The Interior Wall boundary condition can only be applied on interior boundaries.

It is similar to the Wall node available on exterior boundaries except that it applies on both sides of an interior boundary. It allows discontinuities of the velocity potential across the boundary. The Interior Wall boundary condition can be used to avoid meshing thin structures by applying a no-penetration condition, u ⋅ n = Ubnd, on interior curves and surfaces instead. Slip conditions and conditions for a moving wall can also be prescribed.

The Normal wall velocity setting controls the normal wall velocity Ubnd. The list is per default set to Automatic from frame. The physics automatically detects if the spatial frame moves. This can for example happen if an ALE interface is present in the model component. If there is no movement Ubnd = 0. If the frame moves, Ubnd becomes equal to the frame movement projected on the wall normal. Ubnd is accounted for in the actual no-penetration boundary condition prescribed, so that u ⋅ n = Ubnd.

Select Zero (Fixed wall) from Normal wall velocity selection list to always prescribe Ubnd = 0.