Unsteady surface pressures measured at a pitching Lambda wing with vortex dominated flow and transonic effects

Abstract

In the Transonic Wind Tunnel Goettingen, pitch oscillations were performed with a Lambda wing to study unsteady pressure distributions of vortex dominated flow including transonic effects. The free stream Mach number was varied between 0.3 and 0.7. Small pitching amplitudes of 0.08–0.4 deg at excitation frequencies up to 40 Hz were used. In this paper, particularly the data of unsteady Pressure Sensitive Paint measurements and unsteady pressure sensors are analyzed. With increasing angle of attack, a suction peak and a shock occur near the leading edge. Then a shock-induced Separation triggers the development of a vortex at the main wing. The unsteady pressures show: for lower angles of attack, the transonic influences are dominant. For higher angles of attack, the influence of the vortex becomes of similar Magnitude and dominates the behavior of the pressure variations. The shock exhibits, with increasing angle of attack, an inverse motion. For angles of attack beyond the maximum lift, the unsteady pressure distributions and the lift show a significant phase lag, already at very low oscillation frequencies. Compared to subsonic cases, the supersonic region shifts the vortex induced pressures downstream.