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Instructions for specifying the necessary information for the primary and secondary phases and their interaction in a VOF calculation are provided below.
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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.
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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.
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Defining the Primary Phase
To define the primary phase in a VOF calculation, perform the following steps:
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If you make changes to the properties, remember to click
Change before closing the
Edit Material dialog box.
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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
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
or We
.
Several surface tension options are provided through the text user interface (TUI) using the solve/set/surface-tension command :
solve
set
surface-tension
The surface-tension command prompts you for the following information:
The default value is no indicating that cell-based smoothing will be used for the VOF calculations.
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 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).
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.
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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.
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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).
Perform the following steps to include surface tension (and, if appropriate, wall adhesion) effects along the interface between one or more pairs of phases:
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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.
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The contact angle
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,
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.