= Mixing length
{title2=$l$}
A mixing length is the characteristic transverse displacement over which a turbulent parcel retains its streamwise momentum before mixing with its surroundings. Taylor expansion of a mean velocity profile gives a fluctuation of magnitude $l|U_y|$. Combining comparable transverse fluctuations with down-gradient <Reynolds stress> transport gives the closure $\overline{u'v'}=-l^2|U_y|U_y$. This is a phenomenological approximation, not a consequence of the <Navier-Stokes equation>. In a free jet, a prescribed local <mixing length> must also be compatible with centreline regularity and vanishing outer momentum flux.
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