Active control of dynamic stall: A review

The active control of dynamic stall on helicopter blades, wind turbine blades, fixed-wing aircraft, and flapping-wing flyers is reviewed. Different dynamic stall problems are identified, stall types are defined, control metrics are established, and the parameter space is delineated. For helicopter and wind turbine blades, as well as small unmanned aerial vehicles, the primary objective is generally to eliminate or mitigate the effects of dynamic stall. However, in certain fixed-wing, flapping-wing, and energy-generation applications, the goal is to utilize or exploit dynamic stall effects. A major emphasis is placed on nominally incompressible flows, including dynamic stall management, passive control devices, steady and unsteady active control techniques, scheduled control, iterative learning control, closed-loop control, and the role of dynamic stall control in flapping-wing flight. Significant control authority can be achieved by either suppressing or inducing dynamic stall through low-frequency leading-edge slot blowing on thick airfoils, which is particularly suitable for wind turbine blade load control. High-frequency unsteady dynamic stall control—typically achieved using fluidic, combustion-based, or plasma-based devices—is effective because the resulting stall-controlling leading-edge vortices are generated more than an order of magnitude faster than dynamic stall vortices. Simple reactive control can be achieved through the adaptation of stall warning methods, and a feedforward/feedback control architecture is shown to be both practical and effective for gust alleviation in small vehicles. In flapping-wing flyers, the role of dynamic stall control differs between propulsive and hovering flight. In propulsive flight, active flow control may enhance propulsive efficiency. In hovering flight, a relationship exists between Strouhal numbers associated with insect flight, leading-edge forcing on flat-plate airfoils and low-aspect-ratio wings, and vortex shedding. Under subsonic compressible conditions, dynamic stall is driven by shock-induced separation. In such cases, leading-edge modifications—including vortex generators, discrete wall-normal jets or micro-jet blowing, and dielectric barrier discharge actuation—demonstrate significant control potential. Computational methods play a vital role in advancing the application and evaluation of active control strategies by enabling system-level studies that assess aerodynamic, aeromechanical, energetic, and structural benefits.