Andrea Sciacchitano

Collaborative framework for PIV uncertainty quantification: comparative assessment of methods

A posteriori uncertainty quantification of particle image velocimetry (PIV) data is essential to obtain accurate estimates of the uncertainty associated with a given experiment. This is particularly relevant when measurements are used to validate computational models or in design and decision processes. In spite of the importance of the subject, the first PIV uncertainty quantification (PIV-UQ) methods have been developed only in the last three years. The present work is a comparative assessment of four approaches recently proposed in the literature: the uncertainty surface method (Timmins et al 2012), the particle disparity approach (Sciacchitano et al 2013), the peak ratio criterion (Charonko and Vlachos 2013) and the correlation statistics method (Wieneke 2015). The analysis is based upon experiments conducted for this specific purpose, where several measurement techniques are employed simultaneously. The performances of the above approaches are surveyed across different measurement conditions and flow regimes.

PIV uncertainty propagation

This paper discusses the propagation of the instantaneous uncertainty of PIV measurements to statistical and instantaneous quantities of interest derived from the velocity field. The expression of the uncertainty of vorticity, velocity divergence, mean value and Reynolds stresses is derived. It is shown that the uncertainty of vorticity and velocity divergence requires the knowledge of the spatial correlation between the error of the x and y particle image displacement, which depends upon the measurement spatial resolution. The uncertainty of statistical quantities is often dominated by the random uncertainty due to the finite sample size and decreases with the square root of the effective number of independent samples. Monte Carlo simulations are conducted to assess the accuracy of the uncertainty propagation formulae. Furthermore, three experimental assessments are carried out. In the first experiment, a turntable is used to simulate a rigid rotation flow field. The estimated uncertainty of the vorticity is compared with the actual vorticity error root-mean-square, with differences between the two quantities within 5-10% for different interrogation window sizes and overlap factors. A turbulent jet flow is investigated in the second experimental assessment. The reference velocity, which is used to compute the reference value of the instantaneous flow properties of interest, is obtained with an auxiliary PIV system, which features a higher dynamic range than the measurement system. Finally, the uncertainty quantification of statistical quantities is assessed via PIV measurements in a cavity flow. The comparison between estimated uncertainty and actual error demonstrates the accuracy of the proposed uncertainty propagation methodology.

Collaborative framework for PIV uncertainty quantification: the experimental database

The uncertainty quantification of particle image velocimetry (PIV) measurements has recently become a topic of great interest as shown by the recent appearance of several different methods within the past few years. These approaches have different working principles, merits and limitations, which have been speculated upon in subsequent studies. This paper reports a unique experiment that has been performed specifically to test the efficacy of PIV uncertainty methods. The case of a rectangular jet, as previously studied by Timmins et al (2012) and Wilson and Smith (2013b), is used. The novel aspect of the experiment is simultaneous velocity measurements using two different time-resolved PIV systems and a hot-wire anemometry (HWA) system. The first PIV system, called the PIV measurement system (?PIV-MS?), is intended for nominal measurements of which the uncertainty is to be evaluated. It is based on a single camera and features a dynamic velocity range (DVR) representative of typical PIV experiments. The second PIV system, called the ?PIV-HDR? (high dynamic range) system, features a significantly higher DVR obtained with a higher digital imaging resolution. The hot-wire is placed in close proximity to the PIV measurement domain. The three measurement systems were carefully set to simultaneously measure the flow velocity at the same time and location. The comparison between the PIV-HDR system and the HWA provides an estimate of the measurement precision of the reference velocity for evaluation of the instantaneous error in the measurement system. The discrepancy between the PIV-MS and the reference data provides the measurement error, which is later used to assess the different uncertainty quantification methods proposed in the literature. A detailed comparison of the uncertainty estimation methods based on the present datasets is presented in a second paper from Sciacchitano et al (2015). Furthermore, this database offers the potential to be used for comparison of the measurement accuracy of existing or newly developed PIV interrogation algorithms. The database is publicly available on the website www.piv.de/uncertainty.

Spatio-temporal and modal analysis of unsteady fluctuations in a high-subsonic base flow

The high-subsonic flow over an axisymmetric backward facing step is investigated at a Mach number of 0.7, with high-speed particle image velocimetry operating at acquisition frequency up to 20 kHz to resolve the time dependent behavior of the separated flow in the longitudinal plane. The statistical flow properties such as the approaching boundary layer thickness, mean and fluctuating velocity field and shear layer growth are quantified. It is found that the shear layer growth is nearly linear up to x/D = 0.6; downstream this location, the reattachment surface influences the shear layer growth rate, causing it to decrease. The visualization of the high-speed measurements shows a quasi-cyclic behavior of the separated region characterized by a fluctuation of the flow reattachment location. The growing and shrinking of the separated region is accompanied by large-scale fluctuations that dominate the flow motions and the momentum exchange across the shear layer. The analysis of the large-scale fluctuations based on proper orthogonal decomposition (POD) of the velocity field indicates that the separated region exhibits pulsating behaviour as associated to the highest energy mode. The second and third modes are approximately in phase quadrature and can be associated to the growth of large azimuthal vortices that undulate the shear layer approaching the mean reattachment location. Conditional averaging of the data indicates that the second/third mode is associated to either the entrainment of high momentum fluid or ejection of low momentum fluid from the separated region depending on the phase. The high-speed measurements enable the spectral analysis of the velocity; the power spectral density of the POD time coefficients for the first three modes is evaluated. The first mode associated to pulsatile growth and collapse of the separated region yields a dominant frequency at Str D = 0.13 (f = 585 Hz) while the second mode features a broader frequency distribution in the range from 1 to 3 kHz (0.22 < Str D < 0.67) with a distinguishable peak at Str D = 0.4. The dynamical correlation between these two modes shows that time derivative of the first mode temporal coefficient and second mode coefficient are strongly correlated. This suggests that the entrainment of high momentum fluid into the separated region is responsible for the wake growth phenomenon.

