Particle Image Velocimetry Velocity Field Research Articles

Strategies for Computational Fluid Dynamics Validation Experiments

Abstract The Benchmark Validation Experiment for Reynolds-averaged Navier–Stokes (RANS)/large eddy simulation (LES) Investigations (BeVERLI) aims to produce an experimental dataset of three-dimensional non-equilibrium turbulent boundary layers with various levels of separation that, for the first time, meets the most exacting requirements of computational fluid dynamics validation. The application of simulations and modeling in high-consequence engineering environments has become increasingly prominent in the past two decades, considerably raising the standards and demands of model validation and forcing a significant paradigm shift in the design of corresponding validation experiments. In this paper, based on the experiences of project BeVERLI, we present strategies for designing and executing validation experiments, hoping to ease the transition into this new era of fluid dynamics experimentation and help upcoming validation experiments succeed. We discuss the selection of a flow for validation, the synergistic use of simulations and experiments, cross-institutional collaborations, and tools, such as model scans, time-dependent measurements, and repeated and redundant measurements. The proposed strategies are shown to successfully mitigate risks and enable the methodical identification, measurement, uncertainty quantification, and characterization of critical flow features, boundary conditions, and corresponding sensitivities, promoting the highest levels of model validation experiment completeness per Oberkampf and Smith [1]. Furthermore, the applicability of these strategies to estimating critical and difficult-to-obtain bias error uncertainties of different measurement systems, e.g., the underprediction of high-order statistical moments from particle image velocimetry velocity field data due to spatial filtering effects, and to systematically assessing the quality of uncertainty estimates is shown.

Biomimetic crossflow filtration with wave minimal surface geometry for particulate biochar water treatment

Wave minimal surfaces (WMSs) are mathematically defined structures that are commonly observed in nature. Their unique properties have allowed researchers to harness their potential for engineering applications. Since WMSs can be represented by mathematical equations, the geometry can be parametrized and studied using computational fluid dynamics (CFD) for particle separation. Low energy particle separation in water treatment can yield low-carbon footprint technology approaches such as biochar water treatment where removal and recovery of adsorbed N and P on biochar can address water pollution, climate change and food security. The objective of this work was to demonstrate the capability of WMS as a crossflow filtration system to remove particulates in water. For this purpose we used CFD to optimize WMS geometry and studied the performance of the 3D-Printed (3DP) optimized WMS using experimental fluid dynamics (EFD) in a water tunnel. CFD studies quantified planar vorticity, fluid filtrate flux, and particle behavior of WMS. For inflow velocities of 0.2–0.4 m/s, CFD results showed that a reverse wave filter design with convex shape leading-edge, angle of incidence of 90o, and maximum width of n = 1.0 captured 15–25% of upstream velocity at the filter port. CFD analysis showed more than 95% separation efficiency at velocities and pressures of 0.2–0.32 m/s and 5–35 kPa, respectively. Particle Image Velocimetry (PIV) was used for EFD fluid flow measurements with an optimized wave minimal surface (OMWS). Comparison of OMWS CFD and PIV velocity fields showed good agreement with a root-mean-square error of less than 10%. Particle size analysis showed that the 3DP OMWS could filter particle sizes ranging from 1–30 μm with at least 50% particle count reduction in the filtrate. Thus, we successfully demonstrated a novel framework for analyzing a crossflow water filtration system from conceptual design to initial benchtop experiments using iterative CFD, 3DP, and EFD.

Investigating Shear-Layer Instabilities in Supersonic Impinging Jets Using Dual-Time Particle Image Velocimetry

