Surface pressure measurements at a Lambda wing at steady and pitching angle of attack at vortex dominated flow including transonic effects

In the Transonic Wind tunnel Gottingen, different test campaigns were performed with an oscillating lambda wing to study the unsteady pressure distributions at vortical flow conditions including transonic influences. The free stream Mach number was varied between 0.3 and 0.7. Small pitching amplitudes of 0.08° - 0.4° 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, at first 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 the following: For lower angles of attack, the shock is dominant. For higher angles of attack, the effect of the vortex is of similar magnitude and becomes dominant. The shock exhibits an inverse motion with increasing angle of attack. For high angles of attack, beyond the maximum lift, the unsteady pressure distributions and the lift exhibit a significant phase lag, already at very low oscillation frequencies.