Thin airfoil load control during post-stall and large pitch angles using leading-edge trips

Abstract

Dynamic stall significantly impacts the performance of wind turbines. This article describes the effects of a leading-edge trip wire passive flow-control device, applied to a NACA 0012 airfoil undergoing dynamic stall to high angles of attack. The Reynolds number is 20,000. Surface pressure measurements were conducted to determine unsteady lift. Ceasing high rotation-rate airfoil motion, an increase in flow at the leading edge occurs, due to the momentum imparted on the fluid by the airfoil during rotation. The increased flow at the leading edge leads to a severe adverse pressure gradient, resulting in immediate leading-edge flow separation. For high rotation rate conditions, the leading-edge trip wire displayed limited abilities in controlling boundary layer separation, where decreases in the maximum lift and post-stall lift fluctuation were observed. Trip wire benefits are limited by the formation of vortex shedding at the leading edge, reminiscent to bluff body separation, which significantly dwarf vortex structures generated by the trip wire. Under post-stall flow conditions, the trip wire has little impact on the large-scale von-Karman vortex development, where load fluctuations occurred. The application of a trip wire resulted in reducing the maximum lift, but variations in its diameter did not alter the stall angle of attack.