Fluid entrainment
= Fluid entrainment
= Entrainment
{synonym}
<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.