The VOF formulation in ANSYS FLUENT is generally used to compute a time-dependent solution, but for problems in which you are concerned only with a steady-state solution, it is possible to perform a steady-state calculation. A steady-state VOF calculation is sensible only when your solution is independent of the initial conditions and there are distinct inflow boundaries for the individual phases. For example, since the shape of the free surface inside a rotating cup depends on the initial level of the fluid, such a problem must be solved using the time-dependent formulation. On the other hand, the flow of water in a channel with a region of air on top and a separate air inlet can be solved with the steady-state formulation.
The VOF formulation relies on the fact that two or more fluids (or phases) are not interpenetrating . For each additional phase that you add to your model, a variable is introduced: the volume fraction of the phase in the computational cell. In each control volume, the volume fractions of all phases sum to unity. The fields for all variables and properties are shared by the phases and represent volume-averaged values, as long as the volume fraction of each of the phases is known at each location. Thus the variables and properties in any given cell are either purely representative of one of the phases, or representative of a mixture of the phases, depending upon the volume fraction values. In other words, if the q fluid’s volume fraction in the cell is denoted as aq, then the following three conditions are possible:
- aq = 0 : The cell is empty (of the fluid).
- aq = 1 : The cell is full (of the fluid).
- 0 < aq < 1 : The cell contains the interface between the fluid and one or more other fluids.
Based on the local value of aq, the appropriate properties and variables will be assigned to each control volume within the domain.
ANSYS FLUENT TUTORIAL – Multiphase Flow