Geometric and constitutive dependence of Maxwell's velocity slip boundary condition

The general form of Maxwell’s velocity slip boundary condition for rarefied gas flows depends on both the geometry of the surface and the constitutive relations used to relate the viscous stress to rate of strain. The dependence on geometry is often overlooked in current rarefied flow calculations, and the generality of the constitutive dependence means the condition can also be usefully applied in regions where the Navier-Stokes equations fail, e.g. rarefied flows close to surfaces. In this paper we give examples illustrating the importance of both these dependencies and show, therefore, that implementing the general Maxwell condition produces substantially different results to conventional implementations of the condition. Finally, we also investigate a common numerical instability associated with Maxwell’s boundary condition, and propose an implicit solution method to overcome the problem.