Elimination of PIV light reflections via a temporal high pass filter

A novel approach for PIV image pre-processing is proposed to deal with the undesired effect of laser light reflections from solid walls in wind tunnel experiments. The method can be applied for both stationary interfaces as well as when the image of the interface is moving due to vibration of either the model or the imaging system. The working hypothesis is that the motion of the interface is resolved temporally, which is typically the case when employing high-speed PIV systems. The method is based on the decomposition of the pixel intensity in the frequency domain. The high-frequency content of the signal is due to the transit of seeding particles, whereas undesired reflections will appear in the low-frequency range. Applying a high-pass filter on the light intensity time history retains only the contribution of the seeding particles and rejects the undesired light reflections. Two experiments show the application of the method. In the low-speed flow regime around a pitching airfoil, the trace of the laser impinging on the moving surface can be mostly eliminated, enabling cross correlation analysis of the flow closer to the wall. In the transonic regime, experiments are performed in an industrial wind tunnel around the base region of the ARIANE V launcher model. Here, the high-pass filter eliminates all secondary reflections and enhances the particles peak intensity relative to the reflections and the background light.

Experimental investigation of propeller induced ground vortex under headwind condition

During the ground operation of propeller aircraft, the interaction between the ground and the flow field produced by the propeller may lead to the generation of a vortex originating on the ground. Such ground induced vortex results in non-uniform inflow into the propeller disk. Furthermore, the vortex may raise debris from the ground that may damage the blades, decreasing their life time. For this phenomenon, the flow field near the ground is of interest because it provides information on the strength of the vortex before it is being ingested by the propeller. To obtain insight into the origins and the development of the ground vortex, velocity measurements have been performed in the flow field produced by a scaled propeller model using a three-component PIV system on a plane parallel to the ground and in the proximity of it. The velocities in the test plane suggest that a horseshoe vortex is formed above the ground which is built to help understand the flow topology of ground vortices. The vortex/no-vortex domain boundary which is relied upon the presence of node and saddle points in the time averaged flow is presented. The effect of varying the thrust coefficient of the propeller has been assessed under headwind conditions. The averaged strength of the maximum wall normal vortex is compared at different thrust coefficients. The results show that average levels of vortex strength increases up to a maximum and then decreases to achieve a plateau. The pressure under the propeller is measured and the mean value of pressure gradient magnitude keeps increasing as thrust coefficient increases. Vortex wandering is observed at all test conditions in both the lateral and the longitudinal directions for all test conditions. The influence of the ground induced non-uniform inflow upon the propeller performances is found to be negligible.

Spatial-temporal and modal analysis of propeller induced ground vortices by particle image velocimetry

During the ground operation of aircraft, there is potentially a system of vortices generated from the ground toward the propulsor, commonly denoted as ground vortices. Although extensive research has been conducted on ground vortices induced by turbofans which were simplified by suction tubes, these studies cannot well capture the properties of ground vortices induced by propellers, e.g., the flow phenomena due to intermittent characteristics of blade passing and the presence of slipstream of the propeller. Therefore, the investigation of ground vortices induced by a propeller is performed to improve understanding of these phenomena. The distributions of velocities in two different planes containing the vortices were measured by high frequency Particle Image Velocimetry. These planes are a wall-parallel plane in close proximity to the ground and a wall-normal plane upstream of the propeller. The instantaneous flow fields feature highly unsteady flow in both of these two planes. The spectral analysis is co...

The Ring of Fire for in-Field Sport Aerodynamic Investigation

A novel measurement system, the Ring of Fire, is deployed which enables the aerodynamic drag estimation of transiting cyclists. The system relies upon the use of large-scale stereoscopic PIV and the conservation of momentum within a control volume in a frame of reference moving with the athlete. The rider cycles at a velocity of approximately 8 m/s, corresponding to a torso based Reynolds number of 3.2 × 105. The measurements upstream and in the wake of the athlete are conducted at a rate of 2 kHz within a measurement plane of approximately 1000 × 1700 mm2. The non-dimensional, ensemble-averaged streamwise velocity fields compare well to literature and the ensemble-averaged drag area shows a rather constant value along the wake with an uncertainty of 5%. A comparison with wind tunnel force balance measurements shows discrepancies which may be partly attributed to the bike supports and stationary floor in the wind tunnel measurements. The 25% drag difference measured between a rider in upright and time-trial position, however, matches literature well.

Analysis of propeller-induced ground vortices by particle image velocimetry

The interaction between a propeller and its self-induced vortices originating on the ground is investigated in a scaled experiment. The velocity distribution in the flow field in two different planes containing the self-induced vortices is measured by particle image velocimetry (PIV). These planes are a wall–parallel plane in close proximity to the ground and a wall–normal plane just upstream of the propeller. Based on the visualization of the flow field in these two planes, the occurrence of ground vortices and its domain boundary are analysed. The elevation of the propeller from the ground and the thrust of the propeller are two parameters that determine the occurrence of ground vortices. The main features of the propeller inflow in the presence of the ground vortices are highlighted. Moreover, the analysis of the non-uniform inflow in the azimuthal direction shows that with increasing the propeller thrust coefficient and decreasing the elevation of the propeller above the ground, the variation of the inflow angle of the blade increases.Graphical Abstract