Free surface flow refers to a multiphase flow situation where the phases are separated by a distinct interface. Examples of free surface flows include open channel flow, flow around ship hulls, tank filling problems, Pelton turbines, and many others.

Free surface flow with the homogeneous model should be used when possible. Free surface flow with the inhomogeneous model can be used to enable the two phases to separate. This will be required if entrainment of one phase within another occurs and you want to allow the phases to separate again.