Flow unsteadiness in supersonic impinging jets due to the aeroacoustic feedback can lead to practical problems like ground erosion in short takeoff and vertical landing aircraft, high noise levels, and high unsteady structural loading. The shear-layer instabilities of the jet play an important role in the flow dynamics. A full study of these instabilities requires time-resolved measurements of the flow velocity field. However, experimentally obtaining time-resolved velocity fields, derived from particle image velocimetry (PIV) techniques, has its challenges in high-speed flow applications due to the simultaneous requirement of high spatial and temporal resolution. A novel approach is presented in this paper, which uses time-unresolved dual-time digital PIV coupled with dynamic mode decomposition (DMD) analysis (DT–DPIV–DMD) of the velocity field pairs and applies it to study the dynamics of the shear-layer instabilities in supersonic impinging jets. DT–DPIV–DMD involves the acquisition of a pair of instantaneous velocity fields separated by a small time delay using two covisual PIV systems. The time delay must be sufficiently small to provide the necessary bandwidth to capture all the relevant dynamics of the flow being investigated. This is followed by DMD analysis of all acquired digital PIV velocity field pairs. A large-eddy simulation of an underexpanded impinging jet at Reynolds number 60,000, with nozzle-to-wall distance was first used to investigate the required measurement scales for the application of DT–DPIV–DMD. Using the optimum measurement scales, the technique was then experimentally demonstrated using DT–DPIV–DMD for two supersonic impinging jets cases and validated with acoustic measurements.

Experimental and Computational Studies of Peristaltic Flow in a Duodenal Model

We study peristaltic flow in a C-shaped compliant tube representing the first section of the small intestine—the duodenum. A benchtop model comprising of a silicone tube filled with a glycerol-water mixture deformed by a rotating roller was created. Particle image velocimetry (PIV) was used to image flow patterns for deformations approximating conditions in the duodenum (contraction amplitude of 34% and wave speed 13 mm/s). Reversed flow was present underneath the roller with fluid moving opposite to the direction of the peristaltic wave propagation. Deformations of the tube were imaged and used to construct a computational fluid dynamics (CFD) model of flow with moving boundaries. The PIV and CFD vorticity and velocity fields were qualitatively similar. The vorticity field was integrated over the imaging region to compute the total circulation and there was on average a 22% difference in the total circulation between the experimental and numerical results. Higher shear rates were observed with water compared to the higher viscosity fluids. This model is a useful tool to study the effect of digesta properties, anatomical variations, and peristaltic contraction patterns on mixing and transport in the duodenum in health and disease.

Experimental Validation of Enhanced Magnetic Resonance Imaging (EMRI) Using Particle Image Velocimetry (PIV).

Flow-sensitive four-dimensional Cardiovascular Magnetic Resonance Imaging (4D Flow CMR) has increasingly been utilised to characterise patients' blood flow, in association with patiens' state of health and disease, even though spatial and temporal resolutions still constitute a limit. Computational fluid dynamics (CFD) is a powerful tool that could expand these information and, if integrated with experimentally-obtained velocity fields, would enable to derive a large variety of the flow descriptors of interest. However, the accuracy of the flow parameters is highly influenced by the quality of the input data such as the anatomical model and boundary conditions typically derived from medical images including 4D Flow CMR. We previously proposed a novel approach in which 4D Flow CMR and CFD velocity fields are integrated to obtain an Enhanced 4D Flow CMR (EMRI), allowing to overcome the spatial-resolution limitation of 4D Flow CMR, and enable an accurate quantification of flow. In this paper, the proposed approach is validated in a U bend channel, an idealised model of the human aortic arch. The flow patterns were studied with 4D Flow CMR, CFD and EMRI, and compared with high resolution 2D PIV experiments obtained in pulsatile conditions. The main strengths and limitations of 4D Flow CMR and CFD were illustrated by exploiting the accuracy of PIV by comparing against PIV velocity fields. EMRI flow patterns showed a better qualitative and quantitative agreement with PIV results than the other techniques. EMRI enables to overcome the experimental limitations of MRI-based velocity measurements and the modelling simplifications of CFD, allowing an accurate prediction of complex flow patterns observed experimentally, while satisfying mass and momentum balance equations.

Spatial Spectrum From Particle Image Velocimetry Data

We present a simple method of obtaining the turbulence kinetic energy spectrum from spatially resolved particle image velocimetry (PIV) data without need of use of Taylor hypothesis of frozen turbulence. Additionally, this method allows us to extract the velocity field components related to individual length scales resolved in the spectrum. We convolute the measured PIV velocity field with a band-pass filter, i.e., a difference of two Gauss functions with different widths, and the energy content of a such product we relate to the relevant length-scale of the used band. This is a “kind of wavelet transformation”, which, in respect to Fourier transformation, gives a good physical meaning to individual components.

