Eddy transfer

In the previous section, the molecular basis for the processes of momentum transfer, heat transfer and mass transfer has been discussed. It has been shown that, in a fluid in which there is a momentum gradient, a temperature gradient or a concentration gradient, the consequential momentum, heat amd mass transfer processes arise as a result of the random motion of the molecules. For an ideal gas, the kinetic theory of gases is applicable and the physical properties ji/p, k/CpP and D, which determine the transfer rates, are all seen to be proportional to the product of a molecular velocity and the mean free path of the molecules. 

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Equation 12.35 applies only in those regions where eddy transfer dominates, i.e. outside both the laminar sub-layer and the buffer layer (see below). 

Molecular processes only Molecular and eddy transfer together Eddy transfer predominating... 

Craig, R. A., Vertical Eddy Transfer of Heat and Water Vapour in Stable... 

Eddies (vortices, whorls) are thus superimposed on the average flow direction, and this causes a strong mixing effect. The size of the largest eddies is of the order of the smallest dimension of the vessel (e.g., the pipe diameter). These large eddies transfer their kinetic energy to smaller ones, which transfer it to still smaller eddies, etc. With decreasing eddy size, the... 

J.T. Davies, Turbulence Phenomena. An Introduction to the Eddy Transfer of Momentum, Mass and Heat, Particularly at Interfaces, Academic Press, New York, 1972. 

FIGURE 16.7 Eddy transfer in a turbulent shear flow. 

Davies IT. (1972) Turbulence phenomena an introduction to the eddy transfer of momentum, mass, and heat, particularly at interfaces. Academic Press, New York, USA. 

When a solute A is dissolving from a solid surface there is a high concentration of this solute in the fluid at the surface, and its concentration, in general, decreases as the distance from the wall increases. However, minute samples of fluid adjacent to each other do not always have concentrations close to each other. This occurs because eddies having solute in them move rapidly from one part of the fluid to another, transferring relatively large amounts of solute. This turbulent diffusion or eddy transfer is quite fast in comparison to molecular transfer. 

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