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) 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 a interior stationary wall this means that u = 0 on both sides of the wall.

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, , 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.

For more information, see the Nonlinear Navier Slip option in the Wall feature.

When Hatzikiriakos slip is selected, a slip velocity is applied on each side of the interior wall when the tangential stress on the corresponding side is larger than the specified yield stress, τy,

For more information, see the Hatzikiriakos Slip option in the Wall feature.

The Hatzikiriakos Slip option is not available for the Viscoelastic Flow interface.

For more information, see the Asymptotic Slip option in the Wall feature.

The Asymptotic Slip option is not available for the Viscoelastic Flow interface.

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 Default, Use pointwise constraints, or Use DG constraints. Use mixed constraints can be selected when imposing a no slip condition exactly.