Mechanism of Transonic Buffet on Symmetric Airfoils at Zero Incidence Angle

The buffet phenomenon suffered by airfoils in transonic flow has drawn much attention of researchers in the recent decades. For an airfoil with a fixed shape, the transonic buffet occurs at proper Mach numbers, Reynolds numbers, and incidence angles. In this paper, we focus on the mechanism of the buffet in transonic flows around the NACA 0012 airfoil at zero incidence angle, in which case the unsteady flow structures repeat alternatively on the upper and lower surfaces of the airfoil. The discussions are based on computational results obtained with an in-house code for solving the unsteady Reynolds-averaged Navier-Stokes equations (URANS) with a second-order gas-kinetic scheme. The dynamic process in a buffet cycle is presented and analyzed. In addition to the interaction between the shock wave and the boundary layer separation induced by it, the bifurcation of the topological structure composed by the trailing edge and the saddle point around it is found to be responsible for the occurrence and sustaining of the buffet phenomenon. The conclusions can be qualitatively extended to the buffet on other symmetric airfoils at zero or very small incidence angles.