8. Dispersion processes
This section looks at the longitudinal spreading of clouds through dispersion, a process distinct from diffusion. Dispersion is the spreading of a cloud by any process that causes different chemical particles to move at different velocities. The non-uniformity of particle velocity allows some particles to move quickly while others are held back, and can be caused by boundaries (which create shear in the flow), dead zones and absorption reactions (for example, to a porous medium). In environmental systems, dispersion is generally far more important than diffusion in the lengthening of chemical clouds. In this chapter, it is shown that dispersing clouds eventually reach a Fickian limit, at which point they have a Gaussian concentration profile in the longitudinal direction. It is also shown that longitudinal dispersion is maximized when lateral diffusion is small.
The example problems ask the user to make qualitative statements about the dispersion expected in various flows and to predict the shape of a dispersing cloud.
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