Particle Image Velocimetry Images Research Articles

Numerical and experimental study of shock waves emanating from an open-ended rectangular tube

We examine the dynamics of a high-speed shock-induced flow near the open end of a shock tube using the particle image velocimetry (PIV) and the background oriented schlieren (BOS) methods along with two- and three-dimensional numerical simulations. In experiments, planar shock waves (\(M=1.3\)–1.6) are discharged from a rectangular (\(24\,\hbox {mm} \times 48\,\hbox {mm}\)) low-pressure section of a shock tube open to the atmosphere. Due to the rectangular exit geometry, the resulting flow is highly three-dimensional and, thus, more complicated, compared to well-studied circular/axisymmetric geometries. The study focuses on the spatio-temporal flow structure up to 1 ms after the shock wave diffraction. PIV and BOS visualization techniques share the same post-processing principle, and the iterative multi-step cross-correlation algorithm applied in the PIV software is adapted here for the calculation of background pattern displacement on the BOS images. Particular attention is given to the resolution of flow regions where sharp gradients are present, such as a diffracted shock front or embedded shocks. Computational fluid dynamic simulations of the problem are also conducted to validate the experimental results and methods and to gain more insight into the three-dimensional flow dynamics. PIV and BOS images are found to be consistent with the corresponding numerical flow visualizations.

Investigation of combustion dynamics in a cavity-based combustor with high-speed laser diagnostics

The dynamics of the flame/flow interaction produced in an optically accessible, premixed, and staged cavity-based combustor was investigated with high-speed particle image velocimetry (PIV) and OH-planar laser-induced fluorescence (OH-PLIF) . The combined PIV and OH-PLIF images were recorded at 2.5 kHz to assess stabilization mechanisms occurring between the cavity and the mainstream. Dynamic pressure and global heat-release rate fluctuations were complementary measured. Important characteristics were identified for two operating conditions, differing from the ratio of momentum J (taken between the mainstream and the cavity jet): a high ratio of momentum (\(J = 7.1\)) produced a “stable” flow, whereas a lower one (\(J = 2.8\)) displayed “unstable” conditions. Analysis of the “unstable” case revealed an intense flow instability, primarily due to premixed flow rate fluctuations inside the cavity. This effect is confirmed from a proper orthogonal decomposition analysis of PIV data, which illustrates the prominent role of large-scale flow oscillations in the whole combustor. Furthermore, the simultaneous analysis of flow velocities and gas state (either unburned or burned) displayed important fluctuations inside the shear layer, reducing effective flame-holding capabilities. By contrast, the increase in the ratio of momentum in the “stable” case reduces significantly the penetration of the cavity flow into the mainstream and consequently produces stable properties of the shear layer, being valuable to considerably improve flame stabilization.

Diffusion Process of High Concentration Spikes in a Quasi-Homogeneous Turbulent Flow

When a mass spreads in a turbulent flow, areas with obviously high concentration of the mass compared with surrounding areas are formed by organized structures of turbulence. In this study, we extract the high concentration areas and investigate their diffusion process. For this purpose, a combination of Planar Laser Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV) techniques was employed to obtain simultaneously the two fields of the concentration of injected dye and of the velocity in a water turbulent channel flow. With focusing on a quasi-homogeneous turbulence in the channel central region, a series of PLIF and PIV images were acquired at several different downstream positions. We applied a conditional sampling technique to the PLIF images to extract the high concentration areas, or spikes, and calculated the conditional-averaged statistics of the extracted areas such as length scale, mean concentration, and turbulent diffusion coefficient. We found that the averaged length scale was constant with downstream distance from the diffusion source and was smaller than integral scale of the turbulent eddies. The spanwise distribution of the mean concentration was basically Gaussian, and the spanwise width of the spikes increased linearly with downstream distance from the diffusion source. Moreover, the turbulent diffusion coefficient was found to increase in proportion to the spanwise distance from the source. These results reveal aspects different from those of regular mass diffusion and let us conclude that the diffusion process of the spikes differs from that of regular mass diffusion.

