The Laminar Two-Phase Flow, Level Set () interface contains an optional multiphysics coupling boundary feature, Wetted Wall. It is an exclusive feature that overrides the Wall feature in the Laminar interface as well as the No Flow feature in the Level Set interface. It cannot be used in boundaries that have a Flow Continuity feature in the flow physics interface or a Continuity feature in the level set physics interface.

The Wetted Wall boundary condition is suitable for walls 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, when the Wall condition is set to Navier slip, this boundary condition enforces the no-penetration condition u ⋅ nwall = 0 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. The boundary condition does not set the tangential velocity component to zero. However, the extrapolated tangential velocity component is 0 at a distance β outside the wall (see Figure 4-1). When the Wall condition is set to Nonlinear Navier slip or Asymptotic slip, the frictional force has a nonlinear dependency on the velocity (see Wall).If the surface tension force is included in the momentum equation in The Two-Phase Flow, Level Set Coupling Feature, the Wetted Wall boundary condition also adds the following boundary force to enforce the contact angle:

The contact angle θw is defined as the angle between the fluid interface and the wall (see Figure 4-1). For more information, see Ref. 4 and Ref. 5. In order to properly account for direction and yield stress, when using Hatzikiriakos slip in the Wall condition, the surface tension contribution has to be combined with the viscous stress in the expression for the slip velocity,

where , K is the viscous stress tensor, and twall is a unit vector tangential to the wall, in the direction of the projected interface normal.

The contact angle θw can be defined directly or from Young's equation, which considers the components of the forces in the plane of the surface:

where γs1 is the surface energy density on the fluid 1 — solid (wall) interface and γs2 is the surface energy density on the fluid 2 — solid (wall) interface.

The Label is the default multiphysics coupling feature name.

The default Name (for the first Wetted Wall multiphysics coupling feature in the model) is ww1.

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

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 Fluid flow or Moving interfaces lists.

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.

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.

Translational velocity can also be set to Zero (Fixed wall) to force utr = 0. This is typically used in problems where the mesh is deforming or rotating, but we want the boundary to keep zero velocity. For example, in fixed walls tangential to a Rotating Domain.