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Density Currents and Upstream Intrusions

Density currents are buoyancy driven far-field flows that are defined by transverse horizontal spreading while being advected downstream by an ambient current. As illustrated by the image on the right, these spreading processes can intrude into the ambient flow, forming a buoyant upstream wedge and stagnation point. These flows are caused by the density difference of the mixed flow relative to the ambient density.Density currents are preceded by turbulent jet mixing in the near-field and are followed by passive diffusion in the far-field.

An upstream density current and stagnation point forms in this plan view of near-field to far-field transition within a surface boundary interaction. This in a laboratory experiment is shown in plan view from he top (Source: G. Jirka).

Density currents may or may not form upstream intrusions, depending upon the crossflow magnitude and internal buoyancy at boundary interaction.

Surface Buoyant Jet/Density Current Mixing

Positively buoyant jets discharged horizontally along the water surface from a laterally entering channel or pipe as shown in the left image bear some similarities to the more classical submerged buoyant jet. For a relatively short initial distance, the effluent behaves like a momentum jet spreading both laterally and vertically due to turbulent mixing.

Typical buoyant surface jet mixing flow patterns under stagnant or flowing ambient conditions.

After this stage, vertical entrainment becomes inhibited due to buoyant damping of the turbulent motions, and the jet experiences strong lateral spreading. During stagnant ambient conditions, ultimately a reasonably thin layer may be formed at the surface of the receiving water; that layer can undergo the transient density current buoyant spreading motions (Image a).

In the presence of ambient crossflow, buoyant surface jets may exhibit any one of following three types of flow features: They may form a weakly deflected jet that does not interact with the shoreline (Image b, above). When the crossflow is strong, they may attach to the downstream boundary forming a shore-hugging plume (Image c). When a high discharge buoyancy flux combines with a weak crossflow, the buoyant spreading effects can be so strong that an upstream intruding plume is formed that also stays close to the shoreline (Image d).

A tributary mixing zone (freshwater) creates and upstream density current intrusion in a saline coastal ambient. (Photo: I. Wood, Univ. of Canterbury).

Buoyant Spreading Models in CORMIX

Density current are effective transport mechanisms that can quickly spread a mixed effluent laterally over large distances in the transverse direction, particularly in cases of strong ambient stratification. In this case, effluent of considerable vertical thickness at the terminal level can collapse into a thin but very wide layer unless this is prevented by lateral boundaries.

Buoyant spreading processes downstream of the near-field region (example of spreading along the water surface).

If the discharge is non-buoyant, or weakly buoyant there is no buoyant spreading region in the far-field, only a passive diffusion region.

Depending on the type of near-field flow, ambient density stratification, and boundary interaction process, several types of density current buoyant spreading may occur: (i) spreading at the water surface, (ii) spreading at the bottom, (iii) spreading at a sharp internal interface (pycnocline) with a density jump, or (iv) spreading at the terminal level in continuously (e.g. linearly) stratified ambient.

A slaughterhouse wastewater discharge (freshwater density) creates a surface density current with upstream intrusion and stagnation point. (Source: Unknown)

Additional Mechanisms in Density Current Mixing

Two additional density current mixing mechanisms may be considered within CORMIX. These mehanisms effect the density difference between the flow and the ambient environment. These mechanisms are considered for the following pollutant types:

A warm tributary spring runoff produces a sediment laden density current on a cold lake. (Source: G. Jirka).
1. Heated Discharge: A heat loss coefficient may be specified for additional buoyancy loss due to surface heat exchange .
2. Sediment Discharge: Particle settling will cause additional buoyancy loss due to particle settling. Sediment discharge can be simulated with CORMIX-GTS Advanced Tools Sediment Release.

Additional Images of Boundary Interaction and Density Current Mixing

Ambient density stratification causes boundary interaction with internal plume trapping and stratified layer density current formation in a stagnant ambient (Source: Fan, CIT).
Buoyant spreading at the surface of a wastewater discharge into the ocean (Source I. Wood).
Orange County Outfall for municipal wastewater, Southern California, Source: Unknown

CORMIX Simulation and Visualization of Surface Density Currents

A CorVue plan (top) view of TDZ and RMZ locations for a CORMIX1 simulation of flow class V5 (larger image).
Corresponding CorVue side view of TDZ and RMZ locations for a CORMIX1 simulation of flow class V2 (larger image).
Corresponding CorVue 3-D view of TDZ and RMZ locations for a CORMIX1 simulation of flow class V5 (larger image).