Eddy diffusion in the central region of turbulent flows in pipes and between parallel plates

Abstract

In the Reynolds number range 25,800–74,900, time averaged concentration distributions were measured at two distances downstream from a point source, located at the axis of a fully developed highly turbulent flow of water in a 2 in. pipe. Eddy diffusivities of mass in the core were calculated from these distributions using Taylor's theory of eddy diffusion by continuous movements. The dimensionless ratio of the eddy diffusion coefficient to the product of friction velocity and pipe diameter, E D / u*D, was found to be independent of Re and equal to 4·0 x 10 −2. An assessment of related investigations reported in the literature showed that E M / u* D, E H/ u* D, and E D / u* D are in of Re in the core region for Re > 20,000. Consequently Pr T and Sc T are independent of Re. Molecular diffusion proved to have a slight but definite influence on the values of Pr T and Sc T . The values of Pr T and Sc T in a flow between two parallel plates seem to differ significantly from those in pipe flow.