Influence on wind shear and turbulence in flow over obstacles

A wind tunnel study of speed-up effects above the very crest of a sharp-edged escarpment and a hill peak in a simulated atmospheric boundary layer has been carried out. It was desired to do a part-deep simulation of an atmospheric boundary that could be found above sea or coastal area exposed to the open sea. Because of the limited work section length was it used a modified roughness, barrier and mixing-device developed by Counihan to accelerate the boundary layer growth. The mean velocity, integral length scales, power spectrum and turbulence intensity in the simulated boundary layer were compared with full scale empirical data. It showed good agreement except for the turbulence intensity which was too low. Speed-up effects for the mean horizontal velocity and the longitudinal turbulence intensity above the very crest of an escarpment and a hill peak were investigated in the simulated atmospheric boundary layer. From the results it was observed that the speed-up effect gave a decrease in the turbulence intensity and a more uniform profile with height. A considerably increase of the horizontal mean velocity in the lowest part of the flow was also observed. Scaled-up data from the wind tunnel experiment were compared with estimations from the Norwegian standard and potential flow with varying degree of agreement