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This work describes the development of a particle tracking velocimetry (PTV) algorithm designed to improve three-dimensional (3D), three-component velocity field measurements using a single plenoptic camera. Particular focus is on mitigating the longstanding depth uncertainty issues that have traditionally plagued plenoptic particle image velocimetry (PIV) experiments by leveraging the camera's ability to generate multiple perspective views of a scene in order to assist both particle triangulation and tracking. 3D positions are first estimated via light field ray bundling (LFRB) whereby particle rays are projected into the measurement volume using image-to-object space mapping. Tracking is subsequently performed independently within each perspective view, providing a statistical amalgamation of each particle's predicted motion through time in order to help guide 3D trajectory estimation while simultaneously protecting the tracking algorithm from physically unreasonable fluctuations in particle depth positions. A synthetic performance assessment revealed a reduction in the average depth errors obtained by LFRB as compared to the conventional multiplicative algebraic reconstruction technique when estimating particle locations. Further analysis using a synthetic vortex ring at a magnification of − 0.6 demonstrated plenoptic-PIV capable of maintaining the equivalent of 0.1–0.15 voxel accuracy in the depth domain at a spacing to displacement ratio of 5.3–10.5, an improvement of 84–89% compared to plenoptic-PIV. Experiments were conducted at a spacing to displacement ratio of approximately 5.8 to capture the 3D flow field around a rotor within the rotating reference frame. The resulting plenoptic-PIV/PTV vector fields were evaluated with reference to a fixed frame stereoscopic-PIV (stereo-PIV) validation experiment. A systematic depth-wise (radial) component of velocity directed toward the wingtip, consistent with observations from prior literature and stereo-PIV experiments, was captured by plenoptic-PTV at magnitudes similar to the validation data. In contrast, the plenoptic-PIV did not discern any coherent indication of radial motion. Our algorithm constitutes a significant advancement in enhancing the functionality and versatility of single-plenoptic camera flow diagnostics by directly addressing the primary limitation associated with plenoptic imaging. [ABSTRACT FROM AUTHOR]

A new phosphor particle streak velocimetry (phosphor-PSV) diagnostic with high spatial resolution was recently demonstrated (Fan et al. in Opt Lett 46:641, 2021), where individual phosphor particles, excited by a short pulse laser, form streaks as a results of their displacement by the flow during their relatively long luminescence decay. The local flow velocity is derived by fitting each phosphor streak as a two-dimensional linearly-moving point source with a mono-exponential decaying emission. This single-pulse, single-exposure technique yields a vector for each particle, as in particle tracking velocimetry, avoiding the spatial filtering associated with particle image velocimetry (PIV). The wavelength-shifted luminescence also allows rejection of unwanted reflected laser light, and macroscopic measurements in Fan et al. (Opt Lett 46:641, 2021) were obtained at a distance of 30 μ m from a wall. In this manuscript, we establish by a combination of experiments and luminescent particle imaging simulations the performance of the technique in a range of flow conditions and imaging parameters. A new image segmentation algorithm is developed and applied to phosphor streak images to improve the density of measurements to 0.004 vectors per pixel (vpp). Two phosphors with decay times of 564.2 μ s ( Gd 2 O 2 S: Tb 3 + ) and 0.92 μ s ( BaMgAl 10 O 17 : Eu 2 + ), are used to perform measurements in slow liquid flows ( < 20 cm/s) and fast turbulent jets (up to 85 m/s), respectively. To assess the uncertainty of the approach for various experimental parameters (phosphor particles, flow velocity, particle image size, signal-to-noise ratio, and streamline curvature), synthetic streaks are generated and validated against experimental data. Finally, the impact of out-of-plane motion is investigated experimentally. This study paves the way for a wide implementation of the new phosphor-PSV technique in flow research and, in particular, to study two-phase flows and confined or semi-confined flows near solid boundaries. [ABSTRACT FROM AUTHOR]

