Diffusivity, eddy experimental determination

The background of experimental data available at present does not appear to be sufficient for the effect of the several variables upon the eddy diffusivity to be determined with accuracy however, it is worth while to consider certain relationships by analogy with thermal transport. 

There are many transport conditions where experiments are needed to determine coefficients to be used in the solution. Examples are an air-water transfer coefficient, a sediment-water transfer coefficient, and an eddy diffusion coefficient. These coefficients are usually specific to the type of boundary conditions and are determined from empirical characterization relations. These relations, in turn, are based on experimental data. 

The plate theory assumes that an instantaneous equilibrium is set up for the solute between the stationary and mobile phases, and it does not consider the effects of diffusional effects on column performance. The rate theory avoids the assumption of an instantaneous equilibrium and addresses the diffusional factors that contribute to band broadening in the column, namely, eddy diffusion, longitudinal diffusion, and resistance to mass transfer in the stationary phase and the mobile phase. The experimental conditions required to obtain the most efficient system can be determined by constructing a van Deemter plot. 

Assuming that these conditions are met and that the eddy diffusivities Kh and Kv are specified as functions of space and time. Equation (7) provides adequate representation of mean concentrations. Because the diffusivities are essentially empirical parameters to be determined from experimental data, the accuracy of (7) depends upon the degree to which atmospheric conditions at a location of interest correspond to the conditions under which the diffusivities were measured. 

A fundamental problem in performing a turbulent flow analysis involves determining the eddy diffusivities as a function of the mean properties of the flow. Unlike the molecular diffusivities, which are strictly fluid properties, the eddy diffusivities depend strongly on the nature of the flow they can vary from point to point in a boundary layer, and the specific variation can be determined only from experimental data. 

Eddy Diffusivity Models. The mean velocity data described in the previous section provide the bases for evaluating the eddy diffusivity for momentum (eddy viscosity) in heat transfer analyses of turbulent boundary layers. These analyses also require values of the turbulent Prandtl number for use with the eddy viscosity to define the eddy diffusivity of heat. The turbulent Prandtl number is usually treated as a constant that is determined from comparisons of predicted results with experimental heat transfer data. 

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