Particle Image Velocimetry Setup Research Articles

Uncovering wall-shear stress dynamics from neural-network enhanced fluid flow measurements

Accurate prediction and measurement of wall-shear stress dynamics in fluid flows is crucial in domains as diverse as transportation, public utility infrastructure, energy technology and human health. However, we still lack adequate experimental methods that simultaneously capture the temporal and the spatial behaviour of the wall-shear stress. In this contribution, we present a holistic approach that derives these dynamics from particle-image velocimetry (PIV) measurements using a deep optical flow estimator with physical knowledge. While the experimental measurements resemble state-of-the-art PIV set-ups, the established particle image processing is replaced by a deep neural network specifically tailored to extract velocity and wall-shear stress information. Since this WSSflow framework operates at the original image resolution, it provides the respective flow field information at a much higher spatial resolution compared with state-of-the-art PIV processing. The results show that this per-pixel approach is essential for an accurate wall-shear stress estimation. The validity and physical correctness of the derived flow quantities are demonstrated with synthetic and real-world experimental data of a turbulent channel flow, a wavy turbulent channel flow and an elastic blood vessel flow. Where baseline data are available for comparison, the instantaneous and time-averaged wall-shear stress predictions accurately follow the ground truth data.

Characterization of the Endwall Flow in a Low-Pressure Turbine Cascade Perturbed by Periodically Incoming Wakes, Part 1: Flow Field Investigations with Phase-Locked Particle Image Velocimetry

Particle image velocimetry (PIV) measurements were performed inside a low-pressure turbine cascade operating at engine-relevant high-speed and low-Re conditions to investigate the near-endwall flow. Of particular research interest was the dominant periodic disturbance of the flow field by incoming wakes, which were generated by moving cylindrical bars at a frequency of 500 Hz. Two PIV setups were utilized to resolve both (1) a large blade-to-blade plane close to the endwall as well as midspan and (2) the wake effects in an axial flow field downstream of the blade passage. The measurements were performed using a phase-locked approach in order to align and compare the results with comprehensive CFD data that are also available for this test case. The experimental results not only support a better understanding and even a quantification of the wake-induced over/under-turning inside and downstream of the passage, they also enable the tracing of the ‘negative-jet-effect’, which is widely known in the CFD branch of the turbomachinery community but is seldom visualized in experiments. The results also reveal that the bar wake periodically widens the blade wake by up to 165%, while the secondary flow is less affected and exhibits a phase lag with respect to the 2D-flow effects. The results presented here are an essential basis for the subsequent investigation of the near-endwall blade suction surface effects using unsteady pressure-sensitive paint in the second part of this two-part publication.

PIV analysis of opaque flow without using high-tech equipment

The current study reports PIV (particle image velocimetry) analysis of the transparent and opaque flow using a smartphone camera in ambient light. A typical PIV setup consists of high-speed cameras, synchronizers, and high-energy light sources like pulsed lasers or high-power LEDs, whereas in the present work, we demonstrate that PIV analysis can be possible without sophisticated instruments. Two essential parameters, viz recording speed and interrogation window size, have been optimized for the flow recorded in the mobile camera. The present work describes methods for optimization of recording speed (fps-frames per second) and interrogation window (IW) size for the transparent (water) vortex flow. Further, the velocity profile derived at optimal recording speed and interrogation window size using the PIV analysis have been compared with the Burger vortex model to validate the optimization methods and the parameters. The agreement validates the optimization methods and the parameters. Moreover, the optimized parameters have been used to record and analyze the opaque flow to extend the application potentiality. The finding suggests that the PIV- extracted data of the opaque flow can be compared with the transparent flow, which suggests that PIV can also be used to analyze opaque flow.

Inexpensive multi-plane particle image velocimetry based on defocusing: Proof of concept on two-component measurement

This paper presents a method for simultaneous particle image velocimetry (PIV) on parallel planes offset in depth. The method places images from two planes onto a different half of a camera sensor by using image splitting optics with variable optical path lengths. A shallow depth of field is achieved to ensure only one plane is in focus on each half of the sensor. Without needing additional lasers, the method is designed as an inexpensive means to increase the number of measurement plane(s) of single/multi-plane PIV setups and can be combined with existing plane discrimination approaches such as polarization and wavelength. The method is useful for studying instantaneous flow correlations on different planes while retaining high in-plane spatial resolution of typical planar PIV measurement. The measurement uncertainty caused by crosstalk from out-of-focus images is discussed. Experimental results from a laminar flow rig test indicate that the average measurement error of each velocity component is lower than 0.1 pixels per time step, with a 20 mm plane separation in depth and a 35 × 54 mm2 field of view. As an application with varying background scatter and out-of-plane flow motions, in-cylinder flow measurements in an optically accessible internal combustion engine were performed on two swirl planes simultaneously. Characteristics of the proposed method performing stereoscopic PIV measurements will be studied in future.