Comparisons of experimental measurements and large eddy simulations for a helium release in a two vents enclosure

This work takes place in the context of potential hazards in the use of hydrogen in fuel cells. The present article describes comparisons between PIV measurements performed on a two vented cavity with an helium injection and Large Eddy Simulation of the same configuration. A two vented cavity is chosen because a quasi state is reached rapidly and it facilitates both CFD calculations by reducing the CPU costs and also enables statistical treatment of the data, the temporal averaging being possible at steady state. At the same time, this configuration is close to fuel cell designs, except for the set-up reduced size. We also describe the experimental set-up and the care which has to be taken to produce Particle Image Velocimetry velocity fields. The final goal of the paper is to validate a L.E.S approach as a good replacement to experiments, since access to both velocity and concentration fields is required to improve existing simplified models. Indeed, most of the 2 vents models rely on simplified assumptions such as a constant entrainment coefficient, a bi-layer formation which is not always the case in real situations.

Comparison of experimental and simulation results on interior subchannels of a 61-pin wire-wrapped hexagonal fuel bundle

Abstract As part of a joint U.S. Department of Energy project, Framatome, Argonne National Laboratory, Terrapower, and Texas A&M University have collaborated to produce experimental data and computational results to characterize the flow behavior of a 61-pin wire-wrapped hexagonal fuel bundle. In this paper, comparisons are made between Texas A&M’s experimental particle image velocimetry velocity field measurements and Argonne National Laboratory’s large-eddy simulation results in an interior subchannel for a Reynolds number of 19,000. It can be concluded that the shape and magnitude of the mean vertical velocity component is in excellent agreement for the selected horizontal line plots. Maximum relative errors are less than 10% until the subchannel walls are approached. The shape and, to a lesser extent, magnitude of the mean horizontal velocity component is in satisfactory agreement between the particle image velocimetry and large-eddy simulation results.

Aero-Propulsive and Propulsor Cross-Coupling Effects on a Distributed Propulsion System

This study aims to characterize the complex propulsive-airframe and cross-propulsor interactions that occur on an overwing distributed propulsion system with boundary-layer ingestion. Wind tunnel experiments were performed on a NACA airfoil model with an array of five ducted fans integrated across the upper-surface trailing edge. Net forces lift, drag, and pitching moment characteristics, as well as pressure distributions and particle image velocimetry (PIV) velocity fields, were experimentally characterized as a function of fan throttle setting. Additionally, various mixed-throttle cases were investigated with certain fans inoperative windmilling. The experimental results revealed nonlinearities in the changes of the streamwise and stream-normal forces lift, drag, and pitching moment polars as a function of and throttle setting. These nonlinear variations in the airfoil model forces and moments were attributed to variation in the magnitude and direction of the fan thrust vector, which is tied to changes in the airfoil pressure distribution and an induced circulation effect. The ingested boundary-layer height was seen to decrease linearly with a uniform increase in throttle setting. The fan-out cases showed that a windmilling inoperative fan on the edge of the array was more detrimental to performance than a windmilling inoperative fan located within the fan array, and a highly three-dimensional flow was observed from the PIV velocity fields and surface pressure distributions.

Effect of gap flow on the shallow wake of a sharp-edged bluff body—Coherent structures

This experimental study was carried out to investigate the turbulent wake generated by a vertical sharp-edged flat plate suspended in a shallow channel flow with a gap near the bed. This paper is the third part of an extensive study to characterize the gap-flow effects and is primarily focused on the characteristics of vortices in the wake flow. Two different gap heights were studied which were compared to the no-gap flow case. The Reynolds number based on the water depth was 45 000. Extensive measurements of the flow field in the vertical and horizontal planes were made using a particle-image velocimetry (PIV) system. The large vortices were exposed by analyzing the PIV velocity fields using the proper orthogonal decomposition (POD) method. Only a few modes for the POD reconstruction were used to recover ∼50% of the energy content. A vortex identification algorithm was then employed to quantify the properties of the exposed vortices. A statistical analysis of the distribution of number, size, and strength of the identified vortices was carried out to explore the characteristics of the vortices. The results revealed that the gap flow causes an increase in the number of vortices up to ∼81% higher than the no-gap flow case, but a decrease in the vortex size and strength. The occurrence of pairing and tearing processes is observed in the wake flow. In addition, a mathematical model was used to predict the relationship between the vortex circulation and vortex size, which is found to be a function in several parameters.

