Study of Upstream TravellingWaves in Transonic Buffet

This paper investigates the appearance, propagation and orientation of upstream travelling waves (UTWs) in transonic buffet. Their role has been studied experimentally on a supercritical airfoil (OAT15A) in the transonic-supersonic wind tunnel of TU Delft at Mach number of 0.7, and an angle of attack of 3.5. In the experiments Background Oriented Schlieren (BOS) and Particle Image Velocimetry (PIV) have been used, to observe the suction side of the airfoil in a streamwise-spanwise oriented plane. The experiments detected the frequency and propagation velocity of the UTWs, which have been confirmed to behave as acoustic waves that therefore travel at the speed of sound relative to the flow. The particular set-up configuration used confirmed the two-dimensionality of the velocity field and of the shock wave but showed that the UTWs propagate in the plane of measurement with a non-zero orientation.