| Improving the water measurement facilities in irrigation districts and realizing the accurate measurement of agricultural water use are of great significance for promoting the agricultural water price reform and the agricultural water conservation in China.U-shaped channels are widely used in irrigation districts,which means that there are many cross-sections to be measured.Therefore,mobile flow measurement facilities are recommended to measure the discharge.However,the existing mobile flumes used in U-shaped channels are still not suitable enough.Combining the advantages of mobile thin plate flumes and cylindrical flumes,a platecolumn mobile flume is developed.The prototype experiments and numerical simulation experiments were carried out based on the analysis of the flow mechanism of the plate-column flume.The experiments include five different slopes of the U-shaped channel,five different plate-column mobile flumes with different contraction ratios and seven different flow rates ranging from 15 L/s to 45 L/s.According to the results,the flow measurement formula was concluded with the dimension analysis method.In addition,based on the numerical simulation experiments,the flow characteristics of the plate-column flume were compared with the existing mobile flumes used in U-shaped channels,including the cylindrical flume,the watermeasuring column with round head,the elliptical flume and the pier-shaped flume.The main conclusions are as follows:(1)The plate-column portable flume has high precision and can meet the flow measurement requirements of the irrigation area.During the prototype tests,the water depth in the cylinder of the flume was used to replace the water depth of the stagnation point,which improved the flow measurement accuracy.The results show that the plate-column flume has a good relationship between water depth and flow rates.Based on the principle of dimensional analysis,the regression analysis is applied to the flow measurement formula for different slopes and different contraction ratios.The main physical parameters affecting the flow through the plate-column portable flume are: the slope of the channel bottom,the contraction ratio of the plate-column portable flume,the water depth at the stagnation point,the water surface width at the throat section of the plate-column portable flume,and the acceleration of gravity.The upstream water depth of the plate-column portable flume is between 1.85 and 13.69 cm,the critical submergence is between 0.70 and 0.91,and the Froude number(Fr)is less than 0.5;the average relative error is 2.07%.(2)The numerical simulation results are in good agreement with the prototype tests.The numerical simulation can accurately reflect the condition of the flow in the plate-column portable flume.The upstream flow velocity and the Froude number(Fr)of the plate-column portable flume are lower,the streamline distribution is more uniform,and the head loss is smaller;when the water flows through the plate-column portable flume,the flow velocity and the Froude number(Fr)is increased,the streamline is obviously bent,the head loss is increased,and the Froude number(Fr)increases to 1 in the maximum contraction section of the platecolumn portable flume,forming a critical flow;the water flow through both sides of the platecolumn portable flume meets and collides downstream of the plate-column portable flume,and forms a recirculation zone and vortex zones,causing a large head loss;The downstream flow velocity,the Froude number(Fr)and head loss of the plate-column portable flume are small,and the streamline distribution is relatively uniform.(3)Compared with the conventional mobile flumes,the plate-column portable flume has the advantages of small size and high portability.The hydraulic jump in water-measuring column with round head is significantly longer than other flumes.The upstream drowning height and the upstream and downstream head loss are both pier-shaped measuring flumes > plate-column portable flumes > elliptical measuring flumes > water-measuring columns with round head > cylindrical flumes.The Froude number(Fr)of the stagnation points of the stagnation point is pier-shaped measuring flumes > plate-column portable flumes > cylindrical flumes > elliptical measuring flumes > water-measuring columns with round head.The mobile flume only needs to be temporarily installed in the channel during the flow measurement,and the flow measurement time is generally less than 5 minutes.Although the upstream water depth and the head loss are large,the plate-column portable flume will not cause problems such as sedimentation and reduced water capacity through the channel.When the flow rate is large,plate-column portable flumes with larger contraction ratios can be used to prevent the upstream water depth from exceeding the safety of the channel. |
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