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Measurement of discharge in open channels is one of the main concerns in hydraulic engineering. The structures used for this aim should be accurate, economical and easy to use. Weirs are among the oldest and most convenient hydraulic structures that have been used in both laboratory and field for flow measurement in open channels. Due to limitations of simple sharp crested weirs, recently compound sharp crested weirs have attracted great attention of civil engineers. In general, use of compound sharp crested weirs can be an appropriate solution when the discharge should be measure accurately with a reasonable sensitivity over a wide range of flows. The aim of this research is three-dimensional simulation of flow on contracted compound arched rectangular sharp crested weirs by using FLUENT software. For multiphase flow simulation, VOF method is used and for simulation of turbulent flow, RNG k-ɛ turbulence model is used and the result of numerical model is compared with experimental data. The results of this study indicate that FLUENT simulate flow on contracted compound arched rectangular sharp crested weirs with high accuracy and we can use this software for determine the discharge coefficient on contracted compound weirs.

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In this research, different types of weirs have been numerically investigated to determine the optimal design based on two hydraulic and structural criteria. FLOW 3D and ABAQUS software were utilized for the hydraulic and structural analysis, respectively. The accuracy of the numerical models was verified with the available experimental and numerical results. In the hydraulic investigation, 18 models of different types of weirs including rectangular (6 models), square, triangular (3 models), circular, ogee (3 models), and labyrinth (4 models) weirs were examined. In the structural study of weirs, there are 13 models, including rectangular, square, triangular (3 models), circular, ogee (3 models), and labyrinth (4 models) weirs were analyzed. The results of hydraulic analyzes showed that the dimensions of the rectangular weir significantly affect the output velocity. In triangular weirs, the highest energy dissipation will occur with an apex angle of 45°, and with the increase of the apex angle in the ogee weir, more turbulence is observed in the downstream flow. In labyrinth weirs, by changing the shape of the weir from triangular to rectangular, the output velocity and also turbulence of the flow will be much less. According to the findings of the structural analyses, the increase of the apex angle in triangular weirs, the weir will be more critical, but the situation will be more suitable in ogee weirs. Additionally, the rectangular labyrinth weir performs the best structurally among the labyrinth weirs.