### 7.5.2. The Homogeneous Model

The homogeneous model can be viewed as a limiting case of `Eulerian-Eulerian` multiphase flow in which the interphase transfer rate is very large. This results in all fluids sharing a common flow field, as well as other relevant fields such as turbulence. It is valid in the following circumstances:

- In a flow under gravity, where the phases have completely stratified, for example, a free surface flow where the interface is well defined. In this case, the volume fractions of the phases are equal to one or zero everywhere except at the phase boundaries, and it makes sense to use a single velocity field.

- If the flow is drag dominated, that is, is very large, and there are no body forces, the phase velocities will tend to equalize over very short spatial length scales. This can occur in dispersed flows of extremely small particles.

- The approximation does not in general apply to drag dominated multiphase flow under gravity that is not stratified, for example, droplets falling under gravity in a gas. In this case, the droplets will quickly attain a fixed slip velocity relative to the continuous phase, where the interphase drag balances the differences in body forces. In this case, the homogeneous model should only be used if the resulting slip velocities are very small relative to the mean flow.

Free surface flow is a common application of the homogeneous model. The section provides additional details for this specific application. For details, see Free Surface Flow.

To choose the homogeneous model for the momentum equation, select `Homogeneous Model` under Multiphase Options. Once this is done, the bulk momentum equation is solved to obtain the shared velocity field. The use of inhomogeneous turbulence models is also disabled.

When the homogeneous model (under Multiphase Options) is selected, a model for interphase momentum transfer is no longer needed, although the Interphase Transfer Model setting (on the Fluid Pairs tab) may still need to be specified due to other processes that require interphase transfer modeling. Some examples are:

- The heat transfer model may be inhomogeneous. This is required when applying the `Thermal Phase Change` model for interphase mass transfer. For details, see Thermal Phase Change Model.

- Other mass transfer models (other than cavitation) also require use of the interfacial area density.

- `Interphase Additional Variable Transfer` models. For details, see Additional Variable Interphase Transfer Models.

In these situations, the Interphase Transfer Model setting for fluid pairs (on the Fluid Pairs tab, for a particular pair of fluids) must be set to `Particle Model`, `Mixture Model` or `Free Surface Model`. Additional information on these models is available; for details, see

- The Inhomogeneous (Interfluid Transfer) Model

- Interfacial Area Density in the CFX-Solver Theory Guide.

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