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

It is similar to the Wall boundary condition available on exterior boundaries except that it applies on both sides of an interior boundary. It allows discontinuities (velocity, pressure, and turbulence variables) across the boundary. The Interior Wall boundary condition can be used to avoid meshing thin structures by applying no-slip conditions on interior curves and surfaces instead. Slip conditions and conditions for a moving wall can also be prescribed. It is compatible with laminar and turbulent flow.

The following Boundary condition options are available.

The No slip condition models solid walls. No slip walls are walls where the fluid velocity relative to the wall is zero. For an interior stationary wall this means that u = 0 on both sides of the wall.

For turbulent flows, the no slip conditions may either be prescribed exactly or modeled using automatic wall treatment or wall functions depending on the Wall Treatment setting in the Turbulence section of the interface settings.

When Wall Treatment is set to Wall functions, the Apply wall roughness option becomes available. When Apply wall roughness is selected, a Sand roughness model, derived from the experiments by Nikuradse, is applied. Select Generic roughness in order to specify more general roughness types.

The Slip condition prescribes a no-penetration condition, u ⋅ n = 0. It implicitly assumes that there are no viscous effects on either side of the slip wall and hence, no boundary layer develops. From a modeling point of view, this can be a reasonable approximation if the important effect is to prevent the exchange of fluid between the regions separated by the interior wall.

The Navier slip condition enforces no-penetration at the wall, u ⋅ nwall = 0, and adds a tangential stress on each side of the wall

where β is a slip length. For more information, see the Navier Slip option in the Wall feature.

The Translational velocity setting controls the translational wall velocity, utr. 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 utr = 0. If the frame moves, utr becomes equal to the frame movement. utr is accounted for in the actual boundary condition prescribed in the Boundary condition section.

Select Zero (Fixed wall) from Translational velocity selection list to prescribe utr = 0.

Select Manual from Translational velocity selection list in order to manually prescribe Velocity of moving wall, utr. This can for example be used to model an oscillating wall where the magnitude of the oscillations are very small compared to the rest of the model. Specifying translational velocity manually does not automatically cause the associated wall to move. An additional Moving Mesh interface needs to be added to physically track the wall movement in the spatial reference frame.

This section is displayed by clicking the Show button () and selecting Advanced Physics Options. The Constraints settings can be set to Automatic, Pointwise constraints, or Nitsche constraints. Mixed constraints can be selected when imposing a no slip condition exactly.