based on Particle Tracking Velocimetry and high-speed shadow imaging technology, we propose a method termed High-speed Droplet Tracking Velocimetry for analyzing spray particle size. Prior to conducting our study, we validated the reliability of High-speed Droplet Tracking Velocimetry through comparisons with the Malvern and Phase Doppler methods. This study utilized long working distance micro high-speed shadow imaging technology in a visualized constant volume combustion chamber to investigate the spray micro-characteristics of pentanol blended with highly reactive fuels, including Fatty Acid Methyl Ester, Hydrogenated Catalytic Biodiesel, and Diesel. The results indicate that under all operating conditions, the droplets of P20H80 (the volume fraction of 20 % n-pentanol mixed with 80 % Hydrogenated Catalytic Biodiesel) are smaller and more uniformly distributed, demonstrating the minimum Sauter Mean Diameter, characteristic diameter, and width of droplet size distribution. The droplet velocities of P20H80 are the lowest, followed by P20Di80 (the volume fraction of 20 % n-pentanol mixed with 80 % Diesel), and P20B80 (the volume fraction of 20 % n-pentanol mixed with 80 % Fatty Acid Methyl Ester) exhibits the highest velocities. These velocity differences among the three fuels generally remain within 10 %-20 % across various injection pressures. P20Di80 shows the highest Reynolds and Weber numbers, followed by P20H80, with P20B80 having the smallest values due to its intermediate physical properties. The gaps between each fuel type range from 20 % to 30 %.

Cluster-based particle tracking velocimetry algorithm combining the quasi-parallel correction in granular motions reconstruction

Cluster-based particle tracking velocimetry algorithm combining the quasi-parallel correction in granular motions reconstruction