Experimental Study of a Periodical Flapping Flag

A flag flapping in the wind is a classical fluid-structure interaction problem that concerns the interaction of elastic bodies with ambient fluid. The fluid-flag interaction can give rise to three self-sustained oscillation states under certain conditions, i.e., stretched-straight state, periodic state, and chaotic state. This paper reports an experimental study of a cantilevered polydimethylsiloxane (PDMS) flag flapping in a uniform flow at a periodic state. A heavy flag is well designed, with metal strips imbedded in the PDMS sheet. Immersing the elastic but self-sustaining flag into the water flow, we use the time-resolved particle image velocimetry (PIV) and visualization techniques to obtain the whole flow field around the midspan of the flapping flag. A unique PIV image processing method is used to measure the near-wall flow velocities around the flap-ping elastic flag at the periodic state. The image processing technique adopts a radon transform technology to determine the moving interface in the particle images. The interface velocity distribution is subsequently calculated. Artificial particles of uniform size with the interface velocity are added into the flag region. Therefore, the whole velocity field over a flapping period is accurately obtained, giving the basic data to analyze the flag flapping. It is found that there exists an inflection point in the envelope curve of the flag flapping. Based on the analyses of the flapping states and velocity fields, a unified flapping Strouhal number (St = 2Af/U) is proposed by choosing the amplitude of the inflection point as the characteristic length, which is similar to the Strouhal number of the flow around a circular cylinder over the same range of Reynolds number.

Time-Resolved Particle Image Velocimetry Measurements with Wall Shear Stress and Uncertainty Quantification for the FDA Nozzle Model.

We present advanced particle image velocimetry (PIV) processing, post-processing, and uncertainty estimation techniques to support the validation of computational fluid dynamics analyses of medical devices. This work is an extension of a previous FDA-sponsored multi-laboratory study, which used a medical device mimicking geometry referred to as the FDA benchmark nozzle model. Experimental measurements were performed using time-resolved PIV at five overlapping regions of the model for Reynolds numbers in the nozzle throat of 500, 2000, 5000, and 8000. Images included a twofold increase in spatial resolution in comparison to the previous study. Data was processed using ensemble correlation, dynamic range enhancement, and phase correlations to increase signal-to-noise ratios and measurement accuracy, and to resolve flow regions with large velocity ranges and gradients, which is typical of many blood-contacting medical devices. Parameters relevant to device safety, including shear stress at the wall and in bulk flow, were computed using radial basis functions. In addition, in-field spatially resolved pressure distributions, Reynolds stresses, and energy dissipation rates were computed from PIV measurements. Velocity measurement uncertainty was estimated directly from the PIV correlation plane, and uncertainty analysis for wall shear stress at each measurement location was performed using a Monte Carlo model. Local velocity uncertainty varied greatly and depended largely on local conditions such as particle seeding, velocity gradients, and particle displacements. Uncertainty in low velocity regions in the sudden expansion section of the nozzle was greatly reduced by over an order of magnitude when dynamic range enhancement was applied. Wall shear stress uncertainty was dominated by uncertainty contributions from velocity estimations, which were shown to account for 90-99% of the total uncertainty. This study provides advancements in the PIV processing methodologies over the previous work through increased PIV image resolution, use of robust image processing algorithms for near-wall velocity measurements and wall shear stress calculations, and uncertainty analyses for both velocity and wall shear stress measurements. The velocity and shear stress analysis, with spatially distributed uncertainty estimates, highlights the challenges of flow quantification in medical devices and provides potential methods to overcome such challenges.

