Numerical study of the effect of upstream Froude number on the flow characteristics along a rectangular sharp-crested side weir

Diversion structures in side wall of open channel are to be used in order to divert flow from main channel. Side weirs, side orifices and side sluice gates are diversion structures which have wide application in dams, irrigation, drainage systems and combined sewer systems. Side weirs are one of the most important and applicable hydraulic structures for water controlling and directing systems that requires careful review and accurate designing due to their critical importance .In this paper, the flow over a sharp-crested rectangular side weir in open channel was simulated by FLOW-3D software. RNG turbulence model was used to apply the Navier-Stokes equations and the VOF method was used to model the free surface profile changes. In the present study, at first, results related to the distribution of the different components of velocity and angle of the outlet jet adjacent to the crest of the rectangular sharp crest side weir at various heights of the weir crest are validated with experimental data from Bagheri and Heidarpour (2012) research. Then the effects of upstream Froude number on flow pattern and free surface around the side weir were investigated. The results of this study indicated that with decreasing Froude number, maximum and minimum longitudinal velocity along the side weir increases and decreases, respectively. The maximum lateral velocity along the side weir increased with decreasing Froude number. That is why the discharge through the side weir increased with decreasing Froude number. The maximum vertical velocity along the side weir increased with decreasing the upstream Froude numbers because more flow is diverted to the side weir with decreasing the upstream Froude numbers. Distribution of the different components of velocity in the direction of flow height in the middle of the weir crest indicated that by reducing the upstream Froude number, longitudinal velocity distribution in the depth direction became more non uniform, especially near the weir crest. For different Froude numbers, lateral velocity would increased up to a certain elevation from the weir crest and then decreased toward the free surface. By increasing Froude number, the maximum lateral velocity occurs at greater heights above the side weir crest. The maximum vertical velocity for different Froude numbers occurred near the weir crest. Vertical velocity gradually decreased toward the free surface. By reducing the upstream Froude number, the vertical velocity near the free surface of flow is negative due to flow motion to downward. Free surface of the flow experienced extreme changes at the end of the side weir when upstream Froude number was increasing. By increasing the Froude number, the angle of inclination of the outlet jet along the side weir increases. Also by increasing the Froude number, the separation zone around the side weir in near the main channel bed, especially in the channel cross-section decreases.