Comparative Study of the Two-Dimensional and Three-Dimensional Transonic Shock Buffet

A computational comparative study of the shock buffet phenomenon is presented. A comparison between the two-dimensional airfoil buffet and the three-dimensional swept-wing buffet is made at pre-buffet and developed buffet flow conditions to shed further light on the distinct mechanisms that characterize the phenomena. Based on the results near buffet onset, we show that the onset of both airfoil and wing buffet is associated with shock-induced perturbation. Modal analysis using Dynamic Mode Decomposition at developed buffet conditions is used to explain the distinct mechanisms that control the 2D and 3D shock buffet. The 2D buffet involves shock-wave-boundary-layer interaction resulting in a large chordwise shock excursion distance, while the 3D buffet is the result of the spanwise propagation of the perturbation originating in an inboard span section, close to the merging point of the front and rear branches of the lambda-shock. Thus, the 3D shock excursion distance is significantly smaller than the 2D, while the average chordwise shock wave velocities are shown to be comparable between the 2D and 3D buffet. This provides a physical explanation for the swept-wing buffet frequencies that are higher than those of the airfoil buffet.