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Boundary Interactions

Boundary interaction occurs when the flow contacts the surface, bottom, or forms a terminal layer in a density-stratified ambient environment. Boundary interaction also determines if mixing is controlled by stable or unstable discharge source conditions.

Boundary interaction generally provides the transition from near-field (discharge source controlled) to far-field (ambient controlled) mixing processes as illustrated by the image on the right.

However, boundary interaction in the form of dynamic plume attachments to the bottom are considered near-field mixing processes.

Illustrative near-field and far-field regions for a stable positively buoyant discharge without near-field bottom attachments. Surface boundary interaction separates near-field from far-field mixing processes.
After surface boundary interadtion occurs, this plume shows lateral shoreline boundary interaction.
A CorVue visualization of lateral boundary interaction.

Boundary Interaction: Near-field Dynamic Attachments

Dynamic flow attachments occur when the discharge interacts strongly with a boundary in the near-field. These near-field boundary interactions present the possibility of high pollutant concentrations and undesirable benthic impacts. These flows can also exhibit subsequent buoyant lift-off with an unstable near-field. Attachments are indicated by the CORMIX (..) An flow class suffix in the CORMIX flow classification , which also describes dynamic attachment mechanisms and lift-off conditions.

The image on the left shows a LIF image of a laboratory experiment with near-field dynamic flow attachment. Often these near-field attachments are avoidable with proper outfall design facilitated by the CorVue, CorSpy, and CorSens advanced analysis tools.

Wake attachment in the near-field . This set of false-color laser-induced fluorescence images illustrates boundary interaction behavior for a multiport diffuser in crossflow. Crossflow velocity (left to right) is increased from top series (a, d) to the bottom (c, f) causing plume wake attachment (c, e, f). The greater discharge port height on the left (a, b, c) resists bottom attachment as crossflow velocity increases. Wake attachment is indicated by a CORMIX (..) A2 flow classification, and can have undesirable and avoidable benthic ecological impacts (Photo: S. Monismith).

CORMIX Boundary Interaction / Discharge Stability

Examples of the four types of boundary interaction modeled by CORMIX appear in the image on the below. The analyst can use CorVue to visualize these boundary interaction processes in relationship to regulatory mixing zones. In CORMIX, mixing conditions at the impingement point can take on one of the following 4 types:

  1. A turbulent buoyant jet is bent-over by a cross-flow, it will gradually approach the surface, bottom or terminal level and will undergo a smooth, stable transition with little additional mixing (Image a). However, a jet impinging normally, or near-normally, on a boundary will rapidly spread in all directions.
  2. The flow has sufficient buoyancy and ultimately forms a stable layer at the surface (Image b). In the presence of weak ambient flow this will lead to a density current upstream intrusion and stagnation point against the ambient current.
  3. The buoyancy of the flow is weak or its momentum very high, unstable recirculation phenomena can occur in the discharge vicinity (Image c). This local recirculation leads to re-entrainment of already mixed water back into the buoyant jet region.
  4. The intermediate case; a combination of unstable localized vertical mixing and density current upstream spreading with a stagnation point may result (Image d).
Types of boundary interactions modeled by CORMIX (larger image).

Laboratories Images of Boundary Interaction / Discharge Stability

Ambient density stratification causes boundary interaction in the form of terminal level formation
(Image Source: Hofer, Kurt (1978) - Eine verbesserte Theorie turbulenter Freistrahlen im stratifizierten Medium und ihr Vergleich mit dem Experiment. Mitteilung VAW-ETH No. 31).
A buoyant jet interacts in a stable manner with the water surface. (Source Unknown)
An upstream density current forms in this plan view. (Image: G. Jirka, DeFrees Hydraulics Lab)
>Boundary interaction occurs for stable flow trapped by a density-stratified crossflow. (Source Unknown)
Boundary interaction occurs for a buoyant jet in stable flow trapped by an ambient density stratification in a stagnant ambient. (Photo: L. Fan)

Field Images of Boundary Interaction / Discharge Stability

A density current forms with buoyant upstream spreading in this coastal discharge. (Source Unknown)
This surface discharge exhibits characteristic of a CORMIX3 PL1 flow classification. (Photo: I. Wood, Univ. of Canterbury)
This near-shore discharge exhibits lateral boundary interaction. Photo: I. Wood, Univ. of Canterbury)

CorVue Visualizations of Boundary Interactions

CorVue visualization of CORMIX1 flow class S1 trapping by a density-stratified crossflow followed by density current far-field mixing. Enhanced image has ambient boundaries, and flow module regions with RMZ and TDZ locations highlighted (enhanced image).
CorVue CORMIX1 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 CorVue 3-D visualization of a unidirectional multiport CORMIX2 MU1V flow classification showing water surface boundary interaction in the near-field, surface density current upstream intrusion and stagnation point (Enhanced image).
CorVue 3-D plume visualization of near-field dynamic Coanda attachment for CORMIX1 flow class H3A4 (Click for a larger PDF image).
A CorVue 3-D view of CORMIX1 flow class V5 flow mixing zone locations for regulatory TDZ and RMZ values 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 plan (top) view of TDZ and RMZ locations for a CORMIX1 simulation of flow class V5 (larger image).
A CorVue 3-D view of TDZ and RMZ locations for a CORMIX1 simulation of flow class V2 (larger image).
Corresponding CorVue side view of TDZ and RMZ locations for a CORMIX1 simulation of flow class V2 (larger image).
Corresponding CorVue plan (top) view of TDZ and RMZ locations for a CORMIX1 simulation of flow class V2 (larger image).