### 3.2.19. Initial Conditions for a Multiphase Simulation

For a multiphase simulation, the initial conditions for some variables must be the same for all fluids. For example, in CFX, the pressure is assumed to be common for all fluids. However, for other variables, the initial conditions can be specified for each fluid individually.

Initial conditions for parameters common to both fluids are set in the same way as for single-phase simulations. The initial conditions that are fluid specific are set on the Fluid Specific Initialization frame. For details, see Fluid Specific Initialization in the CFX-Pre User's Guide. As with single-phase simulations, the initial conditions can be set to `Automatic` or `Automatic with Value`. For details, see Modeling Advice for Multiphase Flow.

#### 3.2.19.1. Volume Fraction

If a CFX-Solver default value is used for the Fluid Volume Fraction, it is distributed evenly among the fluids for which a volume fraction has not been assigned. For example, for two phase flow, an initial Fluid Volume Fraction of `0.5` would be assigned to each fluid if the defaults are used. For many simulations, including those containing a dilute fluid, using the Solver default values is not recommended. Instead, set a sensible value for the initial Fluid Volume Fraction using `Automatic with Value`. Solver default values are used when the `Automatic` option is selected without an initial values file, or when `Default` is selected. If values are entered for the Fluid Volume Fraction, they must sum to 1.

#### 3.2.19.2. Velocity

The initial velocity fields for each fluid should be slightly different to ensure a non-zero slip velocity. You should set the difference to a fraction of the expected terminal slip velocity.

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