Numerical simulation of the influence of water keeping weir body type parameter on flow capacity
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Abstract:
Water keeping weir is a new type of overflow weir, characterized by small pressure in two sections of the transmission line, and simple hydraulic control. In order to enhance the water conveyance capacity of water keeping weir in large water diversion projects. First of all, based on the actual operation of the project, the RNGk-ε turbulent flow model is used to close the control equations and establish a three-dimensional isometric numerical simulation model of the water keeping weir, and the simulated head and flow velocity of the pressure measuring tube are in good agreement with the measured data, and on the premise of ensuring the accuracy of the flow pattern in the simulated water keeping weir, the influence of the water keeping weir body parameters on the over-flow capacity of the box culvert - water keeping weir water conveyance system is investigated. It revealed the mechanism of the generation and development of annular vortex zones in the weir section, and determined that the dispersion of high velocity water flow zone caused by the height of the water keeping weir and the presence of annular vortex zones are the main bottlenecks affecting the improvement of flow capacity. Thus, the proposed solution of reducing the height of the water keeping weir concentrate the high velocity water flow zone and reduce the scope of the annular vortex zone to reduce the energy dissipation in the weir section, so as to achieve the purpose of improving the overflow capacity of the water conveyance system. Numerical calculations were carried out for 11 options of reducing the weir height of water keeping weirs, and the flow velocity vector distribution and the extent of the annular vortex zone in the weir section as well as the variation of turbulent kinetic energy dissipation and overflow flow were analyzed. The results show that the overflow of the water conveyance system can be enhanced by the weir reduction scheme, and by compared between different weir height schemes, the optimal weir height interval that increases its overflow capacity is finally found to be [1.0 P1≥P≥0.7P1] (where P is water keeping weir height). When the weir height is within this range, the compression effect on the high velocity flow area is weakened by lowering P, resulting in smaller flow vector elevation angle in front of the weir and the annular vortex zone behind the weir, and significantly lower energy dissipation, lowering 0.1P1 flow rate increases by 2.1% on average, where the weir height P is lowered to 0.7P1 when the flow rate is increased up to 6.3%, which can basically meet the water demand requirements of the project; Outside this interval, although lowering P reduces the annular vortex zone after the weir and the flow velocity vector angle becomes smaller, the energy dissipation is reduced, but the rapid expansion of the annular vortex zone before the weir consumes part of the water flow energy, resulting in the flow lift decay, at this time, lowering 0.1P1 flow rate only increases by 0.5% on average. In consideration of the project modification cost and the effect of overflow capacity enhancement, the structure of the water keeping weir is modified in the optimal weir height interval proposed to provide reference for the overflow capacity enhancement of the water transmission system.
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