Document Type : Original Article
Authors
1
Associate Professor, Department of Hydraulic Structures, Faculty of Water and Environmental Engineering, Shahid Chamran University of Ahvaz
2
M.Sc. in Hydroinformatics, Department of Hydraulic Structures, Faculty of Water and Environmental Engineering, Shahid Chamran University of Ahvaz , Ahvaz, Iran
3
Professor, Department of Hydraulic Structures, Faculty of Water and Environmental Engineering, Shahid Chamran University of Ahvaz , Ahvaz, Iran
Abstract
Weirs are among the most common and simplest structures for measuring and regulating flow, installed along the channel axis perpendicular to the flow direction. The duckbill weir, with its trapezoidal plan section, increases flow-conveyance capacity by extending the effective crest length relative to the channel width. In this study, the three-dimensional flow field over duckbill weirs with central angles of 15, 30, 45, 60, and 75 degrees was numerically investigated using FLOW-3D under both free- and submerged-flow conditions. Validation against experimental data showed that the trend and magnitude of the computed discharge agreed well with the measured values, with an average error of 2.8%. Comparison of the water-surface profile prediction errors confirmed the superiority of the RNG turbulence model over the k-ε model; the RMSE and AME values were 0.0825 and 0.0631 m for the RNG model and 0.0934 and 0.0887 m for the k-ε model, respectively. Under free-flow conditions, the discharge coefficient (Cd) decreased as the central angle increased, and this reduction became more pronounced at higher hydraulic heads; the greatest reduction occurred for the 75-degree weir, attributed to its longer crest length and the interference of flow jets in the apex region. Conversely, under submerged-flow conditions, the 60- and 75-degree weirs exhibited greater resistance to submergence owing to their longer effective length, such that at a constant submergence ratio, the discharge over the 75-degree weir was on average about 30% greater than that over the 45-degree weir.
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