The Interior Wetted Wall Coupling Feature

It is similar to The Wetted Wall Coupling Feature available on exterior boundaries of the (

)and(

) multiphysics interfaces, except that it applies on both sides of an interior boundary. It is an exclusive feature which overrides the feature in the Laminar or Turbulent Flow interface as well as the feature in the Level Set interface. It is available for laminar flow and turbulent flow with wall functions or automatic wall treatment.

The coupling feature allows discontinuities (velocity, pressure, level set variable, and turbulence variables) across the boundary and can be used to avoid meshing thin structures in contact with the fluid-fluid interface. When this boundary condition is used, the fluid-fluid interface can move along the wall.

For laminar flow, this boundary condition enforces the no-penetration condition u ⋅ nwall = 0 on both sides of the wall, and adds a tangential stress on the form

where

and K is the viscous stress tensor. β is the slip length. For numerical calculations, a suitable choice is β = h, where h is the mesh element size.

For turbulent flow, the wall is modeled using automatic wall treatment or wall functions depending on the setting in the section of the turbulent flow interface settings.

If the surface tension force is included in the momentum equation, the boundary condition also adds the following boundary force on both sides of the walls to enforce the contact angle:

Note that the contact angle is not enforced on boundaries adjacent to porous domains. For more information, see The Wetted Wall Coupling Feature.

The is the default multiphysics coupling feature name.

The default (for the first multiphysics coupling feature in the model) is iww1.

This section controls which individual interfaces are coupled by the current coupling feature. If a physics interface is deleted and then added to the model again, then in order to reestablish the coupling, you need to choose the correct physics interface again from the or lists.

This section contains controls to describe the wall movement relative to the lab (or spatial) frame.

The setting controls the translational wall velocity, utr. The list is per default set to . 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.

Select from selection list in order to manually prescribe , 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. For 2D axisymmetric components when is selected in the physics interface properties, the

component of utr may also be specified.

When nodes are added from the context menu, select (the default) or select from the list to choose specific boundaries.

Specify the following two properties for the interior wetted wall:

•

The setting is per default set to . The slip length β is then defined as

, where

is the smallest element side (corresponds to the element size in the wall normal direction for boundary layer elements) and

is a user input. Select from selection list in order to manually prescribe β (SI unit: m).

•

Select an option from the — (the default) or . For enter a θw (SI unit: rad). The default is π/2 radians. For enter values or expressions for γs1 (SI unit: J/m2) and γs2 (SI unit: J/m2).