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Similarity of the streamwise velocity component in very-rough-wall channel flow

Published online by Cambridge University Press:  03 December 2010

DAVID M. BIRCH*
Affiliation:
Department of Aeronautics, Imperial College, London, SW7 2AZ, UK
JONATHAN F. MORRISON
Affiliation:
Department of Aeronautics, Imperial College, London, SW7 2AZ, UK
*
Present address: Faculty of Engineering and Physical Sciences, University of Surrey, Guildford, GU2 7XH, UK. Email address for correspondence: [email protected]

Abstract

The streamwise velocity component is studied in fully developed turbulent channel flow for two very rough surfaces and a smooth surface at comparable Reynolds numbers. One rough surface comprises sparse and isotropic grit with a highly non-Gaussian distribution. The other is a uniform mesh consisting of twisted rectangular elements which form a diamond pattern. The mean roughness heights (±) the standard deviation) are, respectively, about 76(±42) and 145(±150) wall units. The flow is shown to be two-dimensional and fully developed up to the fourth-order moment of velocity. The mean velocity profile over the grit surface exhibits self-similarity (in the form of a logarithmic law) within the limited range of 0.04≤y/h≤0.06, but the profile over the mesh surface does not, even though the mean velocity deficit and higher moments (up to the fourth order) all exhibit outer scaling over both surfaces. The distinction between self-similarity and outer similarity is clarified and the importance of the former is explained. The wake strength is shown to increase slightly over the grit surface but decrease over the mesh surface. The latter result is contrary to recent measurements in rough-wall boundary layers. Single- and two-point velocity correlations reveal the presence of large-scale streamwise structures with circulation in the plane orthogonal to the mean velocity. Spanwise correlation length scales are significantly larger than corresponding ones for both internal and external smooth-wall flows.

Type
Papers
Copyright
Copyright © Cambridge University Press 2010

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