Start Date

2018 8:20 AM

Creative Commons License

Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 License.

Abstract

Compact intake structures are used for diverting turbulent supercritical flow on steep catchments in the hinterland of the urban area of Hong Kong to an underground flood diversion system through a number of vortex dropshafts. Bottom racks are placed at the entrance of the intake to exclude debris from entering the system. The rack bars interact with the supercritical inflow and creates a highly turbulent air-water mixture. This paper presents a three-dimensional (3D) computational fluid dynamics (CFD) modeling study to predict the complex flow details and air concentration of the bottom rack intake structure, using the volume-of-fluid (VOF) technique. Numerical simulations are conducted with different inflow rates and bottom rack bar shapes. The water depth, velocity and air concentration agree well with experimental measurement. Model results show that the rack interception induces an energy loss and increase the flow depth above the rack. The rack interception also gives rise to a sheet jet beneath the rack and results in air entrainment. In the rack chamber, the flow consists of a wall jet that impinges on a spiral circulation of aerated flow, inducing significant turbulence and air entrainment. The average air concentration in the rack ranges from 20% - 50% and decreases with increasing discharge. The air concentration in the chamber appears to be little affected by the presence of bottom rack or the shape of rack.

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May 17th, 8:20 AM

3D CFD Modeling of a Supercritical Bottom Rack Intake

Compact intake structures are used for diverting turbulent supercritical flow on steep catchments in the hinterland of the urban area of Hong Kong to an underground flood diversion system through a number of vortex dropshafts. Bottom racks are placed at the entrance of the intake to exclude debris from entering the system. The rack bars interact with the supercritical inflow and creates a highly turbulent air-water mixture. This paper presents a three-dimensional (3D) computational fluid dynamics (CFD) modeling study to predict the complex flow details and air concentration of the bottom rack intake structure, using the volume-of-fluid (VOF) technique. Numerical simulations are conducted with different inflow rates and bottom rack bar shapes. The water depth, velocity and air concentration agree well with experimental measurement. Model results show that the rack interception induces an energy loss and increase the flow depth above the rack. The rack interception also gives rise to a sheet jet beneath the rack and results in air entrainment. In the rack chamber, the flow consists of a wall jet that impinges on a spiral circulation of aerated flow, inducing significant turbulence and air entrainment. The average air concentration in the rack ranges from 20% - 50% and decreases with increasing discharge. The air concentration in the chamber appears to be little affected by the presence of bottom rack or the shape of rack.