Standard Particle Image Velocimetry Research Articles

The effect of permeability on the flow structure of porous square cylinders

This study experimentally investigates the wake structure of a porous square cylinder in terms of permeability over two decades of $Da$ (i.e. $2.4 \times 10^{-5} < Da < 2.9 \times 10^{-3}$ ). The porous cylinder, featuring a simple cubic lattice structure, was fabricated using an additive manufacturing technique. This unique method, combined with a periodic and scalable lattice structure, effectively isolates permeability from porosity, making it suitable for an in-depth parametric study. The key parameter, permeability, was directly estimated by measuring the pressure drop and superficial velocity for each porous case in an open-loop pipe flow system. The downstream flow fields were obtained using standard planar particle image velocimetry measurements in an open-loop wind tunnel. Based on the experimental data, structural modifications in the near wake were examined in relation to permeability, leading to the identification of four distinct flow regimes depending on $Da$ . Additionally, the downstream flow adjustment length ( $L_i$ ) was assessed by introducing a permeability-based source term into the momentum equation, facilitating the development of an analytical model for $L_i$ . The present experimental data support this analytical model, and our results further confirmed that $L_i$ plays a crucial role as a characteristic length scale in the near wake.

A framework to facilitate development and testing of image‐based river velocimetry algorithms

AbstractImage‐based methods have compelling, demonstrated potential for characterizing flow fields in rivers, but algorithms like particle image velocimetry (PIV) must be further tested and improved to enable more effective use of these techniques. This paper presents a framework designed for this exact purpose: Simulating Hydraulics and Images for Velocimetry Evaluation and Refinement (SHIVER). The approach involves coupling a hydrodynamic model with a synthetic particle generator to advect particles between frames, as dictated by local velocity vectors and thus construct a plausible image sequence specific to the reach of interest. The resulting time series can then be used as input to a velocimetry algorithm to compare image‐derived estimates with known (modelled) velocities to perform an exhaustive, spatially distributed accuracy assessment. As an example application of SHIVER, we examined the effects of interrogation area (IA) size, frame rate, flow velocity, and image sequence duration on the performance of a standard PIV algorithm. This analysis indicated that image‐derived velocities were generally in close agreement with those from the flow model (root mean square error <10% and mean bias <3%), except when small IAs were coupled with low frame rates. Velocity estimates were most accurate for the lowest modelled discharge ( at baseflow) and became less reliable as the mean flow velocity increased ( for an intermediate discharge and at bankfull). Accuracy was essentially independent of image sequence duration, implying that long occupations might not be necessary. Errors were concentrated along channel margins, where PIV‐based velocities tended to be greater than those from the flow model. Small IAs led to underpredictions of velocity, while larger IAs led to overpredictions. SHIVER is highly modular and could be updated to make use of different hydrodynamic models or image simulators. The framework could also facilitate more thorough sensitivity analyses and comparison of various velocimetry algorithms.

Simultaneous two-plane flame front detection using PIV based on defocusing.

This Letter presents a simultaneous two-plane flame front detection method. It is based on a standard single-camera single-plane particle image velocimetry (PIV) system in combination with an inexpensive and compact image splitting device. The image splitting optics places images from two depth-offset planes onto the two halves of a camera sensor. A shallow depth of field ensures only one plane is in focus on each half of the sensor. By using a high-pass filter and a novel two-step filter we have devised, the out-of-focus particle images are effectively removed, while the in-focus particle images remain, allowing the turbulent flame fronts on two planes to be detected simultaneously. Our approach could be combined with conventional polarization/wavelength discrimination methods to achieve simultaneous multi-plane flame front reconstruction with similarly high in-plane spatial resolution to single-plane measurement and is suitable for practical combustion devices with limited optical access.

