Origins of atypical shock buffet motions on a supercritical aerofoil

Abstract

To expand the allowable flight regime of contemporary aircraft, a fundamental understanding of edge-of-the-envelope flow phenomena, such as transonic shock buffet, is necessary. In this study, Unsteady Reynolds-Averaged Navier-Stokes simulations are employed to probe the evolution of shock motions throughout the buffet envelope. At deep-buffet freestream conditions, atypical shock motions are shown to develop, in which an alternating strong/weak shock formation is evident with significant upstream pressure wave propagation. The transition between typical, single-frequency shock oscillations and these high-periodicity aerodynamic responses with increasing incidence is found to exhibit characteristics of a period-doubling route to chaos. Analysis of these flows through Higher-Order Dynamic Mode Decomposition reveals the exotic buffet dynamics emerge from the interplay between a dominant oscillatory buffet mode and its related even sub- and superharmonics, which appear with equivalent modal energies at high-incidence conditions. Correspondingly, the tendency for upstream pressure wave propagation is found to be correlated to a reversal in the suction surface leading edge phase gradient of the dominant oscillatory modes, which also develops with increasing angle of attack.