Near-surface flows over random (urban) roughness

Cheng, H. and Castro, I.P. (2000) Near-surface flows over random (urban) roughness. UWERN Annual Conference 2000, Leeds, UK. 18 - 20 Dec 2000.

Abstract

Comprehensive sets of vertical mean velocity and turbulence quantities have been obtained at 64 spatial locations within a unit (plan) area of random roughness. The roughness consisted of rectangular blocks arranged in a regular staggered pattern covering 25% of the wall surface. Each element had the same square cross-section in the horizontal but the heights were randomised. The dispersive stress arising in the roughness sublayer from the spatial inhomogeneity in the mean flow profiles was calculated and is shown to be negligible compared with the usual Reynolds stresses. The data are also compared with corresponding data for a uniform-height roughness of the same pattern and total volume. It is shown that the inertial sublayer, although having an upper limit which is almost identical to that of the uniform surface, is much thinner (i.e. the roughness sublayer was much thicker for the random surface than for the uniform surface). We conclude that the inertial sublayer may not exist at all in some urban areas in which the dominant features are high, irregular and heterogeneous roughness elements.

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