The Wall node has boundary conditions available that describe the existence of a solid wall.

Select a Mixture boundary condition for the wall.

No slip is the default boundary condition for the liquid. A no slip wall is a wall where the mixture velocity relative to the wall velocity is zero. For stationary walls, it sets the mixture velocity to zero at the wall:

For turbulent flows, the no slip condition 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 functions are selected, the Apply wall roughness option becomes available. Click to select the Apply wall roughness check box to apply the roughness correction. When the check box is selected, the default Roughness model is Sand roughness, which is derived from the experiments by Nikuradse. Select Generic roughness to specify more general roughness types.

Select Slip to set the velocity component normal to the wall to zero j ⋅ nwall = 0.

This boundary condition enforces no-penetration at the wall, j ⋅ nwall = 0, and adds a friction force of the form

where β is a slip length, and jtang = j − (j ⋅ nwall) nwall is the velocity tangential to the wall. 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 of the wall. In cases where the wall movement is nonzero, select the Account for the translational wall velocity in the friction force check box to use j − ubnd − ((j − ubnd) ⋅ nwall) nwall instead of jtang in the friction force. Then, the extrapolated tangential velocity component is ubnd at a distance β outside of the wall. Note that the Velocity of sliding wall uw is always accounted for in the friction force.

The Navier Slip boundary condition is suitable for walls adjacent to a free surface when solving for laminar flow. Applying this boundary condition, the contact line (fluid-fluid-solid interface) is free to move along the wall. Note that in problems with contact lines, the tangential velocity of the wall typically represents the movement of the contact line but the physical wall is not moving. In such cases, Account for the translational wall velocity in the friction force should not be selected.

The Slip length setting is per default set to Factor of minimum element length. The slip length β is then defined as β = fhhmin, where hmin is the smallest element side (corresponds to the element size in the wall normal direction for boundary layer elements) and fh is a user input. Select User defined from the Slip length list to manually prescribe β.

Select a Dispersed phase boundary condition — No dispersed phase flux (the default), Dispersed phase concentration, Dispersed phase outlet, Dispersed phase flux, or Symmetry.

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 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.

The Sliding wall option is appropriate if the wall behaves like a conveyor belt; that is, the surface is sliding in its tangential direction. A velocity is prescribed at the wall and the boundary itself does not have to actually move relative to the reference frame.

This section is displayed by clicking the Show More Options button () and selecting Advanced Physics Options.