Mechanism of periodic oscillation in low-Reynolds-number buffet around an airfoil at angle of attack 0

In this study, the low-frequency oscillations found in the compressible low-Reynolds-number regime, defined as low-Reynolds-number buffet, were investigated by numerical calculations and modal analysis. Dynamic mode decomposition (DMD) and compressive sensing methods were employed to extract periodic flow structures. Numerical simulation results showed low-Reynolds-number buffet and Kármán vortex shedding. Low- and high-frequency oscillations St=O(0.01), O(1.0) were extracted by DMD and named the buffet mode and vortex shedding mode, respectively. Low-Reynolds-number buffet does not necessarily exhibit supersonic regions or shock wave. Simulation results show that the thickness of the separated shear layer changes significantly under low-Reynolds-number buffet. The change in the thickness of the separated shear layer was confirmed by the buffet mode of DMD results. Two types of compression pressure waves, advecting upstream, were identified. DMD indicated that they resulted from vortex shedding. According to simulation and DMD results, the origin of the shock waves appears to be the condensation of compression pressure waves due to the vortex shedding mode and expansion in the supersonic region due to the buffet mode. The formation of the shock waves seems to be subordinated to the vortex shedding mode and the buffet mode. A feedback model for low-Reynolds-number buffet inherent in separated shear layers, which does not require supersonic regions or shock waves, was proposed. Supersonic regions highly condensed the compression pressure waves, inducing a larger separation region and amplifying the oscillation. The role of supersonic regions in determining oscillation amplitudes was evidenced, although supersonic regions are not essential to the mechanism of low-Reynolds-number buffet.