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Near-Field Flow Processes

The hydrodynamics of an effluent continuously discharging can be conceptualized as a mixing process occurring in two separate regions.

In the first region, the initial jet characteristics of momentum flux, buoyancy flux, and outfall geometry influence the jet trajectory and mixing. This region is referred to as the "near-field", and encompasses the jet subsurface flow and any surface or bottom interaction, or in the case of a stratified ambient, terminal layer interaction. In this region, outfall design can usually affect the initial mixing characteristics through appropriate manipulation of design variables. In particular, designs with dynamic bottom attachments should be avoided.

As the turbulent buoyant jet travels further away from the source, the source characteristics become less important. Conditions existing in the ambient environment will control trajectory and dilution of the flow through density current buoyant spreading motions and passive diffusion due to ambient turbulence. This region is referred to as the "far-field".

This experiment shows ambient density stratification causes near-field flow trapping and subsequent density current formation in a stagnant ambient.
CorVue S5 flow classification visualization shows a near-field flow trapped by stratified crossflow, buoyant density current upstream spreading, and subsequent far-field mixing (enhanced image).
A single port buoyant jet in crossflow is deflected by the ambient current and exhibits stable near-field behavior without dynamic attachments (Photo: L. Fan, Keck Lab, CIT).
At a larger crossflow velocity, the single port buoyant jet shows more rapid deflection while still exhibiting a stable near-field behavior without dynamic attachments (Photo: L. Fan, Keck Lab, CIT).