Slip ratio (also called *velocity ratio*) is a term used in gas liquid flow to denote the ratio of the velocities of the gas and liquid phases, respectively. Thus:

(1)

where U_{0} and U_{L} are the respective phase *superficial velocities*, ε_{G} is the void fraction (fraction of the crosssection occupied by the gas phase), ρ_{L} and ρ_{G} the liquid and gas densities, and x the quality (fraction of the total mass flow which is gas). Equation (1) can be rearranged to give a relationship between void fraction and slip ratio as follows:

(2)

Correlations for slip ratio are given in the article on Void Fraction.