Wall

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:

Select to set the velocity component normal to the wall to zero

This boundary condition enforces no-penetration at the wall,

, and adds a friction force of the form

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. In cases where the wall movement in non-zero, check to use

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 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, should not be selected.

The setting is per default set to . 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 from the list to manually prescribe β.

Select a — (the default), , , , or .

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For enter a

(dimensionless). The default is 0. When is selected under the section, click either the n0 (SI unit: 1/m3) or dd (SI unit: m) button and enter a value or expression for n0 (the default is 5·106 1/m3) or dd (the default is 1 mm).

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For enter values or expression for the

(SI unit: m/s) and, if is selected under the section, the Νn (SI unit: 1/(m2⋅s)). The defaults are 0 m/s and 0 1/(m2⋅s), respectively.

	Dispersed Phase Boundary Condition Equations

This section contains controls to describe the wall movement relative to the lab (or spatial) frame.

The setting controls the translational wall velocity, utr. The list is per default set to . 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 section.

Select from selection list in order to manually prescribe , 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 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.

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For 3D components, values or expressions for the uw should be specified. If the velocity vector entered is not in the plane of the wall, COMSOL Multiphysics projects it onto the tangential direction. Its magnitude is adjusted to be the same as the magnitude of the vector entered.