Modeling separation in rarefied gas flows

Abstract

In this paper, we study rarefied internal gas flows subject to strong adverse pressure gradients and separation in the slip flow regime (Knudsen number Kn < 0.1), with the objective of investigating the validity of continuum-based slip models under separation. Such conditions are encountered in typical components of Micro-Electro-Mechanical Systems (MEMS) and in low pressure/vacuum applications. Solutions of compressible Navier-Stokes equations subject to various velocity slip and temperature jump boundary conditions are compared against predictions of direct simulation Monte Carlo (DSMC) method, employing variable hard sphere (VHS) model. Sudden changes of the flow conditions in backwards-fac ing step geometry result in significant variations of the mean-free path of the gas molecules, with corresponding variations in the wall shear stress affecting the slip velocity directly. Unlike the straight channel flow, significant cross-flow variations are obtained. In addition, the change of characteristi c length scale from the inlet channel height to larger height downstream presents extra difficulties in choosing the proper scaling.