FREE-Surface Flows (*i.e.* flows that involve the presence and interaction of two or more
fluids, separated by sharply defined interfaces) can be simulated by PHOENICS. Two
different methods are provided, namely:

- the Scalar Equation Method (SEM);
- the Height of Liquid (HOL)technique.

Both HOL and SEM employ a one-velocity-set solution procedure, and the different fluids separated by the distinct interface have only one value of each velocity component, temperature, concentration, etc for each computational cell. The relevant governing equations are solved in the conventional single-phase manner and the two fluids are accounted for through the specification of the physical properties (density, viscosity, etc).

The applicability and limitations of these options can be summarized as follows:

- The SEM technique deduces the interface position from the solution of a conservation
equation for a scalar "fluid-marker" variable, and in this respect can suffer
from numerical diffusion in coarse grids. The SEM is applicable to unsteady incompressible
flows only, in one-, two- or three- dimensions. It can simulate convoluted and overturning
surfaces. Since the SEM employs a fully explicit formulation it is constrained by the
Courant criterion for time-step increment for the stability of the solution. The SEM
generally works well for highly non- orthogonal grids with NONORT set to TRUE. It can also
cope with heat transfer between the fluids, and with conjugate heat transfer between the
fluids and surrounding and immersed solids.
See the entry on S-E-M (Scalar Equation Method) for instructions on how to activate it.

- The HOL method determines the location of the interface from the solution of the liquid-balance equations. The HOL method is applicable to both steady and unsteady incompressible isothermal flows, in one-, two- or three-dimensions. It is restricted to flows that are not convoluted (in interconnected blocked regions for example) and exhibit no "overturning" of the interface. The HOL method is fully implicit and therefore suffers no restrictions, for unsteady cases, on the time-step increments. The HOL method does not generate numerical diffusion which implies less emphasis on grid-size constraints. However, for highly non-orthogonal grids convergence problems may occur with the HOL method.

See the entry for H-O-L for instructions on how to activate it in PHOENICS.

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