Experimental analysis of upstream traveling waves in transonic buffet on a supercritical airfoil*

The transonic flow past airfoils and wings is associated with shock-wave/boundary layer interactions that induce flow separation. For particular combinations of Mach number and angle of attack, buffet (a large-scale periodic and self-sustained oscillation of the shock wave) occurs. As this highly unsteady phenomenon severely limits the aerodynamic performance and flight envelope, its understanding is crucial for the design of aircraft. Explanatory models predicting the phenomenon name pressure waves generated at the airfoil trailing edge as an integral driving force to the governing mechanism that maintains the shock oscillation. We study the propagation velocity of pressure waves along the airfoil upper surface, as well as the associated shedding frequency, on the basis of high-speed focusing-schlieren measurements. We captured the airfoil and a portion of the wake at an acquisition rate of 20 kHz. A thorough analysis of the pressure waves regarding the inherent variation in strength, propagation velocity, and frequency within the buffet cycle gives insight into the role of the waves regarding the back-and-forth motion of the shock wave.

We gratefully acknowledge the support of the German Research Foundation DFG in the framework of the Research Unit FOR 2895 (project number 406435057).