In Sweden, the dam-safety guidelines call for an overhaul of many existing bottom outlets. During the opening of an outlet gate,\nunderstanding the transient air-water flowis essential for its safe operation, especially under submerged tailwater conditions.Threedimensional\nCFD simulations are undertaken to examine air-water flow behaviors at both free and submerged outflows. The gate,\nhoisted by wire ropes and powered by AC, opens at a constant speed. A mesh is adapted to follow the gate movement. At the free\noutflow, the CFD simulations and model tests agree well in terms of outlet discharge capacity. Larger air vents lead to more air\nsupply; the increment becomes, however, limited if the vent area is larger than 10m2. At the submerged outflow, a hydraulic jump\nbuilds up in the conduit when the gate reaches approximately 45% of its full opening.The discharge is affected by the tailwater and\nslightly by the flowwith the hydraulic jump. The flow features strong turbulentmixing of air andwater,with build-up and break-up\nof air pockets and collisions of defragmented water bodies. The air demand rate is several times asmuch as required by steady-state\nhydraulic jump with free surface.
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