Stabilization of high-speed boundary layer using porous coatings of various thicknesses

Ultrasonically absorptive coatings (UAC) can suppress the second-mode instability in hypersonic boundary layer and thereby delay laminar-turbulent transition. Theory and numerical simulations indicate that this stabilization effect essentially depends on the UAC thickness. It is expected that optimal coatings may be several times thinner than it was assumed before. To validate these findings, the UAC thickness effect is investigated on a sharp cone in the Mach=6 wind tunnel. The coatings comprise several layers of a stainless still wire mesh. Their microstructure mimics textile materials frequently used for thermal protection. The wall-pressure disturbances are measured upstream and downstream from the coated region. It is shown that the coatings stabilize the second mode and its higher harmonics in accord with the UAC laminarization concept. The experimental growth rates are compared with predictions of the linear stability theory and direct numerical simulations of 2D disturbances. It is found that an optimal coating is approximately five times thinner than UAC tested in previous experiments. This may facilitate manufacturing and integration of UAC into actual thermal protection systems.