In the present study, we introduce new bubble velocimetry methods based on the optical flow, which were validated (compared) with the conventional particle tracking velocimetry (PTV) for various gas–liquid two-phase flows. For the optical flow algorithms, the convolutional neural network (CNN)-based models as well as the original schemes like the Lucas-Kanade and Farnebäck methods are considered. In particular, the CNN-based method was re-trained (fine-tuned) using the synthetic bubble images produced by varying the density, diameter, and velocity distribution. While all models accurately measured the unsteady velocities of a single bubble rising with a lateral oscillation, the pre-trained CNN-based method showed the discrepancy in the averaged velocities in both directions for the dilute bubble plume. In terms of the fluctuating velocity components, the fine-tuned CNN-based model produced the closest results to that from PTV, while the conventional optical flow methods under- or over-estimated them owing to the intensity assumption. When the void fraction increases much higher (e.g., over 10%) in the bubble plume, the PTV failed to evaluate the bubble velocities because of the overlapped bubble images and significant bubble deformation, which is clearly overcome by the optical flow bubble velocimetry. This is quite encouraging in experimentally investigating the gas–liquid two-phase flows of a high void fraction. Furthermore, the fine-tuned CNN-based model captures the individual motion of overlapped bubbles most faithfully while saving the computing time, compared to the Farnebäck method. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Mapping tidal currents is important for a variety of coastal and marine applications. Deriving current maps from in-situ measurements is difficult due to spatio-temporal separation of measurement points. Therefore, low-cost remote sensing tools such as drone-based surface velocimetry are attractive. Previous application of particle image velocimetry to tidal current measurements demonstrated that accuracy depends on site and environmental conditions. This study compares surface velocimetry techniques across a range of these conditions. Various open-source tools and image pre-processing methods were applied to six sets of videos and validation data that cover a variety of site and weather conditions. When wind-driven ripples are present in imagery, it was found a short-wave celerity inversion performed best, with mean absolute percentage error (MAPE) of 5–6% compared to surface drifters. During lower wind speeds, current-advected surface features are visible and techniques which track these work best, of which the most appropriate technique depends on specifics of the collected imagery; MAPEs of 9–21% were obtained. This work has quantified accuracy and demonstrated that surface current maps can be obtained from drones under both high and low wind speeds and at a variety of sites. By following these suggested approaches, practitioners can use drones as a current mapping tool at coastal and offshore sites with confidence in the outputs. • Drones collected video data at 5 UK sites during differing weather conditions. • Various surface velocimetry techniques are used to estimate currents. • Velocimetry outputs are compared to surface drifters and ADCP measurements. • Mean absolute error of 5–6% can be obtained if the most suitable technique is used. • Guidelines for the appropriate technique for a given set of conditions are developed. [ABSTRACT FROM AUTHOR]

Emerging time-resolved volumetric PIV techniques have made simultaneous measurements of velocity and pressure fields possible. Yet, in many experimental setups, satisfying the spatial and temporal resolution requirements is a challenge. To improve the quality of sparse and noisy data, this paper introduces a constrained cost minimization (CCM) technique, which interpolates unstructured particle tracks to obtain the velocity, velocity gradients, material acceleration, hence the pressure, on a Eulerian grid. This technique incorporates physical constraints, such as a divergence-free velocity field and curl-free pressure gradients. The performance is evaluated using synthetic particle tracks for an unsteady double gyre and direct numerical simulations data for a turbulent channel flow, with varying particle concentrations and added errors. The errors in pressure, calculated using omni-directional integration, and correlations with the original data are compared to those obtained using the singular value decomposition (SVD) interpolation technique. The CCM errors are mostly lower, and the correlation is higher and less sensitive to particle sparsity and added errors compared to those of SVD. The synthetic particle traces are also projected onto four planar images to evaluate the performance of the new procedure together with shake-the-box (STB) particle tracking. A comparison of pressure spectra and correlation with the original data show very good agreement for the CCM method. Hence, CCM appears to be an effective method for improving the interpolation of sparse data. Sample experimental data obtained in the shear layer behind a backward-facing step demonstrate the application of STB and CCM to resolve the pressure field in coherent vortex structures. [ABSTRACT FROM AUTHOR]

