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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

Missing Image:img src="SP7-d_files/81difuse.gif" height="36" width="74"

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. </dd>
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. [[/a>|/a> ]]


This article is based on NASA's Dictionary of Technical Terms for Aerospace Use