Entrainment is the transport of fluid across an interface between two bodies of fluid by a shear-induced turbulent flux. Entrainment is important in turbulent jets, plumes, and

gravity current In fluid dynamics, a gravity current or density current is a primarily horizontal flow in a gravitational field that is driven by a density difference in a fluid or fluids and is constrained to flow horizontally by, for instance, a ceiling. Typ ...

s and is a topic of current research.

History

The entrainment hypothesis was first used as a model for flow in plumes by G. I. Taylor. He was studying the use of oil drum fires to clear fog from airplane runways during

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...

. It became a common model of turbulence closure used in environmental and geophysical

fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics of fluids ( liquids, gases, and plasmas) and the forces on them. It has applications in a wide range of disciplines, including mechanical, aerospace, civil, chemical and ...

Applications

Eductors or

eductor-jet pump A vacuum ejector, or simply ejector is a type of vacuum pump, which produces vacuum by means of the Venturi effect. In an ejector, a working fluid (liquid or gaseous) flows through a jet nozzle into a tube that first narrows and then expands in ...

s make use of entrainment. They are used onboard ships to pump out flooded compartments: 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 A pump-jet, hydrojet, or water jet is a marine system that produces a jet of water for propulsion. The mechanical arrangement may be a ducted propeller ( axial-flow pump), a centrifugal pump, or a mixed flow pump which is a combination of bot ...

, which is used for marine propulsion. Jet pumps are also used to circulate reactor coolant in several designs of

boiling water reactor A boiling water reactor (BWR) is a type of light water nuclear reactor used for the generation of electrical power. It is a design different from a Soviet graphite-moderated RBMK. It is the second most common type of electricity-generating nu ...

s. In power generation, this phenomenon is used in steam jet air ejectors to maintain condenser vacuum by removing non-condensible gases from the condenser.

References

Fluid dynamics Turbulence {{Fluiddynamics-stub