Drag model for convex surfaces in high speed free molecule flow

The objective of this study is to establish a theoretical framework that provides a methodology for the development of a new computer solver for pressure, shear and heat flux on convex surfaces in high speed free molecule flow. In this study, a classical gas-surface interaction model is developed for engineering use that allows for the calculation of gas scattering in high speed free molecule flow without the use of surface accommodation coefficients. The gas scattering information obtained is used to calculate the surface accommodation coefficients which are needed to evaluate the pressure, shear and heat flux on the surface when one uses only the free molecule theory. With an eye on high gas temperature environments as encountered by transatmospheric spaceships, a hard cube gas-surface interaction model is developed which is adequate for representing the gas-surface interation in high speed free molecule flow for the calculation of drag. A new drag model for high speed free molecule flow that encompasses equilibrium surface adsorption is developed serving as a basis for the new gas reduction model for gas-surface scattering. The drag model developed in this work provides a method for the direct evaluation of the surface adsorption for the calculation of the surface area involved in random emission in free molecule flow. This is a new approach and is not available in the open literature. The gas reduction model presented in this study fills a void in the engineering literature by offering a closure model for the calculation of pressure, shear and heat flux to convex surfaces in free molecule flow. The gas reduction model provides a way for the rational calculation of the normal and tangential surface accommodation coefficients in situations where there is no experimental data available on the gas-surface combination or where only the thermal accommodation coefficient is known.