Stabilization of High-Speed Boundary Layer Using Porous Coatings of Various Thicknesses

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