Bubble curtain dynamics in lock-exchange flows*

Bubble curtains are commonly used in shipping locks to mitigate saltwater intrusion that occurs when the lock gate is opened for ships. The opening of the lock gate results in a lock-exchange flow where the denser salty water flows underneath the lighter freshwater in the form of a gravity current. The vertical momentum of the bubble curtain impedes the gravity current flow. We employed Euler-Euler large eddy simulations to model bubble curtains in a lock-exchange configuration and varied the problem’s parameters (water height –  H , density difference – △ρ  and air flow rate – q_air)  covering a wide range of Froude air number Fr_air = (gq_air)^1/3 (g'H)^-1/2 values, with g the gravitational acceleration and g' the reduced gravity. The effectiveness of a bubble curtain is characterized by the amount of salt water that is blocked by it in comparison with the case without a bubble curtain. We compare the effectiveness as a function of Fr_air from simulations with recent experimental results (Bacot et al., JFM 941, A1, 2022) , showing a very good agreement. The results are further used to construct and test an analytical model for the effectiveness, and scaling relations for the typical velocity, length and mixing time scales of the recirculation cells present on each side of the bubble curtain.