Sustainable water resources management plans require advanced instrumentation to measure accurately both the water velocity and the discharge of the aquatic bodies. An innovative way to achieve this goal is with the use of non-contact, cost-effective, and efficient image-based methods. A studied water surface can be captured by subsequent images while surface velocity can be estimated by the movement of tracers on the water’s surface. Various software have been developed in order to utilize such tracing algorithms and to monitor the surface velocity of fluids. The objective of this study was to present the current status of these software, describe the main parameters, and provide applied examples of image-based velocimetry application. To achieve this firstly, a comprehensive review was conducted that focused on the scientific publications that have utilized particle image and tracking velocimetry methods on water bodies. Secondly, the main scope of this research was to record and analyze these publications based on the software that were used. This paper presents 30 software with their main features and few of their application examples. This should help other researchers choose the appropriate image-based velocimetry software to measure water velocity and discharge based on the needs of their study. [ABSTRACT FROM AUTHOR]

Abstract Most dry slab avalanches occur during or immediately following loading by snowfall or wind deposition. In the absence of further loading avalanche activity decreases over time. This suggests that loading favorably changes snow cover properties for avalanche release over short time scales (e.g., minutes and hours), and that changes over longer time scales (days or longer) help to stabilize the snowpack. This study quantifies both: 1) the effect of increasing load on the interaction of the slab and weak layer over short time scales, and 2) the longer term stabilizing changes following loading. We developed a field method to rapidly increase the load on existing weak layers, and conducted two different sets of experiments. For the first set of experiments we used a cardboard frame the dimensions of a standard Propagation Saw Test (PST) and added 5, 10, 15, or 20 cm of disaggregated snow on top of PST columns on 11 sampling days. We allowed the added snow to sinter for approximately 30 to 60 min before completely isolating the block and performing a PST. In the second set of experiments we used the same technique to add 10 cm of disaggregated snow on over 30 isolated columns. We then conducted PSTs in the minutes, hours and days following isolation, with tests ranging from 15 min to 4 days. For both experiments we filmed each test at 120 fps for particle tracking velocimetry analysis. We also utilized a model simulating the experiments to better interpret our results. In the first set of experiments, critical crack lengths dramatically decreased with increasing load while crack propagation speed increased, a finding consistent with previous work. In the second set of experiments, we found that critical crack lengths increased rapidly at first and then more slowly over time. Simulations of the experiments suggest that changes in critical crack length over time are caused by an increase in slab elastic modulus in the first hours following loading, and then caused by both increasing slab elastic modulus and weak layer specific fracture energy in days following loading. Overall, our results help to illustrate changes in critical crack lengths immediately after and in the days following loading. Our results are consistent with field observations of increasing avalanche activity during and immediately following loading events and decreasing avalanche activity afterwards. [ABSTRACT FROM AUTHOR]

In this study, a centrifugal pump with a specific speed of 39.12 m×min-1×m3s-1 is treated to analyze the flow instability under part-load conditions by numerical simulation and experimental test. For calculations, the RANS method, coupled with the k-ω SST turbulence model, is adopted. Numerical results at different operation points are compared with available experimental data, such as hydraulic performance and flow field information by particle image velocimetry. The numerical and experiment results agree well. The flow simulation indicates a strong reverse flow at the passage upstream impeller inlet, and the energy loss in the impeller is the largest under part-load conditions among all flow components in the pump. In one impeller revolution, one blade-to-blade flow passage is always nearly blocked off by the rotating stall occurring at the impeller inlet for each instant, and the blockage induces a jet flow with large velocity at the next blade-to-blade flow passage along the rotational direction of the impeller. The blockage and the jet flow in the blade-to-blade flow passages will make the flow unstable inside the impeller and cause performance breakdown and pressure vibration under part-load conditions for the pump. [ABSTRACT FROM AUTHOR]