Phase-locked measurements of linear and weakly nonlinear interfacial waves in a stratified turbulent gas–liquid pipe flow

The present work reports an experimental characterization of linear and weakly nonlinear interfacial waves in a stratified air–water horizontal pipe flow. An oscillating paddle was employed to generate controlled waves at the liquid interface. The driving signal of the oscillating paddle was controlled and synchronized with image acquisitions, enabling phase-locked measurements and the application of ensemble averaging techniques. Velocity field measurements in the liquid and gas phases were performed simultaneously using an off-axis particle image velocimetry setup and shadowgraphy. The combined techniques allowed us to extract the coherent part of flow fluctuations related to the excited waves. This was done for a range of flow rates and wave frequencies. The selected conditions are close to the transition from stratified to slug/plug flow regimes. In the presence of linear waves, the coherent disturbances in both phases were weakly dependent on near-wall disturbances. Flow changes in the presence of weakly nonlinear waves were also investigated. In these cases, noticeable modifications in the mean flow and in turbulence distribution were observed near the interface, whereas close to the wall, the flow was weakly affected. This investigation follows the work of Farias et al. [“Characterization of interfacial waves in stratified turbulent gas-liquid pipe flow using Particle Image Velocimetry and controlled disturbances,” Int. J. Multiphhase Flow 161, 104381 (2023)], where the threshold for linear and weakly nonlinear waves was studied. Here, a clear comparison between wave-induced disturbances in linear and weakly nonlinear regimes is reported in the literature for the first time for stratified turbulent gas–liquid pipe flows. The methodology proposed is relatively simple and can contribute to describe wave-related phenomena in stratified pipe flows.

Large-Eddy Simulation of Boundary-Layer Transition over a Zigzag Trip

Wall-resolved large-eddy simulations (WR-LES) are conducted to study transitional and turbulent boundary-layer properties and flow structures downstream of a zigzag trip on the low-Reynolds-number SD7032 airfoil at and and angle of attack . Without a zigzag trip, the airfoil features a prominent laminar separation bubble on the suction side. First, a verification and validation study of the LES method is conducted for the zigzag trip at a roughness-height-based Reynolds number . The boundary-layer velocity profiles and the integral parameters are compared with experimental data acquired using a long-distance -particle image velocimetry setup in a wind tunnel. In the LES, if the zigzag tape is thick enough to trigger transition just behind the tape, the transition to turbulence is generally quicker than what was observed in the experiment. Although the general phenomenology and the development of the turbulent boundary layer after transition are consistent with the experimental data, there remain some differences in the exact length of the transitional region and also in the associated airfoil drag. A zigzag tape with or thicker induces transition quickly behind the trip through a shedding process of the shear layer created by the tape. The boundary layer behind the zigzag tape features prominent three-dimensional vortical structures emanating from the zigzags that persist over long chordwise distances and well into the turbulent boundary layer. Nevertheless, in a spanwise-averaged sense, the integral thicknesses of the newly created turbulent boundary layer are actually quite similar to a two-dimensional boundary-layer theory solution, given that the tape has a suitable thickness. In contrast, an overly thick tape will generate a thicker turbulent boundary layer, where the increased displacement and momentum loss are sustained and also contribute to total drag.

Use of low-cost accelerometers for landslides monitoring: results from a flume experiment

Early Warning Systems (EWS) are non-structural measures for landslides disaster prevention. They are based on the detection of impending failure signals. The results of a landslide simulation experiment where accelerometers were used to identify pre-failure signals are presented in this paper. Landslide was simulated in a tilting flume filled with sandy soil. During the experiment, the flume was fixed at 30° inclination and water percolated through the soil until it slid. Accelerometers were embedded into the soil and recorded acceleration data from the beginning of the experiment until failure. Acceleration data were analyzed in time domain aiming at estimating translational velocity of the movement. Angular variation was also estimated from acceleration data. The experiment was recorded with a camera and pictures were used for Particle Image Velocimetry (PIV) analysis, in order to validate the estimated translational velocity. Results showed that accelerometers can identify prefailure signals before any macroscopic movement could indicate impending failure in fast to very fast landslides, showing their potential to be used in EWS. Validation of estimated velocities was not always possible due to PIV setup constraints and the velocity of the mass movement simulated. In fact, the estimated translational velocities seem to be unreliable. On the other hand, the results suggest that acceleration data and angular position variation trend and rate can be incorporated into EWS.

