Experimental Study of Boundary Layer flow over a Zero Pressure Gradient Convergent Riblet Plate

Reducing the friction force in turbulence flows is an important research topic in fluid mechanics. Shark skin has less drag than smooth skins, therefore shark skin surfaces have been widely used. However, some turbulence mechanisms are not completely understood and the drag reduction mechanism of real shark skin has not been thoroughly understood. Many researchers have experimentally studied bio-inspired riblet surfaces to explain some specified phenomena for the sharkskin drag reduction effect. The objectives of this study were to investigate the effect of the converging-diverging riblet-type surface roughness on a zero pressure gradient turbulent boundary layers, to investigate the effect of the surface roughness on the large-scale features and to confirm that converging-diverging riblet generate large-scale counter-rotating vortices. The wind tunnel experiments involved laminar and turbulent flow at various Re were performed. Hot-wire anemometry was used to investigate near-wall flow structure over smooth and riblet surfaces. The results showed that the convergent riblet plate significantly impacted the boundary layer properties. Furthermore, the turbulent skin friction drag over the convergent riblet was higher than on a smooth surface. The results agree well with previous studies.