ABSTRACT
The difference being that in shear flow dispersion, both advection and diffusion are considered together, whereas in ordinary molecular diffusion, only molecular diffusion is considered. Taylor made further assumptions and approximations to obtain what is generally known as the longitudinal dispersion equation. Dispersion coefficient by the change of moment method is given as where is the variance of the concentration-distance profile. The dispersion coefficient based on the diffusive transport method can be estimated using the Lagrangian time scale as where is the squared spatial velocity deviation from the cross-sectional mean. General details of the finite element method can be found in several textbooks. Methods of estimating the dispersion coefficient as well as solution of the dispersion equation using different numerical methods including comparison with known analytical solutions for specific cases of the governing equation are given. The chapter ends with a brief introduction of dispersion through porous media and some general-purpose water quality models.
