The Wall node includes a set of boundary conditions describing fluid-flow conditions at stationary, moving, and leaking walls. For turbulent flow, the description may involve wall functions and/or asymptotic expressions for certain turbulence variables.

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.

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 Physical Model section of the interface settings.

When Wall Treatment is set to Wall functions, the Apply wall roughness option becomes available. When Apply wall roughness is selected, a Sand roughness model, derived from the experiments by Nikuradse, is applied. Select Generic roughness in order to specify more general roughness types.

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.

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.

Slip velocity is available when Turbulence Model in the Physical Model section of the interface is set to None.

This boundary condition enforces no-penetration at the wall, and adds a frictional force of the form

where β is a slip length. The slip length is defined as , where is the smallest element side (corresponds to the element size in the wall normal direction for boundary layer elements). 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.

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.

Leaking Wall is available when Turbulence Model in the Physical Model section of the interface is set to None.

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 button () and selecting Advanced Physics Options.