This work focuses on analysing the collection process in flotation by means of a simultaneous time-resolved measurement of particle and bubble trajectories. We introduced a new method that determined the probability of collision and attachment using a 3D particle-tracking method with high temporal resolution (1000 fps) and spatial ( 0.03  mm/pixel) resolution in a dense particle flow ( 5000 particles/ml). A 4D particle image tracking device with three high-speed cameras recorded the three-phase flow in a rectangular bubble column (2 mm, bubble chain). Particles made of fluorescent polystyrene were employed so that particles appeared bright and bubbles dark on the captured images. An attachment occurred if the trajectory of a particle coincided with that of a bubble. The recovery was calculated based on the number of particles attached to a bubble compared to the total number of particles within a reference volume. With this method, the true flotation depending on the particle diameter ( 30 – 100   μ m) was investigated and the results compared with an existing model of the bubble-particle collection microprocess. [ABSTRACT FROM AUTHOR]

Interstitial flows at the middle height of a freely draining foam column were measured by microparticle image velocimetry (μPIV), and superficial flow was measured from the bottom of the foam by a digital weighing scale. In addition, Plateau border widths were measured from the μPIV images. Plateau borders were selected for μPIV particle-tracking analysis based on their orientation being nearly vertical. Interstitial liquid velocity in the vertically oriented Plateau borders was found to depend linearly on the square of the width consistent with Poiseuille flow. Three foams were examined including a commercial fluorinated Aqueous Film Forming Foam (AFFF), a commercial non-fluorinated firefighting foam (Re-healing Foam Type 6; RF6), and a simple lab-mixed foam composed from sodium dodecyl sulfate (SDS) in water. The interstitial velocity and superficial velocity measurements were compared with theoretical models that assumed either channel-dominated (CD) or node-dominated (ND) viscous dissipation, i.e. rigid or mobile interfaces. The measurements were found to fall in between the two limiting cases for AFFF and RF6, with RF6 showing closer agreement with the CD case than the ND case. Flow measurements in SDS foam agreed approximately with the ND model, as expected, prior to foam breakdown. An approximate liquid volume fraction at the middle height of the foam column was also obtained from the ratio of interstitial flow to superficial flow. [ABSTRACT FROM AUTHOR]

River system measurement and mapping using UAVs is both lean and agile, with the added advantage of increased safety for the surveying crew. A common parameter of fluvial geomorphological studies is the flow velocity, which is a major driver of sediment behavior. Advances in fluid mechanics now include metrics describing the presence and interaction of coherent structures within a flow field and along its boundaries. These metrics have proven to be useful in studying the complex turbulent flows but require time-resolved flow field data, which is normally unavailable in geomorphological studies. Contactless UAV-based velocity measurement provides a new source of velocity field data for measurements of extreme hydrological events at a safe distance, and could allow for measurements of inaccessible areas. Recent works have successfully applied large-scale particle image velocimetry (LSPIV) using UAVs in rivers, focusing predominantly on surficial flow estimation by tracking intensity differences between georeferenced images. The objective of this work is to introduce a methodology for UAV based real-time particle tracking in rivers (RAPTOR) in a case study along a short test reach of the Brigach River in the German Black Forest. This methodology allows for large-scale particle tracking velocimetry (LSPTV) using a combination of floating, infrared light-emitting particles and a programmable embedded color vision sensor in order to simultaneously detect and track the positions of objects. The main advantage of this approach is its ability to rapidly collect and process the position data, which can be done in real time. The disadvantages are that the method requires the use of specialized light-emitting particles, which in some cases cannot be retrieved from the investigation area, and that the method returns velocity data in unscaled units of px/s. This work introduces the RAPTOR system with its hardware, data processing workflow, and provides an example of unscaled velocity field estimation using the proposed method. First experiences with the method show that the tracking rate of 50 Hz allows for position estimation with sub-pixel accuracy, even considering UAV self-motion. A comparison of the unscaled tracks after Savitzky-Golay filtering shows that although the time-averaged velocities remain virtually the same, the filter reduces the standard deviation by more than 40% and the maxima by 20%. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