Development of Axial Flows in Wake Vortices due to End Effects

Experiments on the behavior of wake vortices after the end of lift production (end effects) are conducted in a towing tank. The experiments are a first step toward investigating the behavior of the wake vortices after the landing of an aircraft. The model used for the experiments is the DLR, German Aerospace Center F13, well known from previous investigations. After a straight translation at a constant speed, the F13 model in only wing configuration is decelerated at a constant rate to a complete stop to cease the lift production. After the beginning of the deceleration and the halt of the wing, two different disturbances are observed that travel through the cores of the wake vortices. These disturbances and their influence on tangential and axial flows in the wake vortices are investigated using conventional 2-D and stereo particle image velocimetry setups. First, wakelike axial flow is observed in the vortex core, then switching to jetlike axial flow, initiated by the first of the two disturbances (vortex bursting). After the first disturbance, the jetlike flow prevails and increases in magnitude until the second disturbance arrives (helical-like). The spatial–temporal behavior of the disturbances, including their propagation velocities, is presented and discussed.

A compact in vitro test bench for cardiovascular flow analysis

A low-cost particle image velocimetry set-up that allows to investigate the fluid dynamics inside realistic coronary artery phantoms has been implemented. The proposed smart test bench for experimental characterization of arterial hemodynamics also in the presence of implanted devices represents a low-cost equipment that can be easily implemented in non-expert laboratories for research as well as educational applications.

A compact in vitro test bench for cardiovascular flow analysis

A low-cost particle image velocimetry set-up that allows to investigate the fluid dynamics inside realistic coronary artery phantoms has been implemented. The proposed smart test bench for experimental characterization of arterial hemodynamics also in the presence of implanted devices represents a low-cost equipment that can be easily implemented in non-expert laboratories for research as well as educational applications.

High Resolution 2.5D PIV Measurements of the Flow Fields Generated by Small Fans

Abstract The free jets of an axial and a centrifugal fan have been scanned by a specialized particle image velocimetry (PIV) set-up, which allows for volumetric scans of the time-averaged velocity field. Both of these fans have similar dimensions of approximately 70 mm x 70 mm x 25 mm. A classic PIV set-up was combined with a precise linear stage to move the fans through the laser fan beam in small steps, creating a dense array of measurement planes. Two components of the time-averaged velocity field are captured by the first 2.5D scan. Another scan, with the fan rotated by 90° about its outlet surface normal, captures the missing third velocity component. This article describes the details of the measurement set-up, and mentions measures concerning seeding, reflections, and calibration. In the signal processing stage, two independent sets of gathered image data have to be processed, producing two sets of velocity image frames. These are subsequently combined using gridded interpolation in order to obtain a 3D velocity field. Specifically devised software tools allow for a CFD-like analysis and visualization of the flow field. Typical parameters of the generated jets, like the spreading and rotation rates, are calculated from the measurement data and details of the outlet flow fields are investigated. The interpolated data are also used to analyze the influence of an assumed coarser measurement grid resolution on the results for the obtained outlet flow fields.

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.

Examination of laminar Couette flow with obstacles by a low-cost particle image velocimetry setup

For many technical applications, a detailed analysis of the fluid mechanical properties is necessary, for which computational fluid dynamics (CFD) simulations are used. However, even though flow simulations are becoming faster and more accurate, validation through experimentation is essential. One way of validation is to use Particle Image Velocimetry (PIV), an imaging technique that can visualize the flow field and measure flow velocities. Since the measuring equipment of commercial systems is very expensive, we propose a low-cost PIV setup that is also affordable for small scientific institutions. In addition to the quality of the acquired images, the reliability and comparability between experiment and simulation are also important issues. Therefore, in this work, we compare the image acquisition quality of the proposed low-cost PIV system with two- and three-dimensional CFD simulations for a laminar Couette flow and a laminar flow around square and hexagonal obstacles with very good agreement. In addition, we analyzed the transferability of 2D and 3D CFD simulations with experiments by measuring the velocity field and found that experimentally determined flow velocities often cannot be used to validate idealized (2D) simulations due to the spatial flow that occurs. However, if the non-ideal conditions of the experiment are considered in the (3D) simulation, a good comparability is given and an experimental validation is possible, for which the presented low-cost PIV system is well suitable.

