Estuary Processes and Managment - River Dynamics and Integrated River Management - page 515

round buoyant jet in a stratified cross flow, with a 3D trajectory. JETLAG tracks the evolution of the
average properties of a plume element by conservation of horizontal and vertical momentum, conservation
of mass accounting for shear and vortex entrainment, and conservation of solute or tracer mass/heat. For a
given set of ambient conditions (vertical profile of horizontal velocity, density, and tracer concentration),
the jet trajectory, jet velocity and radius, and dilution (entrainment) can be predicted. The turbulent
entrainment along the jet trajectory can then be obtained from the discharge point up to a terminal level
(free surface, bottom, or trap level in the presence of ambient density stratification). In addition, the boundary
conditions for the near-field model depend on the external (intermediate or far field) flow, which can be
obtained from a 3D circulation model.
Fig. 8.56 Structure of the VISJET system
The simulation engine in the PC-based VISJET system is fully integrated with visualization technology,
which makes it convenient for practical applications such as outfall design, impact assessment, and risk
analysis of pollutant or natural environmental discharges. The jet trajectory data as output by the JETLAG
module is readily available from the visualization results—e.g., the user may locate the point of interest
to interactively retrieve the required data values. The realistic modeling and rendering of the surrounding
environment, such as the sea bed and the sea surface in the case of an ocean outfall study, is used to
enhance the user's sense of presence in a 3D environment so that the user can better understand the context
of the simulated phenomena. Figure 8.57 shows an example of a jet trajectory computed by VISJET
compared with the experimental observation for the Wah Fu Sewage Outfall, Hong Kong.
In many practical situations, the wastewater is discharged from an outfall riser in the form of rosette jet
groups in the presence of a tidal current. A rosette jet group discharging into an ambient current is a complex
flow involving mixing, merging and interaction of co-flowing, cross-flowing, and counter-flowing jets with
three-dimensional trajectories. Figure 8.58 shows an example of VISJET simulation and experimental
observation of multiple buoyant jets of such rosette buoyant jet groups from an ocean outfall riser. Figure
8.59 shows the near field jet mixing of a number of rosette risers mounted on a submerged ocean outfall;
it is noted that the plumes from adjacent risers can interact and overlap significantly; the dilution of a jet
group is, hence, less than that if jet interference is not present. The “composite dilution” of such jet groups
can be determined from the degree of merging of these jet groups (Lai et al., 2007).
Near and far field coupling —Near-field models give predictions of pollutant concentrations in the
initial mixing zone; while turbulent plume entrainment is properly modeled, important mixing characteristics
such as the gravitational spreading layer and its coupling with plume entrainment cannot be predicted by