Relaxation of asymmetric 2-D channel flow into symmetric state caused by a step change in wall roughness (1st report, Characteristics of mean velocity field)

Abstract

Experiments have been conducted in the 7m channel where one of the walls in the upstream half was roughened and all the remaining parts were left smooth, providing a fully developed asymmetric flow at the midsection and a fully developed symmetric flow at the channel exit. Extensive measurement of the mean velocity, Reynolds normal and shear stresses made downstream of the step change in wall roughness indicate that the readjustment to a symmetric flow requires a streamwise distance of 50 times the channel height 2h; whereas the evaluation of the momentum equation shows that for x/h6gt;20 the pressure gradient balances practically with the wall shear stress. It is found also that the adjustment process is quite different between the rough to smooth and the smooth to smooth sides. Non-monotonical adjustment is essential only in the rough to smooth side.