Is transonic buffet a misnomer?*

Self-sustained, periodic flow oscillations over wings in the transonic regime are characteristic of a flow phenomenon referred to as transonic buffet which can be detrimental to aircraft performance. Although the physical mechanisms underlying this phenomenon are unclear, it is generally assumed to be shock-induced and unique to the transonic regime. This assumption is shown to be incorrect here by performing large-eddy simulations of infinite wing sections at different freestream Mach numbers, M, and incidence angles, α. Whereas transonic buffet is observed at high M, sustained periodic oscillations that resemble it are observed at lower M even though the entire flow field remains subsonic at all times. A spectral proper orthogonal decomposition shows that the mode shapes associated with these oscillations are essentially the same at all M. By examining higher α, connections are also made between the present results and low-frequency oscillations that have been reported in incompressible flows at α close to stall. These results indicate that the physical mechanisms underlying "transonic" buffet are essentially subsonic in nature. This insight could be useful in reformulating mitigation strategies for "transonic" buffet, by suggesting a shift in focus away from shock waves.