Can face masks offer protection from airborne sneeze and cough droplets in close-up, face-to-face human interactions?-A quantitative study.

Day-to-day observations reveal numerous medical and social situations where maintaining physical distancing is either not feasible or not practiced during the time of a viral pandemic, such as, the coronavirus disease 2019 (COVID-19). During these close-up, face-to-face interactions, a common belief is that a susceptible person wearing a face mask is safe, at least to a large extent, from foreign airborne sneeze and cough droplets. This study, for the first time, quantitatively verifies this notion. Droplet flow visualization experiments of a simulated face-to-face interaction with a mask in place were conducted using the particle image velocimetry setup. Five masks were tested in a snug-fit configuration (i.e., with no leakage around the edges): N-95, surgical, cloth PM 2.5, cloth, and wetted cloth PM 2.5. Except for the N-95 mask, the findings showed leakage of airborne droplets through all the face masks in both the configurations of (1) a susceptible person wearing a mask for protection and (2) a virus carrier wearing a mask to prevent the spreading of the virus. When the leakage percentages of these airborne droplets were expressed in terms of the number of virus particles, it was found that masks would not offer complete protection to a susceptible person from a viral infection in close (e.g., <6 ft) face-to-face or frontal human interactions. Therefore, consideration must be given to minimize or avoid such interactions, if possible. This study lends quantitative support to the social distancing and mask-wearing guidelines proposed by the medical research community.

Aberration correction for flow velocity measurements using deep convolutional neural networks

Optical imaging-based flow measurement techniques, like particle image velocimetry, are vulnerable to optical distortions caused by inhomogeneous refractive index or fluctuating phase boundaries. These distortions can lead to blurred particle images and uncertain tracer particle position assignment, resulting in a degradation of velocity measurement accuracy. In order to improve the measurement accuracy, adaptive optics system can be applied to correct distortions. For imaging metrology in fluid mechanics, the optical distortions have features of large frequency range, high spatial frequency and large dynamic range. Actuator-based approaches are limited by its performances. In our work, a novel intelligent adaptive optic system was applied to flow measurement, a learning-based aberration correction method without wavefront corrector was demonstrated, which was used to correct distortions in imaging-based flow measurement. A particle image velocimetry setup which can measure wavefront aberration was built to generate training and test dataset for deep neural network, and also the distortion caused PIV image degradation model. The correction performance of the trained neural network was quantitatively evaluated by corrected PIV image quality and flow measurement result.

In vivo experimental study of acute intraocular hypertension in rabbit based on particle image velocimetry technique

At present, there are few in vivo experimental studies on anterior chamber flow field, and the relevant technologies are not mature. This study explores the experimental method and key techniques of particle image velocimetry (PIV) for the in vivo measurement of anterior chamber flow field with slow flow velocity in the rabbit with acute intraocular hypertension. The experimental process can be divided into three parts: model construction of rabbit eye with acute intraocular hypertension, in vivo eyeball preparation, and PIV setup. The following key techniques were mainly investigated: the optimal injection strategy of fluorescent particles and the correction strategy for image acquisition errors caused by the effects of image refraction and respiration. The results showed that the best injection method was that 15 μL of fluorescent particles solution was slowly injected into the anterior chamber through the lower part of iris and then the rabbit was released and waited for 13 h. In this way particles were completely distributed in the anterior chamber with the help of the aqueous humor circulation, and then in vivo PIV experiment could be performed. The eyeball should be covered with a square flume filled with ultrasonic coupling gel for the sake of imaging during the experiment. The Maximal Information Coefficient algorithm could be applied to correct the measured results before post-processing calculation. The results indicated that feasible injection strategy of fluorescent particles and the correction strategy for image acquisition are critical to obtain nice experiment effects for the in vivo PIV measurement of anterior chamber flow field in the rabbit with acute intraocular hypertension.

Influence of Surfactant Additives on the Electrolyte Flow Velocity and Insoluble Gas Bubbles Behavior within a Lead-Acid Battery

Improving the flooded-electrolyte batteries performance in fast charging and discharging processes has attracted many researchers. Due to electrochemical reactions during charging and discharging process in these batteries, many insoluble gas bubbles are produced within the electrolyte. These bubbles have a major effect on the performance of the electrodes and the rate of electrochemical reactions. On the other hand, the electrolyte flow rate plays an important role on the performance of the battery. In the present investigation, the effect of surface tension on the production of insoluble bubbles and velocity of electrolyte flow have been investigated experimentally when different types of surfactants are added to the electrolyte. In addition, a Particle Image Velocimetry setup has been used in order to measure the velocity of electrolyte flow and the behavior of bubbles. The results declared that the capacity of battery enhances about 16% and 10% by adding Triton X100 and SDS surfactants, respectively. Hence, the averaged electrolyte velocities during charging process reduces about 11% and 13% when Triton X100 and SDS, repectively; are used as surfactants. Also, the effect of adding different amounts of SDS surfactant on the mean diameter, rising velocity and production rates of bubbles have been illustrated comprehensively.