PIV tracer behavior on propagating shock fronts

The present work was aimed at the quantitative particle image velocimetry (PIV) measurement of a velocity field near the front of a propagating shock wave and the study of the dynamics of liquid tracers crossing the shock front. For this goal, a shock tube with a rectangular cross-section (48 × 24 mm) was used. The flat shock wave with Mach numbers M = 1.4–2.0 propagating inside the tube channel was studied as well as an expanding shock wave propagating outside the channel with M = 1.2–1.8 at its main axis. The PIV imaging of the shock fronts was carried out with an aerosol of dioctyl sebacate (DEHS) as tracer particles. The pressures of the gas in front of the shock waves studied ranged from 0.013 Mpa to 0.1 MPa in the series of experiments. The processed PIV data, compared to the 1D normal shock theory, yielded consistent values of wake velocity immediately behind the plain shock wave. Special attention was paid to the blurring of the velocity jump on the shock front due to the inertial particle lag and peculiarities of the PIV technique. A numerical algorithm was developed for analysis and correction of the PIV data on the shock fronts, based on equations of particle-flow interaction. By application of this algorithm, the effective particle diameter of the DEHS aerosol tracers was estimated as 1.03 ± 0.12 μm. A number of different formulations for particle drag were tested with this algorithm, with varying success. The results show consistency with previously reported experimental data obtained for cases of stationary shock waves.

Abstract 12954: A Novel-Particle Image Velocimetry (PIV) Assay to Measure the Maturity and Functional Cardiomyocytes Contractility

Introduction: The most important functional property of cardiomyocytes (CMCs) from a therapeutic standpoint is the ability to produce contractile force. Current methods of assessing the same, such as patch clamping and confocal calcium imaging, are labor-intensive, invasive or require fluo-dye in culture medium, which can affect the cells’ contractility. No optimal method is currently available to assess CMC functionality. Hypothesis: We hypothesized that a novel particle image velocimetry (PIV) method would provide accurate assessment of CMC contractility and maturity. Methods: We generated induced pluripotent stem cells (iPSCs) from adult human cells using a highly efficient viral-free transfection of DNA and mRNA combination. iPSCs were further differentiated into functional CMCs with defined culture conditions. Results: To analyze the contractility of iPSC-derived CMCs (iCMCs), we recorded the same areas of culture plate at various time-points using high frame-rate video microscopy (Fig. A, n=11). A novel image analysis technique provided beat patterns time series data of cell displacement, measured relative to a resting reference state (Fig. B, C). As beat patterns of recordings in video microscopic images revealed, the contractility of early CMC nodes was asynchronous in space and irregular in time (Fig. D1-D2, three beating fields are marked as red, green and blue). During subsequent days after the onset of beating, however, the spatially distinct contractile centers became synchronous even as their frequency remained unstable (Fig. D3-D6). The transmission electron microscopic images confirmed a well-developed pattern of sarcomeres in late-stage (day 30) CMCs. Conclusions: This PIV image analysis is a novel method that enables assessment of CMC contractility multiple times, if necessary, without jeopardizing the biology of cells. This method may also prove beneficial for drug screening and detection of cardiotoxicity in iPSC-derived CMCs.

Flow field measurement around vortex cavitation

Models for the center frequency of cavitating-vortex induced pressure-fluctuations, in a flow around propellers, require knowledge of the vortex strength and vapor cavity size. For this purpose, stereoscopic particle image velocimetry (PIV) measurements were taken downstream of a fixed half-wing model. A high spatial resolution is required and was obtained via correlation averaging. This reduces the interrogation area size by a factor of 2–8, with respect to conventional PIV measurements. Vortex wandering was accounted for by selecting PIV images for a given vortex position, yielding sufficient data to obtain statistically converged and accurate results, both with and without a vapor-filled vortex core. Based on these results, the low-order Proctor model was applied to describe the tip vortex velocity outside the viscous core, and the cavity size as a function of cavitation number. The flow field around the vortex cavity shows, in comparison with a flow field without cavitation, a region of retarded flow. This layer around the cavity interface is similar to the viscous core of a vortex without cavitation.

