### 2.7.3. Wall Contact Model

The Wall Contact Model setting controls how the wall area fraction is calculated for the purpose of partitioning the wall values of total shear stress, and other variables, into variables for the phase-specific contributions.

The options are:

- `Use Volume Fraction`

  For the momentum equation, the volume fraction of a given phase is multiplied by the total wall shear stress to determine the component of wall shear stress that is due to that phase. For energy and other scalar equations, an analogous procedure is applied.

- `Specify Area Fraction`

  For the momentum equation, the area fraction of a given phase is multiplied by the total wall shear stress to determine the component of wall shear stress that is due to that phase. For energy and other scalar equations, an analogous procedure is applied.

  This option is suitable, for example, in the case of a thin liquid film whose thickness is smaller than the nearest node distance from the wall. In this case, the liquid volume fraction may be very small, but the area fraction of the liquid is one. You may set area fractions of one or zero to model films.

  For a case where the wall is partially covered by liquid film, the liquid area fraction may be a complicated function of liquid volume fraction. In such a case, you can use CEL to implement a complex model for the wall contact area fraction.

For details on the area contact model, see Area Contact Model.

Note: If the RPI boiling model is used then the Wall Contact Model option is used for the momentum equation but not the energy equation. For details, see Using the Wall Boiling Model.

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