The propagation of modified flow downstream of a change in roughness

Abstract

Abstract Using published atmospheric and wind tunnel data from a variety of sources, it is established that the rate of propagation of modified flow downstream of a change in surface roughness is determined essentially by the rougher surface. For the data examined neither the ratio of the upstream to downstream roughness lengths nor the ratio of boundary layer height to roughness element height is of itself a significant parameter. Based on an empirical analogy with Kazanskii and Monin's plume propagation concept, a simple method is proposed for predicting the depth of modified flow. The predictions include an origin correction and are shown to agree satisfactorily with measurements in neutrally stratified boundary layers.