The following general form is used to model interphase drag force acting on phase due to phase :

(5–35)

Note that and . Hence, the sum over all phases of all interphase transfer terms is zero.

In this section, you will learn how the coefficients may be computed from a knowledge of dimensionless drag coefficients. The range of models available for drag coefficients is also described.

The total drag force is most conveniently expressed in terms of the dimensionless drag coefficient:

(5–36)

where is the fluid density, is the relative speed, is the magnitude of the drag force and is the projected area of the body in the direction of flow. The continuous phase is denoted by and the dispersed phase is denoted by .