Fluid entrainment is incorporation of surrounding fluid into a moving current, jet or buoyant plume. A downstream increase in the flow's volume flux can therefore occur without an additional localized source. The added fluid generally changes the mean velocity, temperature or constituent concentration of the moving region.
For a steady jet whose surroundings initially carry little axial momentum, entrainment increases transported volume while the conserved axial momentum flux constrains how the jet slows and spreads. In a warm buoyant plume, entrained ambient air similarly dilutes the temperature anomaly while increasing the total flow. Measuring only a centreline speed cannot determine the entrainment rate; one needs integrated transport across successive sections or a suitable boundary-flux estimate. Entrainment is different from molecular diffusion, although both can contribute to mixing and are often coupled in an observed flow.
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