Effect of porous wall on Mach 6 boundary-layer transition over a flat plate

Stabilization of the Mack second mode is of great importance to control hypersonic boundary-layer transition, and the porous wall is one of the promising passive techniques. In this paper, the transition onset of a two-dimensional Mach 6 boundary layer over a flat plate with porous coating is predicted by the e^N method based on the linear stability theory. The results show that the second mode instability over the smooth wall can be significantly stabilized by the porous coating. As a price, the other instabilities, mostly the lower-frequency first mode instability, are destabilized, which is more prominent over a hotter wall. Consequently, the transition onset over the porous wall is not always delayed obviously compared to the smooth wall, and even a premature transition is obtained for some porous walls, especially for a hot wall. To achieve an effective delay of transition onset over a hot wall, a sectionalized strategy is proposed in this paper. By partially using the porous wall at downstream location, the destabilization of first mode instability is relieved while the significant stabilization of second mode instability is retained for the most unstable wave, and thus the transition onset is delayed more effectively over a hot wall.