Dispersion of a solute by flow

Dispersion of a solute by flow combines molecular diffusion with spatially varying advection, often producing an enhanced large-scale longitudinal diffusivity.

The Péclet number compares advection over length $L$ at speed $U$ with molecular diffusion of diffusivity $D$.

Taylor dispersion is enhanced longitudinal spreading produced when transverse molecular diffusion repeatedly moves solute between streamlines of different velocity.

For $u(y)=Uy/h$ between reflecting walls at $y=\pm h$, the flow-induced diffusivity is