A subclass of viscoelastic surfactant fluids are investigated as flow-dependent proppant carrying fluids. Shear-induced structure networks have the potential to eliminate the need for cross linkers and viscosity breakers associated with traditional fluids because they shear-thicken and shear-thin depending on the fluid pump rates in the fracture. Particle tracking velocimetry is used to visualize proppant settling velocity inside a fracture flow cell. We show that settling velocities in high shear flows are reduced by a factor 20 compared to settling in low shear ones and fluids at rest. Couette cell and oscillatory rheometry are used to investigate the induced viscoelasticity. The critical shear rate required for fluid thickening and the succeeding viscosity contrast ratio determine the extra proppant carrying capacity. [ABSTRACT FROM AUTHOR]

This study presents experimental results of velocity distribution and water depth at specific locations obtained from simplified urban district experiments accounting for the effect of building blockage. Laboratory experiments are performed inside a flume (0.8m×0.8m×18m) for various arrangements of pervious and impervious building blocks. The Digital Particle Velocimetry System (DPIV) was used for recording velocity distributions behind the building blocks under three upstream discharge ratios. The relationship between building blockage and flow pattern is determined as well as the pervious effect for wake flow characteristics behind the blocks. The obtained experiment data can be readily used to validate or calibrate numerical models for urban flood simulations. [ABSTRACT FROM AUTHOR]

The novel “travelling fluidized bed” (TFB), operated under identical conditions, was deployed to compare alternate experimental measurement techniques for the investigation of solid motion in gas-fluidized beds operating in the square-nosed slugging regime. Measurements of particle velocity obtained by radioactive particle tracking (RPT — non-invasive at the Ecole Polytechnique de Montréal), positron emission particle tracking (PEPT — non-invasive at University of Birmingham), optical fibre probes (invasive at UBC) and borescopic high speed particle image velocimetry (invasive at PSRI) are compared for sand particles of mean diameter of 292 μm. Significant differences between the time-average radial profiles of particle velocity are observed in many cases. The results provide valuable insights into the merits and challenges of advanced particle velocity measurement techniques. [ABSTRACT FROM AUTHOR]

We present an experimental technique, that we refer to as ‘thermographic particle velocimetry’ (TPV), which is capable of the simultaneous measurement of two-dimensional (2-D) surface temperature and velocity at the interface of multiphase flows. The development of the technique has been motivated by the need to study gravity-driven liquid-film flows over inclined heated substrates, however, the same measurement principle can be applied for the recovery of 2-D temperature- and velocity-field information at the interface of any flow with a sufficient density gradient between two fluid phases. The proposed technique relies on a single infrared (IR) imager and is based on the employment of highly reflective (here, silver-coated) particles which, when suspended near or at the interface, can be distinguished from the surrounding fluid domain due to their different emissivity. Image processing steps used to recover the temperature and velocity distributions include the decomposition of each original raw IR image into separate thermal and particle images, the application of perspective distortion corrections and spatial calibration, and finally the implementation of standard particle velocimetry algorithms. This procedure is demonstrated by application of the technique to a heated and stirred flow in an open container. In addition, two validation experiments are presented, one dedicated to the measurement of interfacial temperature and one to the measurement of interfacial velocity. The deviations between the results generated from TPV and those from accompanying conventional techniques do not exceed the errors associated with the latter. [ABSTRACT FROM AUTHOR]

By employing the Nanoparticle-based Planar Laser Scattering method and the particle image velocimetry, structural responses of a Mach 2.95 turbulent boundary layer to two concave curvatures (radiuses are 113 mm and 350 mm, respectively) are experimentally investigated. Large scale vortices formed in the flat plate region are observed to break up into smaller ones immediately after they flow into the concave region. Larger curvature tends to introduce more immediate breakup of the vortices. Fractal analysis reveals that after the turning angle and impulse of bulk dilatation reach 11° and 0.6, respectively, the breakup of the vortices slows down evidently. [ABSTRACT FROM AUTHOR]

