The Wall node includes a set of boundary conditions describing fluid-flow conditions at stationary, moving, and leaking walls.

Select a Boundary condition for the wall.

No slip is the default boundary condition to model solid walls. A no slip wall is a wall where the fluid velocity relative to the wall velocity is zero. For a stationary wall that means that u = 0.

The Slip option prescribes a no-penetration condition, u·n=0. It is implicitly assumed that there are no viscous effects at the slip wall and hence, no boundary layer develops. From a modeling point of view, this can be a reasonable approximation if the main effect of the wall is to prevent fluid from leaving the domain.

When an electric field drives a flow along the boundary, the components for the Electric field E along with the Electroosmotic mobility μeo should be defined. The Built-in expression for the Electroosmotic mobility requires values or expressions for the Zeta potential ζ and the Relative permittivity εr.

In the microscale range, the flow condition at a boundary is seldom strictly no slip or slip. Instead, the boundary condition is something in between, and there is a Slip velocity at the boundary. Two phenomena account for this velocity: noncontinuum effects and the flow induced by a thermal gradient along the boundary.

When the Use viscous slip check box is selected, the default Slip length Ls is User defined. Another value or expression may be entered if the default value is not applicable. For Maxwell’s model values or expressions for the Tangential momentum accommodation coefficient av and the Mean free path λ should be specified. Tangential accommodation coefficients are typically in the range of 0.85 to 1.0 and can be found in G. Kariadakis, A. Beskok, and N. Aluru, Microflows and Nanoflows, Springer Science and Business Media, 2005.

When the Use thermal creep check box is selected, a thermal creep contribution with Thermal slip coefficient σT is activated. Thermal slip coefficients are typically between 0.3 and 1.0 and can be found in G. Kariadakis, A. Beskok, and N. Aluru, Microflows and Nanoflows, Springer Science and Business Media, 2005.

where β is a slip length, and 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.

The Slip Length setting is per default set to Factor of minimum element length. 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 User defined from Slip Length selection list in order to manually prescribe β (SI unit: m).

In cases where the wall movement is nonzero, check Account for the translational wall velocity in the friction force to use instead of in the friction force. Then, the extrapolated tangential velocity component is 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 fluid-fluid interface or 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 checked.

This boundary condition may be used to simulate a wall where fluid is leaking into or leaving the domain with the velocity u = ul through a perforated wall. The components of the Fluid velocity ul on the leaking wall should be specified.

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.

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 in the Show More Options dialog box. The Constraints settings can be set to Default, Use pointwise constraints, Use DG constraints, or Use weak constraints. Use mixed constraints can be selected when imposing a no slip condition exactly.