Stabilization of hypersonic boundary layers by porous coatings

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Stabilization of hypersonic boundary layers by porous coatings

Fedorov, A.V. / Malmuth, N.D. / Rasheed, A. / Hornung, H.G.

The ability to stabilize a hypersonic boundary layer and increase its laminar run is of critical importance in the hypersonic vehicle design. Early transition causes significant increases in heat transfer and skin friction. Higher heating requires an increased performance thermal protection system (TPS), active cooling, or trajectory modification. This translates to higher cost and weight of hypersonic vehicles due to increased TPS weight. Moreover, with the low payload mass fraction, even small savings in TPS weight can provide a significant payload increase. Vehicle maintainability and operability are also affected by transition. Robust metallic TPS have temperature limits lower than ceramic TPS. Laminar flow control (LFC) can help meet these more severe constraints. For a streamlined vehicle with large wetted area, viscous drag becomes important. A second-mode stability analysis has been performed for a hypersonic boundary layer on a wall covered by a porous coating with equally spaced cylindrical blind microholes. Massive reduction of the second mode amplification is found to be due to the disturbance energy absorption by the porous layer. This stabilization effect was demonstrated by experiments recently conducted on a sharp cone in the T-5 high-enthalpy wind tunnel of the Graduate Aeronautical Laboratories of the California Institute of Technology. Their experimental confirmation of the theoretical predictions underscores the possibility that ultrasonically absorptive porous coatings may be exploited for passive laminar flow control on hypersonic vehicle surfaces.