Diffusivity
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Diffusivity
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A measure of the rate of diffusion of
a substance, expressed as the diffusivity coefficient K. When K
is constant, the diffusion equation is
q is the substance diffused; Missing Image:img src="SP7-d_files/del.gif"^{2} is
the Laplacian operator; and t is time. The diffusivity has dimensions
of a length times a velocity; it varies with the property diffused, and for
any given property it may be considered a constant or a function of
temperature, space, etc., depending on the context. Also called coefficient
of diffusion. See conductivity,
kinematic
viscosity, exchange
coefficients.
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In the case of molecular diffusion the length dimension is the mean free
path of the molecules. By analogy, in eddy diffusion, length becomes the
mixing
length. The coefficient is the called then eddy diffusivity, and is
in general several orders of magnitude larger than the molecular diffusivity.
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References
This article is based on NASA's Dictionary of Technical Terms for Aerospace Use