A method to characterize viscoelastic fluids in transient shear is proposed based on spatiotemporal flow velocimetry. Particle tracking velocimetry and ultrasound velocity profiling are functionally applied to obtain shear rate and shear strain of a fluid within a Couette system. The ratio of the cylinder radii is set large differently from ordinary Couette systems so that shear stress and shear strain in their designed ranges are available without changing the rotational speed. Simultaneously performing a torque measurement of the rotating cylinder with flow velocimetry, the spatiotemporal fluid response is converted into a triadic relation among shear rate, shear strain, and shear stress. The relationship is graphically represented as a “flow surface” in the three-dimensional parameter space. For an aqueous polyacrylamide solution, the elementary features, such as yield stress, shear wave, and shear-thinning trend, of the viscoelastic fluid reflected in the flow surface are read off. Finally, the experimental flow surface is directly applied for momentum conservation equation to simulate the viscoelastic flow structure as a demonstration. [ABSTRACT FROM AUTHOR]

The experimental study of integral characteristics of extinguishing liquid (water) droplet evaporation in flaming combustion area has been held. Optical methods of two-phase and heterogeneous mixtures diagnostics (Particle Image Velocimetry and Interferometric Particle Imaging) have been used for heat and mass transfer process investigation. It was established that small-size solid particles, for example carbon particles in droplet structure can enhance water evaporation in flame area. It was shown that the rate of evaporation process depends on concentration and sizes of solid particles in a water droplet. The correlations have been determined between the sizes of solid particles and water droplets for maximum efficiency of fire extinguishing. The physical aspects of the problem have been discussed. [ABSTRACT FROM AUTHOR]

To improve current methods for the experimental validation of numerical simulations in corrugated-plate heat exchangers (CPHEs), a full-field quantitative velocimetry of fluid flow is required. This paper investigates the application of particle-tracking velocimetry (PTV) to modified CPHEs. For this, an experimental CPHE unit with a transparent, corrugated upper plate was built. We show that by viewing through a corrugated, refractive interface, a complex periodic optical distortion is introduced, that affects and corrupts the estimated particle trajectories. As this problem cannot be addressed using existing calibration techniques, we propose a novel calibration algorithm for periodic optical distortion. The algorithm relies on the automatic detection of nonlinear distortion using a checkerboard target place within the CPHE unit. The calibration is first made on a coarse grid and subsequently refined by a set of low-order periodic basis functions in order to seize the periodic nature of the deformation field. The proposed algorithms have been applied to a test case with known, uniform particle velocities in order to demonstrate the performance. When applied to a real case, a reduction in the systematic positional error by 35 % was demonstrated. [ABSTRACT FROM AUTHOR]

In this paper, a novel streak-resolving algorithm is proposed for particle streak velocimetry (PSV) to resolve velocity fields for a single image frame with multiple particle streaks. A model streak function, which was based on the temporal integral of the particle image intensity moving along its trajectory during the exposure period, was approximated using a multivariable least-square fit procedure to reconstruct the streak information and the corresponding particle trajectory. Lagrangian tracking was achieved, and the velocities were evaluated by differentiating the resolved particle trajectory with respect to time. Two types of flows, accelerating parallel flow and the rotational flow of Hill's vortex, were used to generate synthetic streaks for the performance tests. Three types of relative error were defined and used to evaluate the performance of the algorithm in terms of statistical mean and standard deviation (SD) errors. The accuracy of the fitted streak parameters, such as particle image intensity and diameter, were also evaluated and compared. The results reveal that the error and SD were low if the image noise is below 1.0%; for noise levels of 5.0%, the error was up to 10% with an SD of up to 12%. The error and SD of the particle image intensity and particle image diameter for both flow types were also 0%–7% for clean and up to 12% for noisy images. The processing results for experimental streak images of flow past a cylinder reveal that these images can be resolved using the proposed algorithm with a residual mean error of 4.38 and an SD of 9.48. These results suggest that the proposed novel approach can be used to resolve velocity fields with only a single image frame and without expensive hardware for high-speed imaging and thus is suitable for diverse applications. • A post-processing algorithm for Particle Streak Velocimetry (PSV) to resolve particle streak images formed by long camera exposure time. • Extract spatial and temporal information to fully reconstruct particle trajectory as a function of time. • Obtain tangential velocity from the reconstructed trajectory at any point on a streak. • Validated by both synthetic and experimental streak images. • Errors of resolving the streak images quantified and the error sources discussed. [ABSTRACT FROM AUTHOR]

