Entrainment is the transport of fluid across an interface between two bodies of fluid by a shear induced turbulent flux.
The entrainment hypothesis was first used as a model for flow in plumes by G. I. Taylor when studying the use of oil drum fires to clear fog from aeroplane runways during World War II. It has gone on to be a common model of turbulence closure used in environmental and geophysical fluid mechanics.
Entrainment is important in turbulent jets, plumes and gravity currents and is a topic of current research.
Eductors or eductor-jet pumps are an example of entrainment. They are used onboard many ships to pump out flooded compartments: in the event of an accident, seawater is pumped to the eductor and forced through a jet, and any fluid at the inlet of the eductor is carried along to the outlet and up and out of the compartment. Eductors can pump out whatever can flow through them, including water, oil, and small pieces of wood. Another example is the pump-jet, which is used for marine propulsion. Jet pumps are also used to circulate reactor coolant in several designs of boiling water reactors.
In power generation, this phenomenon is used in steam jet air ejectors to maintain condenser vacuum by removing non-condensible gases from the condenser.