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Dynamic Near-field Attachments

Dynamic plume attachments occur when the discharge flow interacts strongly with a boundary in the near-field. Such near-field boundary interactions present the possibility of high pollutant concentrations and undesirable benthic impacts. Often near-field attachments are avoidable with proper outfall design. The image on the right shows a laboratory experiment where boundary interaction occurs in the near-field. This flow also exhibits a subsequent buoyant lift-off and an unstable near-field which is indicated by a CORMIX (..A4) flow class suffix as described below.

A buoyant single port discharge exhibits near-field Coanda attachment, subsequent lift-off, unstable near-field, and surface density current formation. (Source Unknown)

Wake and Coanda Attachments

Two types of attachment are considered by CORMIX; wake attachment forced by the crossflow or Coanda attachment forced by the entrainment demand of the effluent jet itself. A physical description of these processes is given below.
  • Wake Attachment: In wake attachment the presence of the discharge outfall structure and the jet efflux interrupts the ambient velocity field and causes a recirculation region in the wake downstream from the discharge.
  • Coanda Attachment: When a jet discharges close to a parallel boundary located nearby, a rapid dynamic attachment can occur. This process is referred to as a "Coanda effect". It occurs because of the entrainment demand of the jet flow at its periphery. If a boundary limits the approach flow of ambient water then low pressure effects cause the jet to be deflected towards that boundary thereby forming a wall jet. Thus the mixing process of Coanda attached flow is governed by wall jet dynamics.
Examples of crossflow-induced wake attachment and Coanda attachment conditions for flows discharging near boundaries in CORMIX1 (larger image).

CORMIX Flow Attachment Classification

The CORMIX methodology assigns a flow classification suffix to denote flows in which near-field dynamic attachments occur. These flows have the A1, A2, . . ., A5 suffix designations e.g. V1A1, NH3A2, and S3A4 among others. The suffixes are defined as follows:
  • A1: Wake attached with recirculation regions and buoyant lift-off.
  • A2: Wake attached with recirculation regions and without buoyant lift-off.
  • A3: Coanda attached wall jet with stable near-field and buoyant lift-off.
  • A4: Coanda attached wall jet with unstable near-field and buoyant lift-off.
  • A5: Coanda attached wall jets without buoyant lift-off.
The CORMIX1 classification for attached flows.

Sensitivity Studies and Outfall Design

The multiport diffuser discharge image on the left shows how near-field attachments vary with port height and ambient velocity.

The CorSens sensitivity analysis tool can analyze boundary interaction and flow classification stability for a range of discharge and source conditions. The CorVue tool can visualize boundary interaction, density current upstream buoyant spreading, and regulatory mixing zone boundaries.

Near Field Wake Attachment. 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 class, and can have undesirable and avoidable benthic ecological impacts. (Photo: S. Monismith)
A tributary mixing zone creates a density current upstream intrusion and near-field shoreline attachment in a coastal ambient. (Photo: I. Wood, Univ. of Canterbury)