Study on flow noise characteristic of transonic deep buffeting over an airfoil

Transonic buffeting can induce strong noise and reduce aircraft lifespan. In view of the complexity of the transonic buffeting flow, this study combines the highly accurate Delayed-Detached Eddy Simulation and Discrete Frequency Response method to analyze the flow field and sound propagation law in different buffeting states and also investigates its noise-generating characteristics by Dynamic Mode Decomposition and Pearson correlation. It is found that the low-frequency and small-amplitude shock oscillation of the light buffeting state is insufficient to trigger large separated flow. Besides, the reattachment phenomenon occurs in the trailing edge, which is the second mode of boundary layer separation, corresponding to the lower Sound Pressure Levels (SPL). In the deep buffeting state, however, the shock oscillates with high frequency and large amplitude, producing large separated bubbles without the reattachment phenomenon, which is the first mode of boundary layer separation. Moreover, there is a large-scale vortex structure with high energy content in the recirculation zone, which develops toward the trailing edge under the action of convection and produces strong Upstream Traveling Waves (UTWs). The collision occurs between UTWs and the shock wave oscillation. In this process, they promote each other, which increases the shock wave oscillation frequency and SPL. This state is not the superposition effect of buffeting and stall. And its main sound sources are shock oscillation and the von Kármán mode.