Experiment and simulation of factors affecting flow measurement of water-measuring column with round head in U-shaped channel

Abstract: In this paper, water-measuring column with a round head was proposed to measure flow based on the cylindrical flow around theory. In the column, the cylindrical flume was improved by adding a V-shaped empennage behind it to reduce head loss, improve the accuracy of flow measurement and increase sediment transport capacity. The water-measuring column with a round head was installed vertically in a U-shaped channel and its symmetry plane was overlapped with the centerline of the channel bottom. By controlling the length of the V-shaped empennage, it could make the flow pattern steady and reduce head loss. Based on the cylindrical flow around theory and RNG k-ε turbulence model, hydraulic performance of the whole flow field of water-measuring column with a round head in 18 somatotypes under 5 working conditions were numerically calculated. And we obtained the time-averaged flow field, sectional velocity distribution, flow pattern behind the column as well as the measured values and the simulated values of the related hydraulic parameters. The comparison of simulated and measured hydraulic parameters values showed that they had good consistency. When the channel slope was 1/1000, discharge was 0.045m3/s, the contraction ratio was 0.50, and the aspect ratio was 2, the relative error of the maximum velocity of cross-section at a stagnation point between the simulated value and the measured value was 1.51%, and the relative error of the maximum velocity in a profile at 15 cm upward to water surface between the simulated value and the measured value was 0.45%. Therefore, simulation results can provide valuable information for rational design of a water-measuring column with a round head and its analysis of flow field measurements. On the one hand, an oversized contraction ratio may cause excessive upstream backwater and affect the normal operation of the channel. On the other hand, it was difficult to form a critical flow if the contraction ratio was too small. Based on the experimental data, a suitable range of contraction ratio was between 0.50 and 0.70, and it was ultimately determined by the channel slope. When the channel slope was small, the contraction ratio should be bigger. The analysis on the measuring effect of V-shaped empennage in model tests and numerical simulation revealed that V-shaped empennage of suitable aspect ratio could effectively improve the flow pattern. The recommended ranges of the aspect ratio ? were in 3/2~2 when the contraction ratios changed between 0.50 and 0.75. In addition, the aspect ratio should correspondingly increase but should not be more than 5/2 when the contraction ratio was less than 0.50. The flow formula with higher accuracy was fitted by regression analysis, which is based on stagnation-point, contraction ratio, surface width and channel slope. The maximum measuring error was 4.95% and the average error was only 0.10% when the contraction ratio was 0.63. The formula was useful for the further application of the water-measuring column with a round head in a terminal irrigation system in North China, for the reason that it was concise, practical, in line with the principle of dimensionless harmony, and can meet the flow measurement accuracy.