Numerical study on the release process of supersaturated total dissolved gas considering free surface fluctuations...

Studying the release of total dissolved gas (TDG) supersaturation downstream of high dams is crucial for preventing gas bubble trauma (GBT) in fish. Previous numerical calculations on the TDG supersaturation release typically used the Volume of Fluid (VOF) method to simulate the free surface of the water flow and then introduced the gas-liquid mass transfer based on the rigid-cap assumption. However, this method neglects the fluctuations of the free surface, involves a complicated establishment process, and has low calculation accuracy, necessitating optimization. This study establishes a three-dimensional (3D) unsteady two-phase flow model integrating VOF and TDG transport equations to explicitly resolve free-surface fluctuations and their interaction with TDG release. This model captures the shape and evolution of the free surface with higher computational accuracy and efficiency compared to models that do not account for free surface fluctuations. The findings indicate that free surface fluctuations increase the gas-liquid contact area, thereby accelerating the release rate of TDG supersaturation. When the water surface area is increased by 0.15% to 1.32% compared to the non-fluctuating scenario, the outlet TDG saturation is reduced by 0.86% to 2.80%. The proposed framework reduces computational time by 56% while improving accuracy by 1%. This makes it highly suitable for real-time hydropower operations, enabling rapid assessment of TDG mitigation strategies to protect downstream fisheries.