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Equilibrium and relaxation in turbulent wakes

Published online by Cambridge University Press:  29 March 2006

R. Narasimha  and
A. Prabhu
R. Narasimha
Affiliation:
Department of Aeronautical Engineering, Indian Institute of Science, Bangalore Present address: Department of Mathematics, University of Strathclyde, Glasgow.
A. Prabhu
Affiliation:
Department of Aeronautical Engineering, Indian Institute of Science, Bangalore
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Abstract

In order to study the memory of the larger eddies in turbulent shear flow, experiments have been conducted on plane turbulent wakes undergoing transition from an initial (carefully prepared) equilibrium state to a different final one, as a result of a nearly impulsive pressure gradient. It is shown that under the conditions of the experiments the equations of motion possess self-preserving solutions in the sense of Townsend (1956), but the observed behaviour of the wake is appreciably different when the pressure gradient is not very small, as the flow goes through a slow relaxation process before reaching final equilibrium. Measurements of the Reynolds stresse show that the approach to a new equilibrium state is exponential, with a relaxation length of the order of 103 momentum thicknesses. It is suggested that a flow satisfying the conditions required by a self-preservation analysis will exhibit equilibrium only if the relaxation length is small compared with a characteristic streamwise length scale of the flow.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 54 , Issue 1 , 11 July 1972 , pp. 1 - 17
Copyright
© 1972 Cambridge University Press

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