Concept and Operation of a Wind Turbine Driven by Dynamic Stall

Dynamic stall is exploited to produce positive torque on a large-chord/radius-ratio (c/R) vertical-axis wind turbine. An elementary kinematic analysis shows that the blades experience large variations in angle of attack that produce deep dynamic stall, combined with large relative dynamic pressure variations, that are phase shifted by approximately 90 deg. The phase shift allows dynamic lift-overshoot effects associated with the formation of a dynamic stall vortex to drive the turbine when the relative dynamic pressure is high, whereas lift stall associated with shedding of the vortex occurs when the relative dynamic pressure is low. Blades also experience variable virtual camber (or virtual morphing) effects and favorable chordwise pressure gradients, where the former produces a virtual leading-edge droop that reduces the leading-edge angle of attack. Open jet wind-tunnel tests of a two-bladed model turbine revealed maximum power coefficients of 16% that were almost independent of c/R. Flow visualization confirmed that dynamic stall, with the associated dynamic stall vortex, is the mechanism that drives the turbine to maximum power. Until reliable design tools are developed, either empiricism combined with dimensional analysis or high-fidelity computational fluid dynamics are proposed for optimization and analysis.