An Adaptive PIV Software for Pedagogy‐Oriented Practice and a Closed‐Loop Framework for Flow Experiments

PIV analysis of opaque flow without using high-tech equipment

The effects of spatial resolution of planar particle image velocimetry (PIV) on vortex size, swirling strength, circulation and population density characterisation are analysed using a series of experimental and numerical databases. The databases comprise a PIV database of an adverse-pressure-gradient turbulent boundary layer (APG TBL), a PIV database of a zero-pressure-gradient (ZPG) TBL in streamwise-wall-normal planes and streamwise-wall-normal slices of a direct numerical simulation (DNS) of a ZPG TBL. The effects of interrogation window and mesh sizes on the vortex parameters are analysed in the outer region of these flows using different qualitative and quantitative approaches. The quantitative analysis mainly capitalises on the possibility of mimicking the PIV data-sets with the DNS one. These approaches allow us to not only isolate the effects of mesh size and the interrogation window size but also to deduce the combined effects of other measurement errors in PIV. Typical values of mesh size and interrogation window size (0.01–0.03 of the boundary layer thickness) and typical levels of measurement uncertainties have significant effects on the vortex parameters. Moreover, each PIV error source affects the vortex parameters in different and frequently opposite manners. Hence, an optimal selection of measurement parameters such as the interrogation window size is indispensable in order to minimise the effects of spatial resolution and other measurement errors on the vortex parameters. Guidelines are presented in the Conclusions section of this paper. Finally, it is found that all the vortex parameters, when averaged across the outer region, are reasonably comparable in the ZPG and APG TBLs despite the fact that these are very different flows.

In this study, the corrugated channel flow was investigated by using an open-source particle image velocimetry (PIV) software. The open-source software called OpenPIV was first verified by using images of an earlier experimental work of a vortex ring formation. The corrugated channel flow images were taken with 200 W power LED light source and a high speed camera and those images were analysed with these spatial and temporal tools of OpenPIV. Laminar, transient and turbulent flow regimes were identified when Reynolds number was below 1100, in between 1100 and 2000 and higher than 2000, respectively. The velocity vectors were found to be about 20% lower than the previous study results. The flow inside the grooves was also investigated with OpenPIV and flow characteristics at the grooves were captured when interrogation window size was lowered. The visualization of the flow was presented for different Reynolds numbers with the relative scale values. As a result of this study, OpenPIV software was determined as promising open source PIV analysis software.

A high resolution digital cinematic Particle Image Velocimetry(PIV) has been developed. The system consists of a high speed CCD camera, a continuous Ar-ion laser and a computer with camera controller. To improve the spatial resolution, we adopt a Recursive Technique for velocity interrogation. At first, we obtain a velocity vector fur a larger interrogation window size based on the conventional two-frame cross-correlation PIV analysis using the FFT algorithm. Based on the knowing velocity information, more spatially resolved velocity vectors are obtained in the next iteration step with smaller interrogation windows. When the correct velocity vector at the first step is found to be critical, a Multiple Correlation Validation(MCV) technique is applied to decrease the spurious vectors. The MCV technique turns out to improve SNR(Signal to Noise Ratio) of the correlation table. The developed cinematic PIV method has been applied to the measurement of the unsteady flow characteristics of a Rushton turbine mixer. A total of 3,245 instantaneous velocity vectors were successfully obtained with 4 ms time resolution. The acquired spatial resolution corresponds to the conventional high resolution digital PIV system using a 1K 1K CCD camera.

