Sebastien Coudert

Back-projection algorithm with misalignment corrections for 2D3C stereoscopic PIV

In stereoscopic PIV mapping and warping algorithms are used to back-project the image information on the object plane. The mapping algorithm back-projects each pixel, whereas the warping algorithm back-projects the 2D displacement vectors. In this paper, both algorithms and the misalignment of the laser sheet regarding the object plane are considered. Then, a new algorithm using both mapping and warping algorithms is described for a 2D3C angular stereoscopic PIV set-up with standard lenses and the so-called Scheimpflug condition. The accuracy of the new algorithm, that takes into account laser sheet misalignment, is compared to the previous algorithm on the same sets of images. These sets are recorded from a paper pattern mounted on a 3D translation and rotation stage and from a turbulent pipe flow. The tests show that the accuracy benefit is one order of magnitude on these sets. The accuracy gain depends on the laser sheet misalignment magnitude, which can be measured using the mapping algorithm.

Two-point correlations in high Reynolds number flat plate turbulent boundary layers

Two-point correlations of turbulent boundary layer are presented for Reθ of 9800 and 19,100. The results are based on wind tunnel measurements performed in the 30 cm thick boundary layer of the 21.6 m long LML (Laboratoire de Mécanique de Lille) boundary layer research facility. Simultaneous hot-wire probe measurements of the entire boundary layer at 143 different points on an array are used for computation of the two-point correlations. The two-point correlations in the streamwise–spanwise plane at 11 different wall-normal positions covering one boundary layer thickness in the spanwise direction show that the maximal extension of the correlations in the streamwise direction is bounded within ±3.5δ for both of the Reynolds number tested. The shapes of the positive correlations in the streamwise–spanwise plane at different wall-normal positions are similar throughout the boundary layer from nearly the freestream to the wall. The correlations in the streamwise–wall-normal plane for 11 different wall-normal reference positions also show that the correlations in some cases cover the entire boundary layer. The streamwise extent of the correlations in the streamwise–wall-normal plane is about 7–8 boundary layer thicknesses. Two-point correlation maps for the streamwise–wall-normal plane reveal the existence of non-zero correlations between even the intermittent region and near-wall region.

Two-step cross correlation–based algorithm for motion estimation applied to fertilizer granules’ motion during centrifugal spreading

Imaging systems are progressing in both accuracy and ro- bustness, and their use in precision agriculture is increasing accordingly. One application of imaging systems is to understand and control the cen- trifugal fertilizing spreading process. Predicting the spreading pattern on the ground relies on an estimation of the trajectories and velocities of ejected granules. The algorithms proposed to date have shown low ac- curacy, with an error rate of a few pixels. But a more accurate estimation of the motion of the granules can be achieved. Our new two-step cross- correlation-based algorithm is based on the technique used in particle image velocimetry (PIV), which has yielded highly accurate results in the field of fluid mechanics. In order to characterize and evaluate our new algorithm, we develop a simulator for fertilizer granule images that ob- tained a high correlation with the real fertilizer images. The results of our tests show a deviation of <0.2 pixels for 90% of estimated velocities. This subpixel accuracy allows for use of a smaller camera sensor, which decreases the acquisition and processing time and also lowers the cost. These advantages make it more feasible to install this system on existing centrifugal spreaders for real-time control and adjustment. C 2011 Society of

Stereoscopic particle image velocimetry using telecentric lenses

Telecentric imaging is presented in the general frame of digital stereoscopic particle image velocimetry (DSPIV). A 2D?3D mapping approach is combined with telecentric imaging in order to obtain a user-friendly DSPIV technique. This technique is tested on translation tests then in a round free jet.

The WALLTURB Joined Experiment to Assess the Large Scale Structures in a High Reynolds Number Turbulent Boundary Layer

Experiments, involving the joint effort of three European teams and aiming at using the state-of-the-art techniques to study the dynamics of the high Reynolds turbulent boundary layer, have been performed in June 2006 in the LML large wind tunnel. A set of four stereoscopic PIV systems and a rake of 143 hot wires were used to provide synchronised measurements. This paper summarises these experiments, presents sample results of both PIV and hot-wire rake data and illustrates the complementarity of such a coupled approach that combines the advantages of each technique.

