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Unstable Near-field Mixing

The assessment of near-field stability, i.e. the distinction of stable or unstable conditions, is a key aspect of effluent dilution analyses and mixing zone modeling. It is especially important for understanding the behavior of the two-dimensional plumes resulting from multiport diffusers, as shown by some examples in the image on the right. Turbulent buoyant jet integral models are not in general applicable to unstable flow conditions. It is therefore essential to determine flow stability before any simulation is attempted.

The flow classification system of CORMIX determines the stability of each discharge/environment situation under consideration. The resulting flow class assures that the correct combination of regional flow modules are used to simulate the mixing zone.

Examples of near-field stability and instability conditions for submerged discharges in limited water depth (larger image).
  • Stable discharge conditions: Usually occur for a combination of strong buoyancy, weak momentum and deep water. Thus they are often referred to as "deep water" conditions (Image cases a, c)
  • Unstable discharge conditions: Occurs when a recirculation phenomena appears in the discharge vicinity (Image cases 2.3c, d). This local recirculation leads to re-entrainment of already mixed water back into the buoyant jet region. Unstable may be considered synonymous to "shallow water" conditions.

CORMIX Tools for Simulation and Visualization

The CORMIX classification scheme accounts for discharge stability and boundary attachments.

The CorSens sensitivity analysis tool can analyze boundary interaction and 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.

A laboratory demonstration of near-field dynamic Coanda attachment and subsequent buoyant lift-off.
A thermograph of the far-field surface plume from the San Onofre Generating Plant. This flow results from an unstable near-field. (Image: Fischer, et. al. 1979).
A CorVue visualization of stable H3A4 Coanda boundary attachment with subsequent buoyant lift-off.