Nonlinear Aspects of Hypersonic Boundary-Layer Stability on a Porous Surface

The nonlinear aspects of the stabilization of the second-mode disturbance using a passive, ultrasonically absorptive coating (UAC) of regular microstructure are studied using bispectral analysis. The experimental data consist of hot-wire measurements made in artificially excited wave packets that are introduced into the hypersonic boundary layer on both solid and porous surfaces. The bispectral measurements show that the subharmonic and harmonic resonances of the second mode are significantly modified. The harmonic resonance, which is quite pronounced in the latter stages of the hypersonic boundary layer on solid surfaces, is completely absent on the porous surface. The degree of nonlinear phase locking that is associated with the subharmonic resonance and identified on the solid surface is substantially weakened on the porous surface. This nonlinear interaction persists farther downstream on theporous surface than on the solid surface; however, unlike on the solid surface, there are no strongly preferred interaction modes. The spectral measurements, made in previous work, show that the first mode is moderately destabilized on the porous surface. The bispectral measurements presented here identify a nonlinear interaction that is associated with the destabilized first mode; however, this is observed to be a very weak nonlinear interaction that has no deleterious effect on the performance of the UAC.