Hydraulic performance experiments and numerical simulation of portable water measuring flume of imitating airfoil shape

Abstract: Accurate, simple, and rapid water measurement in the field is highly required for the planned water use and water-saving agriculture in most irrigation areas, particularly in the last small cross-section irrigation districts. In this study, a measuring device was proposed in an airfoil-like shape with a simple structure, according to the conventional airfoil-shaped flow measuring flume. The simple structure of the airfoil-shaped measuring device was applied to the small cross-section flow measurement of the rectangular channel. A hydraulic model test was carried out in the Laboratory of Hydraulic Engineering and Hydraulics of Northwest A&F University of China in 2020. A plexiglass rectangular channel flume was designed with a size of 17 m×70 cm×100 cm. Flow flume models were all made of hollow wooden materials. A total of 42 test schemes were designed, including 1 set of airfoil length, 6 sets of contraction ratios, and 7 sets of flow rates. A better hydraulic performance was presented in the numerical simulation than before, such as Froude number and flow streamline. A FLOW-3D software was also used to supplement several sets of models to evaluate the simulation accuracy, and the hydraulic performance. Specifically, the upstream Froude number (less than 0.5) was verified to meet the national standard requirements. The position of the throat produced the critical flow, whereas, the downstream resumed the slow flow. The average height of backwater was 3.25 cm, while the maximum was 6.32 cm, and the minimum was 0.46 cm. A total of 84 datasets were obtained from the hydraulic model test and numerical simulation. The flow formula of the portable airfoil-shaped flume was achieved, where the average relative error was 6.34%, meeting the requirements of the last small cross-section channel. The flow rate range of the flume was 15.3-47.2 L/s. An optimal combination of hydraulic parameters was achieved, where the range of shrinkage ratio from 0.606 to 0.709, with an average error of 6.95% under shrinkage ratio of 0.606, while 4.91% under shrinkage ratio of 0.709. The airfoil length ranged from 65 to 80 cm, all of which were suitable for the specific irrigation area. Correspondingly, it was suggested that the airfoil length ranged from 65 to 80 cm, when the channel width was 70 cm in the practical field. In addition, the double channel in the flow measurement device demonstrated that the entrance was flush, and the central axis was parallel in the center of the rectangular water tank. There was no gap between the bottom of the fixture and the channel surface during installation. The symmetrical model was well sealed at the joint of the central axis of the channel for a higher accuracy of flow measuring precision. Consequently, the simplified airfoil-shaped flume can be expected for the higher safety of measuring channel, concurrently remaining the excellent performance. This finding can provide a promising economic and practical support to the higher performance of measuring water and the efficiency of water use, particularly to the popularization of small-section portable flow measurement flume in most irrigation districts.