A simultaneous measurement technique based on planar laser-induced fluorescence imaging (PLIF) and particle image/tracking velocimetry (PIV/PTV) is described for the investigation of the hydrodynamic characteristics of harmonically excited liquid thin-film flows. The technique is applied as part of an extensive experimental campaign that covers four different Kapitza ( Ka ) number liquids, Reynolds ( Re ) numbers spanning the range 2.3–320, and inlet-forced/wave frequencies in the range 1–10 Hz. Film thicknesses (from PLIF) for flat (viscous and unforced) films are compared to micrometer stage measurements and analytical predictions (Nusselt solution), with a resulting mean deviation being lower than the nominal resolution of the imaging setup (around 20 μm). Relative deviations are calculated between PTV-derived interfacial and bulk velocities and analytical results, with mean values amounting to no more than 3.2% for both test cases. In addition, flow rates recovered using LIF/PTV (film thickness and velocity profile) data are compared to direct flowmeter readings. The mean relative deviation is found to be 1.6% for a total of six flat and nine wavy flows. The practice of wave/phase-locked flow-field averaging is also implemented, allowing the generation of highly localized velocity profile, bulk velocity and flow rate data along the wave topology. Based on this data, velocity profiles are extracted from 20 locations along the wave topology and compared to analytically derived ones based on local film thickness measurements and the Nusselt solution. Increasing the waviness by modulating the forcing frequency is found to result in lower absolute deviations between experiments and theoretical predictions ahead of the wave crests, and higher deviations behind the wave crests. At the wave crests, experimentally derived interfacial velocities are overestimated by nearly 100%. Finally, locally non-parabolic velocity profiles are identified ahead of the wave crests; a phenomenon potentially linked to the cross-stream velocity field. [ABSTRACT FROM AUTHOR]

Oxygen supplementation [hyperoxia, increased fraction of inspired oxygen (FiO2)] is an indispensable treatment in the intensive care unit for patients in respiratory failure. Like other treatments or drugs, hyperoxia has a riskbenefit profile that guides its clinical use. While hyperoxia is known to damage respiratory epithelium, it is unknown if damage can result in impaired capacity to generate cilia-driven fluid flow. Here, we demonstrate that quantifying cilia-driven fluid flow velocities in the sub-100 μm/s regime (sub-0.25 in./min regime) reveals hyperoxia-mediated damage to the capacity of ciliated respiratory mucosa to generate directional flow. Flow quantification was performed using particle tracking velocimetry optical coherence tomography (PTV-OCT) in ex vivo mouse trachea. The ability of PTV-OCT to detect biomedically relevant flow perturbations in the sub-100 μm/s regime was validated by quantifying temperature- and drug-mediated modulation of flow performance in ex vivo mouse trachea. Overall, PTV-OCT imaging of cilia-driven fluid flow in ex vivo mouse trachea is a powerful and straightforward approach for studying factors that modulate and damage mammalian respiratory ciliary physiology. [ABSTRACT FROM AUTHOR]

Recent advances in development of in situ video settling columns have significantly contributed toward fine-sediment dynamics research through concurrent measurement of suspended sediment floc size distributions and settling velocities, which together also allow inference of floc density. Along with image resolution and sizing, two additional challenges in video analysis from these devices are the automated tracking of settling particles and accounting for fluid motions within the settling column. A combination of particle tracking velocimetry (PTV) and particle image velocimetry (PIV) image analysis techniques is described, which permits general automation of image analysis collected from video settling columns. In the fixed image plane, large-particle velocities are determined by PTV and small-particle velocities are tracked by PIV and treated as surrogates for fluid velocities. The large-particle settling velocity (relative to the suspending fluid) is determined by the vector difference of the large and small-particle settling velocities. The combined PTV/PIV image analysis approach is demonstrated for video settling column data collected within a dredge plume in Boston Harbor. The automated PTV/PIV approach significantly reduces uncertainties in measured settling velocity and inferred floc density. [ABSTRACT FROM AUTHOR]