The present work is an experimental and numerical investigation of the small-scale motions in turbulent free-shear flows. In the far-field turbulence of a jet at high Reynolds number (Re? = 350) hot-wire anemometry (HWA) is applied to measure time series of flow velocity. By filtering these time series, large- and small-scale velocity fluctuations are obtained. Both the amplitude and the frequency of the small-scale signals are locally stronger (weaker) for positive (negative) fluctuations of the large-scale signal, which is refered to as amplitude and frequency modulation. The local amplitude and frequency of the small-scale signals increase monotonically with the strength of the large-scale velocity fluctuations. The same flow is also investigated with long-range ?PIV (microscopic Particle Image Velocimetry). The measurement is validated against the HWA signals by comparison of the turbulence statistics. A validation based on the topological content is also performed. The coherent structures of vorticity and of intense dissipation are adequately resolved, and their characteristic size is assessed. It is found that the size of the vortical structures does not change significantly when conditioned on strongly-positive or strongly-negative large-scale velocity fluctuations. Using the PIV results the amplitude and frequency modulation observed from HWA signals is explained as an inhomogeneous distribution of the small-scale structures within the flow. In particular, the analysis of ?PIV data reveals that the intense vortical and dissipation structures tend to be preferentially located in high-velocity regions, hence they are characterized by convection velocities higher than the mean velocity of the flow. Furthermore, the spatially resolved velocity vector fields allow to quantify amplitude modulation directly in physical space. From this direct estimation in physical space, amplitude modulation is only 25% of the value measured from hot-wire anemometry. The remaining 75% comes from the fixed spectral band filter used to obtain the large- and the small-scale signals, which does not consider the local convection velocity (Taylor hypothesis of frozen turbulence). A very similar overestimation of amplitude modulation when quantified in the time-frame is also confirmed analytically. Based on the experimental analysis on the jet an explanation for amplitude and frequency modulation is developed, which can be extended to other free-shear flows. The validity of this interpretation is assessed based on the analysis of Direct Numerical Simulations of a mixing layer, at the Reynolds number based on the Taylor microscale (Re? =) of 250. The local vorticity rms, taken as a measure of the small-scale activity, is found to be modulated by the large-scale velocity fluctuations depending on the position within the flow. In particular, on the low-speed side of the mixing layer, positive large-scale velocity fluctuations correspond to a stronger vorticity rms, whereas on the high-speed side, they correspond to a weaker vorticity rms. This is consistent with previous studies on a mixing layer. Important differences are found in the strength of the scale interaction from time series and in physical space, consistent with the predictions developed from the analysis of the jet. On the high-speed side of the mixing layer, amplitude modulation from time series largely underestimates the value obtained from spatial series, and overestimates it on the low-speed side. Therefore, the interaction between large-scale velocity fluctuations and small scales is dependent on the flow position within the mixing layer, similar to a turbulent boundary layer. Nonetheless, when the vorticity rms is correlated with the large-scale shear velocity gradients, the correlation coefficient is found to be nearly constant throughout the mixing layer, and close to unity. This reveals that the large and the small scales present a strong interaction independent of the position when the large-scale shear velocity gradients are considered, instead of the large-scale velocity fluctuations, as in the existing literature on amplitude modulation. The strong correlation between the large-scale gradients and the small scales suggests to investigate possible evidence of the so called “scale invariance” (Meneveau and Katz 2000). The alignment between the local vorticity and the large-scale vorticity is examined within the vortical tubes. It is found that the vorticity from unfiltered (representing the small scales) and from low-pass-filtered velocity vector fields (representing the larger scales) tend to be aligned within the vortical tubes. This suggests that the direction of vorticity does not vary significantly across the scales. Therefore, the anisotropy of the large scales is partially preserved at the small-scale level, which is in contrast with the Kolmogorov’s hypothesis of local isotropy.

New results from surface PIV (Particle Image Velocimetry) measurements are presented. Surface PIV can potentially provide researchers with a cheap and versatile method for mapping 2D flow fields. This technique was evaluated in a laboratory flume with a random distribution of rigid plastic straws, to simulate flows through emergent vegetation. Velocities were computed via an open-source tool for conventional PIV, and a sensitivity analysis conducted, in which the factors seeding particle size, particle image density, size of interrogation window, number of passes and contrast were evaluated. Results show that, with the appropriate settings, 98.7\(\%\) of data points were considered to be reliable. It was found that the best quality velocity maps were obtained with small seeding particles and intermediate window resolutions (16\(\,\times \,\)16 pixels). The practical use of this technique is illustrated by using the data to identify the portion of flow through vegetation occupied by wakes. For this, a straightforward criterion, related to the incident flow conditions and generated vorticity, is proposed. Further refinements of this research can lead to applications in several branches of fluid mechanics, such as in situ measurements of the flow field and analysis of scalar dispersion processes in ecohydraulics.

