24.3.4 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  24.3.6 and 24.4.3 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 Phases list.

2.   Click Edit... to open the Primary Phase dialog box (Figure  24.3.5) .

Figure 24.3.5: The Primary Phase Dialog Box

3.   In the Primary Phase dialog box, 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 Edit Material dialog box will open.

(b)   In the Edit Material dialog box, check the properties, and modify them if necessary. (See Chapter  8 for general information about setting material properties, Section  24.3.6 for specific information related to compressible VOF calculations, and Section  24.3.7 for specific information related to melting/solidification VOF calculations.)

If you make changes to the properties, remember to click Change before closing the Edit Material dialog box.

6.   Click OK in the Primary Phase dialog box.

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 Phases list.

2.   Click Edit... to open the Secondary Phase dialog box (Figure  24.3.6) .

Figure 24.3.6: The Secondary Phase Dialog Box for the VOF Model

3.   In the Secondary Phase dialog box, 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 dialog box.

Including Surface Tension and Wall Adhesion Effects

As discussed in this section in the separate Theory Guide , the importance of surface tension effects depends on the value of the capillary number, Ca (defined by this equation in the separate Theory Guide), or the Weber number, We (defined by this equation in the separate Theory Guide). Surface tension effects can be neglected if Ca  $\gg 1$ or We  $\gg 1$.

Several surface tension options are provided through the text user interface (TUI) using the solve/set/surface-tension command :

solve $\rightarrow$ set $\rightarrow$ surface-tension

The surface-tension command prompts you for the following information:

• whether you require node-based smoothing

  The default value is no indicating that cell-based smoothing will be used for the VOF calculations.

• the number of smoothings

  The default value is 1. A higher value can be used in case of tetrahedral and triangular meshes in order to reduce any spurious velocities.

• the smoothing relaxation factor

  The default is 1. This is useful in the cases where VOF smoothing causes a problem (e.g., liquid enters through the inlet with wall adhesion on).

• whether you want to use VOF gradients at the nodes for curvature calculations

  With this option, ANSYS FLUENT uses VOF gradients directly from the nodes to calculate the curvature for surface tension forces. The default is yes which produces better results with surface tension compared to gradients that are calculated at the cell centers.

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. ANSYS 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 this section in the separate Theory Guide.

If you want to include the effects of surface tension along the interface between one or more pairs of phases, as described in this section in the separate Theory Guide , click Interaction... to open the Phase Interaction dialog box (Figure  24.3.7).

Figure 24.3.7: The Phase Interaction Dialog Box for the VOF Model ( Surface Tension Tab)

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.   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 this section in the separate Theory Guide 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 dialog box. 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  24.2.5 for details.

3.   If you want to include wall adhesion, enable 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  24.2.9).

The contact angle $\theta_w$ is the angle between the wall and the tangent to the interface at the wall, measured inside the phase listed in the left column under Wall Adhesion in the Momentum tab of the Wall dialog box. For example, if you are setting the contact angle between the oil and air phases in the Wall dialog box shown in Figure  24.3.8, $\theta_w$ is measured inside the oil phase, as seen in Figure  24.3.9. For more information, refer to this section in the separate Theory Guide.

Figure 24.3.8: The Wall Dialog Box for a Mixture in a VOF Calculation with Wall Adhesion

Figure 24.3.9: Measuring the Contact Angle