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Ambient Density Stratification

Information about the density distribution in the ambient water body is very important for the correct prediction of effluent discharge plume behavior.

If the ambient water is fresh and above 4 degrees C, the CORMIX system provides the option of entering ambient temperature data so that the ambient density values can be internally computed from an equation of state. This is the recommended option for specifying the density of fresh water, even though ambient temperature per se is not needed for the analysis of mixing conditions.

Boundary Interaction
Ambient density stratification can cause boundary interaction as shown here by a buoyant jet trapping and density current formation in density stratified crossflow.

CORMIX Ambient Density Profiles

When conditions are non-uniform, CORMIX requires that the actual measured vertical density distribution be approximated by one of three schematic stratification profile types illustrated. These are:

  1. Type A: Linear density profile.
  2. Type B: Two-layer system with constant densities and density jump.
  3. Type C: Constant density surface layer with linear density profile in bottom layer separated by a density jump.

Corresponding profile types exist for approximating a temperature distribution when it is used for specifying the density distribution.

Types of profiles used for ambient density specification with CORMIX (larger image) .

CORMIX Sensitivity to Density Profile Specification

When in doubt about the specification of the ambient density values it is reasonable to first simplify as much as possible. The sensitivity of a given assumption can be explored in subsequent CORMIX simulations. For instance, the CorSens sensitivity analysis tool can automatically create cases with variation of Type A linear ambient density profiles.

Furthermore, if CORMIX flow classification indicates indeed a flow configuration (flow class) with near-field flow behavior which is stable, additional studies with the post-processor option CorJet can be performed to investigate any arbitrary density distribution.

After selecting the stratification approximation to be used, the user then enters all appropriate density (or temperature) values and pycnocline heights (HINT) to fully specify the profiles. The pycnocline is defined as zone or level of strong density change that separates the upper and lower layers of the water column. The program checks the density specification to insure that stable ambient stratification exists (i.e. the density at higher elevations must not exceed that at lower elevations).

Note that a dynamically correct approximation of the actual density distribution should keep a balance between over-and under-estimation of the actual data similar to a best-fit in regression analysis. If simulation results indicate internal plume trapping, then it is desirable to test --through repeated use of CORMIX using CorSens-- different approximations (i.e. with different stratification types and/or parameter values) in order to evaluate the sensitivity of the resulting model predictions.

Specification of Ambient Density for Seawater

In the case of salt water conditions, the figure on the right can be used as a practical guide for specifying the density if "salinity values" in parts-per-thousand (ppt) are available for the water body. Typical open ocean salinities are in the range 33 - 35 ppt.

In CORMIX, the user then specifies whether the ambient density (or temperature) can be considered as uniform or as non-uniform within the water body, and in particular within the expected discharge flow regions. As a practical guide, vertical variation in density of less than 0.1 kg/m3 or in temperature of less than 1 degrees C can be neglected. For uniform conditions, the average ambient density or average temperature must be specified.

Density-temperature-salinity diagram for seawater (larger image).