Unsteady characteristics of near-wall turbulence using high repetition stereoscopic particle image velocimetry (PIV)

This study is part of a project that is aimed at building dynamic boundary conditions near a solid wall, in order to reduce the large eddy simulation spatial resolution that is necessary in this region. The objective is to build a low-order dynamical system in a plane parallel to the wall, which will mimic the unsteady behaviour of turbulence. This dynamical system should be derived from a POD decomposition of the velocity field. The POD decomposition is to be applied on an experimental database of time-resolved velocity fields. In order to obtain the experimental database, a specific experiment of high-speed stereoscopic particle image velocimetry (PIV) has been performed. This experiment was carried out in the turbulent boundary layer of the LML wind tunnel. The plane under study was parallel to the wall located at 100 wall units. This database is validated via comparison with hot-wire anemometry (HWA). Despite some peak locking observed on the streamwise velocity component, the PDF and the power spectra are in very good agreement with the HWA results. The two-point spatial correlations are also in good agreement with the results from the literature. As the flow is time-resolved, space-time correlations are also computed. The convection of the flow structure is observed to be the most important effect at this wall distance. The next step is to compute the dynamical system and to couple it to a large eddy simulation.

Vortex Generator Induced Flow in a High Re Boundary Layer

Stereoscopic Particle Image Velocimetry measurements have been conducted in cross-planes behind three different geometries of Vortex Generators (VGs) in a high Reynolds number boundary layer. The VGs have been mounted in a cascade producing counter-rotating vortices and the downstream flow development was examined. Three VG geometries were investigated: rectangular, triangular and cambered. The various VG geometries tested are seen to produce different impacts on the boundary layer flow. Helical symmetry of the generated vortices is confirmed for all investigated VG geometries in this high Reynolds number boundary layer. From the parameters resulting from this analysis, it is observed at the most upstream measurement position that the rectangular and triangular VGs produce vortices of similar size, strength and velocity induction whilst the cambered VGs produce smaller and weaker vortices. Studying the downstream development in the ensemble and spanwise averaged measurements, it is observed that the impact from the rectangular and triangular VGs differs. For the rectangular VGs, self-similarity in the streamwise component was confirmed.

Digital microscopic holography for micrometer particles in air

The development of a specific holographic setup designed for providing three-dimensional imaging of micrometer particles in a very small volume inside a large air-flow facility is described. Study of a 1.5 mm(3) volume is made possible with the use of a microscope objective for magnification of the object field. Particles that are too small to be detected with a standard in-line hologram (about 1 μm in diameter or less) are illuminated laterally, and the 90° scattered field is magnified and recombined with a reference wave for in-line recording. A calibration procedure relates reconstruction space coordinates to measurement volume coordinates. Analysis of the results shows that particle images reconstruct with very good axial accuracy. Preliminary tests show that this approach should allow measurements of fluid velocity very close to the wall in a wind-tunnel flow.

Calibration of the WALLTURB Experiment Hot Wire Rake with Help of PIV

In the WALLTURB LML experiment presented in Delville et al. (Stanislas, M., Jimenez, J., Marusic, I. (eds.) Progress in Wall Turbulence: Understanding and Modeling. Proceedings of the WALLTURB International Workshop Held in Lille, France, April 21–23, 2009. ERCOFTAC Series, vol. 14. Springer, Dordrecht, 2011), the calibration of the hot wire rake had to be performed on site, due to the large number of wires on the rake (143). For that purpose, the position of the wires with respect to the wall was carefully measured using two complementary techniques. The blockage effect due to the rake was characterized using StereoPIV and modeled using a simple potential flow model. The results show that the blockage affects mostly the mean flow and not the turbulence itself. Finally, the calibration data were built using the StereoPIV data in a plane located 1 cm upstream of the wires plane. The mean velocity, the second and third moments were provided. The calibration was checked on the fourth moment of the fluctuating velocity.

Subpixel characterization of a PIV-CCD camera using a laser spot

We present a simple method for charge-coupled device (CCD; or CMOS) sensor characterization by using a subpixel laser spot. This method is used to measure the variations in sensitivity of the 2D sensor array systems equipped with a microlens array. The experimental results show that there is variation in the sensitivity for each position on the CCD of the camera, and the pixel optical center error with respect to the geometrical center is in the range of one-tenth that of a pixel. The disparity observed is attributed to the coherence of the laser light used that generates interference at the scale of the pixel. This may have significant consequences for coherent light imaging using CCD (or CMOS) such as particle image velocimetry.