Optimal porosity and pore radius of porous surfaces for damping the second-mode instability

It is reported that micro pores, micro grooves and ultrasonically absorptive surface can be used to suppress second-mode waves in hypersonic boundary layers. In this study, the boundary conditions of porous surfaces are modeled with the theory of acoustic wave propagating within thin and long tubes, and the temporal linear stability theory is used to study the stabilization effect on boundary layer of porous surfaces. The investigation of a Mach 6 hot flat plate boundary layer shows that porous surfaces can delay neutral points as well as suppress the growth rate of the most unstable second-mode waves. In order to find out the optimal porosity and pore radius for stabilizing the boundary layer, we investigate a wide range of parameters. The optimal parameters along the plate surface are discovered. Furthermore, the nonparallel (wall-normal velocity) effect of the base-flow is studied on the optimal parameters and control effect.