Abstract
Diffusion of a scalar quantity (ammonia gas) from a steady line source within a twodimensional turbulent boundary layer is studied. Using a long 6 × 6 ft square test section, the boundary-layer thickness varied from 5 to 11 in for distances of 3 to 43·5 ft downstream with air speeds from 9 to 16 ft/s. Measurements of mean concentrations of ammonia in air are reported, analysed and compared with a few measurements of heat transfer in similar conditions. Formulation of the results takes into consideration the transverse non-homogeneity of the velocity field and also divides the downstream diffusion field into four zones. Measurements of the mean velocity field and the mean concentration field permit the flux of mass in the vertical direction to be calculated through the equation of mass conservation. The use of an eddy-diffusivity coefficient to describe the processes of turbulent diffusion is discussed and it is shown that for a long distance downstream of the source, such a coefficient cannot be related to the local Eulerian variables of the velocity field.
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