A comprehensive evaluation of various mean-free-path forms in hypersonic rarefied gas flows

Abstract Simulations of the hypersonic rarefied gas flows are essential when designing high-speed vehicles such as re-entry vehicles. Computational Fluid Dynamics (CFD) and Direct Simulation Monte Carlo (DSMC) are typical approaches to simulate these flows. The latter has a higher computational cost than the CFD, which solves the Navier-Stokes-Fourier (NSF) equations. Appropriately treating the slip and jump conditions is critical for comparing the DSMC data and the CFD results. The accuracy of the NSF simulations is typically determined by the slip and jump boundary conditions (BCs) used to predict the surface properties, such as the slip velocity and the surface gas temperature. The mean free path (MFP) of gas molecules is vital in the slip-and-jump BCs to generate precise simulation results that enhance hypersonic vehicle design and performance. The current work comprehensively evaluates two forms of the MFP: the Hard-Sphere (HS) and the Variable-Hard-Sphere (VHS). The objective is to determine the model most suitable for hypersonic rarefied gas flow simulations that predict surface properties accurately. The research adopts basic aerodynamic configurations, including a sharp leading-edge flat plate, a wedge, and a cylinder in cross-flow, to validate the two MFP models. The Mach numbers examined in this study range from 6 to 10, with argon and nitrogen as the working gases. The results of the simulations show that the VHS mean free path form gives better results than the HS one compared to the DSMC data.