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- See entrainment for other types.
Entrainment is the movement of one fluid by another.
One fluid moving in another can push or pull the other along with it. Eductors or eductor-jet pumps are an excellent example. 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 (BWRs).
In power generation, this phenomenon is used in steam jet air ejectors to maintain condenser vacuum by removing non-condensible gases from condenser.
Entrainment due to laminar velocity shear or turbulent diffusion of momentum is also important in laminar and turbulent jets and plumes. Sometimes the plume will develop a unique radio signature, similar to background radiation, which is known as the "Plume Train".[full citation needed] The plume train is still under study with several scientists questioning the diversity of variable of the plume train with some defining two variables "train length" (ς) and "train frequency" (ψ).[full citation needed] Turner, in her study questions the limitations of using these two variables and proposes the "plume static effect" (ρ).[full citation needed] This effect is described as the unusable background effect of the plume, which indicates its initial strength and plume width. The plume train links into entrainment through the particles that are polarised by the waves which are then taken into the plume. Plume Trains are still a field in development 
- Scase, M. J.; Caulfield, C. P., Dalziel, S. B. & Hunt, J. C. R. (2006). "Time-dependent plumes and jets with decreasing source strengths". J. Fluid Mech 563: 443–461.
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