= Eddy diffusivity
{title2=$K$}
= Turbulent diffusivity
{synonym}
An eddy diffusivity is a transport closure representing unresolved <turbulence> by a mean scalar flux proportional to a negative mean <concentration> gradient. A local estimate is $K\sim U\ell$, where $U$ and $\ell$ are turbulent speed and length scales. In a <line plume> with constant characteristic speed and width proportional to $z$, this gives $K\propto z$. Its coefficient is an independent closure parameter; <fluid entrainment> conservation alone does not determine it.
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