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Far-Field Flow Processes

After the effluent flow has interacted with the water surface, bottom, pycnocline, or terminal layer and has thus completed its near-field mixing phase, far-field mixing begins.

The far-field region consists of one or two mixing processes and is simulated within CORMIX with the following two conditions:

  1. If it contains sufficient buoyancy there will be a density current region followed by a passive diffusion region.
  2. If it is non-buoyant or weakly buoyant there is no density current region, only a passive diffusion region.

The density current region is characterized by dynamic horizontal spreading and gradual vertical thinning of the mixed effluent flow while being advected by the ambient current.

Buoyant upstream intrusions may occur with stagnation points. Vertical boundary interaction may occur, and the flow may contact one or both lateral boundaries (shorelines). Heated discharges loose buoyancy due to heat transfer while buoyancy diminishes in sediment laden flows due to particle settling.

In the passive diffusion region, the dilution is controlled by the turbulent mixing action of the flowing ambient water body.

Again, boundary interaction may occur, and the flow may become both laterally and vertically fully mixed within this region.

A buoyant jet trapped in a density-stratified crossflow.
A buoyant jet trapped in a density-stratified crossflow. A stratified terminal level forms with a subsequent density current (Source Unknown).

A CorVue visualization of flow class S1 trapping by a density-stratified crossflow.
A CorVue visualization of flow class S1 trapping by a density-stratified crossflow followed by density current far-field mixing. Enhanced image has ambient boundaries, flow module regions and mixng zone regulatory RMZ and TDZ locations highlighted.
Far-field Prediction Methods

In contrast to the near-field flow there is no general need for an advanced classification scheme with CORMIX to determine the behavior of the far-field flow for a given discharge/environment situation.

Since the flow in the far-field is always advected in the direction of the ambient flow, the various interaction processes are simply calculated as part of the downstream modeling process of the applicable far-field solutions.

This also applies also to the transition between the density current and passive ambient diffusion mixing regions.

CORMIX Far-Field Plume Locator

Although the main emphasis of CORMIX is on the near-field mixing behavior of discharges it can also be used for providing plume predictions at larger distances in the far-field provided the flow is not highly irregular with pronounced re-circulating zones and eddies in the ambient flow.

The FFLOCATR is a simple method for interpreting the schematized CORMIX far-field plumes within the actual flow patterns in natural rivers and estuaries.

This procedure, based on the cumulative discharge method, by the image on the right.

The cumulative discharge method, first proposed by Yotsukura and Sayre is a convenient approach of dealing with lateral mixing in natural irregular (but not highly irregular with recirculating zones!) channels.

In such channel geometry the passive far-field plume that is vertically mixed, or approaches vertical mixing, will be positioned around the "streamline", or more precisely the "cumulative discharge line", that passes through the plume.

Dye release to study far-field mixing.
Dye release to study far-field mixing.
A dye is released to study far-field plume mixing.

A buoyant jet trapped in a stratified crossflow
A buoyant jet trapped in a stratified crossflow with higher velocity. (Source: Unknown)

Far Field Locator
Illustration of the cumulative discharge method for translating the CORMIX predicted far-field plume to the actual flow characteristics in winding irregular rivers or estuaries.