### 10.3.1. Setup

The choice of using a steady-state or transient simulation is problem-dependent. There are two models available for free surface flow: homogeneous and inhomogeneous. The homogeneous model can be used when the interface between the two phases remains well defined and none of the dispersed phase becomes entrained in the continuous phase. An example of homogeneous free surface flow is flow in an open channel. A breaking wave is one example of an inhomogeneous flow case.

The same choice of turbulence model as for single phase simulations is appropriate. When using the inhomogeneous model, you should use the homogeneous turbulence option in CFX-Pre. The Buoyancy Reference Density should be set to the density of the least dense fluid.

When setting boundary conditions, the volume fractions can be set up using step functions to set the liquid height at each boundary. An outlet boundary having supercritical flow should use the Supercritical option for Mass And Momentum. This requires that you set the relative pressure of the gas above the free surface at the outlet.

For most free surface applications, the initial conditions can use step functions to set the volume fractions of the phases as a function of height. The initial condition for pressure should be set to hydrostatic conditions for the specified volume fraction initialization and the buoyancy reference density.

The timestep for free surface flows should be based on a L/U (Length/Velocity) scale. The length scale should be a geometric length scale. The velocity scale should be the maximum of a representative flow velocity and a buoyant velocity, which is given by:

In addition, it is sometimes helpful to reduce the timestep for the volume fraction equations by an order of magnitude below that of the other equations.

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