A two-phase flow regime in a pipe or tube characterized by the oscillatory transition from a continuous liquid phase to a continuous and predominant gas phase. It occurs when gas bubbles coalesce and liquid becomes entrained in the bubbles. At high gas velocities, Taylor bubbles that have been formed in plug flow break down into an unstable pattern in which there is a churning or oscillatory motion of liquid. Churn flow may be regarded as a breaking-up of plug flow with an occasional bridging across the pipe by the liquid phase. At high gas flow rates, it may be considered as a degenerative form of annular flow with the direction of the film flow changing and large waves being formed on the interface for which the term semi-annular flow is occasionally used.