Study On Flow Measurement Method And Numerical Simulation Of Novel Trapezoidal Weir

Accurate flux gauging in irrigated areas is the basis and premise of water-saving irrigation and scientific management of water resources.the implementation of high precision,strong applicability and economical and practical quantity of water facilities is of great significance for agricultural water-saving and alleviation of water resources shortage.Trapezoidal measuring weir due to its convenient measurement,unsophisticated structure,and high accuracy,has become one of the most widely used flux-gauging facilities.Despite the sedimentation on the upstream side of weir,and the mismatch between gauging range of thin-wall weir and channel flux,most studies in the field of thin-wall weir have only focused on improving the gauging accuracy,For this reason,in view of the problems existing in the trapezoidal water measuring weir in the irrigation area,a novel water measuring facility with strong applicability and simple structure is proposed.In this study,model test and numerical simulation was used to investigate the performance of the proposed weir under different flow conditions and structural parameters.The main research contents and conclusions are as follows:(1)In light of the flow range in specified irrigation area,novel trapezoidal weirs with different structural parameters were designed based on the standard weir.In view of two commonly used flow measurement methods,the basic form of weir flow measurement formula for the proposed weir was obtained.Based on the principle of high flux-gauging accuracy and high flexibility of the formula,the optimal measuring formula(Q=m(?))was found,and its average relative error was 2.63%,which met the requirements of the precision of agricultural water measurement in China.(2)The numerical simulation of the proposed weir was established,the model parameters of which were then calibrated reasonably considering the model test results.Through the comparison between the former head of weir and over-current flow pattern of the result of simulation and model test,it was shown that the average relative error was 2.53%,which indicate the high degree of match and the reliability of simulation based on Fluent.Based on the above models and simulation parameters,the hydraulic characteristics such as weir flow rate,longitudinal and transverse flow velocity distribution,turbulent kinetic energy and water surface profile under different working conditions are deeply analyzed.The distribution law of hydraulic characteristics of the novel weir flow under the influence of pore flow is obtained.(3)Taking the novel weir with the crest width b=250 mm as an example,the distribution of hydraulic characteristics of weir flow under different hole heights was analyzed,and the suitable hole height(z=50mm)was determined.The change of velocity distribution along Z direction of water flow is taken as an index to determine the appropriate hole height,and the change of velocity distribution along Z direction of water flow under different specifications of trapezoidal weirs with different hole heights is analyzed.The suitable hole height ratio(z≈P/2)of trapezoidal weir with different specifications was found.(4)The water measuring performance of the novel trapezoidal water measuring weir in various aspects is studied.The results show that:When the standard trapezoidal weir with width b=250 mm is taken as an example,a suitable hole(z=50 mm)is opened at the bottom.The flow measurement range is increased by458%compared with standard weir and 34%compared with improved weir.The average velocity of flow near the orifice is 0.7～1.2 m/s,which is higher than the incipient velocity of common sediment in the irrigation area and can effectively avoid sediment deposition in front of the weir.In the northern region,the bottom orifice can be used to remove the water in front of weir after autumn irrigation,which eliminates the condition of frost heaving damage of the canal.For a channel with a bottom slope greater than 1/1500,the novel weir can save at least 8.1%of the channel lining works compared with the trapezoidal weir under the same conditions.The improvement of the performance reported here shed new light on the promotion and application of the novel trapezoidal weir in irrigation areas.