The Slip Flow (slpf) interface (), found under the Rarefied Flow branch () when adding a physics interface, is used to model thermal and isothermal flows within the slip flow regime. In the slip flow regime, the Navier-Stokes equations can be used to model the flow of the gas, except within a thin layer of rarefied gas adjacent to the walls (known as the Knudsen layer). The effect of the Knudsen layer on the continuum part of the flow can be modeled by means of modified boundary conditions for the Navier Stokes equations. Thermal effects are also important in this regime, with effects such as thermal creep or transpiration often playing a significant role. For this reason the Slip Flow interface includes the heat flow equations. Typically slip flow applies at Knudsen numbers between 0.01 and 0.1.

When this physics interface is added, these default nodes are also added to the Model Builder: Fluid, External Slip Wall, and Initial Values. Then, from the Physics toolbar, add other nodes that implement, for example, boundary conditions and volume forces. You can also right-click Slip Flow to select physics features from the context menu.

The Label is the default physics interface name.

The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.

The default Name (for the first physics interface in the model) is slpf.

Select the Neglect inertial term (Stokes flow) check box to model flow at very low Reynolds numbers where the inertial term in the Navier-Stokes equations can be neglected. When this option is checked COMSOL Multiphysics solves the linear Stokes equations for the fluid flow. The Stokes flow or creeping flow regime frequently applies in microfluidic devices, where the flow length scales are very small. Enter a Reference pressure level pref. The default value is 1[atm].

To display this section, click the Show More Options button () and select Advanced Physics Options. Normally these settings do not need to be changed. The Default model is Fluid.

Select the Use pseudo time stepping for stationary equation form check box to add pseudo time derivatives to the equation when the Stationary equation form is used. When selected, also choose a CFL number expression — Automatic (the default) or Manual. Automatic calculates the local CFL number (from the Courant–Friedrichs–Lewy condition) from a built-in expression. For Manual enter a Local CFL number CFLloc.

By default the Enable conversions between material and spatial frames check box is selected.