An intermittent two-phase gas–liquid flow regime found in vertical pipes or tubes often referred to as slug flow characterized by bullet-shaped bubbles, which tend in size towards the diameter of the pipe. In the continuous liquid phase, the higher-velocity gas bubbles coalesce to form rising plugs or slugs. A Taylor bubble is the liquid film around the plug that may move downwards at low velocity. The liquid between Taylor bubbles often contains a dispersion of smaller bubbles.