Hypersonic Boundary Layer Analysis Comparison of Velocity Measurements and Monte-carlo Calculations

The study of gas-solid accommodation at low pressure is of paramount importance to several areas of scientific research and industrial development. In Chemical Vapor Deposition (CVD) processes for instance, ONERA has quantified the accommodation for several diatomicsolid pairs and shown that, near or below 100 Pa, the gas can be substantially colder than the surface being grown, at a distance less than one mean free path from that surface [1, 2]; this drastically lowers the rate of chemical reactions in the reactor volume with respect to the case where accommodation would be complete. In supersonic aerodynamics, a domain which is important for flight mechanics at high altitude, e.g. in the reentry of a space shuttle, boundary layers may slip, here causing vehicle performance to deviate from that which is computed using models that do not account for it. Work recently performed at Los Alamos indicates that momentum accommodation can be determined using a torsion balance [3]. Kinetics of molecules rebounding off a surface were carefully studied for many years [4, 5]. This expertise is key to the use of Monte Carlo simulations of flow slip next to the wall. In accommodation studies, it is customary to define coefficients for the thermal accommodation related to the translational and internal energy and for the tangential momentum accommodation. Those are very delicate to measure, and it has often been impossible to obtain reliable values that could be used for numerical simulations. In aerodynamics, the momentum accommodation also matters. Depending on values of the accommodation coefficients one can find a corresponding violation of continuum at the surface becoming apparent in a velocity slip and a temperature jump, influencing the aerodynamic drag and heat transfer. This may be particularly important in wind tunnel tests where pressure is low and Mach number high. Measuring the coefficients in the flow is a result of confrontation between theory and experiment. The computational modeling of the experimental situation is the only way to extract accommodation coefficients in the transition regime and close to continuum. The main objectives of this paper are threefold: 1) measurement of velocity profile in a supersonic flow over a flat plate, 2) comparison of experimental data and numerical results and 3) analysis of the effect of accommodation coefficients on the boundary layer. Section 1 is devoted to the experiment: the basic principle of the velocimetry technique is presented and the experiment described. In a second section, the gas/surface interaction model is described. Experimental results are reported and compared with calculations in section 3.