6 - Shear Flow Dispersion

Summary

This chapter is the most crucial part of the book, the fundamental building block of the concept of dispersion in porous media and macrodispersion in the field-scale aquifers. It unravels the myth of macrodispersion, anomalous dispersion, scale-dependent dispersion, dual-domain, and other recently developed dispersion models for solute transport in aquifers (Chapters 9 and 10). This chapter first explains how the concept of dispersion evolves from molecular diffusion to account for the effects of fluid-dynamics-scale velocity variation in solute migration in a pipe. The relationship between dispersion and the concentration averaged over the cross-section of a pipe is visited. Further, this chapter illustrates the molecular and fluid-dynamics-scale velocity variations, the interaction between diffusion and dispersion, and scale issues associated with the shear flow dispersion. Finally, it discusses the limitations of extending Fick’s law for molecular-scale velocity variations to describe the effects of fluid-dynamics-scale velocity variations.