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Shear Stress Behaviour on Complex Rough Surfaces

J.A. Gillies and W.G. Nickling


A newly developed drag plate was used in a wind tunnel to examine the behaviour of surface shear stress generated by fully developed turbulent boundary flow. Three different complex rough gravel surface configurations were tested. Roughness was measured with a laser profiling system. The measured surface shear stress (s) obtained with a newly designed drag plate was found to scale in proportion to the shear stress (0) determined from wind profile measurements using Pitot tubes, as a function of increasing surface roughness. In addition, increased surface roughness caused increased variability in s as quantified by the standard deviation which scaled as a function of boundary Reynolds number (Reb). It was also noted that the distribution of measured surface shearing stress changed with increasing roughness. The s data showed different responses for each of the surface roughness configurations to similar ranges of wind profile measured shear stress. As roughness increased, s increased, due to the creation of more energetic turbulent activity, increased wall bursting, and increased bluff body effects caused by the elements protruding higher into the boundary layer flow.