| Water gauging in canals is a foundation of the water management and a critical technique. It is of great significance for water saving and water utilization in irrigation districts efficiently and sustainably. Studying the water measuring facility with simple structure, easy operation, low head loss, high accuracy and clear discharge formula is one of the irrigation management techniques urgently needed in irrigation districts. Because of the complicated conditions in the irrigation districts, it is difficult to apply a uniform water measuring facility. At present, water-measuring flumes cannot fulfill the demand of water gauging in irrigation districts. Developing water measuring facility and technique suited to current economic development level and specific requirements to meet the water-gauging demand, therefore, is the springboard of the research, especially for a large number of sublateral and farm canals and gentle slope canals. The streamlined Airfoil-shaped measuring flume, which is characterized by its simple structure and smooth flow, can be studied as a new water-measuring device. According to indoor experiments in accordance with theoretical analysis, numerical calculations and irrigation field tests, hydraulic characteristics of Airfoil-shaped measuring flume are studied systemically based on the preliminary studies.1. This article summarizes the facility and technique of water gauging in irrigation districts including the latest developments of water measuring flume, and describes the water-gauging principle of critical water-depth flume in details. It also obtains discharge formula by the method of dimensional analysis and traditional hydraulics in the U-shaped, rectangular and trapezoidal canal and compares it with the experiment data. Simple, clear and practical discharge formula obtained by the two methods can meet the demand of the accuracy requirement.2. This article, taking the Airfoil-shaped measuring flume that used in rectangular canal for example, qualitatively discusses and summarizes flow pattern of flow passing through the flume. It provides theoretical basis to understand short-throat flume's flow pattern and guides the design of the water-measuring flume. As for the Airfoil-shaped measuring flume built in the even canal, the critical flow occurs at the throat inlet cross section, which has the minimum local contraction. Certain engineering measures can be taken to make the cross section's ratio of local contraction changed from big to small values, then the flow in the flume can present a sub-critical, critical or super-critical flow patterns successively. Provided that flow in the water measuring flume can be controlled to be a super-critical flow, a high critical submergence will be obtained.3. The laboratory tests on various water measuring characteristics of the Airfoil-shaped measuring flume built in the three different canals are conducted. Critical submergence, backwater height and Froude number of the upstream cross section are analyzed to provide technical parameters for the application of water measuring flume in the irrigation districts. The critical submergence of the Airfoil-shaped measuring flume built in the three different canals is high enough so that the flume can work under free flow condition over a considerable extent even in the plain irrigation districts. Compared with the others, the upstream water of Airfoil-shaped measuring flume built in the rectangular canal is fluctuant more smoothly Meanwhile velocity distribution is more evenly and upstream Froude number is small relatively. The backwater heights obtained among the laboratory tests can fulfill the demand of the irrigation districts.4. This article analyzes the mechanism and effect factors of the head loss of the flow passing through the Airfoil-shaped measuring flume. The head loss includes losses of the contraction section in the upstream,expansion section in the downstream and the hydraulic jump section. The total head loss can be taken as local head loss. The relation between the local head loss coefficient and the discharge, cross section contraction ratio and submergence is obtained by the data analysis. The change laws of head loss coefficient of Airfoil-shaped measuring flume in the U-shaped and trapezoidal canals are similar, but significant different from that of flume in rectangular canal.5. A serial of calculations also are performed on how the meshes, turbulence models influence the numerical simulation. It provides reasonable simulation parameters and mathematic models for the later numerical simulations. The 3D water flow numerical calculations are carried out on the Airfoil-shaped measuring flume in the U-shaped canal. It mainly focuses on the change of water surface curves, streamlines, velocity field and submergence which controlled by the downstream water level under the free flow. The unsteady numerical simulations also are run to simulate the flow in the measuring flume and the instant change of water level. Compared the numerical simulation results with the experiment data, it is concluded that the CFD simulation technique can simulate the flow in the measuring flume so that forecast simulation also can be run to simulate flow in other Airfoil-shaped measuring flumes which have different sizes. It breaks the restraint of physical model test and expands the study extent.6. Application research also are conducted in the irrigation districts that located at the south of Huang-river in the city of Erdos, Inner Mongolia and main channel of diversion project from Daqing river to Qinwangchuan. The result verifies that the Airfoil-shaped measuring flume has a small water head loss, high critical submergence and it is easy to be constructed. The article summarizes the practical experience on the design, construction and calibration, and then gives reasonable advice to the application of Airfoil-shaped measuring flume.The indoor experiments of Airfoil-shaped measuring flume that constructed in three different canals and the field test indicate that the Airfoil-shaped measuring flume can guide the new developmental direction of the measuring flume in the irrigation districts. |
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