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Side weirs are a type of hydraulic structures used for different purposes in water transition systems, water supply, flow diversion and flood control important. Side weir, is a key structure in transition of urban sewage; the advantage of this  structure in urban sewage is the pre treatment of the diverted flow due to side weir height which is in environmental engineering. The flow on these structures is spatially varied flow type with decreasing discharge. Spatially varied flow is a type of steady flow with decreasing or increasing discharge along the channel. To analyze this flow, its necessary to know the velocity distribution and the values of the  kinetic energy correction factor (α) and the momentum correction factor (β).However due to complexities concerned with this type of flow and experimental limitations there hasn't been enough study on the velocity distribution  for this kind of flow . In this research the velocity distribution in a rectangular side weir has been investigated using a commercial software. Before performing the numerical analysis it's necessary to check the software's ability in modeling the  3D flow on the side weir. Experimental data of Jalili Ghazizadeh (1994) has been used for verification. In these experiments side weir lengths 20,30,45,75 (cm) and side weir heights 1, 10, 19 (cm) has been used while discharge in the main channel varied from 43 to 90 (lit/s). The simulation boundary conditions are volume flow rate discharge for upstream boundary,the "wall" for wall and "symmetry" boundary conditions for water surface. The only difference in boundary conditions for subcritical and supercritical flow is in downstream boundary condition which is "specified pressure" for supercritical flow and "specified velocity" for subcritical flow used respectively. Turbulence model is RNG in all simulations. Comparing the results shows that the software is capable of calculating  the discharge passing the rectangular side weir with a good accuracy for both subcritical and supercritical flows. Therefore, based on obtained results we can conclude that the commercial software is capable of simulating 3D flow on rectangular side weir and the results obtained from performing analysis with this software can be cited. Velocity distribution, correction factors for kinetic energy and momentum were studied in detail . In the case of subcritical flow on the side weirs, water in the main channel and downstream area of the side weir has been observed to seperate in the opposite direction of the main channels, there for it is important to study these areas. A noticeable point is that although large amounts of simulation points have (α) and (β) close to one, simulation results show that (α) and (β) can not be considered  equal to one for the  whole cases. The variation of (α) and (β) in side weirs length in this research were ascending. Based on existing simulation results, new equation between (α) and (β) for subcritical and supercritical flow and quantification of separating area were proposed. Results of this research can help side weir designers to have a better understanding of the complex 3D flow on side weirs.
 
 

Keywords: Rectangular Side Weir, Energy Flux Correction Factor, Momentum Flux Correction Factor, Separation Zone, Numerical simulation, FLOW-3D Software., Energy Correction Factor, Momentum Correction Factor, Numerical Simulation

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