Experimental investigation of the flow features around an elliptical Ahmed body

The standard Ahmed body (SAB) with a rectangular rear end is a simplified, canonical bluff body geometry that shares similarities with generic road vehicles. It is known to produce flow separations and strong vortices at the rear end. The aerodynamic performance of vehicles is significantly affected by such three-dimensional flow structures. This work proposed a modified Ahmed body with an elliptical rear end called the elliptical Ahmed body. The study examines in detail the near-field characteristics of the flow over the elliptical Ahmed body at a slant angle of 25° with a low Reynolds number of 4.31×104 based on the model height. The principal flow features are analyzed using time-resolved and standard particle image velocimetry. The time-averaged and time-resolved analyses revealed a fully detached flow at the rear end that eliminated the slant separation bubble. Furthermore, a significant wake flow restructuring is found to curb the lower recirculation bubble and shift the upper recirculation bubble toward the slant surface. Thus, the elliptical curvature shifts the critical slant angle of 30° in SAB to 25°, which indicates transformation of the high-drag flow structures into low-drag. Several distinct Strouhal numbers are found over the slant surface (St = 0.55–0.88), which are higher than that of the SAB, while no significant change is found in the wake region (St = 0.23–0.50). In addition, the effect on coherent structures is evaluated using proper orthogonal decomposition and dynamic mode decomposition, which shows a considerable difference between the standard and elliptical Ahmed body wake structures. The results of this investigation show that body modification of the Ahmed body using elliptical curvature at the rear end can significantly affect the overall flow features and help improve the aerodynamic performance of the vehicles.

Reactive PLIF Method for Characterisation of Micromixing in Continuous High-Throughput Chemical Reactors

This work aimed to test and optimise reactive Planar Laser-Induced Fluorescence (PLIF) methods for the visualisation of the micromixing regions in chemical reactors using standard PLIF and Particle Image Velocimetry (PIV) equipment with the laser source 512 nm. Two methods were tested: (i) an acid–base reaction with fluorescein as the reaction-sensitive tracer and (ii) Fenton’s reaction, with Rhodamine B as the reaction tracer. Both test-reactions were studied in stopped-flow equipment to define suitable operational conditions, namely the chemical composition of the inflow streams, the concentration of reagents and fluorophore, and suitable excitation light wavelength. The visualisation of the micromixing regions was tested in a continuous flow reactor with a T-jet geometry. A laser light sheet emitted from an Nd:YAG laser illuminated the axial section of the demonstration reactor. The mixing dynamics and the reaction course were visualised with the acid–base reactive PLIF images. Fenton’s reactive PLIF method showed the overall distribution of mixing and reaction regions. The main contribution of this work is benchmarking two methods with costs that enable the visualisation of micromixing regions in continuous high-throughput reactors.

A microfluidic study of transient flow states in permeable media using fluorescent particle image velocimetry

Velocity fields in flow in permeable media are of great importance to many subsurface processes such as geologic storage of CO2 , oil and gas extraction, and geothermal systems. Steady-state flow is characterized by velocity fields that do not change significantly over time. The flow field transitions to a new steady state once it experiences a disturbance such as a change in flow rate or in pressure gradient. This transition is often assumed to be instantaneous, which justifies the expression of constitutive relations as functions of instantaneous phase saturations. This work examines the evolution of velocity fields in a surrogate quasi-2D permeable medium using a microfluidic device, a microscopy system, and a high-speed camera. Tracer particles are injected into the medium along with Deionized water. The evolution of the velocity field is examined by tracing these particles in the captured images using the standard high-density particle image velocimetry algorithm founded on cross-correlation. The results suggest that the transition between steady states for an incompressible fluid takes a finite and non-negligible amount of time that is independent of the magnitude of the change in pressure gradient. The existence of transient states and the nature of the response during these states are readily interpreted by the principle of least action where flow gradually establishes an optimal configuration such that energy dissipation is minimized. The findings provide evidence against the applicability of the assumption that flowing phases relax instantaneously to their steady states and, hence, against the accuracy of the classical multiphase extension of Darcy’s law. Cited as: Sun, J., Li, Z., Furtado, F., Aryana, S. A. A microfluidic study of transient flow states in permeable media using fluorescent particle image velocimetry. Capillarity, 2021, 4(4): 76-86, doi: 10.46690/capi.2021.04.03

Stereoscopic particle image velocimetry in inhomogeneous refractive index fields of combustion flows.

