Eddy diffusion, eddy dispersion, multipath, or turbulent diffusion is any diffusion process by which substances are mixed in the atmosphere or in any fluid system due to eddy motion.[1][2] In other words,[3] it is mixing that is caused by eddies that can vary in size from the small Kolmogorov microscales to subtropical gyres. The size of eddies decreases as kinetic energy is lost, until it reaches a small enough size for viscosity to control, resulting in kinetic energy dissipating into heat.[4] The concept of turbulence or turbulent flow causes eddy diffusion to occur.[5] The theory of eddy diffusion was developed by Sir Geoffrey Ingram Taylor.[6]

Because the microscopic processes responsible for atmospheric mixing are too complex to model in detail, atmospheric modelers generally treat atmospheric mixing as a macroscopic "eddy" diffusion process. In this approach, the diffusion rate at each pressure level is parameterized by a quantity known as the eddy diffusion coefficient, K[7] (also sometimes called eddy diffusivity, with units of m2 s−1).The eddy diffusion coefficient is used in the diffusion equation, to describe the dispersion or mixing of solutes in fluid or atmospheric systems.[8]

In natural systems, the vertical component of eddy diffusion, is an important parameter for modeling and describing atmospheric variations due to turbulence and convection.[9] This process of eddy diffusion is the foundation for the structure of the atmospheric boundary layer.[9] In rivers, the eddies cause fluctuations in the river's velocity and their size determines the magnitude of eddy diffusion.[4] The size is usually limited by the depth of the river, due to a rivers depth is usually much smaller than its width.[4]

References

Science world, wolframe
Chanson, Hubert (2004), "Introduction: Turbulent Mixing and Dispersion in Rivers and Estuaries: An Introduction", Environmental Hydraulics of Open Channel Flows, Elsevier, pp. 35–36, doi:10.1016/b978-0-7506-6165-2.50062-x, ISBN 978-0-7506-6165-2
Srinivasan, Kaushik; Young, W. R. (June 2014). "Reynolds Stress and Eddy Diffusivity of β-Plane Shear Flows". Journal of the Atmospheric Sciences. 71 (6): 2169–2185. doi:10.1175/jas-d-13-0246.1. ISSN 0022-4928.
Kalinske, A. A.; Pien, C. L. (1944-03-01). "Eddy Diffusion". Industrial & Engineering Chemistry. 36 (3): 220–223. doi:10.1021/ie50411a008. ISSN 0019-7866.
Chamberlain, J.W. & Hunten, D.M. (1987). Theory of Planetary Atmospheres: An Introduction to Their Physics and Chemistry. New York: Academic Press. pp. 75 and 90.
"Studying the effect of vertical variation of wind speed and eddy diffusivity on the advection-diffusion equation". Applied Science Reports. 14 (3). 2016-05-05. doi:10.15192/pscp.asr.2016.14.3.250257. ISSN 2311-0139.
Kumar, Pramod; Sharan, Maithili (2012-03-01). "Parameterization of the eddy diffusivity in a dispersion model over homogeneous terrain in the atmospheric boundary layer". Atmospheric Research. 106: 30–43. doi:10.1016/j.atmosres.2011.10.020. ISSN 0169-8095.

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