Transitory Control of Dynamic Stall on a Pitching Airfoil

Transitory attachment of the flow over a stalled, 2-D airfoil is investigated in wind tunnel experiments using pulsed actuation. Actuation is provided by a spanwise array of momentary, combustion-based actuator jets having a characteristic time scale O[1 ms] that is an order of magnitude shorter than the convective time scale of the flow. It is shown that a single actuation pulse results in transitory flow attachment that is manifested by rapid increase in the global circulation and aerodynamic forces and persists for about ten convective time scales before the flow becomes fully stalled again. Large-scale changes in vorticity accumulation that are associated with repetitive, burst-modulated actuation pulses are exploited for significant extension of the streamwise domain and duration of the attached flow with a corresponding increase in circulation. The effects of the transitory actuation are further amplified when the airfoil is mounted on a dynamic 2DOF (pitch and plunge) traversing mechanism and the actuation is tested with pitch oscillations beyond the stall limit. In this configuration, the actuation is nominally two-dimensional within a spanwise domain measuring 0.21S that is bounded by end fences. It is shown that pulse actuation significantly increases the lift not only at post-stall but also at angles of attack that are below stall (ostensibly by trapping vorticity over the entire oscillation cycle).