Numerical Simulation Of Bubble Screens For Mitigating Salt Intrusion Through Sea Locks

Abstract

Salt intrusion through sea locks causes the mixing of fresh and salt water due to a gravity current. This process may occur gradually over tens of kilometers and affects the required quality of the inland water. A bubble screen alongside the locks is one of the available mitigating measures for this undesired phenomenon. However, current numerical simulations on bubble screens to mitigate salt intrusion are relatively scarce, and the validation of such models has not yet been thoroughly checked. The objective of this thesis is to study the performance of bubble screens for mitigating the salt intrusion using an Euler-Euler Computational Fluid Dynamics (CFD) model in Fluent 17.2. The work was conducted at TU Delft in collaboration with Deltares, a hydraulics institute in Delft in The Netherlands. First, a simulation of a gravity current is conducted to study the mixing of fresh and salt water in the absence of a bubble screen. The mass transport equation for the salt concentration, together with a linear approximation of the concentration-density relation, is involved into the governing equations. The results are validated with empirical predictions and experiments at Deltares of gravity currents to study the buoyancy effects of different turbulence submodels. Second, simulations of bubble screens in the fresh-fresh water system are performed and validated with particle image velocimetry (PIV) measurements conducted at Deltares to investigate the bubble screen dynamics. Larger flow circulations are generated with increasing air flow rates, while a constant surface current thickness of around 0.3 times the water depth is found for all cases. Lastly, bubble screens simulations for six different Froude air numbers $Fr_{air}$ in the fresh-salt water system are conducted and validated with the dye measurements. The dimensionless number $Fr_{air}$ serves as a ratio of the kinetic energy of the rising bubble plume to the potential energy of the gravity current. The salt transmission factor, defined as a ratio of the salt intrusion with mitigating measures to that without any measures, shows that $Fr_{air}$ in the range of 0.93 - 1.08 is most efficient for mitigating salt intrusion. At a lower $Fr_{air}$, the salt water generally intrudes at the bottom area as a salt tongue, whereas the salt intrudes through the surface current for larger $Fr_{air}$ owing to a higher rate of liquid entrainment into the bubble screen.