SUMMARY

The dynamics of transitory 2-D and 3-D flow attachment effected by pulsed actuation on the separated flows over a 2-D airfoil model are investigated in wind tunnel experiments. Actuation on time scales that are an order of magnitude shorter than the characteristic convective time scale is effected by momentary pulsed jets that are generated by a spanwise array of combustion-based actuators integrated into the airfoil. The effects of transitory actuation on the aerodynamic characteristics of the airfoil are assessed using force and torque measurements along with surface pressure measurements, and planar and stereoscopic particle image velocimetry (PIV) obtained phase-locked to the commanded actuation waveform. On a static airfoil, a single spanwise-bounded actuation pulse leads to a strong transitory change in the circulation about the airfoil that is manifested by the 2-D severing of the separated vorticity layer and the subsequent shedding of a large-scale stall vortex. Successive actuation pulses extend the flow attachment over the airfoil and enhance its global aerodynamic performance by exploiting the actuated flow dynamics that are commensurate with the flow convective time scale. It is shown using unbounded pulsed actuation that spanwise spreading of the induced transitory 3-D flow attachment beyond the spanwise edges of the actuators can enhance the accumulation of vorticity over the airfoil and amplify the aerodynamic performance of the airfoil compared to 2-D, spanwise-bounded actuation. When the airfoil is undergoing time-periodic pitch oscillations beyond its static stall margin, a sequence of staged actuation pulses coupled to the airfoil’s motion can improve the lift hysteresis and also leads to enhanced pitch stability with reduced “negative damping” that are typically associated with the onset of dynamic stall.