Perform Particle Image Velocimetry Research Articles

Three-dimensional flow structures and scalar mixing of a sand dune-inspired jet in crossflow

This paper performed particle image velocimetry (PIV) measurements and delayed detached-eddy simulations (DDES) to comprehensively investigate the flow and mixing behaviors of a sand dune(SD)-inspired JICF (jet in crossflow). PIV measurements were conducted in planes parallel to the mid-plane (Z/D = 0, 0.5, and 1), and three velocity ratios (VR = 0.4, 0.8, and 1.2) were studied. The time-averaged and turbulent flow features behind the sand dune-inspired JICF is strongly three-dimensional: the jet tends to move from the mid-plane to the outer planes and can stay attached well to the wall even at Z/D = 1. Then DDES method was applied to reveal detailed three-dimensional flow structures and mixing characteristics of the SD-inspired JICF, and a cylindrical JICF was also simulated as a benchmark for comparison. The horseshoe vortex, arch-shaped shear layer vortices, and streamwise rollers are mainly responsible for the formation of the anti-counter rotating vortex pair (anti-CRVP) observed in the mean flow field of SD configuration. Analysis of concentration distributions indicates that scalar mixing between the jet and the mainstream is highly correlated to the flow features and vortex structures, and the mixing strength of the sand dune-inspired JICF is generally weaker than that of the cylindrical JICF.

Underwater oscillations of rigid plates with H-shaped cross sections: An experimental study to explore their flow physics

In this work, we present a comprehensive experimental study on the problem of harmonic oscillations of rigid plates with H-shaped cross sections submerged in a quiescent, Newtonian, incompressible, viscous fluid environment. Motivated by recent results on the minimization of hydrodynamic damping for transversely oscillating flat plates, we conduct a detailed qualitative and quantitative experimental investigation of the flow physics created by the presence of the flanges, that is, the vertical segments in the plate cross section. Specifically, the main goal is to elucidate the effect of flange size on various aspects of fluid–structure interaction, by primarily investigating the dynamics of vortex shedding and convection. We perform particle image velocimetry experiments over a broad range of oscillation amplitudes, frequencies, and flange size-to-width ratios by leveraging the identification of pathlines, vortex shedding and dynamics, distinctive hydrodynamic regimes, and steady streaming. The fundamental contributions of this work include novel hydrodynamic regime phase diagrams demonstrating the effect of flange ratio on regime transitions, and in the investigation of their relation to qualitatively distinct patterns of vortex–vortex and vortex–structure interactions. Finally, we discuss steady streaming, identifying primary, and secondary structures as a function of the governing parameters.

Particle Image Velocimetry measurements inside a tapered roller bearing with an outer ring made of sapphire: Design and operation of an innovative test rig

Experimental studies on rolling-element bearing lubrication represent an ambitious engineering challenge. In the present study, the design of a dedicated test rig to perform Particle Image Velocimetry (PIV) measurements on the lubricant inside a tapered roller bearing is presented. Through the usage of a sapphire outer ring, it has been possible to optically access the inside of a bearing without compromising its kinematics. PIV measurements allowed the reconstruction of the lubricant velocity field in the cavity between the cage and the outer ring at different rotational speeds. Results show that as the speed of rotation increases, bubbles due to aeration phenomena tend to appear and modify the behavior of the lubricant considerably.

Particle Image Velocimetry for MATLAB: Accuracy and enhanced algorithms in PIVlab

PIVlab is a free toolbox and app for MATLAB®. It is used to perform Particle Image Velocimetry (PIV) with image data: A light sheet illuminates particles that are suspended in a fluid. A digital camera records a series of images of the illuminated particles. The input images are divided into sub-images (interrogation areas), and for each of these, a cross-correlation is performed. The resulting correlation matrix is used to estimate the most probable displacement within each interrogation area. PIV is extensively used for flow analyses where a thin laser sheet illuminates suspended particles in the fluid, but also for other moving textures, like cell migration or ultrasonic images. This paper presents several improvements that were implemented in PIVlab, enhancing the robustness of displacement estimates. The benefit of these improvements is evaluated using experimental images and synthetic images of particle and non-particle textures. Linear correlation and repeated correlation increase the robustness and decrease bias and root-mean-square (RMS) error of the displacement estimates. Particle images have a significantly lower bias and RMS error than non-particle images.

