Transitory Aerodynamic Control of a Pitching Airfoil using Pulsed Bleed Actuation*

The unsteady aerodynamic loads of a static and pitching 2-D airfoil (Clark-Y, Re_c = 6·10⁵) are regulated without moving control surfaces by impulsive actuation (~4 convective time scales) engendered by pulsed air bleed driven by pressure differences between the pressure and suction surfaces through the airfoil interior. The bleed air is driven through spanwise arrays of ports near midchord on both surfaces (2.3% of planform open area per side) and regulated using integrated louvers on the suction surface that are controlled at prescribed phases relative to the time-periodic pitch cycle. Despite its short duration, the bleed impulse leads to large-scale changes in the flow around the airfoil and consequently to significant temporal variations in the lift (C_L) and pitching moment (C_M). When the airfoil is static, a single bleed impulse leads to momentary changes in C_L (up to 27% at a=10°), and during time-periodic pitch motion yields programmable temporal variations in C_L and C_M that increase C_Lmax by up to 0.2 and reduce hysteresis (up to 30% in C_L and 20% in C_M).  Phase-locked measurements of the flow field in the near wake during dynamic pitch yield the cyclic variation in circulation that scales well with the corresponding changes in C_L.