Effects of roughness on the intermittent maintenance of Reynolds shear stress in pipe flow

Abstract

The presence of a rough wall does not fundamentally alter the intermittent maintenance of Reynolds shear stress as found for turbulent flows over a smooth wall, be it boundary layers or pipe flows. Ejections, and to a lesser extent sweeps, are still the dominant events contributing to the shear stress. The mean period of occurrence of the violent ejections (those whose amplitude exceeds a characteristic threshold) is, however, larger for the rough pipe than it is for the smooth pipe at the same Reynolds number. A scaling based on specific parameters of the largest scale motions is proposed. The mean length scale of the violent ejections is larger than those encountered for a smooth pipe, but still seems to be of the order of the integral length scale of the transverse component of the velocity fluctuation.