The impact of porous surface on hypersonic boundary layer instability

Abstract

The stabilization effect of porous coating on hypersonic boundary layers over flat plates and cones has been studied by theoretical analyses, experiments, and numerical simulations. The results show that porous coating slightly destabilizes the Mack first mode whereas it significantly stabilizes the Mack second mode. In previous numerical simulations, either felt-metal porous coating or regular structure porous coating is considered. Comparison of the two types of porous coatings is only done in experiments. In this paper, the impact of porous surface on hypersonic boundary layer instability is studied, focusing on the effect of different porous coating. Stability simulations are carried out by introducing disturbances corresponding to mode S at a cross-section of the boundary layer near the leading edge. It is found that the two types of porous coatings both destabilize the Mack first mode whereas they stabilize the Mack second mode. For the specific cases considered in current paper, the overall effect of porous surface is destabilizing mode S. The peak amplitude of mode S propagating over porous surface is slightly higher than that of mode S propagating over solid surface. At approximately the same porosity, regular structure porous coating is weaker in first mode destabilization and second mode stabilization than felt-metal porous coating. Therefore, the two types of porous coatings lead to approximately the same peak amplitude of mode S. However, the wall-normal velocity disturbance on the wall induced by regular structure porous coating is significantly weaker than that induced by felt-metal porous coating. For regular structure porous coating, the results also show that decreasing of porosity leads to even weaker first mode destabilization, second mode stabilization, and wall-normal velocity disturbance on the wall.