A wind tunnel investigation of roughness properties over non-homogeneous rough surfaces

Abstract

Laboratory experiments were conducted to study the properties of roughness parameters over surfaces with small-scale heterogeneity, using a wind tunnel fitted with an underlying weighting lysimeter packed with uniform fine sand. Either small columns or sand ridges were distributed homogeneously or non-homogeneously on the surface as roughness obstacles. The roughness lengths for momentum (z 0m ), heat (z 0h ) and vapour (z 0v ) were estimated for each surface by fitting the flux profile equations to the mean profiles observed. The flow over a non-homogeneous rough surface was observed to become homogeneous above a certain height, so that 'effective' parameters can be found to describe the average surface fluxes from the non-homogeneous surface. The roughness properties of the non-homogeneous surfaces are, in general, similar to those of homogeneous surfaces. The 'effective' parameter values, however, suggest that the surface non-homogeneity which is a perturbation part of the obstacle distribution has a significant effect on roughness properties, and that the 'effective' roughness of a non-homogeneous surface is always considerably greater than the actual average roughness. The values of z 0m for the non-homogeneous surfaces are significantly greater than scalar roughness lengths (z 0h and z 0v ). Though homogeneous rough surfaces also have z 0m /z 0m and z 0m /z 0v ) ratios of greater than unity, these ratio values for non-homogeneous surfaces are found to be almost one order of magnitude greater than those for homogeneous surfaces, even when the homogeneous and the non-homogeneous surfaces have the same average roughness. © 1998 John Wiley & Sons, Ltd.