In this work a new, two-fluid model for the hypervelocity rarefied regime is presented. This work avoids many of the shortcomings of the previous multi-fluid models and results in a set of partial differential moment equations which are of the same order of difficulty as the conventional gas dynamic equations.nTo make an "a priori" choice of fluids fur the model, those particles reflected from the surface of the body are considered as the "wall" fluid while all scattered and free-stream particles comprise the "cold" fluid;the assumption being that collisions between "wall" and "cold" particles convert "wall" particles to "cold" particles. Kinetic models are then constructed for both the "wall" and "cold" fluids. To obtain expressions for the absorption and emission terms in the model, the distri¬bution function of each fluid is represented by an expansion in the derivatives of the delta function and the relevant integrations are performed.nTo take advantage of the relative simplicity of the collisional moments a moment method is developed for this regime. For the "wall" fluid we employ the free molecular distribution function with the number density as state-variable for the "wall" fluid. For the "cold" fluid the distribution function is expressed as an expansion in the higher derivatives of the delta function, the expansion parameter being the "Mach number" of the "cold" fluid. The resulting moment equations give a hydrodynamic description of the "cold" fluid and a differential equation for the attenuation of the "wall" fluid number density.nThese equations are analyzed for the case of one-dimensional hypersonic compression (piston problem), and numerical results are presented. The velocity and density profiles are shown as functions of time and distance from the plate, and the speed ratio of the cold fluid is plotted. Although the model does not provide a description of the entire transition between free molecule flow and a fully developed sleek wave, the results obtained for the piston problem do give insight into the initial phase of shock formation.
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