An Extended Numerical Study of Mach 6 Boundary Layer Stabilization by Means of a Porous Surface

Abstract

Direct numerical simulations are carried out of boundary layer flow at Mach 6 over a porous surface, in which a Mack mode of instability is excited. The pores are resolved rather than modeled, allowing an evaluation to be made of the accuracy of simplified analytical models used in previous investigations based on linear stability theory. It is shown that the stabilizing effect of porosity is stronger in the simulations than in the corresponding theory for both two- and three-dimensional pores. From comparisons of spanwise grooves, streamwise slots and square pores it appears that the detailed surface structure is not as important as the overall porosity and the hydraulic diameter is able to collapse the results for different pore shapes to good accuracy. When the porous surface consists of fewer larger pores, the flow is noisier, with sound waves generated at the pore edges. Also the influence of a random distribution of 2D grooves was investigated and the former statements were shown to remain valid. Finally the influence of rarefied gas behavior of the flow inside the pores was taken into account by introducing a slip-boundary condition with finite Knudsen number. The additional damping effect at moderate Knudsen numbers, investigated by other groups, was confirmed.