Encyclopaedia Index

FREE-SURFACE-FLOW simulation in PHOENICS

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. Three different methods are provided, namely:

1. The Volume of Fluid Method (VOF);
2. the Scalar Equation Method (SEM);
3. the Height of Liquid (HOL) technique.

HOL, VOF and SEM all 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 VOF and SEM techniques deduce 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 VOF and SEM are applicable to unsteady incompressible flows only, in one-, two- or three- dimensions. They can simulate convoluted and overturning surfaces. VOF can also handle 3 separate phases.

  Since the VOF and SEM employ a fully explicit formulation they are constrained by the Courant criterion for time-step increment for the stability of the solution, VOF less so than SEM. The VOF and SEM generally work well for highly non- orthogonal grids with NONORT set to TRUE. They 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 VOF and SEM for instructions on how to activate them.

• 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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