Experimental study on roughness effects on turbulent boundary layer flow over a two-dimensional steep hill

Wind tunnel experiments were carried out to study the effects of surface roughness on the turbulent boundary layer flow over a two-dimensional steep hill, accompanied by a relatively steady and large separation, sometimes called a separation bubble. Rough surface conditions were modeled by placing small cubes on the hill surface with an arrangement whose roughness density equaled 4.1%. Vertical profiles of the turbulence statistics over the hill were investigated, and compared with those for an oncoming turbulent boundary layer over a flat surface covered by cubes with the same arrangement. Furthermore, measurements of the turbulent boundary layer flow over a smooth hill of the same shape were taken in order to identify the surface roughness effects. The speed-up ratio above the crest and the turbulence statistics in the wake were focused on. Flow structures in the separation bubble were investigated by measuring the velocity with a split-fiber probe. Measurement with fine resolution in the streamwise direction for near-ground flows was carried out to detect the reattachment point. The results show that the roughness blocks both on the hill surface and on the upstream ground affect the speed-up ratio over the hill. The speed-up ratio above the crest of a rough hill is larger than that of a smooth hill. The separation bubble of a rough hill extends further downstream, resulting in a larger reattachment length than a smooth hill. The location with the maximum turbulence intensity in the downstream profile also moves further downstream for a rough hill. Profiles of reverse-flow intermittency that give a description of the dynamic behavior of the separation bubble are presented. In addition, variations of power spectrum of longitudinal velocity fluctuation downstream of the hill are shown for both surface conditions.