Transient Control of Separating Flow over a Dynamically-Pitching Airfoil

Transitory separation control of the flow over a 2-D airfoil that is dynamically pitching beyond the stall limit is investigated in wind tunnel experiments using pulsed actuation. Actuation is effected 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. The present experiments consider two stall configurations. In the first configuration, the actuation is nominally two-dimensional and the actuator spans a stalled flow domain that is bounded by spanwise fences. In the second configuration the fences are removed and the actuation covers about 20 % of the entire span of a fully-stalled airfoil. In the nominally 2-D configuration, single-pulse actuation can significantly increases the lift over most of the oscillation cycle not only at post-stall but also at angles of attack that are below stall. In the 3-D configuration, a single pulse can increase the lift during a significant fractions of the oscillation cycle despite the massive separation on the outboard segments of the unactuated airfoil. Several actuation pulses distributed during the oscillation cycle can lead to a remarkable increase in lift over most of the cycle including at angles of attack that are below stall ostensibly by controlled trapping vorticity over the entire oscillation cycle. I