Eddy diffusivity ($K$)

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.

The ratio of turbulent momentum diffusivity to turbulent scalar diffusivity. It need not equal the analogous molecular diffusivity ratio, and it is unrelated to the Schmidt number used for quantum entanglement. For ${\kappa }_T={\nu }_T/2$ its value is two.