  Self-sustained low-frequency flow unsteadiness over rigid aerofoils in the
transonic regime is referred to as transonic buffet. Although the exact
physical mechanisms underlying this phenomenon are unclear, it is generally
assumed to be unique to the transonic regime. This assumption is shown to be
incorrect here by performing large-eddy simulations of flow over a NACA0012
profile for a wide range of flow conditions. At zero incidence and sufficiently
high freestream Mach numbers, M, transonic buffet occurs with shock waves
present in the flow. However, self-sustained oscillations that occur at similar
frequencies are observed at lower M for which shock waves are absent and the
entire flow field remains subsonic at all times. At higher incidences, the
oscillations are sustained at progressively lower M. Oscillations were observed
for M as low as 0.3, where compressibility effects are small. A spectral proper
orthogonal decomposition shows that the spatial structure of these oscillations
(i.e., mode shapes) are essentially the same for all cases. These results
indicate that buffet on aerofoils does not necessarily require the presence of
shock waves. Furthermore, the trend seen with increasing incidence angles
suggests that transonic buffet on aerofoils and low-frequency oscillations
reported in the incompressible regime (Zaman et al., 1989, J. Fluid Mech., vol.
202, pp. 403--442) have similar origins. Thus, models which rely specifically
on shock waves to explain transonic buffet are incorrect. These insights could
be useful in understanding the origins of ``transonic" buffet and reformulating
mitigation strategies by shifting the focus away from shock waves.
