To specify the VOF formulation to be used, select the appropriate VOF Scheme under VOF Parameters in the Multiphase Model panel.
The VOF formulations that are available in FLUENT are the Explicit and Implicit schemes.
Note that FLUENT will automatically turn on the unsteady formulation with first-order discretization for time in the Solver panel.
To use this formulation, make sure Explicit is selected as the VOF Scheme in the Multiphase panel, then select Geo-Reconstruct as the Volume Fraction Discretization scheme in the Solution Controls panel.
The Donor-Acceptor scheme is used when Explicit is selected as the VOF Scheme in the Multiphase panel. Initially, this formulation is not available in the GUI. To make it available, use the following text command:
solve set expert
You will be asked a series of questions, one of which is
Allow selection of all applicable discretization schemes? [no]
If your response is yes, then many more discretization schemes will be available for your selection. You can now use this formulation by selecting Donor-Acceptor as the Volume Fraction Discretization in the Solution Controls panel.
While the explicit time-dependent formulation is less computationally expensive than the geometric reconstruction scheme, the interface between phases will not be as sharp as that predicted with the geometric reconstruction scheme. To reduce this diffusivity, it is recommended that you use the second-order discretization scheme for the volume fraction equations. In addition, you may want to consider turning the geometric reconstruction scheme back on after calculating a solution with the implicit scheme, in order to obtain a sharper interface.
| For the geometric reconstruction and donor-acceptor schemes,
if you are using a conformal grid (i.e., if the grid node locations are identical at the boundaries where two subdomains meet), you must ensure that there are no two-sided (zero-thickness) walls within the domain. If there are, you will need to slit them, as described in Section
| The issues discussed above for the explicit time-dependent formulation also apply to the implicit steady-state and time-dependent formulations, described below. You should take the precautions described above to improve the sharpness of the interface.
To use this formulation, select Implicit as the VOF Scheme, and enable an Unsteady calculation in the Solver panel (opened with the Define/Models/Solver... menu item).
To help you determine the best formulation to use for your problem, examples that use different formulations are listed below:
Use the explicit scheme (time-dependent with the geometric reconstruction scheme or the donor-acceptor) if problems occur with the geometric reconstruction scheme.
Use the time-dependent with the implicit interpolation scheme.
Use the steady-state with the implicit interpolation scheme.