Wall

The Wall feature includes a set of boundary conditions describing the fluid flow condition at a wall.

•No Slip

•Slip

•Sliding Wall

•Moving Wall

•Leaking Wall

•Slip Velocity

•Velocity of Moving Wall

•Viscous Slip and Slip Length

•Thermal Creep

•Wall Functions

•Sliding Wall (Wall Functions)

•Moving Wall (Wall Functions)

B O U N D A R Y S E L E C T I O N

From the Selection list, choose the boundaries that represent solid walls.

P A I R S E L E C T I O N

If Wall is selected from the Pairs menu, choose the pair to define. An identity pair has to be created first. Ctrl-click to deselect.

B O U N D A R Y C O N D I T I O N

Select a Boundary condition for the wall. See Table 4-2 and Table 4-3 for available boundary conditions by interface.

TABLE 4-2: SINGLE-PHASE FLOW, LAMINAR WALL BOUNDARY CONDITIONS

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TABLE 4-3: SINGLE-PHASE FLOW, TURBULENT WALL BOUNDARY CONDITIONS

No Slip

No slip is the default boundary condition for a stationary solid wall. The condition prescribes u = 0, that is, that the fluid at the wall is not moving.

Slip

The Slip condition assumes that there are no viscous effects at the slip wall and hence, no boundary layer develops. From a modeling point of view, this may be a reasonable approximation if the important effect of the wall is to prevent fluid from leaving the domain.

• Theory for the Slip Wall Boundary Condition in the COMSOL

Multiphysics User’s Guide

See Also

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Sliding Wall

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. The wall does not have to actually move in the coordinate system.

• Theory for the Sliding Wall Boundary Condition in the COMSOL

Multiphysics User’s Guide

See Also

In 2D, the tangential direction is unambiguously defined by the direction of the boundary, but the situation becomes more complicated in 3D. For this reason, this boundary condition has slightly different definitions in

the different space dimensions. Enter the components of the Velocity of

2D the tangentially moving wall Uw (SI unit: m/s). In axial symmetry, if Swirl flow is selected in the interface properties, also specify a velocity, vw, in thedirection.

Enter the components of the Velocity of the sliding wall uw (SI unit: m/s). If the velocity vector entered is not in the plane of the wall, COMSOL

projects it onto the tangential direction. Its magnitude is adjusted to be the

3D

same as the magnitude of the vector entered.

Moving Wall

If the wall moves, so must the fluid. Hence, this boundary condition prescribes u = uw. Enter the components of the Velocity of moving wall uw (SI unit: m/s).

Specifying this boundary condition 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

Important

frame.

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• The Moving Mesh Interface in the COMSOL Multiphysics User’s

Guide

See Also

Leaking Wall

Use this boundary condition to simulate a wall where fluid is leaking into or leaving through a perforated wall u = ul. Enter the components of the Fluid velocity ul

(SI unit: m/s).

Slip Velocity

In the microscale range, the flow 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: the noncontinuum effect in viscosity and flow induced by a thermal gradient along the boundary.

• Theory for the Slip Velocity Wall Boundary Condition

See Also

Velocity of Moving Wall

Enter the components for the Velocity of moving wall uw (SI unit: m/s).

Viscous Slip and Slip Length

If the Use viscous slip check box is selected, the default Slip length Ls (SI unit: m) is User defined. Enter another value or expression if the default of 1·10 7 m is not applicable.

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If Maxwell’s Model is selected, enter values or expressions for the Tangential momentum accommodation coefficient av and the Mean free path (SI unit: m).

The 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, Tip Microflows and Nanoflows, Springer Science and Business Media, 2005.

Thermal Creep

If the Use thermal creep check box is selected, it activates the thermal creep component of the boundary condition. Enter the fluid’s Temperature T (SI unit: K) and the Thermal slip coefficient T. The default temperature is 293.15 K and the default thermal slip coefficient is 0.75.

The 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

Tip Nanoflows, Springer Science and Business Media, 2005.

M O R E W A L L B O U N D A R Y C O N D I T I O N S F O R T H E T U R B U L E N T F L O W

I N T E R F A C E S

Wall Functions

The Wall Functions boundary condition applies wall functions to a solid walls in a turbulent flow. Wall functions are used to model the thin region near the wall with high gradients in the flow variables.

Sliding Wall (Wall Functions)

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. Enter the component values or expressions for the Velocity of sliding wall uw (SI unit: m/s).

Moving Wall (Wall Functions)

Specifying this boundary condition does not automatically cause the

associated wall to move.

Important

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