Modal decomposition analysis of transonic shock buffet on airfoil

The transonic buffet on the airfoil is a phenomenon of low-frequency normal shock oscillation coupled with shock wave/turbulent boundary layer interaction and flow separation. Although numerous studies have been conducted to investigate the transonic buffet flow and oscillation mechanism, the influence of high-frequency disturbances has not received sufficient attention. To explore buffet flow characteristics at different frequencies, simulations of OAT15A airfoil are performed using unsteady Reynolds-averaged Navier-Stokes (URANS). Parametric analysis of transonic buffet at different attack angles suggests that, separation region increases both in time and space with attack angle, while the buffet frequency is insensitive. Meanwhile, a large attack angle may cause the shock to bend during oscillation. Dynamic mode decomposition (DMD) and spectral proper orthogonal decomposition (SPOD) analysis present typical buffet modes with low-frequency shock oscillation, while the high-frequency SPOD modes further highlight the influence of vortical structures, pressure waves and shock induced disturbances. Additionally, part of the pressure waves induced by trailing edge are found to propagate upstream along lower surface of the airfoil and interact with incoming freestream, which has not been considered in traditional buffet model.