Thick Airfoil Deep Dynamic Stall

Abstract

Recent interest in vertical axis wind turbines (VAWTs) has surged due to their appearance in the built environment and their potential for deep water offshore applications. Their well-known advantages include insensitivity to the wind direction, proximity of the generator to the ground and relatively low noise levels. Furthermore, blade profile uniformity along the span can significantly reduce manufacturing costs, particularly for large scale utility machines. A drawback of VAWTs is the tendency of their blades to stall dynamically when they are pitched beyond their static stall angle. Dynamic stall is characterized by a strong vortex, the dynamic stall vortex (DSV), which forms near the leading-edge. When the DSV is convected downstream, a rapid drop in lift and severe pitching moment fluctuations result [1]. On VAWTs operating at low tip-speed ratios, dynamic stall occurs periodically throughout the rotation of the blades [2]. This situation is unique in that the blades experience pitch oscillations about zero angle of attack and the flow separates alternately on both sides. This results in a number of undesired effects: On the one hand, the full separation on the suction surface produces a sharp drop in cl and thus rotor torque. On the other hand, the unsteady aerodynamic loads cause fatigue damage to the generator and drive train [3].