Effect of film cooling on the aerodynamic performance of an airfoil

The effect of film cooling on the aerodynamic performance of turbine blades is becoming increasingly important as the gas turbine operating temperature is being increased in order to increase the performance. The current paper investigates the effect of blowing ratio on the aerodynamic losses of a symmetric airfoil by pressure measurements and Particle Image Velocimetry (PIV). The test model features 4 rows of holes located on the suction side at 5%, 10%, 15% and 50% of the chord length. The Reynolds number based on the airfoil chord is 1.2×105. Experiments are performed by varying the location of air injection, the angle of attack, and the mainstream velocity. The coolant air is injected at ambient temperature and the blowing ratio is varied from 0 to 1.91. It is observed that the losses due to film cooling increase with blowing ratio of 0 to 0.48, and the wake is shifted towards the suction side. Conversely, the aerodynamic losses decrease when the blowing ratio is increased further from 0.64 to 1.91. This trend has been observed for all the experimental configurations. The effect of blowing ratio on flow separation is investigated with the time-averaged velocity fields obtained from PIV measurements. It is observed that low blowing ratios, the separation point shifts upstream and at high blowing ratios the ejected coolant energizes the flow and delays separation. The pressure field around the airfoil is reconstructed from the integration of the Poisson equation based on the PIV velocity fields. The experimental results can be used for validation of numerical models for predicting losses due to film cooling.

Experimental study on the wind-turbine wake meandering inside a scale model wind farm placed in an atmospheric-boundary-layer wind tunnel

Increasing use of wind energy over the years results in more and larger clustered wind farms. It is therefore fundamental to have an in-depth knowledge of wind-turbine wakes, and especially a better understanding of the well-known but less understood wake-meandering phenomenon which causes the wake to move as a whole in both horizontal and vertical directions as it is convected downstream. This oscillatory motion of the wake is crucial for loading on downstream turbines because it increases fatigue loads and in particular yaw loads. In order to address this phenomenon, experimental investigations were carried out in an atmospheric-boundary-layer wind tunnel using a 3 × 3 scaled wind farm composed of three-bladed rotating wind-turbine models subject to a neutral atmospheric boundary layer (ABL) corresponding to a slightly rough terrain, i.e. to offshore conditions. Particle Image Velocimetry (PIV) measurements were performed in a horizontal plane, at hub height, in the wake of the three wind turbines in the wind-farm centreline. From the PIV velocity fields obtained, the wake-centrelines were determined and a spectral analysis was performed to obtain the characteristics of the wake-meandering phenomenon. In addition, Hot-Wire Anemometry (HWA) measurements were performed in the wakes of the same wind turbines to validate the PIV results. The spectral analysis performed with the spatial and temporal signals obtained from PIV and HWA measurements respectively, led to Strouhal numbers St = fD/Uhub ≃ 0.20 − 0.22.

Drag reduction of a heavy vehicle using a modified boat tail with lower inclined air deflector

The improvement of the fuel efficiency of heavy vehicles has received considerable attention. Thus, various flow-control devices have been implemented to reduce aerodynamic drag exerting on heavy vehicles. In this study, we proposed noble boat tail with lower inclined air deflector (LIAD), as a passive flow-control device on attached at the rear end of heavy vehicles. The aerodynamic performance of the modified boat tail with LIAD is experimentally examined by measuring the drag force, side force exerting on a real-shaped heavy vehicle model at various yawing angles. As a result, the maximum drag reduction effect of the boat tail with LIAD (θ = 45°) is about 9.02% compared to the result without the boat tail, although the length of the bottom tail is half of that of four-way boat tail. Furthermore, particle image velocimetry velocity field measurements were conducted to investigate the modified flow structure are by the attachment of various boat tails. Consequently, the LIAD effectively suppresses the formation of large-scale recirculating zone in the wake behind the rear body of the vehicle, as an effective guide of the underbody flow.