Self-similar jet evolution after drop impact on a liquid surface

Small conical-shaped jets may emanate from a liquid bath a short while after a small drop has hit a liquid pool. Here we perform Particle Image Velocimetry (PIV) measurements of the liquid flow inside upward jets after drop impact and show that fluid elements inside the jets may decelerate up to 5-20 times the gravitational acceleration. The measurements show that both the shape of the jet and the velocity profile are self-similar. A theoretical model including surface tension, fluid inertia, and gravity correctly predicts the self-similar velocity profile and shape of the jet, allowing us to provide the first quantitative explanation of the shape and dynamics of the emanating jets.

Exploration of frequencies peaks observed on local wall pressure measurements by time-resolved velocity field measurements in complex flows

In pressurized water reactors (PWR), fluid–structure interaction provokes vibrations of the fuel rods leading to grid-to-rod fretting. To explore the origins of the local excitations of the rods by the flow, analytical experiments are performed within a 5 × 5 rod bundle maintained by spacer grids. New velocity (laser Doppler velocimetry) and pressure fluctuations measurements, taken for various Reynolds numbers, show the presence of frequency peaks in the spectra over a broad range of Reynolds numbers, i.e. 13,000 to 108,000. The Strouhal numbers obtained vary weakly with the Reynolds number and are in the range of 0.21–0.26. The phenomena being more pronounced downstream the dimples, this manuscript explores the flow at the exit of spacer grids downstream the dimples. To see through aligned rods, refractive index matching is implemented on parts of selected rods. The dilated scale of the experiments permits one to perform particle image velocimetry (PIV; grid of 51 × 89 points with a correlation window size of 0.032 Dh) to identify eddies smaller than the rods in the wake of the dimples. PIV measurements reveal the presence of multi-scale eddies with a periodic street of main eddies. The size of these eddies and the periodic length scale of the street are found to be in agreement with the integral length scale and the length scale built on the frequency peaks of pressure fluctuations. To the authors’ knowledge, this is the first experimental evidence of the existence and the role of coherent eddies streets downstream from dimples of spacer grids with geometries relevant to PWR fuel assemblies.

Velocity distribution in the subchannels of a pin bundle with a wrapping wire (Evaluation of the Reynolds number dependence in a three-pin bundle)

A sodium-cooled fast reactor has been designed to attain a high burn-up core in commercialized fast reactor cycle systems. The sodium-cooled fast reactor adopts a wire spacer between fuel pins. The wire spacer performs functions of securing the coolant channel and mixing between subchannels. In high burn-up fuel subassemblies, the fuel pin deformation due to swelling and thermal bowing may decrease the local flow velocity in the subassembly and influence the heat removal capability. Therefore, understanding the flow field in a wire-wrapped pin bundle is important. This study performed particle image velocimetry (PIV) measurements using a wire-wrapped three-pin bundle water model to grasp the flow field in the subchannel under conditions, including the laminar to turbulent regions. The PIV results confirmed that the normalized flow velocity near the wrapping wire in the low Re number condition was relatively decreased compared to that in the high Re number condition. Meanwhile, in the region away from the wrapping wire, the maximum flow velocity was increased by decreasing the Re number. Accordingly, the PIV measurements using the three-pin bundle geometry without the wrapping wire were also conducted to understand the effect of the wrapping wires on the flow field in the subchannel. The results confirmed that the mixing due to the wrapping wire occurred, even in the laminar condition. These experimental results are useful not only for understanding the pin bundle thermal hydraulics, but also for the code validation.

Improvement of Defogging Performance of Automobile Defroster using Vortex Generators

Fog on the automotive windshield interferes with the sight of the driver and causes serious safety problems; thus, removing it rapidly has been a longstanding goal in the automobile industry. In this study, we propose a novel method for improving the defogging performance of the defroster without its structural alteration by using vortex generators (VGs). To optimize the performance of the VGs, we performed particle image velocimetry experiments, an evaporation measurement experiment, and a numerical simulation for visualizing the airflow from the defroster inlet. The dimensions of the VG (height h = δ and length l = 5δ) were selected according to the boundary-layer thickness, δ, of the lowest-flow rate (ulow = 0.33 m/s) used in the defroster inlet. We employed a dimensionless parameter, i.e., the secondary flow intensity (Se), to quantify the intensity of vortices for several angles of attack (30°, 45°, and 60°). The results indicated that the vortices generated from VGs with a 30° angle of attack retained their maximum intensity until they reached the vision area of the driver. Additionally, we explored two different configurations of VGs—co-rotating and counter-rotating—and concluded that the counter-rotating configuration had higher performance than the co-rotating configuration. The optimized VGs were directly inserted in the defroster inlet and improved the defogging performance by approximately 10%. The proposed method is applicable to various automobile models for enhancing the performance of the defroster regardless of the interior volume, air-ventilation performance, and other conditions, such as the number of passengers and outside temperature.

