PIV And LDV Measurements Behind ABackward Facing Step

Abstract

This experimental work is the result of a collaboration between the Mechanical Engineering Department of Cagliari, Italy, and the von Karman Institute for Fluid Dynamics, Brussels. Here the flow field downstream of a backward facing step in a low speed wind tunnel was experimentally studied by means of a PIV system, and series of images of the instantaneous flow were acquired and averaged to obtain the mean flow field and turbulence information. The same flow region in similarity conditions was experimentally investigated by an optic fibre LDV system, in a similar test facility in Cagliari university. The averaged flow field is characterised by a principal recirculation region downstream of the step, together with a secondary vortex located between the main bubble and the step, while the instantaneous flow field is characterised by a very complicated system of vortices, and a shear layer develops from the backward facing step corner and reaches the bottom wall at the reattachment length. The comparison of LDV and PIV experiments results is quite good, especially for what the horizontal mean velocity component is concerned. The test performed showed that PIV and LDV information should be integrated to give both a temporal and a spatial information for the whole measurement field, which is fundamental for a correct evaluation of the complex atmospheric flows, and allows the analysis of the time evolution of the main flow patterns. Air Pollution VIII, C.A. Brebbia, H. Power & J.W.S Longhurst (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-822-8