Vortex dynamics for flow over a circular cylinder in proximity to a wall

The dynamics of vortical structures in flow over a circular cylinder in the vicinity of a flat plate is investigated using particle image velocimetry (PIV). The cylinder is placed above the flat plate with its axis parallel to the wall and normal to the flow direction. The Reynolds number $Re_{D}$ based on the cylinder diameter $D$ is 1072 and the gap $G$ between the cylinder and the flat plate is varied from gap-to-diameter ratio $G/D=0$ to $G/D=3.0$. The flow statistics and vortex dynamics are strongly dependent on the gap ratio $G/D$. Statistics show that as the cylinder comes close to the wall ($G/D\leqslant 2.0$), the cylinder wake becomes more and more asymmetric and a boundary layer separation is induced on the flat plate downstream of the cylinder. The wake vortex shedding frequency increases with decreasing $G/D$ until a critical gap ratio (about $G/D=0.25$) below which the vortex shedding is irregular. The deflection of the gap flow away from the wall and its following interaction with the upper shear layer may be the cause of the higher shedding frequency. The vortex dynamics is investigated based on the phase-averaged flow field and virtual dye visualization in the instantaneous PIV velocity field. It is revealed that when the cylinder is close to the wall ($G/D=2.0$), the cylinder wake vortices can periodically induce secondary spanwise vortices near the wall. As the cylinder approaches the wall ($G/D=1.0$) the secondary vortex can directly interact with the lower wake vortex, and a further approaching of the cylinder ($G/D=0.5$) can result in more complex interactions among the secondary vortex, the lower wake vortex and the upper wake vortex. The breakdown of vortices into filamentary debris during vortex interactions is clearly revealed by the coloured virtual dye visualizations. For $G/D<0.25$, the lower shear layer is strongly inhibited and only the upper shear layer can shed vortices. Investigation of the vortex formation, evolution and interaction in the flow promotes the understanding of the flow physics for different gap ratios.

Brief review of uncertainty quantification for particle image velocimetry

Metrological studies for particle image velocimetry (PIV) are recent in literature. An attempt to evaluate the uncertainty quantifications (UQ) of the PIV velocity field are in evidence. Therefore, a short review on main sources of uncertainty in PIV and available methodologies for its quantification are presented. In addition, the potential of some mathematical techniques, coming from the area of geometric mechanics and control, that could interest the fluids UQ community are highlighted as good possibilities.“We must measure what is measurable and make measurable what cannot be measured” (Galileo)

Effect of non-Newtonian viscosity on the fluid-dynamic characteristics in stenotic vessels

Although blood is known to have shear-thinning and viscoelastic properties, the effects of such properties on the hemodynamic characteristics in various vascular environments are not fully understood yet. For a quantitative hemodynamic analysis, the refractive index of a transparent blood analogue needs to be matched with that of the flowing conduit in order to minimize the errors according to the distortion of the light. In this study, three refractive index-matched blood analogue fluids with different viscosities are prepared—one Newtonian and two non-Newtonian analogues—which correspond to healthy blood with 45 % hematocrit (i.e., normal non-Newtonian) and obese blood with higher viscosity (i.e., abnormal non-Newtonian). The effects of the non-Newtonian rheological properties of the blood analogues on the hemodynamic characteristics in the post-stenosis region of an axisymmetric stenosis model are experimentally investigated using particle image velocimetry velocity field measurement technique and pathline flow visualization. As a result, the centerline jet flow from the stenosis apex is suppressed by the shear-thinning feature of the blood analogues when the Reynolds number is smaller than 500. The lengths of the recirculation zone for abnormal and normal non-Newtonian blood analogues are 3.67 and 1.72 times shorter than that for the Newtonian analogue at Reynolds numbers smaller than 200. The Reynolds number of the transition from laminar to turbulent flow for all blood analogues increases as the shear-thinning feature increases, and the maximum wall shear stresses in non-Newtonian fluids are five times greater than those in Newtonian fluids. However, the shear-thinning effect on the hemodynamic characteristics is not significant at Reynolds numbers higher than 1000. The findings of this study on refractive index-matched non-Newtonian blood analogues can be utilized in other in vitro experiments, where non-Newtonian features dominantly affect the flow characteristics.

