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)

where σ_turb = – ρw'_xw'_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 component of the velocity vector.
Comment. The term is defined by assuming that fluctuations of the physical properties can be neglected.