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 for a stationary solid wall for laminar flow (and SST, Low Re k-ε, Algebraic yPlus, L-VEL, and Spalart-Allmaras turbulence models). The condition prescribes u = 0; that is, the fluid at the wall is not moving.

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.

The Sliding wall boundary condition 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.

For an arbitrary wall movement, the condition u = uw may be prescribed. In this case, the components of the Velocity of moving wall uw should be specified.

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.

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. The components of Velocity of moving wall: uw should be specified. Zero values are used for a stationary wall.

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.

The Wall functions boundary condition replaces the no-slip condition on solid walls for certain turbulence models such as the k-ε and k-ω models. Wall functions are used to model the thin region with large gradients in flow variables near the wall.

When Apply wall roughness is selected, a Sand roughness model, derived from the experiments by Nikuradse, is applied. More general roughness types can be specified by selecting Generic roughness.

The Sliding wall (wall functions) boundary condition applies wall functions to a wall in a turbulent flow where the velocity magnitude in the tangential direction of the wall is prescribed. The tangential direction is determined in the same manner as in the Sliding Wall feature. The component values or expressions for the Velocity of sliding wall uw should be specified.

The Apply wall roughness option is available for the Sliding wall (wall functions). See Wall Functions for details.

Applying this boundary condition does not automatically cause an associated geometry change. The Moving wall (wall functions) boundary condition applies wall functions to a wall with prescribed velocity uw in a turbulent flow. Component values or expressions for the Velocity of moving wall uw should be specified.

The Apply wall roughness option is available for the Moving wall (wall functions). See Wall Functions for details.

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