Suppression of Dynamic Stall by Steady and Pulsed Upper-Surface Blowing.

Abstract : The Boeing-Vertol VR-7 airfoil was experimentally studied with steady and pulsed upper surface blowing for sinusoidal pitching oscillations described by alpha = alpha sub m + 10 deg sin omega(t). The tests were conducted in the U.S. Army Aeroflightdynamics Directorate's Water Tunnel at NASA Ames Research Center. The experiment was performed at a Reynolds number of 100,000. Pitch oscillations with alpha sub m= 10 and 15 deg and with reduced frequencies ranging from k = 0.005 to 0.15 were examined. Blowing conditions ranged from C sub mu = 0.03 to 0.66 and F+ = 0 to 3. Unsteady lift, drag, and pitching moment loads were measured, and fluorescent dye flow visualizations were obtained. Steady, upper surface blowing was found to be capable of trapping a separation bubble near the leading edge during a portion of the airfoil's upward rotation. When this occurred, the lift was increased significantly and stall was averted. In all cases, steady blowing reduced the hysteresis amplitudes present in the loads and produced a large thrust force. The benefits of steady blowing diminished as the reduced frequency and mean angle of oscillation increased. Pulsed blowing showed only marginal benefits for the conditions tested. The greatest gains from pulsed blowing were achieved at F+ = 0.9.