Imaging flow during impingement of differentially heated jets over a flat surface

The present study investigates thermal fluctuations occurring in the mixing region of two adjacent differentially heated water jets impinging on a horizontal stainless steel surface. The nozzle diameters of the individual jets are equal. The jets are immersed in a pool of water at the ambient temperature. The Reynolds number range considered in the experiments is 2.09×103–2.51×104 for each jet, while the Richardson number is in the range of 9.58×10−3–1.38×10−2. Light intensity fluctuations in the interfacial region of the jets are imaged using the shadowgraph technique. Particle image velocimetry (PIV) has been carried out in a plane passing across the two jets. The effect of various flow rates near the nozzles and the impingement plate are visualized. Time-averaged and RMS temperature fluctuations on the exposed face of the impingement plate are measured using thermocouples. Spectra of light intensity fluctuations determined from the shadowgraph data are correlated with those in wall temperature.Light intensity spectra grow in amplitude in the flow direction, as additional jet instabilities set in. The spectra are broadband with distinct peaks, typical of high Reynolds number jets. Time-averaged and RMS wall temperature distribution are quite distinct from those of light intensity within the impinging jets. Consistent with a wall-jet behavior, the largest RMS wall temperature occurs near the stagnation point and decays away from it. The gradient in the time-averaged wall temperature is the highest for equal velocity jets and occurs at the mid-plane location. This is also the location of the peak in RMS wall temperature. Among all velocity ratios, the equal velocity configuration shows the highest RMS wall temperature. Here, PIV imaging shows a recirculation pattern in the time-averaged velocity near the stagnation point. When the flow rates are unequal, the higher velocity jet provides a blanket on the lower velocity side. The wall temperature fluctuations depend on the interaction between the cold and the hot jets and the location of the impingement plate.JEL Classification: K. Thermal hydraulics

Study of Time-Resolved Vortex Structure of In-Cylinder Engine Flow Fields Using Proper Orthogonal Decomposition Technique

The structure of in-cylinder flow field makes significant impacts on the processes of fuel injection, air–fuel interactions, and flame development in internal combustion engines. In this study, the implementation of time-resolved particle image velocimetry (PIV) in an optical engine is presented. Flow field PIV images at different crank angles have been taken using a high-speed double-pulsed laser and a high-speed camera with seeding particles mixed with the intake air. This study is focused on measuring the flow fields on the swirl plane at 30 mm below the injector tip under various intake air swirl ratios. A simple algorithm is developed to identify the vortex structure and to track the location and motion of vortex center at different crank angles. Proper orthogonal decomposition (POD) has been used to extract the ensemble and variation information of the vortex structure. Experimental results reveal that strong cycle-to-cycle variations exist in almost all test conditions. The vortex center is difficult to identify since multiple, but small scale, vortices exist during the early stage of the intake stroke. However, during the compression stroke when only one vortex center exists in most cycles, the motion of vortex center is found to be quite similar at different intake swirl ratios and engine speeds. This is due to the dominant driving force exerted by the piston’s upward motion on the in-cylinder air.

A modified cross-correlation algorithm for PIV image processing of particle-fluid two-phase flow

PIV (Particle Image Velocimetry) technique for flow field measurement has achieved popular self-identify through over ten years development, and its application range is becoming wider and wider. PIV post-processing techniques have a great influence on the success of particle-fluid two-phase flow field measurement and thus become a hot and difficult topic. In the present study, a Phase Respective Identification Algorithm (PRIA) is introduced to separate low-density solid particles or bubbles and high-density tracer particles from the PIV image of particle-fluid two-phase flow. PTV (Particle Tracking Velocimetry) technique is employed to calculate the velocity fields of low-density solid particles or bubbles. For the velocity fields of high-density solid particles or bubble phase and continuous phase traced by high-density smaller particles, based on the thought of wavelet transform and multi-resolution analysis and the theory of cross-correlation of image, a delaminated processing algorithm (MCCWM) is presented to conquer the limitation of conventional Fourier transform. The algorithm is firstly testified on synthetic two-phase flows, such as uniform steady flow, shearing flow and rotating flow, and the computational results from the simulated particle images are in reasonable agreement with the given simulated data. The algorithm is then applied to images of actual bubble-liquid two-phase flow and jet flow, and the results also confirmed that the algorithm proposed in the present study has good performance and reliability for post-processing PIV images of particle-fluid two-phase flow.

