TABLE 6-5: BOUNDARY CONDITIONS FOR THE LEVEL SET AND PHASE FIELD INTERFACES

1. In addition to the Link to Documentation column, see Boundary Conditions for the Level Set Function. 2. In addition to the link to documentation column, see Boundary Conditions for the Phase Field Function.

• Boundary Conditions for the Single-Phase Flow Interfaces

See Also

Wall

The Wall node represents the walls in a two-phase flow fluid-flow simulation. It includes slip and no-slip boundaries as well as sliding, leaking, moving, and wetted walls. The No-slip boundary condition is the default boundary condition.

See Table 6-6 and Table 6-7 for the available wall boundary conditions for the level set and phase field interfaces. Most of the boundary conditions are described for the flow variables in single-phase flow (see Wall), except for Wetted Wall and Moving Wetted Wall, which are described in this section.

TABLE 6-6: MULTI-PHASE FLOW, LAMINAR WALL BOUNDARY CONDITIONS

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TABLE 6-6: MULTI-PHASE FLOW, LAMINAR WALL BOUNDARY CONDITIONS

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 walls.

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

Select a Boundary condition for the wall (additional two-phase flow conditions only). See Wall for the Laminar Flow interface for most of the settings.

Wetted Wall

This option is available for The Laminar Two-Phase Flow, Level Set

Interface and The Laminar Two-Phase Flow, Phase Field Interface.

Note

The Wetted wall boundary condition is suitable for walls in contact with the fluid-fluid interface. If this boundary condition is used, the fluid-fluid interface can move along the wall. For applications where the interface is fixed on the wall, the No slip condition is suitable.

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The implementation of the Wetted wall is different depending on the method used for tracking the fluid-fluid interface (the level set method or the phase field method).

Level Set For the level set method, this boundary condition enforces the slip condition u nwall and adds a frictional force of the form

Ffr = –-- u

where is the slip length. For numerical calculations it is suitable to set h, where h is the mesh element size. The boundary condition does not set the tangential velocity component to zero; however, the extrapolated tangential velocity component is 0 at the distance outside the wall (see Figure 6-2).

Finally, the boundary condition adds the following weak boundary term:

test u nwall – ncos w dS

The boundary term is a result of the partial integration of the surface tension force in the Navier-Stokes equations. Define the contact angle w (that is, the angle between the fluid interface and the wall). Figure 6-2 illustrates the definition of the contact angle.

Figure 6-2: Definition of the contact angle at interface/wall contact points (left) and illustration of the slip length (right).

With the level set method, define the following two properties for the wetted wall:

•Enter a value or expression for the Contact angle w. The default is pi/2 ( /2) rad.

•Enter a value or expression for the Slip length (SI unit: m). The default is h, which is the variable for the local mesh element size h.

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