PIV investigations of shock-buffet dynamics on a supercritical airfoil with a pitching degree of freedom

The interaction between the transonic flow around a spring mounted OAT15A airfoil model and the resulting pitching motion of the model was investigated with the goal of understanding the dynamics of the occurring phenomena. The experiments were performed at free-stream Mach numbers from 0.70 to 0.77 and at a Reynolds number of Rec ≈ 3 × 10^6. The dominant structural frequency of the airfoil’s pitch motion was adjusted to be in the range of the natural buffet frequency of the flow with inhibited pitching motion of the model. The variation of the Mach number allowed for an investigation in the pre-buffet, buffet and post-buffet regime. A periodic pitch motion with an amplitude of the angle of attack of up to ±1.17° was observed for buffeting flow conditions while it was significantly less in pre-buffet and post-buffet conditions. Velocity field measurements by means of high repetition rate particle image velocimetry (PIV) were used to capture the motion of the shock and to determine the state of the boundary layer flow for the different phases of the model motion. For the Mach number range Ma = 0.72 − 0.75 the change of the angle of attack and the shock position correlate strongly with each other. From the measurements, the delay of both quantities during the coupled motion could be determined.