If you are using the time-dependent VOF formulation in FLUENT, an explicit solution for the volume fraction is obtained either once each time step or once each iteration, depending upon your inputs to the model. You also have control over the time step used for the volume fraction calculation.
To compute a time-dependent VOF solution, you will need to enable the Unsteady option in the Solver panel (and choose the appropriate Unsteady Formulation, as discussed in Section 25.17.1). If you choose the Explicit scheme, FLUENT will turn on the first-order unsteady formulation for you automatically, so you need not visit the Solver panel yourself.
Define Models Solver...
There are two inputs for the time-dependent calculation for the VOF model:
If you want
FLUENT to solve the volume fraction equation(s) at every iteration within a time step, use the text command:
define models multiphase
and select vof as the model. When prompted to solve vof every iteration?, enter yes. When FLUENT solves these equations every iteration, the convective flux coefficients in the other transport equations will be updated based on the updated volume fractions at each iteration. This choice is the less stable of the two, and requires more computational effort per time step than the default choice.
| If you are using sliding meshes, or dynamic meshes with layering and/or remeshing, using the
solve vof every iteration? option will yield more accurate results, although at a greater computational cost.
In the region near the fluid interface, FLUENT divides the volume of each cell by the sum of the outgoing fluxes. The resulting time represents the time it would take for the fluid to empty out of the cell. The smallest such time is used as the characteristic time of transit for a fluid element across a control volume, as described above. Based upon this time and your input for the maximum allowed Courant Number in the Multiphase Models panel, a time step is computed for use in the VOF calculation. For example, if the maximum allowed Courant number is 0.25 (the default), the time step will be chosen to be at most one-fourth the minimum transit time for any cell near the interface.
Note that these inputs are not required when the implicit scheme is used.