An improved particle counting method for particle volume concentration in aeolian sand transport

An improved particle counting method (denoted as M-count method) is proposed to calculate the particle volume concentration in aeolian sand transport in a wind tunnel. In the present M-count method, the thickness of the laser sheet of PIV (particle image velocimetry) system and the variation of particle diameter with height are considered. The laser sheet thickness is determined by an optical method, in which a full width at half maximum (FWHM) is used to express the width of laser sheet on the image. The mean particle diameter calculated by the pixel areas from PIV images at different height layer is also modified. The results show that the present M-count method is an available method for obtaining the particle volume fraction and also verified by the M-flux method in which the particle volume fraction is deduced from the sand mass flux and the mean horizontal particle velocity. The present M-count method is also helpful to study particle concentration in aeolian sand transport.

Characteristics of the wake behind a heated cylinder in relatively high Reynolds number

Thermal effects on the dynamics and stability of the flow past a circular cylinder operating in the forced convection regime is studied experimentally for Reynolds numbers (Red) between 1000 and 4000, and different cylinder wall temperatures (Tw) between 25 and 75°C by means of Particle Image Velocimetry (PIV). In each experiment, to acquire 3000 PIV image pairs, the temperature and Reynolds number of the approach flow were held constant. By adjusting different temperatures in different Reynolds numbers, the corresponding Richardson number was varied between 0.0 and 0.2. With increasing temperature of the wall cylinder, significant modifications of the wake flow pattern and wake vortex shedding process were clearly revealed. By increasing the Richardson number, the high temperature gradient in the wake shear layer creates a type of vorticity with opposite sign to that of the shear layer vorticity. This temperature gradient-vorticity weakens the strength of the shear layer vorticity, causing delay in reaching the recreation point. In addition to the wake characteristics, it is found that, as the Richardson number is increased, the organization of the vortex shedding is altered and the relative position of the first detached vortices with respect to the second one is changed. This change varies the frequency of the shedding process.

PIV measurements of flow around an arbitrarily moving free surface

We present an image preprocessing method for particle image velocimetry (PIV) measurements of flow around an arbitrarily moving free surface. When performing PIV measurements of free surface flows, the interrogation windows neighboring the free surface are vulnerable to a lack, or even an absence, of seeding particles, which induces less reliable measurements of the velocity field. In addition, direct measurements of the free surface velocity using PIV have been challenging due to the intermittent appearance of the arbitrarily moving free surface. To address the aforementioned limitations, the PIV images with a curvilinear free surface can be treated to be suitable for a structured interrogation window arrangement in a Cartesian grid. The proposed image preprocessing method is comprised of a free surface detection method and an image transform process. The free surface position was identified using a free surface detection method based on multiple textons. The detected free surface points were used to transform PIV images of a curvilinear free surface into images with a straightened free surface using a cubic Hermite spline interpolation scheme. After the image preprocessing, PIV algorithms can be applied to the treated PIV images. The fluid-only region velocities were measured using standard PIV method with window deformation, and the free surface velocities were resolved using PIV/interface gradiometry method. The velocity field in the original PIV images was constructed by inverse transforming that in the transformed images. The accuracy of the proposed method was quantitatively evaluated with two sets of synthetic PIV images, and its applicability was examined by applying the present method to free surface flow images, specifically sloshing flow images.