Effect of a radial flow foregoing a capillary die on the behavior of extruded PDMS: velocity field–distorted strand correlation

The present work aimed to investigate the influence of flow geometry on volume instability associated to a linear polydimethylsiloxane (PDMS). To do so, a convergent radial flow was created at the die entrance. The performed particle image velocimetry (PIV) recordings under unstable flow regime, in capillary rheometer, characterized by the new entrance zone, show a new pattern of streamlines above the die and a disappearance of the vortex developed in the corner of the reservoir. Photographs of the extrudate strands obtained at the die exit depict a new morphology of defect which appears with a well-established radial flow. These results led to the correlation agreement between the gross melt fracture and the flow instability at the entrance zone. It also proved the importance of elongational and shear components linked to the upstream flow in the appearance and development of volume distortion.

Experimental investigation of turbulent flow over live mussels

Unionids have been described as ecosystem engineers capable of affecting the local food web as well as the hydrodynamics, passively and actively. In this study, we perform particle image velocimetry measurements to characterize the flow around a live Amblema plicata using natural sediments as tracer particles under two specific flow rates and orientations i.e., facing upstream and downstream, to understand the interaction between the organism and flow surrounding it. The behavior of the mussel is also quantified under all the above experimental conditions using a hall-effect gape sensor that captures valve motion. The flow measurements show clear variations between the two orientations tested, with the presence of a low velocity region behind the mussel extending beyond the tested field of view for the upstream orientation. In contrast, the flow in the wake of the downstream orientation recovers very quickly to the incoming flow strength. The inspection of the vorticity contours around the mussel also highlights clear differences between orientations, with the structure of a shear layer under the upstream orientation changing to a region of concentrated vorticity near the siphons for the downstream cases. The variation of the net flux through the mussel along with the frequency of partial closures captured by the gape sensor measurements illustrate a potential added bioenergetic cost to the mussel feeding when facing downstream, especially at higher flow speeds.

Stereovision Combined With Particle Tracking Velocimetry Reveals Advection and Uplift Within a Restraining Bend Simulating the Denali Fault

Scaled physical experiments allow us to directly observe deformational processes that take place on time and length scales that are impossible to observe in the Earth’s crust. Successful evaluation of advection and uplift of material within a restraining bend along a strike-slip fault zone depends on capturing the evolution of strain in three dimensions. Consequently, we require deformation within the horizontal plane as well as vertical motions. While 3D Digital Image Correlation systems can provide this information, their high costs have prompted us to develop techniques that require only two DSLR cameras and a few Matlab® toolboxes, which are available to researchers at many institutions. Matlab® plug-ins can perform Particle Image Velocimetry (PIV), a technique used in many analog modeling studies to map the incremental displacements fields. For tracking material advection throughout experiments more suitable Matlab® plug-ins perform Particle Tracking Velocimetry (PTV), which tracks the complete two-dimensional displacement path of individual particles. To capture uplift the Matlab® Computer Vision Toolbox™, uses pairs of photos to capture the evolving topography of the experiment. The stereovision approach eliminates the need to stop the experiment to perform 3D laser scans, which can be problematic when working with materials that have time dependent rheology. We demonstrate how the combination of PIV, PTV and stereovision analysis of experiments that simulate the Mount McKinley restraining bend reveal the evolution of the fault system and three-dimensional advection of material through the bend.

Effects of bottom topography on the spin-up in a cylinder

Motivated by better understanding the long-standing issue of the role of topography on the transport of angular momentum in rapidly rotating fluids, we conducted spin-up experiments in a straight cylinder with a regular pavement of rectangular blocks at the bottom. We perform particle image velocimetry measurements to monitor the decay of the initial differential motion generated by the sudden increase of the container rotation rate. We observe that the re-synchronization time, the so-called spin-up time, is shorter in the presence of topography with a minimum at a particular length scale of the topography pattern. We show evidence of energy transport by inertial waves as well as non-linear mechanisms leading to a scaling of the spin-up time significantly different from the classical E−1/2 in the absence of topography.

Benchmark for Numerical Models of Stented Coronary Bifurcation Flow.

