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:
where U0 and UL are the respective phase superficial velocities, εG is the void fraction (fraction of the crosssection occupied by the gas phase), ρL and ρG 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:
Correlations for slip ratio are given in the article on Void Fraction.