A statistical tool called design of experiments (DOEs) is introduced for uncertainty quantification in particle image velocimetry (PIV). DOE allows to quantify the total uncertainty as well as the systematic uncertainties arising from various experimental factors. The approach is based on measuring a quantity (e.g. time-averaged velocity or Reynolds stresses) several times by varying the levels of the experimental factors which are known to affect the value of the measured quantity. Then, using Analysis of Variances, the total variance in the measured quantity is computed and hence the total uncertainty. Moreover, the analysis provides the individual variances for each of the experimental factors, leading to the estimation of the systematic uncertainties from each factor and their contributions to the total uncertainty. The methodology is assessed for planar PIV measurements of the flow over a NACA0012 airfoil at 15 degrees angle of attack considering five experimental factors, namely camera aperture, inter-frame time separation, interrogation window size, laser sheet thickness and seeding density. Additionally, the methodology is applied to the investigation by stereoscopic PIV of the flow at the outlet of a ducted Boundary Layer Ingesting propulsor. The total uncertainty in the time-averaged velocity as well as the constituent systematic uncertainties due to the experimental factors, namely camera aperture, inter-frame time separation, interrogation window size and stereoscopic camera angle, are quantified.

The flow characteristics in the inertial Reynolds number regime are investigated in a mono-dispersed random pack porous media. Time-resolved particle image velocimetry (PIV) is used to visualize the velocity field in a low aspect ratio bed with 15 mm glass beads. An aqueous solution of Ammunium Thiocynante is used as the working fluid to facilitate matching the solid-fluid refractiveindices. In order to illuminate the inertial regime characteristics, two pore Reynolds number of 100 and 270 are examined. Also, due to the random nature of the packing several pore geometries are compared to identify local scaling used to define the inertial regime effects. Discrete vortical flow structures are evaluated using LES (lowpass filtering) decomposition, in conjunction with criticalpoint analysis of the local velocity gradient tensor. The identified scales associated with the vortical elements are compared based on Reynolds number and pore geometry. Implementing circulation as an integral measure of all vortical structures locally at the pore-scale level demonstrated a linear attitude over the range of Reynolds numbers. Evolution of inertial effects within pore-regions are indicated to be the primary driving mechanism for the emergence of swirling structures passing through the PIV field of view at the onset of turbulence.

Vortex structure of steady flow in a rectangular cavity

Particle image velocimetry (PIV) is a measurement technique which is well adapted to the study of the structure of turbulent flows, as it allows us to obtain quantitative information on the spatial structure of the velocity field. This contribution presents an experimental approach to characterize the measurement noise of a PIV system and the spatial response of such a method. This approach is based on a specific spectral analysis of the velocity vector field deduced from several PIV experiments. This study was done in two steps. The first step was to measure the noise level of PIV and to determine a model for the PIV transfer function from a series of displacement fields measured in a quiet liquid. This model shows the effect of the interrogation window size and introduces a spectral noise density which is constant for a given recording set-up. The second step was to compute spectra from velocity fields obtained in a turbulent boundary layer in a plane parallel to the wall. These spectra show that PIV behaves as a band pass filter. This series of experiments allows us to build a model for the prediction of the PIV spectrum knowing the real one. This model confirms that the PIV noise is white. It allows us to optimize the interrogation window size in order to obtain the best compromise between the spectral response and the spatial resolution. The rms value of the noise can be estimated from the noise density, allowing us to quantify the measurement accuracy. The improvement of sub-pixel window shift is also discussed, leading to a small decrease in the noise level. An analysis is proposed to identify the main sources of noise: particles cut by the border of the interrogation window, isolated particles, etc.

In this study, the drag reduction effect is studied for a cylinder with different V-groove depths on its surface using a k-ω/SST (Shear Stress Transport) turbulence model of computational fluid dynamics (CFD), while a particle image velocimetry (PIV) system is employed to analyze the wake characteristics for a smooth cylinder and a cylinder with different V-groove depths on its surface at different Reynolds numbers. The study focuses on the characteristics of the different V-groove depths on lift coefficient, drag coefficient, the velocity distribution of flow field, pressure coefficient, vortex shedding, and vortex structure. In comparison with a smooth cylinder, the lift coefficient and drag coefficient can be reduced for a cylinder with different V-groove depths on its surface, and the maximum reduction rates of lift coefficient and drag coefficient are about 34.4% and 16%, respectively. Otherwise, the vortex structure presents a complete symmetry for the smooth cylinder, however, the symmetry of the vortex structure becomes insignificant for the V-shaped groove structure with different depths. This is also an important reason for the drag reduction effect of a cylinder with a V-groove surface.