Tomographic PIV in a model of the left ventricle: 3D flow past biological and mechanical heart valves

Left ventricular flow is intrinsically complex, three-dimensional and unsteady. Its features are susceptible to cardiovascular pathology and treatment, in particular to surgical interventions involving the valves (mitral valve replacement). To improve our understanding of intraventricular fluid mechanics and the impact of various types of prosthetic valves thereon, we have developed a custom-designed versatile left ventricular phantom with anatomically realistic moving left ventricular membrane. A biological, a tilting disc and a bileaflet valve (in two different orientations) were mounted in the mitral position and tested under the same settings. To investigate 3D flow within the phantom, a four-view tomographic particle image velocimetry setup has been implemented. The results compare side-by-side the evolution of the 3D flow topology, vortical structures and kinetic energy in the left ventricle domain during the cardiac cycle. Except for the tilting disc valve, all tested prosthetic valves induced a crossed flow path, where the outflow crosses the inflow path, passing under the mitral valve. The biological valve shows a strong jet with a peak velocity about twice as high compared to all mechanical heart valves, which makes it easier to penetrate deeply into the cavity. Accordingly, the peak kinetic energy in the left ventricle in case of the biological valve is about four times higher than the mechanical heart valves. We conclude that the tomographic particle imaging velocimetry setup provides a useful ground truth measurement of flow features and allows a comparison of the effects of different valve types on left ventricular flow patterns.

Single calibration multiplane stereo-PIV: the effect of mitral valve orientation on three-dimensional flow in a left ventricle model

The characterization of flow patterns in the left ventricle may help the development and interpretation of flow-based parameters of cardiac function and (patho-)physiology. Yet, in vivo visualization of highly dynamic three-dimensional flow patterns in an opaque and moving chamber is a challenging task. This has been shown in several recent multidisciplinary studies where in vivo imaging methods are often complemented by in silico solutions, or by in vitro methods. Because of its distinctive features, particle image velocimetry (PIV) has been extensively used to investigate flow dynamics in the cardiovascular field. However, full volumetric PIV data in a dynamically changing geometry such as the left ventricle remain extremely scarce, which justifies the present study. An investigation of the left ventricle flow making use of a customized cardiovascular simulator is presented; a multiplane scanning-stereoscopic PIV setup is used, which allows for the measurement of independent planes across the measurement volume. Due to the accuracy in traversing the illumination and imaging systems, the present setup allows to reconstruct the flow in a 3D volume performing only one single calibration. The effects of the orientation of a prosthetic mitral valve in anatomical and anti-anatomical configurations have been investigated during the diastolic filling time. The measurement is performed in a phase-locked manner; the mean velocity components are presented together with the vorticity and turbulent kinetic energy maps. The reconstructed 3D flow structures downstream the bileaflet mitral valve are shown, which provides additional insight of the highly three-dimensional flow.

The technology of processing intensive structured dataflow on a supercomputer

Abstract Modern experimental setups generate prolonged and intense data streams. For example, non-contact measurement techniques PIV (Particle Image Velocimetry), based on continuous image processing, are widely used in the experimental aerodynamics and hydrodynamics. These experimental stereo or volumetric velocimetry 3D PIV setups are able to generate long-lasting and intensive flows of images by using two or more cameras. High computational complexity of the image processing algorithms is the main limiting factor in the conditions of the low computational performance of PIV setups itself. Removal of these limitations by moving image processing tasks on a remote supercomputer by using our proposed technology “Distributed PIV”, which will allow users to apply their new high-precision parallel algorithms in a real-time and implement feedback to the experimental setup. This paper describes the innovative technology for high-performance processing of the intensive flows of the structured data generated by an experimental setup and delivered through a high-speed DWDM backbone directly into computing nodes of a remote supercomputer. A high BDP (Bandwidth-Delay Product) problem case in the design of protocols such as TCP in respect of performance tuning to achieve maximum network throughput is solved by designed middleware.