Particle image velocimetry (PIV) measurements in reactive flows are disturbed by inhomogeneous refractive index fields, which cause measurement deviations in particle positions due to light refraction. The resulting measurement errors are known for standard PIV, but the measurement errors for stereoscopic PIV are still unknown. Therefore, for comparison, the velocity errors for standard and stereoscopic PIV are analyzed in premixed propane flames with different Reynolds numbers. For this purpose, ray-tracing simulations based on the time-averaged inhomogeneous refractive index fields of the studied non-swirled flame flows measured by the background-oriented Schlieren technique are performed to quantify the resulting position errors of the particles. In addition, the performance of the volumetric self-calibration relevant to tomographic PIV is analyzed with respect to the remaining position errors of the particles within the flames. The position errors cause significant standard PIV errors of 2% for the velocity component radial to the burner symmetry axis. Stereoscopic PIV measurements result in measurement errors of up to 3% radial to the burner axis and 13% for the velocity component perpendicular to the measurement plane. Due to the lower refractive index gradients in the axial direction, no significant velocity errors are observed for the axial velocity component. For the investigated flame configurations, the position errors and velocity errors increase with the Reynolds numbers. However, this dependence needs to be verified for other flame configurations such as swirled flame flows.

Experimental Study on the Influence Factors of Flow Velocity Structure and Turbulent Characteristics in Open Channel with Biomimetic Grass

The problem of suspension treatment of subsea oil-gas pipelines has been highly concerned by engineering construction units and researchers. The current research indicates that the bionic sea grass can effectively reduce the flow rate, promote sediment deposition, and control the development of the pipeline suspension area. The velocity distribution of open channel flow with bionic grass is very complex. The height and laying space of bionic grass will affect the flow velocity distribution. At present, the flow velocity in open channels with bionic grass is mainly studied by measuring the velocity variation at the front, middle, and back of bionic grass. Few effective measurements are made for the full velocity field. The velocity field distribution of bionic aquatic grass along the vertical plane is measured by using standard particle image velocimetry (PIV). The effects of height and laying space of bionic grass on probability density distribution, spatial correlation of pulsating velocity, turbulence intensity, Reynolds stress and turbulent kinetic energy in the open channel after the protection section of bionic grass are further analyzed.

Experimental Study on Flow Velocity Structure and Turbulence Characteristics in Open Channel with Biomimetic Grass

The problem of suspension treatment of oil and gas pipelines has been highly concerned by engineering construction units and researchers. Research shows that the bionic grass can effectively reduce the flow rate, promote sediment deposition, and control the development of the pipeline suspension area. The velocity distribution of open channel flow with bionic grass is very complex. The height and spacing of bionic grass will affect the flow velocity distribution. At present, the flow velocity in open channels containing bionic grass is mainly studied by measuring the velocity variation in the front, middle, and back of bionic grass, but few effective measurements are made for the full velocity field. This paper describes the use of modern means of an advanced test, using standard particle image velocimetry (PIV) measurements with bionic grass along the water channel to the vertical plane of the distribution of the velocity field. The probability density distribution, spatial correlation of pulsating velocity, turbulence intensity, Reynolds stress, and turbulent kinetic energy in the open channel after the protection section of bionic grass were further analyzed.

GANs-based PIV resolution enhancement without the need of high-resolution input

A data-driven approach to reconstruct high-resolution flow fields is presented. The method is based on exploiting the recent advances of SRGANs (Super-Resolution Generative Adversarial Networks) to enhance the resolution of Particle Image Velocimetry (PIV). The proposed approach exploits the availability of incomplete projections on high-resolution fields using the same set of images processed by standard PIV. Such incomplete projection is made available by sparse particle-based measurements such as super-resolution particle tracking velocimetry. Consequently, in contrast to other works, the method does not need a dual set of low/high-resolution images, and can be applied directly on a single set of raw images for training and estimation. This data-enhanced particle approach is assessed employing two datasets generated from direct numerical simulations: a fluidic pinball and a turbulent channel flow. The results prove that this data-driven method is able to enhance the resolution of PIV measurements even in complex flows without the need of a separate high-resolution experiment for training.

