Diffusive/advective transport regime

Fluid motion for Re = 1-100 indicates that the flow is laminar and transmission is characterized by laminar-advective transport. A low flow velocity or small length scale also means that molecular diffusion can be an important transport process relative to advective transport (i.e., Pe is low). Flow streamlines in low Re environments do not cross and the odor plume or trail consequently is relatively coherent and characterized by sharp gradients in chemical concentration, particularly in the axial (cross-flow) direction. This regime is important for smaller animals, particularly those in the plankton. Cruising copepods typically have a Re < 10, for instance, and the wake retains high concentration of scent in an isolated region due to the slow diffusive spread and dilution in the laminar regime (Yen et al. 1998). 

Chemical mass is redistributed within a groundwater flow regime as a result of three principal transport processes advection, hydrodynamic dispersion, and molecular diffusion (e.g., Bear, 1972 Freeze and Cherry, 1979). Collectively, they are referred to as mass transport. The nature of these processes and how each can be accommodated within a transport model for a multicomponent chemical system are described in the following sections. 

Taylor s dispersion is one of the most well-known examples of the role of transport in dispersing a flow carrying a dissolved solute. The simplest setting for observing it is the injection of a solute into a slit channel. The solute is transported by Poiseuille s flow. In fact this problem could be studied in three distinct regimes (a) diffusion-dominated mixing, (b) Taylor dispersion-mediated mixing and (c) chaotic advection. 

The transition from a turbulent flow regime with advective and eddy transport to a small scale dominated by viscosity and diffusional transport is apparent when an impermeable solid-water interface such as the sediment surface is approached (Fig. 5.4). According to the classical eddy diffusion theory, the vertical component of the eddy diffusivity, E, decreases as a solid interface is approached according to E = A v where A is... 

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