## 23.3.1 Overview and Limitations of the VOF Model

Overview

The VOF model can model two or more immiscible fluids by solving a single set of momentum equations and tracking the volume fraction of each of the fluids throughout the domain. Typical applications include the prediction of jet breakup, the motion of large bubbles in a liquid, the motion of liquid after a dam break, and the steady or transient tracking of any liquid-gas interface.

Limitations

The following restrictions apply to the VOF model in FLUENT:

• You must use the pressure-based solver. The VOF model is not available with either of the density-based solvers.

• All control volumes must be filled with either a single fluid phase or a combination of phases. The VOF model does not allow for void regions where no fluid of any type is present.

• Only one of the phases can be defined as a compressible ideal gas. There is no limitation on using compressible liquids using user-defined functions.

• Streamwise periodic flow (either specified mass flow rate or specified pressure drop) cannot be modeled when the VOF model is used.

• The second-order implicit time-stepping formulation cannot be used with the VOF explicit scheme.

• When tracking particles in parallel, the DPM model cannot be used with the VOF model if the shared memory option is enabled (Section  22.11.9). (Note that using the message passing option, when running in parallel, enables the compatibility of all multiphase flow models with the DPM model.)

The VOF formulation in 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 fluid's volume fraction in the cell is denoted as , then the following three conditions are possible:

• : The cell is empty (of the fluid).

• : The cell is full (of the fluid).

• : The cell contains the interface between the fluid and one or more other fluids.

Based on the local value of , the appropriate properties and variables will be assigned to each control volume within the domain.

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