Control of Dynamic Stall on a Pitching Airfoil Using High-Frequency Actuation

Abstract

A flow control strategy for the delay of unsteady separation and dynamic stall on a pitching NACA 0012 airfoil is explored by means of high-fidelity large-eddy simulations. The flow fields are computed employing a high-fidelity large-eddy simulation (LES) approach. The flow parameters are freestream Mach number M∞ = 0.1 and chord Reynolds numbers Rec = 5× 10. Both constant-rate and oscillatory pitching motions are considered. For the baseline cases, dynamic stall is initiated with the bursting of a contracted laminar separation bubble (LSB) present in the leading-edge region. This observation motivated a flow control approach employing high-frequency pulsed actuation imparted through a zero-net mass flow blowing/suction slot located on the airfoil lower surface just downstream of the leading edge. For the constant-rate pitching case, both pulsed and harmonic spanwise-nonuniform forcing are considered with a maximum non-dimensional frequency Stf = fc/U = 50.0 which corresponds to a sub-harmonic of the dominant natural LSB fluctuations for a baseline static case used for reference purposes. A significant delay in the onset of dynamic stall is demonstrated with pulsed forcing at high frequencies (Stf = 25.0, 50.0), however, control effectiveness diminishes with decreasing frequency. At Stf = 12.5, pulsed actuation is shown to be superior to harmonic forcing suggesting that the higher harmonic content present in the pulsed mode is still capable of energizing the LSB. For the oscillatory pitching motion, pulsed high-frequency flow control with Stf = 50.0) is considered for two cases exhibiting light and deep dynamic stall respectively. For light dynamic stall, flow actuation is capable of maintaining an effectively attached flow during the entire pitching cycle thereby inhibiting the formation of large-scale leading-edge and shear-layer vortical structures. For deep dynamic stall, control is found to also be very effective in eliminating leading-edge separation and the formation of a dynamic stall vortex. Nonetheless, trailing-edge separation eventually occurs at high incidence. For both cases, actuation provided a significant reduction in the cycle-averaged drag and in the force and moment fluctuations. In addition, the negative (unstable) net-cycle pitch damping found in the baseline cases was eliminated.