In-stent restenosis ails many patients who have undergone stenting. When the stented artery is a bifurcation, the intervention is particularly critical because of the complex stent geometry involved in these structures. Computational fluid dynamics (CFD) has been shown to be an effective approach when modeling blood flow behavior and understanding the mechanisms that underlie in-stent restenosis. However, these CFD models require validation through experimental data in order to be reliable. It is with this purpose in mind that we performed particle image velocimetry (PIV) measurements of velocity fields within flows through a simplified coronary bifurcation. Although the flow in this simplified bifurcation differs from the actual blood flow, it emulates the main fluid dynamic mechanisms found in hemodynamic flow. Experimental measurements were performed for several stenting techniques in both steady and unsteady flow conditions. The test conditions were strictly controlled, and uncertainty was accurately predicted. The results obtained in this research represent readily accessible, easy to emulate, detailed velocity fields and geometry, and they have been successfully used to validate our numerical model. These data can be used as a benchmark for further development of numerical CFD modeling in terms of comparison of the main flow pattern characteristics.

Flow Recirculation in Cartilaginous Ring Cavities of Human Trachea Model.

Despite the prevailing assumption of "smooth trachea walls" in respiratory fluid dynamics research, recent investigations have demonstrated that cartilaginous rings in the trachea and main bronchi have a significant effect on the flow behavior and in particle deposition. However, there is not enough detailed information about the underlying physics of the interaction between the cartilage rings and the flow. This study presents an experimental observation of a simplified Weibel-based model of the human trachea and bronchi with cartilaginous rings. A transparent model and refractive index-matching methods were used to observe the flow, particularly near the wall. The flow was seeded with tracers to perform particle image velocimetry and particle tracking velocimetry to quantify the effect the rings have on the flow near the trachea and bronchi walls. The experiments were carried out with a flow rate comparable with a resting state (trachea-based Reynolds number of ReD = 2650). The results present a previously unknown phenomenon in the cavities between the cartilaginous rings: a small recirculation is observed in the upstream side of the cavities throughout the trachea. This recirculation is due to the adverse pressure gradient created by the expansion, which traps particles within the ring cavity, thus affecting the treatment of patients suffering from lung disease and other respiratory conditions. The detection of recirculation zones in the cartilage ring cavities sheds light on the particle deposition mechanism and helps explain results from previous studies that have observed an enhancement of particle deposition in models with cartilage rings. These results bring to light the importance of including cartilage rings in experimental, numerical, and theoretical models to better understand particle deposition in the trachea and bronchi. In addition, the results provide scientists and medical staff with new insights for improving drug delivery.

Experimental and Numerical Investigation on Windage Power Losses in High Speed Gears

Enhancing the efficiency of gearing systems is an important topic for the development of future aero-engines with low specific fuel consumption. An evaluation of its structure and performance is mandatory in order to optimize the design as well as maximize its efficiency. Mechanical power losses are usually distinguished into two main categories: load-dependent and load-independent losses. The former are all those associated with the transmission of torque, while the latter are tied to the fluid dynamics of the environment, which surrounds the gears. The relative magnitude of these phenomena is dependent on the operative conditions of the transmission: load-dependent losses are predominant at slow speeds and high torque conditions, load-independent mechanisms become prevailing in high speed applications, like in turbomachinery. A new test rig was designed for investigating windage power losses resulting by a single spur gear rotating in a free oil environment. The test rig allows the gear to rotate at high speed within a box where pressure and temperature conditions can be set and monitored. An electric spindle, which drives the system, is connected to the gear through a high accuracy torque meter, equipped with a speedometer providing the rotating velocity. The test box is fitted with optical accesses in order to perform particle image velocimetry (PIV) measurements for investigating the flow field surrounding the rotating gear. The experiment has been computationally replicated, performing Reynolds-averaged Navier–Stokes (RANS) simulations in the context of conventional eddy viscosity models, achieving good agreement for all of the speed of rotations.

Adverse-Pressure-Gradient Effects on Turbulent Boundary Layers: Statistics and Flow-Field Organization

