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Development of a hybrid PIV-PTV super resolution method - Application to separated flows

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VKI PHDT 2004-02, Development of a hybrid PIV-PTV super resolution method - Application to separated flows, ISBN 2-930389-10-9

Development of a hybrid PIV-PTV super resolution method - Applic

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Development of a hybrid PIV-PTV super resolution method - Application to separated flows
By Adel Stitou, published in 2003, ISBN 2-930389-10-9

PhD Thesis from the von Karman Institute/ Université Libre de Bruxelles, Belgium, November 7, 2003

Abstract

Particle Image Velocimetry (PIV) constitutes a powerful technique to perform 2D quantitative measurements for a broad range of flow conditions.  Even though important improvements allow achieving high accuracy and robust performances, nevertheless, one of the major drawbacks of this technique is its limited spatial resolution. Alternatively, the Particle Tracking Velocimetry technique (PTV) tries to follow the individual displacement of the tracers, achieving theoretically the best spatial resolution, but its application is not straightforward, especially in highly seeded flows.

The present works constitutes the study and the development of an integrated PIV-PTV method, which combines the advantages of the two approaches. The technique is a hybrid algorithm that starts by statistical tracer’s displacement evaluation (PIV analysis) and further refines the measurement resolution with the tracking of individual particles (PTV). This concept was introduced by Keane et al. (Measurement Science and Technology, 1995) and referred as the Super Resolution analysis. Other authors have also discussed this subject, focusing on the way to perform the correct association between particles images but this strategy is still not generalized. The present work is addressing the accuracy issue of such methods. The improvement in the determination of particle displacement respect to the classical methods is discussed.  The usual method consists to estimate the position of every tracer in each frame. Another approach that correlates individual particles images is developed and it is referred as the IPC method. It aims at improving the efficiency compared to the classical approach. The accuracy and the spatial response are validated on synthetic images of reference flow fields. The robustness of the method is considered by examining several validation criterions for the association of the tracer’s images. The identification process of identification of the particle image is also considered. After reviewing the available literature, it comes out that no method is universal. A method based on the search of local maximum of intensity in the images and the determination of the background noise of the recordings is investigated in order to be applied to study monophasic flows.

Developments were also made to apply such method to simultaneously measure concentration and velocity, which is a challenging task in many applications: droplet in combustion, the dispersion of pollutant gas…. The advantage is that it proposes an alternative method, that is an extension of classical particle imaging methods, and that it doesn’t require a specific set-up and need less equipment than others techniques such PIV-LIF. The main idea consists to use all the information available on the image to measure the concentration. Image processing provides an instantaneous velocity field for each couple of images and analyzing the spatial distribution of the particle images gives simultaneously a concentration field. A method based on particle counting is investigated and characterized. The effect of the limit in the performance of the extraction algorithm is then modelized, explaining one of the sources of the measurement scatter. The error made on the localization is also identified as another source of scatter. On another hand, systematic tests on synthetic images showed that the tool has a non-linear response. Good agreement is obtained with experiments held in a low speed wind tunnel. It comes out that the mean measurements present self-similar profile. Hence, a correction of the bias is possible if experimental parameters such as the particle image diameter are determined. In a second part, an application of such technique to concentration measurement is also presented. The near field of a seeded jet flow is measured.

The implementation of the investigated methods to real applications is also considered.  The purpose is to understand the behaviour and the reactions of the developed tools in such conditions more than the physical investigation of the flow filed.  Turbulent flows and separated ones are of practical interest. The phenomenon occurring in industrial flow field can be investigated by isolating the dominating features or studying simplified configurations. The flow over a backward facing step (BFS) is a generic case of separated flows occurring in combustion as well in applications in aerodynamics. PIV and SRPIV are compared on instantaneous measurement and averaged ones. An assessment on a 2-dimensionnal flow with isotropic fluctuations is also presented. The ability of the technique to characterise the vortices of a flow field is investigated on synthetic images as well on real recordings. The vortex shedding of the flow over a BFS is analysed. The first international PIV challenge was organised by PIVnet and the Japanese Society of Visualisation (VSJ). The results from this extensive comparison are reported.

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Manufacturer von Karman Institute for Fluid Dynamics

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