Experimental Two-Phase Flow Analysis Inside a Laboratory Scale Jet-Engine Combustion Chamber Using Particle Image Velocimetry

Abstract The characterization of the two-phase kerosene/air flow near the nozzle of an aero-engine combustor is important in order to understand the combustion characteristics of the burner. Typically, particle image velocimetry (PIV) or laser Doppler velocimetry is used to measure velocities inside aero-engine combustors. However, these measurement techniques rely on tracer particles to visualize the flow field and are usually only able to measure the velocity field of one phase at a time. In the case of PIV measurements, both the flow tracers and the kerosene droplets scatter the laser light, and thus appear on the PIV recordings. Depending on droplet size and flow velocity, these kerosene droplets do not necessarily follow the airflow leading to errors in the derived velocity field. This work presents a method on how to separate kerosene droplets from flow tracers depending on their optical characteristics in the PIV recording. This phase separation enables the independent measurement of the flow fields of both the gaseous and liquid phase at the same time as well as the instantaneous slip velocity between droplets and gaseous flow using a standard PIV setup. The method is demonstrated on a laboratory scale aero-engine combustor operated at atmospheric conditions. The obtained results show that in the setup under investigation, gaseous and liquid phase can have significantly different flow fields with kerosene droplets moving in the opposite direction of the recirculating airflow.

Measurement of temperature of a dusty plasma from its configuration

AbstractA new concept called “configurational temperature” is introduced in the context of dusty plasma, where the temperature of the dust particles submerged in the plasma can be measured directly from the positional information of the individual dust particles and the interaction potential between the dust grains. This method does not require the velocity information of individual particles, which is a key parameter to measure the dust temperature in the conventional method. The technique is initially tested using two‐dimensional (2D) OpenMP parallel molecular dynamics and Monte Carlo simulation and then compared with the temperature evaluated from experimental data. The experiment have been carried out in the Dusty Plasma Experimental (DPEx) device, where a 2D stationary plasma crystal of melamine formaldehyde particles is formed in the cathode sheath of a DC glow discharge argon plasma. The kinetic temperature of the dust is calculated using the standard particle image velocimetry technique at different pressures. An extended simulation result for the three‐dimensional case is also presented, which can be employed for the temperature measurement of a three‐dimensional dust crystal in laboratory devices.

Thrust and Hydrodynamic Efficiency of the Bundled Flagella.

The motility mechanism of prokaryotic organisms has inspired many untethered microswimmers that could potentially perform minimally invasive medical procedures in stagnant fluid regions inside the human body. Some of these microswimmers are inspired by bacteria with single or multiple helical flagella to propel efficiently and fast. For multiple flagella configurations, the direct measurement of thrust and hydrodynamic propulsion efficiency has been challenging due to the ambiguous mechanical coupling between the flow field and mechanical power input. To address this challenge and to compare alternative micropropulsion designs, a methodology based on volumetric velocity field acquisition is developed to acquire the key propulsive performance parameters from scaled-up swimmer prototypes. A digital particle image velocimetry (PIV) analysis protocol was implemented and experiments were conducted with the aid of computational fluid dynamics (CFD). First, this methodology was validated using a rotating single-flagellum similitude model. In addition to the standard PIV error assessment, validation studies included 2D vs. 3D PIV, axial vs. lateral PIV and simultaneously acquired direct thrust force measurement comparisons. Compatible with typical micropropulsion flow regimes, experiments were conducted both for very low and higher Reynolds (Re) number regimes (up to a Re number = 0.01) than that are reported in the literature. Finally, multiple flagella bundling configurations at 0°, 90° and 180° helical phase-shift angles were studied using scaled-up multiple concentric flagella thrust elements. Thrust generation was found to be maximal for the in-phase (0°) bundling configuration but with ~50% lower hydrodynamic efficiency than the single flagellum. The proposed measurement protocol and static thrust test-bench can be used for bio-inspired microscale propulsion methods, where direct thrust and efficiency measurement are required.

Three-dimensional extent of flow stagnation in transcatheter heart valves.

The recent unexpected discovery of thrombosis in transcatheter heart valves (THVs) has led to increased concerns of long-term valve durability. Based on the clinical evidence combined with Virchow's triad, the primary hypothesis is that low-velocity blood flow around the valve could be a primary cause for thrombosis. However, due to limited optical access in such unsteady three-dimensional biomedical flows, measurements are challenging. In this study, for the first time, we employ a novel single camera volumetric velocimetry technique to investigate unsteady three-dimensional cardiovascular flows. Validation of the novel volumetric velocimetry technique with standard planar particle image velocimetry (PIV) technique demonstrated the feasibility of adopting this new technique to investigate biomedical flows. This technique was used to quantify the three-dimensional velocity field in the vicinity of a validated, custom developed, transparent THV in a bench-top pulsatile flow loop. Large volumetric regions of flow stagnation were observed in the neo-sinus throughout the cardiac cycle, with stagnation defined as a velocity magnitude lower than 0.05 m s-1. The volumetric scalar viscous shear stress quantified via the three-dimensional shear stress tensor was within the range of low shear-inducing thrombosis observed in the literature. Such high-fidelity volumetric quantitative data and novel imaging techniques used to obtain it will enable fundamental investigation of heart valve thrombosis in addition to providing a reliable and robust database for validation of computational tools.