Dynamic analysis of granite rockburst based on the PIV technique

This paper describes the deep rockburst simulation system to reproduce the granite instantaneous rockburst process. Based on the PIV (Particle Image Velocimetry) technique, quantitative analysis of a rockburst, the images of tracer particle, displacement and strain fields can be obtained, and the debris trajectory described. According to the observation of on-site tests, the dynamic rockburst is actually a gas–solid high speed flow process, which is caused by the interaction of rock fragments and surrounding air. With the help of analysis on high speed video and PIV images, the granite rockburst failure process is composed of six stages of platey fragment spalling and debris ejection. Meanwhile, the elastic energy for these six stages has been calculated to study the energy variation. The results indicate that the rockburst process can be summarized as: an initiating stage, intensive developing stage and gradual decay stage. This research will be helpful for our further understanding of the rockburst mechanism.

Measurement of fluid rotation, dilation, and displacement in particle image velocimetry using a Fourier–Mellin cross-correlation

Traditional particle image velocimetry (PIV) uses discrete Cartesian cross correlations (CCs) to estimate the displacements of groups of tracer particles within small subregions of sequentially captured images. However, these CCs fail in regions with large velocity gradients or high rates of rotation. In this paper, we propose a new PIV correlation method based on the Fourier–Mellin transformation (FMT) that enables direct measurement of the rotation and dilation of particle image patterns. In previously unresolvable regions of large rotation, our algorithm significantly improves the velocity estimates compared to traditional correlations by aligning the rotated and stretched particle patterns prior to performing Cartesian correlations to estimate their displacements. Our algorithm, which we term Fourier–Mellin correlation (FMC), reliably measures particle pattern displacement between pairs of interrogation regions with up to ±180° of angular misalignment, compared to 6–8° for traditional correlations, and dilation/compression factors of 0.5–2.0, compared to 0.9–1.1 for a single iteration of traditional correlations.We apply our FMC algorithm to synthetic computer-generated PIV images with known velocity and vorticity fields, and to an experimentally measured flow field. Our results show that combining FMC with discrete window offset (DWO) or iterative image deformation (IID) algorithms decreases the mean and variance of displacement and vorticity errors compared to traditional correlations, and that FMC accelerates the convergence of IID.

Detachment of a bubble anchored to a vertical cylindrical surface in quiescent liquid and grid generated turbulence

AbstractTurbulence plays a critical role in detachment process of bubble from a solid surface. To investigate this effect, detachment process of a stationary air bubble from a nozzle in both quiescent and turbulent liquid field was studied. A stationary vertical (flat‐ended) needle of ID 1.24 mm was used as a nozzle to generate a bubble which was anchored to the needle tip. Different sizes of bubbles were generated in quiescent liquid. Volume and contact angle for these bubbles were measured precisely using microscopic imaging technique and correlated. In the quiescent case experiments, a constant contact angle of 90° and bubble diameter of 3.05 ± 0.004 mm were obtained consistently. A simple force balance approach was proposed assuming bubble in equilibrium to determine this maximum bubble diameter during detachment. The detached bubble size calculated using this approach agreed fairly well with the experimental results. An oscillating grid device capable of operating at different frequencies was then applied to generate a homogeneous, near‐isotropic turbulent velocity field around the anchored bubble. It was observed that for detachment of smaller bubbles, higher turbulence intensity was indeed necessary. The turbulent flow field was quantified using particle image velocimetry (PIV) technique and resolved into flow structures (eddies) of different length scales using a Gaussian filter. It was concluded that smaller eddies perturbed the bubble interface whilst the larger eddies contributed to weakening of the capillary force causing the bubble detachment. Energy dissipation profile obtained from the PIV images indicated significant energy dissipation near the bubble compared to the bulk fluid which supported the fact that strong interactions between bubble and eddies were indeed responsible for bubble detachment.