This manuscripts presents a study on adverse-pressure-gradient turbulent boundary layers under different Reynolds-number and pressure-gradient conditions. In this work we performed Particle Image Velocimetry (PIV) measurements supplemented with Large-Eddy Simulations in order to have a dataset covering a range of displacement-thickness-based Reynolds-number 2300 <Re_{delta ^{*}}< 34000 and values of the Clauser pressure-gradient parameter β up to 2.4. The spatial resolution limits of PIV for the estimation of turbulence statistics have been overcome via ensemble-based approaches. A comparison between ensemble-correlation and ensemble Particle Tracking Velocimetry was carried out to assess the uncertainty of the two methods. The effects of β, Re and of the pressure-gradient history on turbulence statistics were assessed. A modal analysis via Proper Orthogonal Decomposition was carried out on the flow fields and showed that about 20% of the energy contribution corresponds to the first mode, while 40% of the turbulent kinetic energy corresponds to the first four modes with no appreciable dependence on β and Re within the investigated range. The topology of the spatial modes shows a dependence on the Reynolds number and on the pressure-gradient strength, in line with the results obtained from the analysis of the turbulence statistics. The contribution of the modes to the Reynolds stresses and the turbulence production was assessed using a truncated low-order reconstruction with progressively larger number of modes. It is shown that the outer peaks in the Reynolds-stress profiles are mostly due to large-scale structures in the outer part of the boundary layer.

Relaxation of a highly deformed elastic filament at a fluid interface

We perform experiments to investigate the relaxation of a highly deformed elastic filament at a liquid-air interface. The dynamics for filaments of differing length, diameter and elastic modulus collapse to a single curve when the time-dependence is scaled by a time scale $\tau = 8 \pi \mu L_o^4/B$. The relaxation, however, is completed in a very small fraction of the time $\tau$. Even though the time scale $\tau$ can be obtained by balancing the linear bending and viscous forces, it appears to control the highly nonlinear regime of our experiments. Nonlinear numerical simulations show that the force due to tension along the filament is comparable to the bending force, producing a net elastic restoring force that is much smaller than either term. We perform particle image velocimetry at the liquid-air interface to support the results of the numerics. Finally, we find that when the filament is initialized in asymmetric shapes, it rapidly goes to a shape with symmetric stresses. This symmetrisation process is entirely non-linear; we show that the symmetric curvature state minimizes energy at arbitrarily large deformation.

バックステップを過ぎる粘弾性流体乱流の空間相関関数による解析

In this paper, we carried out experiments on backward-facing step (BFS) turbulent flow to investigate viscoelastic influence. We performed particle image velocimetry (PIV) measurement on turbulent flow passing BFS for Newtonian and viscoelastic fluids. The measurement fields were set in x-y plane and x-z plane (here, x is streamwise direction, y is wall-normal direction and z is spanwise direction). The instantaneous velocity fields show that meandering main flow in x-y plane (not in x-z plane) on certain conditions. We employed analysis to investigate characteristics of turbulent structure using of method combined with 4-quadrant classification method and two-point correlation function. The results show that meandering motion is dominated varicose mode, vortices are generated from shear layer, accompanied with that low speed fluid ejecting from upper wall. High correlation region of streamwise velocity fluctuation corresponding to meandering motion is observed at inflection point of streamwise velocity.

Spontaneous Marangoni Mixing of Miscible Liquids at a Liquid-Liquid-Air Contact Line.

We investigate the flow patterns created when a liquid drop contacts a reservoir liquid, which has implications on various physicochemical and biochemical reactions including mixing in microfluidic systems. The localized vortical flow spontaneously triggered by the difference of surface tension between the two liquids is studied, which is thus termed the Marangoni vortex. To quantitatively investigate the strength of vortices, we performed particle image velocimetry (PIV) experiments by varying the surface tension difference, the gap of the flow cell, the density and viscosity of the reservoir liquid, and the size of the drop. A scaling law that balances the interfacial energy of the system with the kinetic energy of the vortical flows allows us to understand the functional dependence of the Marangoni vortex strength on various experimental parameters.

Coordination of contractility, adhesion and flow in migrating Physarum amoebae.

This work examines the relationship between spatio-temporal coordination of intracellular flow and traction stress and the speed of amoeboid locomotion of microplasmodia of Physarum polycephalum. We simultaneously perform particle image velocimetry and traction stress microscopy to measure the velocity of cytoplasmic flow and the stresses applied to the substrate by migrating Physarum microamoebae. In parallel, we develop a mathematical model of a motile cell which includes forces from the viscous cytosol, a poro-elastic, contractile cytoskeleton and adhesive interactions with the substrate. Our experiments show that flow and traction stress exhibit back-to-front-directed waves with a distinct phase difference. The model demonstrates that the direction and speed of locomotion are determined by this coordination between contraction, flow and adhesion. Using the model, we identify forms of coordination that generate model predictions consistent with experiments. We demonstrate that this coordination produces near optimal migration speed and is insensitive to heterogeneity in substrate adhesiveness. While it is generally thought that amoeboid motility is robust to changes in extracellular geometry and the nature of extracellular adhesion, our results demonstrate that coordination of adhesive forces is essential to producing robust migration.