
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:

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

Defining a Secondary Phase
To define a secondary phase in a VOF calculation, perform the following steps:
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 23.316), or the Weber number, We (defined by Equation 23.317). 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:

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.
