| Measuring water of irrigation district is basic in the popularization and application process of agricultural water-saving irrigation technology, and it's the basic conditions to manage water use in irragation districts . With water saving agriculture development and pricing mechanism reform, it is important to study simple structure and high precision flow measurement devices.Flat V weir combinate advantages of triangular-profile weir and thin triangle weir, has a simple structure, flow smoothly, and high accuracy. In this paper, flat V-weir flow measurement principle is analyzed, methods of flat V-weir numerical simulation are analyzed of selected, and the results of simulation are analyzed. Main conclusions as follow :(1)By comparing the simulations results of different length of channel from the weir , we obtained that with the increase of channel length from the weir, The water level is more stable. Therefore, we need to ensure that the water level to some extent the results close to the case of stable value, select the appropriate calculation domain, saving computation time. This paper selection, 12m from the the weir is more suitable to take in this article numerical simulation model.(2)By comparing the simulation results obtained with different size of mesh for the computational domain, we got that when the grid is reduced to a certain extent, improve the accuracy of simulation results is limited, while the computation time is greatly increased. we need to select the appropriate mesh size to reduce the computing time. By comparison in This paper, we find that using a grid size of 0.2m and the encryption of weir department method to ensure the accuracy of operation, save some computing time and improve efficiency of operations.(3)By using different turbulence model to simulate the results of the comparison, obtained in the numerical simulation of flat V-weir. Standard turbulence model with the results of error slightly larger than Realizable turbulence model and the RNG turbulence model with the error generated. At the same time, taking into account amendments to the RNG turbulence model can handle a greater degree of convective flow of curved lines, so this follow-up using RNG turbulence model simulation.(4)The average flow rate set for the entrance of the simulation, the water level began to be imported from the flow rate gradually decreased, and the cross-sectional flow velocity distribution with the advance of the situation will become more and more uneven, from the energy point of view, we come to the water level decline in the velocity distribution because of the entrance of the section of water kinetic energy, lower than the flow cross-section of non-uniform distribution of water kinetic energy, flow rate adjustment in the process of non-uniform distribution, With the increase of kinetic energy, according to the principle of conservation of energy, potential energy must decrease, the water level will fall. Calculation for future improvements to enhance channel simulation accuracy and provide some ideas.(5)In the numerical simulation of flat V-weir, use RNGk-e turbulence model; use RNGk-e turbulence model; VOF method to track the free water surface mining; PISO solution method, the water level by using different numerical simulations, the water line and the initial value contrast, found that the method is accurate simulation of water flow through a rectangular channel flat V-weir when the three-dimensional flow characteristics, can be flat for the laboratory V-weir design and field measurement volume flow rate of water facilities to provide a reliable basis.
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