Modeling Air Entrainment Downstream of Spillways

One environmental concern of hydropower is the supersaturation of total dissolved gas (TDG), which might induce gas bubble disease in affected fish. The main source of TDG downstream of dams is the dissolution of bubbles entrained during spill events. Direct numerical simulation of the air entrainment process and subsequent bubble transport and dissolution in a tailrace is well beyond current computer capabilities. Current two-phase flow models used to predict TDG are based on a Eulerian approach with the entrained gas volume fraction and bubble size as model parameters. Since direct measurement of air entrainment at the impact region of a spillway is extremely difficult, these parameters are typically calibrated against TDG measurements downstream of the dam. In this study, a mechanistic model is used to compute the air entrained in the spillway. TDG concentration is calculated with a transport equation taking into account that bubble dissolution depends on the bubble interfacial area and depth reached by those bubbles in the tailrace. A bubble number density equation is used to predict bubble size, which can change by dissolution and compression. The model was implemented in the open source code OpenFOAM. Preliminary results in a single spillway bay of Tongjiezi dam in the Dadu River, China show that the model is able to capture the observed TDG during a spill event with the bubble size at the entrainment region as unique parameter.