## 23.10.3 Defining the Phases for the VOF Model

Instructions for specifying the necessary information for the primary and secondary phases and their interaction in a VOF calculation are provided below.

 In general, you can specify the primary and secondary phases whichever way you prefer. It is a good idea, especially in more complicated problems, to consider how your choice will affect the ease of problem setup. For example, if you are planning to patch an initial volume fraction of 1 for one phase in a portion of the domain, it may be more convenient to make that phase a secondary phase. Also, if one of the phases is a compressible ideal gas, it is recommended that you specify it as the primary phase to improve solution stability.

 Recall that only one of the phases can be a compressible ideal gas. Be sure that you do not select a compressible ideal gas material (i.e., a material that uses the compressible ideal gas law for density) for more than one of the phases. See Sections  23.10.5 and 23.11.3 for details.

Defining the Primary Phase

To define the primary phase in a VOF calculation, perform the following steps:

1.   Select phase-1 in the Phase list.

2.   Click Set... to open the Primary Phase panel (Figure  23.10.2) .

3.   In the Primary Phase panel, enter a Name for the phase.

4.   Specify which material the phase contains by choosing the appropriate material in the Phase Material drop-down list.

5.   Define the material properties for the Phase Material.

(a)   Click Edit..., and the Material panel will open.

(b)   In the Material panel, check the properties, and modify them if necessary. (See Chapter  8 for general information about setting material properties, Section  23.10.5 for specific information related to compressible VOF calculations, and Section  23.10.6 for specific information related to melting/solidification VOF calculations.)

 If you make changes to the properties, remember to click Change before closing the Material panel.

6.   Click OK in the Primary Phase panel.

Defining a Secondary Phase

To define a secondary phase in a VOF calculation, perform the following steps:

1.   Select the phase (e.g., phase-2) in the Phase list.

2.   Click Set... to open the Secondary Phase panel (Figure  23.10.3) .

3.   In the Secondary Phase panel, enter a Name for the phase.

4.   Specify which material the phase contains by choosing the appropriate material in the Phase Material drop-down list.

5.   Define the material properties for the Phase Material, following the procedure outlined above for setting the material properties for the primary phase.

6.   Click OK in the Secondary Phase panel.

Including Surface Tension and Wall Adhesion Effects

As discussed in Section  23.3.8, the importance of surface tension effects depends on the value of the capillary number, Ca (defined by Equation  23.3-16), or the Weber number, We (defined by Equation  23.3-17). Surface tension effects can be neglected if Ca  or We  .

 Note that the calculation of surface tension effects will be more accurate if you use a quadrilateral or hexahedral mesh in the area(s) of the computational domain where surface tension is significant. If you cannot use a quadrilateral or hexahedral mesh for the entire domain, then you should use a hybrid mesh, with quadrilaterals or hexahedra in the affected areas. FLUENT also offers an option to use VOF gradients at the nodes for curvature calculations on meshes when more accuracy is desired. For more information, see Section  23.3.8.

If you want to include the effects of surface tension along the interface between one or more pairs of phases, as described in Section  23.3.8, click Interaction... to open the Phase Interaction panel (Figure  23.10.4).

Perform the following steps to include surface tension (and, if appropriate, wall adhesion) effects along the interface between one or more pairs of phases:

1.   Click the Surface Tension tab.

2.   If you want to include wall adhesion, turn on the Wall Adhesion option. When Wall Adhesion is enabled, you will need to specify the contact angle at each wall as a boundary condition (as described in Section  23.9.8).

3.   For each pair of phases between which you want to include the effects of surface tension, specify a constant surface tension coefficient. Alternatively you can specify a temperature dependent, polynomial, piece-wise polynomial, piecewise linear, or a user-defined surface tension coefficient. See Section  23.3.8 for more information on surface tension, and the separate UDF Manual for more information on user-defined functions. All surface tension coefficients are equal to 0 by default, representing no surface tension effects along the interface between the two phases.

 For calculations involving surface tension, it is recommended that you also turn on the Implicit Body Force treatment for the Body Force Formulation in the Multiphase Model panel. This treatment improves solution convergence by accounting for the partial equilibrium of the pressure gradient and surface tension forces in the momentum equations. See Section  23.9.4 for details.

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