In fluids composed of multiple components, e.g., fluid/particles, fluid/bubbles, fluid/fluid mixtures, where the components have different densities, it is observed that the components can assume different flow velocities. Velocity differences arise because the density differences result in non-uniform body forces. Often the differences in velocities can be very pronounced, for example, large raindrops falling through air or gravel sinking in water. Under many conditions, however, the relative velocities are small enough to be described as a “drift” of one component through the other. Examples are dust in air and silt in water.
The “drift” distinction has to do with whether or not the inertia of a dispersed component moving in a continuous component is significant. If the inertia of relative motion can be ignored, and the relative velocity reduced to a balance between a driving force (say gravity or a pressure gradient) and an opposing drag force between the components, then we can speak of a “drift-flux” approximation. Drift velocities are primarily responsible for the transport of mass and energy. Some momentum may be transported as well, but this is usually quite small and has been neglected in the FLOW-3D1 drift model. A more complete analysis of when the “drift” assumption is valid can be found in the Flow Science, Inc. Technical Note, FSI-08-TN77.