Turbulent Dispersion Modeling of Environmental Discharges
This article was contributed by Daniel Valero, FH Aachen, winner of Flow Science’s 35th Anniversary Simulation Contest.
Environmental discharges and outfall structures have been traditionally designed by means of complex, cost-intensive and time-consuming experimental studies. Some models based on an integral approach are commonly employed despite their limitations, but contaminant re-entrainment or strong adverse discharges fall outside the hypothesis of such models. Thus, using a full 3D model for contaminant dispersion may improve knowledge on the real contaminant dispersion in rivers and estuaries. Similarly, bounded jets can be modeled and different diffusor locations can be tested in order to improve the overall environmental water quality and biotic conditions.
In this study, a jet discharge is modeled both experimentally and numerically. Then, an estimation of the turbulent dispersion in the shear region can be obtained. For the turbulence modeling, the k-ε RNG model is employed together with the VOF method for the free surface tracking. A monotonicity-preserving, second-order scheme is employed for contaminant advection ensuring proper modelling of turbulent transport. It is observed that the upstream channel flow deforms the jet, pushing it to the side groin fields where re-circulation takes place. The physical modeling together with FLOW-3D is used to obtain a turbulent Schmidt number, which can then be used in FLOW-3D for other similar contaminant dispersion problems, for example a real groin field in a river.
The bounded jet shows an unsteady behavior even for the statistically steady final solution. For ease of visualization, two iso-concentration surfaces are shown: (reddish) and (transparent white) that show a representative envelope of the contaminant reach. Consequently, the white iso-concentration surface shows a bigger dispersion than the reddish one, with the latter completely contained within the first one. The selection of such boundaries allows the contaminant dispersion to be visualized in a similar way to the one used in the laboratory. The grid spacing is set to 5 cm, as in the experimental model. As found in natural environments, the jet discharge is turbulent and some postprocessing operations are required, such as temporal averaging.
The numerical model accurately reproduces the jet trajectories and the jet entrapment in the shear region. The frequency of the flow oscillation in the shear region is also in agreement with the experimental model, which is linked to the overall re-circulation flow dynamics in the groin field. Simulations may be used to determine a suitable turbulent Schmidt number for this flow configuration to accurately capture turbulent dispersion.