Turbulence characteristics of radially-confined impinging jet flows

Radially confined, axisymmetric impinging jet flows are investigated by using the standard particle image velocimetry experimental technique. The confinement is achieved by placing a confinement block around a jet, co-axially. The inner diameter of the block is successively varied to nine different values. The inlet-based Reynolds number of the jet is kept constant at 5000. The nine diametric values yielded nine different flows of widely different characteristics. Among other usage, an insight into the flow characteristics can be helpful in designing compact impinging jet applications, as such a radially confined flow is equivalent to passing the pre-impingement jet through a hole perforated in a solid wall (i.e. the jet source can be placed behind a wall). The study has revealed that the flows, in general, form two circulation zones, three mixing layers, and two boundary layers. Based on turbulence characteristics of the five shear layers, overall characteristics of the flows are understood systematically. Mean velocity and various turbulence statistics are also presented, and mechanisms underlying behind their variations are explained. Finally, scaling laws are obtained for the mean velocity and for the turbulence statistics, both in the impingement and in the wall jet regions.

PIV adaptive interrogation and sampling with image projection applied to water sloshing

The measurement of fluid velocity in the vicinity of wavy interfaces by means of Particle Image Velocimetry (PIV) still constitutes a challenge. Besides the experimental complexities such as appropriate seeding, reflections due to gradients in refractive indices, aberrations, etc., also the image processing phase constitutes a critical component. Ignoring bias errors introduced by laser reflections near the interface, strong velocity gradients are typically encountered near the curved liquid/gas interface and are detrimental to the common cross-correlation analysis. These effects are exacerbated by the use of traditional rectangular static cross-correlation windows. Moreover, dynamic boundaries suffer from a certain loss in reliability due to the difference in number of particles located in a specific physical region between the two frames. In this paper we present an improved algorithm enhancing the accuracy of the velocity vector detection in dynamic wavy flows. This algorithm divides the fluid domain in subregions (i.e. bulk, intermediate, vicinity and interface) and applies to each region different features (such as image patching, window relocation and forward difference finite schemes) and ad hoc predictors. Its assessment is performed on the basis of synthetic images, which reproduce a wavy motion similar to the one encountered when fluid in a cylindrical reservoir undergoes sloshing. When juxtaposed with a standard PIV code, numerical errors are shown to strongly diminish while improving the spatial resolution approaching the interface. The algorithm is finally applied to experimental PIV images of water sloshing in a cylindrical reservoir to provide more reliable results at the interface, contrary to standard PIV codes.

Sooting limits of non-premixed counterflow ethylene/oxygen/inert flames using LII: Effects of flow strain rate and pressure (up to 30 atm)

An absolute irradiance-calibrated Laser Induced Incandescence (LII) technique and a standard particle image velocimetry (PIV) technique were utilized to collect quantitative data on soot volume fraction and corresponding flow strain rates of diluted ethylene-air non-premixed counterflow flames. Pressures up to 30 atm were explored with increasing dilution with nitrogen or helium to minimize flow strain limits at which incipient soot was detected and to maintain the flame in laminar mode. For weakly strained flames considered, the species and velocity boundary conditions were used to predict the gas-phase flame structure (e.g., temperature and major species). The predicted gas properties, together with soot particle temperature decay rate measured by two-color pyrometry were used in the LII heat transfer model to extract the effective soot particle size and particle number density. Estimates of global activation energy of incipient soot yield with pressure indicated a sudden change around a pressure of 20 atm, which may be attributed to a shift in soot nucleation and growth pathways.

