Slip ratio (gas–liquid flow)
Slip ratio (or velocity ratio) in gas–liquid (two-phase) flow, is defined as the ratio of the velocity of the gas phase to the velocity of the liquid phase. In the homogeneous model of two-phase flow, the slip ratio is by definition assumed to be unity (no slip).
Source: Wikipedia — Slip ratio (gas–liquid flow) (CC BY-SA 4.0)