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Wake Flow Discharge Conditions

Effluent plume exit velocity to ambient cross-flow ratio, R is defined as follows:

R = U0/Ua, where

U0 is the effluent plume exit velocity and
Ua is the ambient cross flow velocity.

WAKE FLOW DISCHARGE CONDITIONS generally occur whenever the effluent discharge exit velocity (U0) is less than or equal to the ambient cross-flow velocity (Ua),
i.e. R = (U0/Ua) ≤ 1.00

Wake flows are characterized by unsteady "patchy" plumes with high,instantaneous effluent concentrations. At any downstream location from the discharge source there are instantaneous, high concentration pools of effluent flowing by, followed by periods of low concentration effluent pools; The effluent mixing is hence un-steady in behavior.

The observed and/or measured and reported steady state Dilution S values and/or plume trajectory values generally represent a time average and do not capture the patchy nature of the effluent mixing in the ambient.

Because of the "patchy" quality of wake flows, measurements of plume trajectory, width, and dilution in laboratory and field studies may be difficult and furthermore may be subject to significant measurement errors.

Vortex Shedding (Wake Flows)
Wake flow animation. Vortex shedding behind a circular cylinder. In this animation, the flow on the two sides of the cylinder are shown in different colors, to show that the vortices from the two sides alternate.
Courtesy, Cesareo de La Rosa Siqueira. Source, WikiPedia

Wake Flow - LIF Imagery
von Karman vortex sheet and shedding typical of Wake Flows Shown by LIF Image.

Additionally, the steady state dilution S values and/or plume trajectory values reported by any steady-state mixing model like PLUMES, OOC or CORMIX generally represents time averaged values and does not capture or represent the patchy nature of the effluent mixing in the ambient.

  • From an effluent mixing standpoint, under such WAKE FLOW CONDITIONS there is no discharge momentum induced mixing. The general mixing characteristics are UNDESIRABLE.
  • From an environmental impact mitigation standpoint, WAKE FLOW CONDITIONS must be AVOIDED since there is possibility of instantaneous high concentration periods leading to possible adverse effects on marine and/or aquatic life and environments.
  • From a regulatory standpoint, WAKE FLOW CONDITIONS should be avoided.
  • From a discharge/outfall design standpoint WAKE FLOW CONDITIONS are generally easily remedied by incorporating an appropriate variable opening check valve. These variable opening check values can increase the effluent plume exit velocity, depending on the discharge flow.

The CORMIX Mixing Zone Model is the only model that alerts/warns the end user to the occurrence of such WAKE FLOW CONDITIONS and provides suitable design recommendations to remedy such discharge conditions.