### 5.5.4. Interphase Drag for the Free Surface Model

In the free surface model, interphase drag is calculated in the same way as for the mixture model (see Interphase Drag for the Mixture Model), except that the interfacial area density is given by:

| (5–68)

When more than two phases are present, this is generalized as follows:

| (5–69)

[The Free Surface Model]

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