Influence of inhomogeneous refractive index fields on particle image velocimetry

The influence of inhomogeneous refractive index fields on particle image velocimetry (PIV) measurements is a well known problem, which leads to an unknown measurement uncertainty in, e.g., flame flows, shock waves and super sonic flows. Previous studies give only rough estimations of the measurement error due to inhomogeneous refractive index fields, and quantitative information is only available for special conditions such as a gradient of the refractive index independent of the viewing direction. Hence, the assessment of the spatial distribution of the quantitative measurement errors inside inhomogeneous refractive index distributions especially for stereoscopic and tomographic PIV is an open question. For this purpose, the flow measurement inside a hot jet flow is considered as an example, and a general analytic description of the error of the measured particle positions inside the hot jet flow is derived, numerically evaluated and finally validated by experiments. In particular, the determination of the particle position with triangulation is investigated, which is performed in stereoscopic and tomographic PIV. As a result, the measurement error is generally larger than for standard PIV without triangulation. The theoretically predicted errors of the measured particle position are validated with experiments and here amount to 11.7 µm for standard PIV and 17.3 µm for triangulation at the same distance from the center of the flow, respectively. Note that the error estimation requires the knowledge of the refractive index field that was determined by temperature measurements in the flow. Furthermore, the error analysis shows that for triangulation the measured particle position depends on the gradient and the curvature of the refractive index field, whereas for standard PIV the measured particle position only depends on the gradient of the refractive index. For the given temperature profile with a maximum temperature of 191 °C, the resulting flow velocity error is maximally 0.8 % with standard PIV, 1.7 % for the in plane and 2.9 % for the out of plane direction with stereoscopic PIV and 1.1 % with triangulation which is performed in tomographic PIV.

A Novel Plasma-Based Fluid for Particle Image Velocimetry (PIV): In-Vitro Feasibility Study of Flow Diverter Effects in Aneurysm Model.

Particle image velocimetry (PIV) is a commonly used method for in vitro investigation of fluid dynamics in biomedical devices, such as flow diverters for intracranial aneurysm treatment. Since it is limited to transparent blood substituting fluids like water-glycerol mixture, the influence of coagulation and platelet aggregation is neglected. We aimed at the development and the application of a modified platelet rich plasma as a new PIV fluid with blood-like rheological and coagulation properties. In standardized intracranial aneurysm silicone models, the effect of this new PIV plasma on the fluid dynamics before and after flow diverter implantation was evaluated and compared with water-glycerol measurements. The flow diverting effect was strongly dependent on the used fluid, with considerably lower velocities achieved using PIV plasma, despite the same starting viscosity of both fluids. Moreover, triggering coagulation of PIV plasma allowed for intra-aneurysmal clot formation. We presented the first in vitro PIV investigation using a non-Newtonian, clottable PIV plasma, demonstrating a mismatch to a standard PIV fluid and allowing for thrombus formation.

Optimized Time-Resolved Echo Particle Image Velocimetry- Particle Tracking Velocimetry Measurements Elucidate Blood Flow in Patients With Left Ventricular Thrombus.

Contrast ultrasound is a widely used clinical tool to obtain real-time qualitative blood flow assessments in the heart, liver, etc. Echocardiographic particle image velocimetry (echo-PIV) is a technique for obtaining quantitative velocity maps from contrast ultrasound images. However, unlike optical particle image velocimetry (PIV), routine echo images are prone to nonuniform spatiotemporal variations in tracer distribution, making analysis difficult for standard PIV algorithms. This study introduces optimized procedures that integrate image enhancement, PIV, and particle tracking velocimetry (PTV) to obtain reliable time-resolved two-dimensional (2D) velocity distributions. During initial PIV analysis, multiple results are obtained by varying processing parameters. Optimization involving outlier removal and smoothing is used to select the correct vector. These results are used in a multiparameter PTV procedure. To demonstrate their clinical value, the procedures are implemented to obtain velocity and vorticity distributions over multiple cardiac cycles using images acquired from four left ventricular thrombus (LVT) patients. Phase-averaged data elucidate flow structure evolution over the cycle and are used to calculate penetration depth and strength of left ventricular (LV) vortices, as well as apical velocity induced by them. The present data are consistent with previous time-averaged results for the minimum vortex penetration depth associated with LVT occurrence. However, due to decay and fragmentation of LV vortices, as they migrate away from the mitral annulus, in two cases with high penetration, there is still poor washing near the resolved clot throughout the cycle. Hence, direct examination of entire flow evolution may be useful for assessing risk of LVT relapse before prescribing anticoagulants.