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Turbulent separation upstream of a forward-facing step

Published online by Cambridge University Press:  29 April 2013

D. S. Pearson ,
P. J. Goulart  and
B. Ganapathisubramani
D. S. Pearson*
Affiliation:
Department of Aeronautics, Imperial College London, London SW7 2AZ, UK
P. J. Goulart
Affiliation:
Department of Aeronautics, Imperial College London, London SW7 2AZ, UK Automatic Control Laboratory, ETH Zürich, 8092 Zürich, Switzerland
B. Ganapathisubramani
Affiliation:
Department of Aeronautics, Imperial College London, London SW7 2AZ, UK Aerodynamics and Flight Mechanics Group, University of Southampton, Southampton SO17 1BJ, UK
*
†Email address for correspondence: [email protected]
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Abstract

The turbulent flow over a forward-facing step is studied using two-dimensional time-resolved particle image velocimetry. The structure and behaviour of the separation region in front of the step is investigated using conditional averages based on the area of reverse flow present. The relation between the position of the upstream separation and the two-dimensional shape of the separation region is presented. It is shown that when of ‘closed’ form, the separation region can become unstable resulting in the ejection of fluid over the corner of the step. The separation region is shown to grow simultaneously in both the wall-normal and streamwise directions, to a point where the maximum extent of the upstream position of separation is limited by the accompanying transfer of mass over the step corner. The conditional averages are traced backwards in time to identify the average behaviour of the boundary-layer displacement thickness leading up to such events. It is shown that these ejections are preceded by the convection of low-velocity regions from upstream, resulting in a three-dimensional interaction within the separation region. The size of the low-velocity regions, and the time scale at which the separation region fluctuates, is shown to be consistent with the large boundary layer structures observed in the literature. Instances of a highly suppressed separation region are accompanied by a steady increase in velocity in the upstream boundary layer.

JFM classification

Boundary Layers: Boundary layer separation Turbulent Flows: Turbulent boundary layers
Type
Papers
Information
Journal of Fluid Mechanics , Volume 724 , 10 June 2013 , pp. 284 - 304
Copyright
©2013 Cambridge University Press 

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