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۱٫ Concepts, Denitions, and the Diusion
Equation
Environmental uid mechanics is the study of uid mechanical processes that aect the
fate and transport of substances through the hydrosphere and atmosphere at the local or
regional scale1 (up to 100 km). In general, the substances of interest are mass, momentum
and heat. More specically, mass can represent any of a wide variety of passive and reactive
tracers, such as dissolved oxygen, salinity, heavy metals, nutrients, and many others. Part I
of this textbook, \Mass Transfer and Diusion,” discusses the passive process aecting the
fate and transport of species in a homogeneous natural environment. Part II, \Stratied
Flow and Buoyant Mixing,” incorporates the eects of buoyancy and stratication to deal
with active mixing problems.
This chapter introduces the concept of mass transfer (transport) and focuses on the
physics of diusion. Because the concept of diusion is fundamental to this part of the
course, we single it out here and derive its mathematical representation from rst principles
to the solution of the governing partial dierential equation. The mathematical rigor
of this section is deemed appropriate so that the student gains a fundamental and complete
understanding of diusion and the diusion equation. This foundation will make the
complicated processes discussed in the remaining chapters tractable and will start to build
the engineering intuition needed to solve problems in environmental uid mechanics.