The stability and resolution of iterative PIV image analysis methods is investigated. The study focuses on the effects of stabilization by means of spatial filtering when implemented into the iterative process. Two filtering approaches are studied: predictor and corrector filtering respectively. A family of convolution filters is proposed, which allows to vary the filtering strength in a systematic way and primarily affects the system stability and to a smaller extent its spatial response. A critical value for the filter parameter is identified which guarantees the stability of the iterative process. A theoretical analysis is provided that determines the asymptotic properties of the iterative method with varying filter parameters. The study is completed with a numerical assessment and concludes with an application to real experiments, showing the consequence of an incorrect implementation of the iterative scheme under experimental conditions.

The experimental approach used for the evaluation of the particle response time across a stationary shock wave is assessed by means of PIV measurements. The study focuses on the experimental requirements for a reliable and unbiased measurement of the particle response time s p and length n p based on a single-exponent decaying law. A numerical simulation of the particle response experiment returns the parameters governing the measurement: namely the normalized spatial and temporal resolution, shock strength, and digital resolution. Representing the velocity decay in logarithmic coordinates it is shown that measurements performed with laser pulse separation time up to s p and interrogation window up to n p still yield unbiased results for the particle response. A set of experiments on the particle response across a planar oblique shock wave was conducted to verify the results from the numerical assessment. Liquid droplets of DEHS and solid tracer particles of silicon and titanium dioxide with different primary crystal size are compared. The resulting temporal response ranges from 2 to 3 ls, corresponding to values commonly reported in literature, to almost 0.3 ls when particles are properly dehydrated and a filter is applied before injection into the wind tunnel. It is the first experimental evidence of particle tracers with a measured response time lower than 0.4 ls. The same procedure is applied to attempt the measurement of individual particle tracers by particle tracking velocimetry to estimate the spread in the distribution of tracer time response. The latter analysis is limited by the particle image tracking precision error, which biases the results introducing a wider broadening of the particle velocity distribution.

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The turbulent wake past an axisymmetric body is investigated with time-resolved stereoscopic particle image velocimetry (PIV) at a Reynolds number ReD = 6.7 × 104 based on the object diameter. The azimuthal organization of the near-wake is studied at different locations downstream of the trailing edge. The time-averaged velocity field features a circular shear layer bounding a region of recirculating flow. Inspection of instantaneous PIV snapshots reveals azimuthal meandering of the reverse flow region with a significant radial offset with respect to the time-averaged position. The backflow meandering appears as the major contribution to the near-wake dynamics in proximity of the base, whereas closer to the rear-stagnation point, the shear layer fluctuations become important. For x/D ≤ 0.75, the time-history and probability distributions of the backflow centroid position allow to identify this motion with an irregular precession about the model symmetry axis occurring at time scales in the order of 103 D/U∞ or higher. The first two modes obtained by snapshot proper orthogonal decomposition of the velocity fluctuations can be related to an anti-symmetric mode of azimuthal wave-number m = 1 reflecting a radial displacement of the separated flow region, while the third and fourth proper orthogonal decomposition modes are identified with a second mode pair m = 2 and are representing wake ovalization. Close to the base, a third axisymmetric mode m = 0 is identified, corresponding to a streamwise pulsation of the reverse flow region. Based on the analysis of the spatial eigen-functions and frequency spectra of the time-coefficients, it is concluded that the anti-symmetric mode m = 1 is associated with the backflow instability in the very-low frequency range StD = 10−4/10−3 close to separation, whereas more downstream it reflects the fluctuations related to the shear layer development.

. The three-dimensional flow organization past a micro-ramp in a supersonic boundary layer. Physics of Fluids, 24(5), 055105. doi: 10.1063/1.4711372 This is the accepted version of the paper.This version of the publication may differ from the final published version. The three-dimensional instantaneous flow organization in the near wake of a micro-ramp interacting with a Mach 2.0 supersonic turbulent boundary layer is studied using Permanent