Similarities Between Cellular Patterns Occurring in Transonic Buffet and Subsonic Stall

Abstract

This paper studies the similarities between the three-dimensional (3-D) cellular patterns that appear in conditions of transonic buffet and subsonic stall, using Reynolds-averaged Navier–Stokes/unsteady Reynolds-averaged Navier–Stokes simulations. The geometries are obtained from the extrusion of a two-dimensional (2-D) airfoil with an added sweep angle, and periodic boundary conditions are imposed at both ends. First, numerical simulations of transonic buffet are performed. The numerical solutions exhibit three-dimensional cellular flow patterns, forming what has been named buffet cells. The latter are convected in the spanwise direction when a sweep angle is added, leading to the superposition of two frequencies. The first one is independent of the sweep angle and is characteristic of the two-dimensional buffet. The second one increases with the sweep angle and is related to the convection of the buffet cells. These cells are reminiscent of the stall cells, which are well known for low-speed flow conditions. Second, solutions of the unsteady Reynolds-averaged Navier–Stokes equations for infinite swept wings in stall conditions at low speed are computed, showing the same convection of the cellular patterns. These results indicate that the discrepancies between 2-D and 3-D buffet are caused by the appearance of stall cells, which are the same as buffet cells.