5. Advection and diffusion of an instantaneous release
The diffusion of a cloud in a static fluid was examined in Chapter 3. This chapter takes things one step further and looks at the simultaneous advection and diffusion of an instantaneous release in a uniform flow. The cloud structure studied in Chapter 3 still holds here, as there is a Gaussian concentration distribution about the cloud's center of mass, which now moves at the flow velocity. The distinction between spatial and temporal records of concentration is also examined. The use of the Peclet number in determining the relative importance of advection and diffusion in scalar transport is strongly emphasized. If systems are dominated by either advection or diffusion, it becomes simple to determine the time taken for a chemical to be observed at a given distance from the release.
The example problems test one's ability to simplify analyses based on the Peclet number, and to distinguish between spatial and temporal records of concentration.
Animation
showing simultaneous advection and diffusion
(these links open in new browser windows)
