In the transitional flow regime there are no simple boundary conditions available for inflow and outflow. The Transitional Flow Interface includes boundary conditions that are applicable either in the Navier–Stokes limit (the Inlet and Outlet conditions) or in the molecular flow regime (for example, the Reservoir condition). If the flow throughout the region of interest is transitional then it is necessary to introduce additional domains on either side of the region of interest to allow the flow to reach a physical solution before it enters the region of interest. When this approach is taken then it is reasonable to use (with care) the boundary conditions available, even if they are not strictly physical. Generally it is best to use the molecular flow boundary conditions because they do not require the fully coupled solver.

The Wall boundary condition is valid across the full range of Knudsen numbers but is limited to the case of diffuse reflection of molecules incident on the boundary.