Correlation of intermittency with preferential transport of heat andchemical species in turbulent shear flows.

A simplified model of transport processes in fully developed free turbulent shear flows is proposed to account for observed differences between the lateral transport rates of mass (heat or chemical species) and momentum. The model incorporates the salient transport features of Townsend's description of the double-structure of free shear flows. It is envisaged that the shear flow zone consists of a central region of continuous turbulence where transport may be described by gradient diffusion, coupled with an outer region of intermittent turbulence where mass transport is effected primarily by bulk convection. The analysis of the model is implemented by use of empirical data on the intermittency factor 7. It is deduced that the apparent eddy diffusivity of heat is roughly 1.5 and 2.0 times that of momentum for axisymmetric and planar flows, respectively. These results are in plausible agreement with a large variety of data quoted by Hinze.