An experimental study was conducted to investigate the flow behavior around a bioinspired corrugated airfoil compared with a traditional streamlined airfoil and a flat plate at the chord Reynolds number of Re 34; 000 to explore the potential application of such bioinspired corrugated airfoils for micro air vehicle applications. The experiments were conducted in a low-speed wind tunnel. A high-resolution particle image velocimetry system was used to conduct detailed flowfield measurements to quantify the transient behavior of vortex and turbulent flow structures around the studied airfoils. The particle image velocimetry measurement results demonstrated clearly that the corrugated airfoil has better performance over the streamlined airfoil and the flat plate in preventing largescale flow separation and airfoil stall at low Reynolds numbers. It was found that the protruding corners of the corrugated airfoil would act as turbulators to generate unsteady vortex structures to promote the transition of the separated boundary-layer flow from laminar to turbulent. The unsteady vortex structures trapped in the valleys of the corrugated cross section would pump high-speed fluid from outside to near-wall regions to provide sufficient kinetic energy for the boundary layer to overcome adverse pressure gradients, thus discouraging large-scale flow separations and airfoil stall. Aerodynamic force measurements further confirmed the possibility of using such bioinspired corrugated airfoils in micro air vehicle designs to improve their flight agility and maneuverability.

The crosse ow structure of the leading-edge vortices on a delta wing is investigated via particle image velocimetry. The transformation of the streamline topology as the vortex breakdown position moves upor downstream of its nominal value is characterized by comparing instantaneous images. While the streamline patterns are distinctly different depending on the location of vortex breakdown relative to the e eld of view, contours of constant vorticity do not exhibit such distinctions. Several classes of instantaneous streamline topology can exist in the crosse ow plane. These topologies, which involve various combinations of limit cycles and foci, indicate the importance of considering the instantaneous nature of both the vorticity contours and streamlines when describing the e ow physics. Nomenclature c0 = centerline chord dI = interrogation window size t = time U = freestream velocity xL = location of laser sheet xvb = location of vortex breakdown, measured along wing centerline from apex Dl = interrogation step size f = roll angle v = out-of-plane vorticity

An experiment was conducted to identify and measure the sources of uncertainty that are associated with the application of particle image velocimetry to the measurement of the vortical wakes trailing from helicopter rotor blades. Phase-resolved, three-component particle image velocimetry measurements were performed in the wake of a subscale rotor operating in hover, and were compared with high-resolution three-component laser Doppler velocimetry measurements obtained with the same rotor under identical operating conditions. This helped formulate the essential experimental conditions that need to be satisfied for particle image velocimetry to accurately resolve the high-velocity gradient, high streamline curvature flows that are present inside the rotor wake and in the blade tip vortices. Uncertainties associated with the calibration, acquisition, and processing of the particle image velocimetry images were analyzed in detail. It was shown that the optimization of laser pulse separation time is fundamental to reduce the errors associated with acceleration and curvature effects. Similarly, the interrogation window size was shown to play a critical role in determining the velocity gradient bias errors. The correlation between the laser Doppler velocimetry and particle image velocimetry measurements of the tip vortex characteristics, such as core radius and peak swirl velocity, were found to be excellent.

The result of a particle image velocimetry (PIV) measurement is a velocity field averaged over interrogation windows. This severely affects the measurement of small-scale turbulence quantities when the interrogation window size is much larger than the smallest length-scale in turbulence, the Kolmogorov length. In particular, a direct measurement of the dissipation rate demands the measurement of gradients of the velocity field, which are now underestimated because the small-scale motion is not resolved. A popular procedure is to relate the statistical properties of the measured, but underresolved gradients to those of the true ones, invoking a large-eddy argument (Sheng et al. in Chem Eng Sci 55(20):4423–4434, 2000). We argue that the used proportionality constant, the Smagorinsky constant, should depend on the window overlap, on the used elements of the strain tensor, and on the way in which derivatives are approximated. Using an analytic description, PIV measurements of velocity fields from a kinematic simulation and experiments in a synthetic jet-driven turbulent flow with zero mean velocity, we propose new values for this constant.