= Entrainment velocity
{title2=$u_e$}
The mean speed, measured normal to a chosen plume or jet boundary, at which ambient fluid enters the turbulent region. The boundary and averaging convention must be specified: this speed is not the plume's axial velocity. In the <Batchelor entrainment hypothesis> for a rising <Boussinesq> plume, $u_e=\alpha w$, where $\alpha$ is the dimensionless <entrainment coefficient> and $w$ the characteristic axial speed. Multiplying by the boundary perimeter gives the entrained <volume flux> per axial length.
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