Study of reflection models of gas molecules on water adsorbed surfaces in high-speed flows

We consider a Couette flow of a rarefied Ar gas with heat transfer between two wall surfaces and investigate the scattering behavior of gas molecules reflected either at a clean Pt surface or at a surface contaminated with adsorbates. Water molecules abundantly present in the atmosphere were adopted as the adsorbates. The reflection of gas molecules on the lower wall surface was simulated by Molecular Dynamics (MD) method to obtain accommodation coefficients and velocity distribution functions of gas molecules. We applied the modified reflection model of gas molecule and investigated the velocity distribution functions of the model by comparing the MD results to verify the validity. The accommodation coefficients obtained by the MD method depend on the number of adsorbed water molecules on the lower wall surface. Specifically, tangential momentum accommodation coefficient (TMAC) tended to increase and then decrease with the increase in adsorbed water molecules, but normal momentum accommodation coefficient (NMAC) tended to decrease monotonically. The velocity distribution functions of the modified reflection model approximately show the good agreement with the MD calculation but the degree of coincidence depends on the speed difference between the upper and lower wall surfaces, and the number of adsorbed water molecules on the surface.