Molecular simulation of boundary layer flow over thermal protection system microstructure

Boundary layer flow, relevant to hypersonic flight, is simulated over Thermal Protection System (TPS) microstructure using the direct simulation Monte Carlo (DSMC) method. Simulation results are compared when the material microstructure is un-resolved (a flat surface) and when it is resolved using realistic, porous fiber-based microstructure. Reaction probabilities for carbon oxidation, specified on surface elements, are taken from molecular beam experimental data as a function of local surface temperature. For relatively high density conditions (low altitudes) the extent of surface chemistry and boundary layer profiles are nearly identical for both flat plate and resolved microstructure simulations. For low density conditions (high altitudes) only moderate differences are found. If surface reaction probabilities are lowered by a few orders of magnitude, significant differences in the extent of surface chemistry and in boundary layer profiles are found between flat plate and microstructure calculations. These results are shown to be qualitatively consistent with the non-dimensional Thiele number, corresponding to each flow condition. The results presented should be considered preliminary, however, under realistic flight conditions and using O atom reaction probabilities with carbon from molecular beam experiments, ablation due to atomic oxygen appears to mainly occur in the surface ablation regime as opposed to the volume ablation regime.