Cilia-driven fluid flow is important for multiple processes in the body, including respiratory mucus clearance, gamete transport in the oviduct, right-left patterning in the embryonic node, and cerebrospinal fluid circulation. Multiple imaging techniques have been applied toward quantifying ciliary flow. Here, we review common velocimetry methods of quantifying fluid flow. We then discuss four important optical modalities, including light microscopy, epifluorescence, confocal microscopy, and optical coherence tomography, that have been used to investigate cilia-driven flow. [ABSTRACT FROM AUTHOR]

The article focuses on a study which investigates the vector-adjusting characteristic of a dual-synthetic-jet (DSJ) actuator with an evaluation method based on schlieren and particle-image velocimetry (PIV) experiments. Topics discussed include the schematic of the DSJ actuator, the experimentally vectoring characteristic of the DSJ at different dimensional displacements and variations of vectoring angle and maximum velocity with the dimensionless displacement.

This study aims to experimentally investigate the impact of vessel stiffness on the flow dynamics of pulsatile vascular flow. Vessel phantoms with elastic walls were fabricated using polyvinyl alcohol cryogel to result in stiffness ranging from 60.9 to 310.3 kPa and tested with pulsatile flows using a flow circulation set-up. Two-dimensional instantaneous and time-dependent flow velocity and shear rate vector fields were measured using ultrasonic particle image velocimetry (EchoPIV). The waveforms of peak velocities measured by EchoPIV were compared with the ultrasonic pulse Doppler spectrum, and the measuring accuracy was validated. The cyclic vessel wall motion and flow pressure were obtained as well. The results showed that vessel stiffening influenced the waveforms resulting from vessel wall distension and flow pressure, and the fields of flow velocity and shear rate. The stiffer vessel had smaller inner diameter variation, larger pulse pressure and median pressure. The velocity and shear rate maximized at peak systole for all vessels. The results showed a decrease in wall shear stress for a stiffer vessel, which can initiate the atherosclerotic process. Our study elucidates the impact of vessel stiffness on several flow dynamic parameters, and also demonstrates the EchoPIV technique to be a useful and powerful tool in cardiovascular research. [ABSTRACT FROM PUBLISHER]

Within its life cycle, a copepod goes through drastic changes in size, shape and swimming mode. In particular, there is a stark difference between the early (nauplius) and later (copepodid) stages. Copepods inhabit an intermediate Reynolds number regime (between ∼1 and 100) where both viscosity and inertia are potentially important, and the Reynolds number changes by an order of magnitude during growth. Thus we expect the life stage related changes experienced by a copepod to result in hydrodynamic and energetic differences, ultimately affecting the fitness. To quantify these differences, we measured the swimming kinematics and fluid flow around jumping Acartia tonsa at different stages of its life cycle, using particle image velocimetry and particle tracking velocimetry. We found that the flow structures around nauplii and copepodids are topologically different, with one and two vortex rings, respectively. Our measurements suggest that copepodids cover a larger distance compared to their body size in each jump and are also hydrodynamically quieter, as the flow disturbance they create attenuates faster with distance. Also, copepodids are energetically more efficient than nauplii, presumably due to the change in hydrodynamic regime accompanied with a welladapted body form and swimming stroke. [ABSTRACT FROM AUTHOR]

The paper describes experimental studies of water droplet motion in turbine cascades using laser diagnostic techniques (particle image velocimetry/particle tracking velocimetry). The investigations were performed at the experimental installation Wet Steam Circuit – 2 (WSC-2) at the Moscow Power Engineering Institute. The facility allows the study of the flow of superheated, saturated and wet steam over a broad range of velocities. The effect of the steam initial conditions on the behaviour of the coarse droplets downstream of a nozzle blade cascade has been studied for subsonic and supersonic flow. The variation of the average droplet velocities and angles across the blade pitch, and the droplet trajectories have been obtained. The relationship between the slip coefficients of droplets having minimal velocities in the blade trailing-edge plane, the initial steam wetness, and the axial distance downstream of the nozzle blade are suggested as the criteria characterising the coarse droplet motion downstream of the cascade. [ABSTRACT FROM PUBLISHER]