Numerical study of bubble screens for mitigating salt intrusion in sea locks

In order to mitigate the effects of salt water intrusion at sea locks, bubble screens are installed which act as a barrier between the dense sea water and the fresh water inland. In order to optimize the design of the bubble screen, in this study a state-of-the-art numerical model is developed based on the Euler-Lagrange CFD method which is expanded with a simple salt balance and concentration-density coupling. The model has been validated by means of experimental results on a laboratory-scale bubble screen. The liquid circulation and entrainment have been investigated for two types of bubble injection methods. It is found that the bubble screen is successful as a separator of salt and fresh water in an initial period of τ_sep=30 seconds but acts more as a mixer at later times due to the swaying of the screen. The rate of the mixing increases with the air flow rate. Two mechanisms of salt intrusion are distinguished; a delayed density current along the bottom and entrained liquid being circulated through the domain back to the screen. An optimum in air flow rate is found at a Froude air number Fr_air=0.91. Bubble screen behaviour is also checked at the lock-scale using lock-scale geometry and simulations. The amount of salt transmitted agrees well with the large-scale field tests up until the reported Fr_air numbers but Fr_air > > 1 need to be tested to check for the optimum as found in the lab-scale tests.