Numerical study of strong shear nonequilibrium phenomenon and its relaxation mechanism in hypersonic Knudsen layer flows based on the direct simulation Monte Carlo method

Abstract

Based on the different flowfield and wall settings, a series of plane Couette flows of rarefied argon gas are simulated by the direct simulation Monte Carlo method, and the relaxation process of shear nonequilibrium has been systematically analyzed. The statistical result of molecular streamwise velocity (vx) shows a qualitatively platform-shaped distribution under the conditions of diffuse-reflection wall, low wall temperature, small wall spacing (roughly within the range of Knudsen layer), and large wall speed and shear rate. The simplified mechanism of this platform-like phenomenon is deduced, which proves that after the collision between two molecules reflected from the different walls (named the dual-wall molecular collision), their vx will be uniformly distributed. Thus, if all reflected molecules occur once dual-wall molecular collision on average, their vx will form the platform-shaped distribution. This conclusion is demonstrated by the verification cases. Similar phenomena can be found in more general flows, such as the hypersonic flow over a flat plate. The nonequilibrium phenomenon in the flat-plate flow can be explained by the equivalent relaxation process of dual-wall molecular collision, which indicates that the platform-like phenomenon is a universal nonequilibrium state in strong-shear rarefied flow, and they share a similar relaxation mechanism. The analysis of this phenomenon will help us to understand and model the relaxation process of hypersonic Knudsen layer flow.