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

Birch, DM and Morrison, JF (2011) Similarity of the streamwise velocity component in very-rough-wall channel flow JOURNAL OF FLUID MECHANICS, 668. 174 - 201. ISSN 0022-1120

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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 structure 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.

Item Type: Article
Additional Information: Copyright 2011 Cambridge University Press.
Uncontrolled Keywords: Science & Technology, Technology, Physical Sciences, Mechanics, Physics, Fluids & Plasmas, Physics, pipe flow boundary layer, turbulent boundary layers, TURBULENT-BOUNDARY-LAYERS, DEVELOPED PIPE-FLOW, SURFACE-ROUGHNESS, SMOOTH, HYPOTHESIS, FRICTION
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Divisions: Faculty of Engineering and Physical Sciences > Mechanical Engineering Sciences
Depositing User: Symplectic Elements
Date Deposited: 20 Dec 2011 13:38
Last Modified: 23 Sep 2013 18:57
URI: http://epubs.surrey.ac.uk/id/eprint/69213

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