Stability of Hypersonic Boundary Layer on Porous Wall with Regular Microstructure

Abstract

Theoretical and experimental studies of hypersonic boundary-layer stabilization using a passive porous coating of regular microstructure are discussed. Propagation of disturbances inside pores is simulated with linear acoustic theory including the gas rarefaction effect. This model provides boundary conditions for stability analysis of boundary-layer disturbances on the porous wall. Experiments were conducted in the Mach 6 wind tunnel on a 7-deg half-angle sharp cone whose longitudinal half-surface is solid and whose other half-surface is covered by a perforated sheet comprising equally spaced cylindrical blind microholes. Hot-wire measurements of natural disturbances and artificially excited wave packets are conducted on both solid and porous surfaces. Natural disturbance spectra indicate that the second mode is a dominant instability. The porous coating stabilizes the second mode and weakly affects the first mode. Measurements of artificially excited wave packets show that the porous coating leads to substantial decrease of the wave-packet growth. The experimental data on phase speeds and amplitudes of the second-mode disturbances are compared with theoretical predictions. Satisfactory agreement is obtained for both solid and porous surfaces. This study confirms the concept of hypersonic boundary-layer stabilization by passive porous coatings, which can be used for laminar flow control.