1. n. []

A multiphase flow regime in which the lighter fluid flows in the center of the pipe, and the heavier fluid is contained in a thin film on the pipe wall. The lighter fluid may be a mist or an emulsion. Annular flow occurs at high velocities of the lighter fluid, and is observed in both vertical and horizontal wells. As the velocity increases, the film may disappear, leading to mist flow or emulsion flow. When the interface between the fluids is irregular, the term wavy annular flow may be used.

See: churn flow,  flow structure,  froth flow