= Eddy diffusion
{title2=$\partial_t\sigma\supset D\nabla^2\sigma$}
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= Turbulent diffusion
{synonym}
Mixing by <turbulence> is represented approximately by a diffusive flux down a mean concentration gradient. Its coefficient is the <eddy diffusivity>. A constant-coefficient model gives a $D\nabla^2\sigma$ term and a smoothing rate $Dk^2$ for a <Fourier mode>. Effective diffusion can suppress slow gravitational concentration even when microscopic diffusion is negligible.
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