Author(s): S. N. Chan; J. H. W. Lee
Keywords: Intake structure; Supercritical flow; Bottom rack; Volume-of-fluid; CFD
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 increases 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.