On the unsteady development of the flow under transonic Buffet conditions: Experimental study of the interaction of shock and pressure waves using PIV and Schlieren visualization

This thesis presents an experimental study of the unsteady phenomenon known as transonic buffet. Completely developed in the High Speed Lab of the Aerospace Faculty at TU Delft (The Netherlands), the experiments were conducted in the transonic/supersonic wind tunnel TST-27. In order to achieve time and spatial resolution of the phenomenon, high-speed Schlieren and time-resolved particle image velocimetry were used as the main measurement techniques over two aerodynamic models, the NACA0012 and the OAT15A. The aim of the project was to achieve fully developed buffet conditions, to study the time evolution of the flow and the dynamic characteristics of the features involved in the phenomenon, to investigate the wave propagation in the flow that maintains the self-sustained character of buffet and to study the origin and dynamic development of these waves. An spectral analysis of the shock wave motion was done over all the experimental test cases to analyze the influence of different aerodynamic parameters (Ma, Re, AoA) over the phenomenon and to come up with the cases were buffet is most fully developed. The results showed that, as expected, there is a range of values of these aerodynamic parameters under which buffet develops, and out of this range it just vanishes. Buffet was fully achieve for the OAT15A model at Ma=0.70, AoA=3.5o and fixed transition at x/c=7%, on the other hand, buffet about the NACA0012 was not fully developed, but it was found to be stronger at Ma=0.70, AoA=4o and free transition. From the PIV results, the instantaneous and the phase-average velocity fields showed how the actual flow development takes place and all the features involved in the buffet cycles. It was shown that under fully developed buffet conditions, massive separation takes place from the shock foot and extends beyond the trailing edge during the upstream motion of the shock wave, whereas reattachment occurs in that region during the downstream motion of the shock. Finally, a correlation analysis of the velocity fields demonstrated the presence of the so called upstream traveling waves shed at around 2000 Hz and traveling upstream at -80 m/s. Furthermore, it was demonstrated that the vortices shed from the shock foot when full separation occurs are not the cause for the upstream traveling waves, instead they are originated by another type of disturbances shed when reattachment occurs at the shock foot. This made possible a reformulation of the hypothetical dynamic evolution of the features involved in the buffet cycles, explaining the links between them and the physical mechanisms that cause their self sustained development.