Distributed Entrainment Sink Approach (DESA) for Modelling Mixing and Transport in the Intermediate Field

In densely populated coastal cities in Asia, wastewater outfalls are often located not far from sensitive areas such as beaches or shellfisheries. The impact and risk assessment of effluent discharges poses particular technical challenges, as pollutant concentration needs to be accurately predicted both in the near field and intermediate field.

The active mixing close to the discharge can be modelled by proven plume models, while the fate and transport far beyond the mixing zone can be well-predicted by 3D circulation models based on the hydrostatic pressure approximation. These models are usually applied separately with essentially one-way coupling; the action of the plume mixing on the external flow is neglected. Important phenomena such as surface buoyant spread or source-induced changes in ambient stratification cannot be satisfactorily addressed by such an approach.

A Distributed Entrainment Sink Approach (DESA) is proposed to model effluent mixing and transport in the intermediate field by dynamic coupling of a three-dimensional (3D) far field shallow water circulation model with a Lagrangian near field plume model. The action of the jet on the surrounding flow is modelled by a distribution of sinks along the jet trajectory and an equivalent diluted source flow at the terminal height. The induced entrainment flows computed by the jet model are included in the far field model as internal forcing conditions, while the velocity and concentration field computed by the 3D model provide the boundary conditions for the near-field jet model. In this way, a two-way dynamic link can be established at grid cell level between the near field and far-field models. Any changes in one model will immediately be passed to the other model to ensure the two-way dynamic coupling between the two models.

The key advantages of DESA include: a) two-way communication between the near field jet model and the 3D far field model which are based on very different modelling concept; b) the jet mixing can be well-represented in the 3D model without a need for ad hoc adjustment of grid size or turbulent diffusivity; and c) the same 3D model grid can be used for different discharge configurations without customization - unlike CFD calculations which would require different high resolution grids for different jet configurations.

The method is demonstrated for a number of complex flows including the interaction of a confined rising plume with ambient stratification, and the mixing of a line plume in cross-flow. Numerical predictions are in excellent agreement with basic laboratory data. The general method can be readily incorporated in existing circulation models to yield accurate predictions of mixing and transport in the intermediate/far field.

 

References

  • Choi, K.W., Lai, C.C.K., and Lee, J.H.W., (2016), "Mixing in the Intermediate Field of Dense Jets in Cross Currents." Journal of Hydraulic Engineering, ASCE, Volume 142, Issue 1.
  • Choi, K.W., Yu D., Lee J.H.W., (2009), "Modelling of dense jet in co-flow and counter flow." In: Zhang C, Tang H (eds) Advances in Water Resources and Hydraulic Engineering, Proc. of 16th IAHR-APD Congress and 3rd Symposium of IAHR-ISHS, Vol 2. Tsinghua University Press, Beijing, pp 603-607.
  • Choi, K.W. and Lee, J.H.W., (2007), "Distributed entrainment sink approach for modelling mixing and transport in the intermediate field", Journal of Hydraulic Engineering, ASCE, Vol. 133, No. 7, pp. 804-815
  • Choi, K.W. and Lee, J.H.W., (2005), "A new approach to effluent plume modelling in the intermediate field", Proc. 31st IAHR Congress, Seoul, Korea, September 11-16, 2005, 4303-4311. [pdf 787KB]

 

Example Applications

  • Round Buoyant Jet in a Confined Region
  • Line Diffuser in a Cross Current
  • Sewage outfall discharge in Tolo Harbour  
    The animation shows the mixing and transport of a sewage effluent plume in an actual unsteady coastal flow in Tolo Harbour, Hong Kong. The concentration field in the intermediate/far field is computed by coupling the VISJET plume model with the EFDC (Hammrick, 1992) shallow water circulation model using the new DESA method. The mixing and gravitational spreading of the buoyant effluent in the unsteady tidal flow and the impact on the shoreline can be clearly seen. The hypothetical multiport diffuser outfall has a source flow of 0.12 m3/s discharged through four vertical ports of diameter 0.71 m and spacing 25 m.


For further information on DESA, please contact Prof. Joseph Lee at jhwlee@ust.hk or Dr. David Choi at dkwchoi@ust.hk.