Three-dimensional CFD Modelling of Mixing and Dispersion from Marine Outfall Discharges

Municipal and industrial wastewater is commonly discharged into coastal waters via ocean outfalls. Accurate prediction of mixing and dispersion of such wastewater is important to prevent significant negative effects on the water quality of the receiving environment. In the near field, mixing is governed by the momentum and buoyancy of the discharge. Such buoyancy is a result of temperature and salinity differences between the effluent and the receiving waters. However in the far field, mixing and dispersion respond to the wider ocean hydrodynamics. Traditional modelling approaches predict dilution in the near field based on length-scale or entrainment models, whereas the far field is simulated using coastal circulation models. CFD approaches have been scarcely used to model both regions, despite their great potential. The fact that the CFD methods have been infrequently used is a result of the length scale disparity between the discharging ports and the receiving water body plus the unknown trajectory of the flow prior its simulation. This means that fine meshing may be required for an accurate solution, making this approach computationally expensive. This study presents the validation of near field simulations from single and multiple ports discharges at laboratory scale using three-dimensional, time-dependent RANS k-epsilon and k-omega turbulence models. Water density was defined as a function of temperature and salinity using the UNESCO seawater state equation. To overcome the difficulties arising from the different length scales of the discharge ports and the receiving water body, a mesh adaptation approach was used. Predictions of flow trajectory, velocity and dilution show good agreement with available experimental values, providing higher accuracy than widely used modelling packages such as CORJET, VISJET and UM3. The validated methodology is then applied to the three-dimensional, time-dependent simulation of a full-scale real-life ocean outfall in Cartagena, Colombia, under varying conditions. The sensitivity of the model to changes in parameters such as discharge flow, current velocity and direction, density stratification, port spacing, and port depth is studied. Results are compared with available water quality monitoring data of the Cartagena near field. Potential approaches for coupling CFD near field model results with far field ocean circulation models are proposed.