In the slip flow regime, the Navier–Stokes equations can still be used with modified boundary conditions to account for rarefaction effects close to the wall. A layer of rarefied gas with a size similar to the mean free path develops close to the wall — this is termed the Knudsen layer. The Navier–Stokes equations are not applicable in this layer, but the flow outside the layer can be described by extrapolating the bulk gas flow toward the wall and applying Maxwell’s slip boundary condition at the wall (Ref. 5). For thermal flows the von Smoluchowski temperature jump boundary condition must also be applied (Ref. 5).

To model flows in the slip flow regime use the Slip Flow interface. That physics interface can be used to model isothermal and nonisothermal flows with or without explicit modeling of thermal processes in adjacent solid domains. To model isothermal flows use the Temperature boundary condition on all external model surfaces, with the temperature set equal to the fluid temperature. The Slip Wall and External Slip Wall boundary conditions are used to model slip on interior and exterior model boundaries respectively. These boundary conditions include thermal creep or transpiration, viscous slip, and the von Smoluchowski temperature jump. The slip coefficients can be specified directly or by means of Maxwell’s model. For further details see The Slip Flow Interface and Theory for the Slip Flow Interface.