PIV Measurement of Flow Field around Film Boiling in He II

Film boiling phenomena in He II were studied using the PIV (Particle Image Velocimetry). Noisy and silent film boiling states were generated on/around a planar or a cylindrical heater. Tracer particles were neutrally buoyant solid H2-D2 particles. The motions of vapour bubble and of tracer particles were PIV-analyzed. The velocity field was found to be composed of AC and DC velocity components. The AC component is caused by the dynamic behaviour of vapour, and the DC results primarily from the thermal counter flow. The time-averaged PIV velocity field provides with the flow field characteristics of each boiling mode.

PIV-based load investigation in dynamic stall for different reduced frequencies

Measuring the aerodynamic loads on dynamic objects in small wind tunnels is often challenging. In this regard, fast-response particle image velocimetry (PIV) data are post-processed using advanced tools to calculate aerodynamic loads based on the control-volume approach. For dynamic stall phenomena, due to the existence of dynamic stall vortices and significant load changes over a short time interval, applying the control-volume technique is difficult in particular for drag estimation. In this study, an examination of the dynamic stall phenomena of an oscillating SD7037 airfoil is reported for a reduced frequency range of \(0.05\le k \le 0.12\) when \(Re=4\times 10^{4}\). A numerical simulation is utilized as an alternative method for comparison and agrees well with the experimental results. The results suggest that loads can be determined accurately if the spatial resolution satisfies the reduced frequency increment. Minimizing the control-volume works best for lift determination. For the drag calculation, it was found that the location of the downstream boundary should be placed where it was not disturbed with wake vortices. The high-velocity gradients of the wake vortices increase the error level along the downstream boundary for the drag calculation but not for the lift estimation. Beside the load calculation, high-resolution PIV velocity fields also reveal insights into the effects of reduced frequency on dynamic flow behavior including the pitch angle range for vortex growth (between vortex generation and pinch-off), phase delay and number of vortices. These observations agree well with the load curve behavior.

Effect of pulsatile swirling flow on stenosed arterial blood flow

The existence of swirling flow phenomena is frequently observed in arterial vessels, but information on the fluid-dynamic roles of swirling flow is still lacking. In this study, the effects of pulsatile swirling inlet flows with various swirling intensities on the flow field in a stenosis model are experimentally investigated using a particle image velocimetry velocity field measurement technique. A pulsatile pump provides cyclic pulsating inlet flow and spiral inserts with two different helical pitches (10D and 10/3D) induce swirling flow in the stenosed channel. Results show that the pulsatile swirling flow has various beneficial effects by reducing the negative wall shear stress, the oscillatory shear index, and the flow reverse coefficient at the post-stenosis channel. Temporal variations of vorticity fields show that the short propagation length of the jet flow and the early breakout of turbulent flow are initiated as the swirling flow disturbs the symmetric development of the shear layer. In addition, the overall energy dissipation rate of the flow is suppressed by the swirling component of the flow. The results will be helpful for elucidating the hemodynamic characteristics of atherosclerosis and discovering better diagnostic procedures and clinical treatments.

Experimental investigation on upstream-injection interaction with a scramjet cavity flameholder

The study has evaluated the coherent structures and the velocity field of gaseous transverse jet upstream of a scramjet flameholder in a supersonic turbulent flow. The high spatiotemporal resolution coherent structures of the jet plume/wakes and the cavity shear layer were obtained by utilizing the nanoparticle-based planar laser scattering technique. Compared to the no upstream injection case, the cavity shear layer downstream of the jet shows significant mass exchange with mainstream. The particle image velocimetry velocity field and the velocity profiles demonstrated that the interaction intensity of the cavity shear layer with the upstream injection is further enhanced as the injection stagnation pressure increases.