Abstract
Dispersion is the enhanced mixing of material through spatial variations in velocity. When it is of interest (when we are not keeping track of the three-dimensional mixing), dispersion is typically one or two orders of magnitude greater than turbulent diffusion. The process of dispersion is associated with a spatial mean velocity, the assumption of plug flow, and a velocity profile. The means used in association with diffusion, turbulent diffusion, and dispersion are identified in Table 6.1.
This chapter was originally published as part of the Encyclopedia of Sustainability Science and Technology edited by Robert A. Meyers. DOI:10.1007/978-1-4419-0851-3
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- Convection:
-
The movement of a constituent with movement of the fluid.
- Diffusion:
-
The spreading of fluid constituents through the motion inherent to atoms and molecules.
- Diffusion coefficient:
-
A coefficient that describes the tendency of molecules to spread a constituent mass.
- Dirac delta:
-
An impulse of a given quantity (mass) that occurs over an infinitely short time or space.
- Dispersion:
-
The process of mixing caused by a variation in velocity and transverse diffusion or turbulent diffusion.
- Dispersion coefficient:
-
A coefficient that can describe the mixing caused by a transverse velocity profile and transverse diffusion or turbulent diffusion. A dispersion coefficient means that some sort of spatial mean velocity is being used to describe the flow. Then, the mixing lateral or longitudinal to the spatial mean velocity due to a combination of a velocity profile and diffusion or turbulent diffusion is described by the dispersion coefficient. The coefficient’s location in the mass transport equation is similar to diffusion coefficients, and the units are similar.
- Laminar flow:
-
Flow that has no turbulent eddies, where the fluid flows in laminas and diffusion creates the mixing of the fluid.
- Turbulent diffusion:
-
The mixing of fluids through turbulent eddies created by convection.
- Turbulent diffusion coefficient:
-
A coefficient that comes from the multiplication of two turbulent velocities of the flow, divided by density of the fluid. The coefficient’s location in the mass transport equation is similar to diffusion coefficients, and the units are similar, so it is called a “turbulent diffusion coefficient.”
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Gulliver, J.S. (2012). Chemicals in the Environment, Dispersive Transport. In: Gulliver, J. (eds) Transport and Fate of Chemicals in the Environment. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-5731-2_6
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