Numerical simulation and test of hydraulic performance for triangle long-throat flume for water measurement in furrow irrigation

Abstract: In view of the lack of effective water measuring facilities for furrow irrigation areas in North China, a portable triangular long-throated flume was proposed as a small flow water measuring equipment in the field. This study was to investigated its hydraulic performance of the triangular long-throated flume based on both test and numerical analysis. The prototype test was carried out in Northwest A & F University in Yangling, Shannxi of China. The flume was installed at 6.25 m far from the inlet of upstream of U-shaped channel. The base slope of the channel was 1/10 000. The triangular weir was installed at the end of the discharge pond downstream of the channel. A total of 14 sections were designed to observe the flow characteristics. The numerical simulation was used to study its hydraulic characteristics of the flume by using the flow-3D-based CFD method. Based on the RNG k(( three-dimensional turbulence model together with the TruVOF technique, experiments and the corresponding simulations were performed for 16 working conditions (the flow rate of 1.03, 3.03, 5.06, 7.07, 9.00, 11.07, 13.07 and 15.05 L/s under free and submerged outflow conditions) on the trapezoidal triangle long-throat flume with flow rate up to 15 L/s. The flow pattern, water depth and Froude number, the longitudinal mean velocity and turbulence intensity were analyzed. The results from the prototype experiment was used to validate the simulation results. The results showed that the maximum relative error between the tested water depth and the simulated value was less than 10% under all the 16 flow rate conditions. The variation of the tested and simulated flow pattern were consistent with each other, indicating that the simulation had a high accuracy. The longitudinal mean flow velocity of the shrinkage section and the throat section of the flume continuously increased along the water flow. The maximum flow velocity existed below the water surface. The longitudinal mean flow velocity increased along the flume at the contraction section and throat section. The maximum flow velocity occurred under the water surface. The position where maximum flow velocity occurred was lowered and the flow velocity distribution was uneven at the position closer to the contraction section and throat section. The turbulence intensity showed a trend of increasing along the flume. The relative water depth with the maximum turbulence intensity of each section was between 0.13 and 0.30, and gradually increased along the flume. Through the derivation of critical flow theory and regression analysis, the flow rate estimation formula for the long-throat flume was obtained based on water depth. The maximum relative error between the calculated result and the actual flow rate was 4.34%, the mean absolute relative error was 1.86%. Under the condition of submerged outflow, the maximum relative error was 3.47% and the mean absolute relative error was 1.94%. It suggested that the model had the high accuracy and could met the common requirements of flow measurement in irrigation areas. The study can guide the further optimization of the shape parameters such as the length of the throat section of the flume and the shrinkage angle of the water surface.