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International audience; We review recent developments made between three departments at ONERA (DTIM, DAFE, DMAE) with regard to 3D imagery used in fluid mechanics experiments. We first discuss 3D PIV (Particle Image Velocimetry) and present original contributions on the modeling of the imaging process, the reconstruction of the 3D volume of particles and the estimation of the 3D motion field between two time instants. These three contributions emphasize the pointwise character of particles in PIV, in contrast with classical Tomo-PIV approaches, and have been shown to outperform state-of-the-art methods, yielding more accurate 3D velocity estimations. Then, we consider 3DBOS (Background Oriented Schlieren), which is aimed at estimating the instantaneous 3D density field of a moving fluid. We have recently proposed an original one-step numerical approach for 3DBOS. This method has been successfully used in several experiments conducted in various ONERA facilities, in particular in the F2 and S1MA wind tunnels. Finally, we draw some important perspectives, especially for the study of compressible flows by combining both techniques.; Nous présentons les résultats récents concernant la mesure volumique 3D par imagerie en mécanique des fluides expérimentale, issus de développements menés entre les trois départements DTIM, DAFE et DMAE de l’ONERA. Nous discutons d’abord de la mesure 3D de vitesse par imagerie de particules (3D PIV). Nos contributions originales concernent le modèle d’imagerie lui-même, la méthode de reconstruction 3D du volume de particules et l’estimation du champ de déplacements 3D. Leur principe est globalement d’exploiter le caractère ponctuel des particules et de chercher à recourir à des méthodes parcimonieuses (sparse). Ces approches nous ont permis d’atteindre ou même de dépasser l’état de l’art en termes de qualité d’estimation et d’efficacité algorithmique. Nous abordons ensuite la mesure 3D instantanée de la masse volumique par des méthodes de strioscopie BOS (Background Oriented Schlieren). Nous avons proposé récemment une structure originale d’estimation en une étape de la masse volumique à partir des mesures de déviation de la lumière traversant un fluide inhomogène. Cette méthode a été utilisée avec succès dans plusieurs installations expérimentales de l’ONERA, notamment dans les souffleries F2 et S1MA. Enfin, nous traçons quelques perspectives de ces développements en particulier pour la caractérisation expérimentale des fluides compressibles.

International audience; Backgroung Oriented Schlieren (BOS) is a Schlieren-like method that can be used as a 3D quantitative method for measuring density in 3D flows. This can be done providing that there are enough points of view, at least 10, and that an accurate calibration of the multi-camera bench is available. We present here a study of multi-camera calibration for 3DBOS. We use a simple printed flat plate with a dotted regular pattern as calibration body (CB). The main issues are the workload, because there are several hundred of images, and the blurring effect. The blur affects the images of the CB when it is positioned in the common part of the fields of views of the cameras. It can not be avoided because of illumination limitation. We present practical solutions and results for both issues. A GPU implementation allows to complete the calibration of a system with 12 cameras in less than 30mn. We also propose an uncertainty analysis using the jackknife method. It is shown that the uncertainty of the estimation of the horizontal field of view (HFoV) is 0.04°. Moreover, HFoV is nearly identical for all cameras and quasi equal to the theoretical value.

6th PSP Workshop, Seattle, Washington, available, unclassified, unlimited

Journée SFT « Thermographie Quantitative », janvier 1997, IFP, Paris, available, unclassified, unlimited

Flow dynamics research is involved in all branches of science and technology: civil and chemical engineering, the automotive industry, energy, etc. Flow dynamics are even more central to aeronautics technology because they provide air lift to planes (non-reacting flow dynamics) as well as the moving impetus through combustion (reacting flow dynamics). The great challenges of flow dynamics require the validation of physical models with relevant lengths ranges spanning a dozen of orders of magnitude, from a few Angstroms, at which atoms and molecules collide and bond (particularly in combustion), to a meter range (the size of a combustion chamber or a wing). Thanks to their non-invasive character, their spectrocopic and imaging possibilities, optical diagnostics are unsurpassed for addressing this enormous challenge., Journal Aerospace Lab, Issue 1, December 2009; ISSN: 2107-6596

We present recent developments in data processing for velocity field estimation and visualization originating from computer vision. We review the current paradigm of PIV data processing, based on window correlation, and the regularization or variational approach which is dominant in optical flow estimation. We propose a novel unifying framework via the optimization of a compound regularized criterion written in terms of a dense displacement (or velocity) field. The paper then focuses on algorithmic issues. A fast iterative window correlation method leading to a highly parallel algorithm termed FOLKI is described. Thanks to a GPU (Graphical Processing Unit) implementation, FOLKI reaches video rate for typical PIV data. Then we present more sophisticated solvers able to deal with the regularization term of the criterion, notably multigrid methods. In our view, these two components form the foundation of a video rate velocity field visualization and interpretation toolbox which, together with recent advances in experimental apparatus and numerical simulation, opens the way to a major development in experimental fluid science., Journal Aerospace Lab, Issue 1, December 2009; ISSN: 2107-6596

In this paper, the principles of two-component Particle Image Velocimetry (PIV) and stereoscopic PIV are first recalled. Recent improvements in the camera calibration procedure for stereoscopic PIV are highlighted. The advantages of the PIV technique are illustrated through different studies performed in Onera laboratories for non-reacting or reacting flows, and various issues raised by the nature of fluid, flow dynamics, spatial and temporal resolutions, as well as data utilization, are examined. Finally, recent advances in the technique, time-resolved PIV and tomographic PIV, as well as potential gains from novel processing and post-processing algorithms, are presented., Journal Aerospace Lab, Issue 1, December 2009; ISSN: 2107-6596