Experimental and theoretical studies of the effect of an ultrasonically absorptive coating (UAC) on hypersonic boundary-layer stability are described. A thin coating of fibrous absorbent material (felt metal) was selected as a prototype of a practical UAC. Experiments were performed in the Mach 6 wind tunnel on a $7^{\circ}$ half-angle sharp cone whose longitudinal half-surface was solid and other half-surface was covered by a porous coating. Hot-wire measurements of ‘natural’ disturbances and artificially excited wave packets were conducted on both solid and porous surfaces. Stability analysis of the UAC effect on two- and three-dimensional disturbances showed that the porous coating strongly stabilizes the second mode and marginally destabilizes the first mode. These results are in qualitative agreement with the experimental data for natural disturbances. The theoretical predictions are in good quantitative agreement with the stability measurements for artificially excited wave packets associated with the second mode. Stability calculations for the cooled wall case showed the feasibility of achieving a dramatic increase of the laminar run using a thin porous coating of random microstructure.

Experimental investigations of the boundary layer receptivity, on the sharp leading edge of a flat plate, to acoustic waves induced by two-dimensional and threedimensional perturbers, have been performed for a free-stream Mach number M ∞ = 5.92. The fields of controlled free-stream disturbances were studied. It was shown that two-dimensional and three-dimensional perturbers radiate acoustic waves and that these perturbers present a set of harmonic motionless sources and moving sources with constant amplitude. The disturbances excited in the boundary layer were measured. It was found that acoustic waves impinging on the leading edge generate Tollmien-Schlichting waves in the boundary layer. The receptivity coefficients were obtained for several radiation conditions and intensities. It was shown that there is a dependence of receptivity coefficients on the wave inclination angles.

Nonlinear processes in a hypersonic boundary layer on a sharp cone are considered using the bicoherence method. The experiments are performed for a Mach number M∞ = 5.95 with introduction of artificial wave packets at the frequency of the second mode. It is shown that the basic mechanism of nonlinear interaction at the location of the maximum r.m.s. voltage fluctuation is the subharmonic resonance; all nonlinear interactions in the maximum r.m.s. voltage fluctuation layer are related to the second mode of disturbances; nonlinear processes above and below that layer are much more intense than those in it. The effect of artificial disturbances on nonlinear interactions in the boundary layer is shown to be insignificant.

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