Effect of Porous Coating on Boundary-Layer Instability

Abstract

The stabilization effect of porous coating on hypersonic boundary layers over flat plates and cones has been successfully demonstrated by theoretical analyses, experiments, and numerical simulations. It has been found that porous coating slightly destabilizes the Mack first mode whereas it significantly stabilizes the Mack second mode. In previous studies, porous coating covers either the entire flat plate or the surface around half the cone circumference. The effect of porous coating locations on boundary layer stabilizations has not been considered. Furthermore, the destabilization effect of porous coating on the Mack first mode has not been studied in detail. In this paper, the stabilization of a Mach 5.92 flat-plate boundary layer using felt-metal porous coating is studied, focusing on the effect of porous coating locations and the destabilization of the Mack first mode. Stability simulations are carried out by introducing disturbances corresponding to mode S at a cross-section of the boundary layer near the leading edge. In a series of simulations, porous coatings are put both upstream and downstream of the synchronization point. It is found that porous coating location has an important effect on the stabilization of mode S. Disturbances are destabilized when porous coating is located upstream of the synchronization point whereas they are stabilized when porous coating is downstream of the synchronization point. For felt-metal porous coating, destabilization of the Mack first mode is significant so that disturbances are slightly destabilized when porous coating are put on the whole flat plate. The results indicate that an efficient way to s hypersonic boundary layers is to put porous coating downstream of the synchronization point.