Turbulent Shear Flows Over a Step Change in Surface Roughness

Abstract

A large body of data now exists on the response of turbulent shear flows to sudden or step changes in surface roughness. Authors have used a variety of methods to reduce and present the data; thus a consistent description of these flows has not yet been presented. This paper presents all available data reduced in a uniform way. As there are extremely few Reynolds stress measurements within this large body of data, the analyses presented here are necessarily based on mean velocity profiles. It is shown that the growth rate of the new internal layer for all types of flow both with and without pressure gradient can be described in terms of a single length scale associated with the new wall condition. It is also shown that all mean velocity profiles after a step change in roughness display semi-logarithmic distributions. However, in the region immediately downstream of a step the constant of proportionality (the von Karman constant) has values different from the usual equilibrium value of 0.41. The differences appear to be large with values for the constant ranging between about 0.2 to 0.8.