## Eddy diffusivity of momentum

A quantity characterizing the intensity of turbulent momentum transfer, defined by the identity
A quantity having the dimensions of velocity and numerically equal to the square root of the ratio of the wall shear stress (*σ*_{w}) to the density of the fluid at the wall (*ρ*_{w})

*σ*

_{turb}= –

*ρ*

*w'*

_{x}

*w'*

_{y}is the turbulent shear stress (i.e., the stress due to turbulent transfer) over a certain unit area in the flow;

*w'*

_{x}and

*w'*

_{y}are the fluctuations of the longitudinal and transverse (with respect to the area) components of the velocity vector; ∂

*w*

_{x}/∂

*y*is the derivative with respect to the normal to the wall of the averaged value of the longitudinal comp ...