Increasing Flashback Resistance in Lean Premixed Swirl-Stabilized Hydrogen Combustion by Axial Air Injection

Since lean premixed combustion allows for fuel-efficiency and low emissions, it is nowadays state of the art in stationary gas turbines. In the long term, it is also a promising approach for aero engines, when safety issues like lean blowout (LBO) and flame flashback in the premixer can be overcome. While for the use of hydrogen the LBO limits are extended, the flashback propensity is increased. Thus, axial air injection is applied in order to eliminate flashback in a swirl-stabilized combustor burning premixed hydrogen. Axial injection constitutes a nonswirling jet on the central axis of the radial swirl generator which influences the vortex breakdown (VB) position. In the present work, changes in the flow field and their impact on flashback limits of a model combustor are evaluated. First, a parametric study is conducted under isothermal test conditions in a water tunnel employing particle image velocimetry (PIV). The varied parameters are the amount of axially injected air and swirl number. Subsequently, flashback safety is evaluated in the presence of axial air injection in an atmospheric combustor test rig and a stability map is recorded. The flame structure is measured using high-speed OH* chemiluminescence imaging. Simultaneous high-speed PIV measurements of the reacting flow provide insight in the time-resolved reacting flow field and indicate the flame location by evaluating the Mie scattering of the raw PIV images by means of the qualitative light sheet (QLS) technique. The isothermal tests identify the potential of axial air injection to overcome the axial velocity deficits at the nozzle outlet, which is considered crucial in order to provide flashback safety. This effect of axial air injection is shown to prevail in the presence of a flame. Generally, flashback safety is shown to benefit from an elevated amount of axial air injection and a lower swirl number. Note that the latter also leads to increased NOx emissions, while axial air injection does not. Additionally, fuel momentum is indicated to positively influence flashback resistance, although based on a different mechanism, an explanation of which is suggested. In summary, flashback-proof operation of the burner with a high amount of axial air injection is achieved on the whole operating range of the test rig at inlet temperatures of 620 K and up to stoichiometric conditions while maintaining single digit NOx emissions below a flame temperature of 2000 K.

PIV measurements of air-core intake vortices

Particle Image Velocimetry (PIV) measurements were conducted in a large-scale hydraulic model to obtain the horizontal velocity field of intake vortices with considerable air entrainment. Vortex wandering and air-core diameter were determined by object detection using PIV images and velocity vector maps. This allowed for the calculation of ensemble- and azimuthally-averaged velocity fields. The results of the horizontal velocity field around the vortex, whose circulation affects a wide area, confirm the applicability of the analytic solution of the 2D Navier–Stokes equation for a potential (free) vortex. The setup and the improvement of the PIV measurements and their limitations are presented. For example, a completely resolved velocity field close to the air-core did not exist due to reflections at the interface between air and water. Vortex characteristics like vorticity and circulation are derived from the velocity fields and turbulence is shown by representing in-plane Turbulent Kinetic Energy (TKE).

POD analysis of the unsteady behavior of a laminar separation bubble

Particle Image Velocimetry (PIV) measurements have been performed in order to analyze the unsteady flow field developing along the separated flow region of a laminar separation bubble. Data have been post-processed by means of Proper Orthogonal Decomposition (POD) to improve the understanding of the physics of this complex phenomenon. The paper shows that the first two POD modes of the normal to the wall velocity component are coupled. Thus, they are representative of a vortex shedding phenomenon which is identified to be induced by Kelvin–Helmholtz instability. The POD allows the phase identification of each PIV image within the vortex shedding cycle. The computed eigenvectors are used to sort the experimental snapshots and then reconstruct a phase-averaged velocity field which highlighted the motion of vortices shed close to the bubble maximum displacement. Moreover, other sources of deterministic fluctuations characterized by frequencies which are different from the one induced by the Kelvin–Helmholtz instability are also revealed. Indeed, the most energetic POD mode of the streamwise velocity component is not related to the shedding frequency, while it describes large velocity fluctuations in the shear layer region upstream of the bubble maximum displacement, where the turbulent activity is not yet present. The POD decomposition presented here identifies the large scale structures within the flow, thus separately accounts for both coherent and stochastic contributions to the overall energy of the velocity fluctuations.