Dynamic Stall Control: A Review

The control of dynamic stall associated with wind turbine and helicopter blades, fixed-wings, flapping wings, as well as alternating flow separation and attachment produced by cyclical control inputs, is reviewed. Different airfoil and wing stall types are described, control metrics are identified, and the generality of harmonic pitch oscillations is clarified. A major emphasis is placed on nominally incompressible flows, and includes dynamic stall management, passive control devices, steady and unsteady active control techniques, scheduled control, iterative learning control, closed-loop control, flapping-wing flight, and computational predictions. Large control authority is attained by suppressing and inducing dynamic stall by means of low-frequency leading-edge slot blowing on thick airfoils, appropriate for horizontal axis wind turbine blade load control. High-frequency unsteady dynamic stall control is effective because its resultant stall-controlling leading-edge vortices are generated at more than an order-of-magnitude faster than the dynamic stall vortices. Alternating flow separation and attachment can be exploited to achieve secondary objectives, like the control of trailing vortices and enhanced turbulent mixing. Simple closed-loop control is achieved by adaptation of stall warning methods, and a feedforward/feedback control architecture is shown to be expedient and practical for gust alleviation on small vehicles. On flapping wing flyers, the roles of dynamic stall and its control are different for propulsive flight and hovering flight. For propulsive flight, active flow control can potentially be used to increase propulsive efficiency. For hovering, a relationship appears to exist between Strouhal numbers associated with insect flight, leading-edge forcing on flat-plate airfoils and low aspect-ratio flat wings, and vortex shedding. Accurate numerical predictions of dynamic stall control, namely, code validations against experimental data, have not been adequately performed, and dedicated experimental test cases for this purpose are proposed. Under subsonic compressible conditions, dynamic stall is driven by shock-induced separation, where passive leading-edge modifications with vortex generators, discrete wall-normal jet or micro-jet blowing, and dielectric barrier discharge actuation all show significant control potential.