Rough wall turbulent boundary layers in shallow open channel flow

An experimental study was undertaken to investigate the effects of roughnes s on the structure of turbulent boundary layers in open channels. The study was carried out using a laser Doppler anemometer in shallow flows for thre e different types of rough surface, as well as a hydraulically smooth surfa ce. The flow Reynolds number based on the boundary layer momentum thickness ranged from 1400 to 4000. The boundary layer thickness was comparable with the depth of flow and the turbulence intensity in the channel flow varied from 2 to 4 percent. The defect profile was correlated using an approach wh ich allowed both the skin friction anti wake strength to vary. The wake par ameter was observed to vary significantly with the type of surface roughnes s in contradiction to the "wall similarity'' hypothesis. Wall roughness als o led to higher turbulence levels in the outer region of the boundary layer . The profound effect of surface roughness on the outer region as well as t he effect of channel turbulence on the main flow indicates a strong interac tion, which must be accounted for in turbulence models. [S0098-2202(00)0080 3-8].