Free surface flows can be simulated when using the inhomogeneous (particle or mixture models) or the homogeneous model. Use the following rules-of-thumb to decide which model is appropriate:
Use the homogeneous model if the interface is distinct and well-defined everywhere.
If the interface is not well-defined in some locations, perhaps because one phase is entrained in the other, an inhomogeneous model may be more appropriate. For example, if there is significant splashing, air becomes entrained in the water and behaves as a dispersed phase. In this case, the particle model, using air as the dispersed phase, can give superior results because the unequal velocity fields allow the air and water phases to separate and form a distinct interface. The disadvantage of the inhomogeneous models is that they require more CPU time and memory to run. Note also that whichever inhomogeneous model you choose has the same limitations as discussed in the previous chapter; for example, the accuracy of the particle model decreases at high dispersed phase volume fractions.