Hydraulic Characteristics And Flow Measurement Performance Analysis Of Portable Fish-shaped Flumes In U-shaped Channels

Strengthening the accurate measurement of irrigation water in the final channel is an important way to achieve agricultural precision irrigation,improve the utilization efficiency of irrigation water and reduce the waste of agricultural water resources.The common fixed measuring flume has the disadvantages of high construction cost,easy to cause local deposition of the channel and cause a large number of measurement errors.The mobile measuring flume does not have such shortcomings as fixed type,but the developed mobile measuring flume type is less,which cannot be popularized and applied in large areas.The structure type of the measuring flume needs further optimization.In this study,bionics and critical flow principle were applied to design a streamlined movable fish-shaped cylinder(the plane figure is fish-shaped)for water measurement.In order to study the hydraulic characteristics and flow measurement performance of portable fish-shaped flume in U-shaped channel,based on dimensional analysis principle and RNG k-ε three-dimensional turbulence model,the prototype test and numerical simulation of fish-shaped flume were carried out under five different channel bottom slopes,five different contraction ratios and flow range of20-45 L / s.FLOW-3D software was used to numerically simulate and analyze the flow fields of five kinds of flumes,and the suitable range of aspect ratio of fish flume was studied.The main conclusions are as follows:(1)Portable fish-shaped flume can be used for flow measurement,which has the advantages of high measurement accuracy,low backwater height,small head loss,small upstream turbulence and high inundation.Based on dimensional analysis,the flow measurement formula of fish-shaped flume suitable for different slopes and contraction ratios was obtained.The average relative error of flow measurement in the portable fish-shaped flume with U-shaped channel is 1.74 %,and the Froude number in the upstream is less than0.5.The backwater height in the upstream of the flume is 1.91 – 7.16 cm,and the critical submergence is 0.75 – 0.94.(2)Using FLOW-3D software,the numerical simulation results based on RNG k-ε threedimensional turbulence model are in good agreement with the prototype experiment.The internal flow field motion law of the fish-shaped measuring flume is: the flow of the front part of the portable fish-shaped measuring flume is very slow,the upstream flow velocity and Froude number of the measuring flume are low,and the head loss is small.The flow in the diffusion section of the measuring flume collides and forms a hydraulic jump.The large flow velocity causes large local head loss.The slow flow pattern was restored downstream of the flume,and the flow velocity,Froude number and head loss were small.The fish-shaped measuring water column with suitable length-width ratio can improve the flow behind the fishshaped measuring water tank,reduce the head loss,and the streamline distribution is homogenous.(3)Compared with other portable U-shaped channels,fish-shaped flume has the advantages of good structural flow linearity and smooth flow.The hydraulic performance characteristics of the five flumes are as follows: upstream backwater height: cylinder < fishshaped cylinder < tailed cylinder < ellipse cylinder < plate-column cylinder;head loss: fish shape cylinder < tailed cylinder < cylinder < ellipse < plate-column cylinder;Froude number before the column: elliptical cylinder < fish-shaped cylinder < cylinder < tailed cylinder <plate-column cylinder.Under the same conditions,the Froude number in front of the fishshaped measuring flume column is small,and the fluctuation of the stagnation point depth is small,which is convenient for the measurement of the stagnation point depth.Fish measuring flume upstream backwater is low,resulting in minimum head loss.Therefore,the contraction section of the measuring water column adopts a curve with greater curvature than the circular to make the upstream flow more stable.The tail wing should be set in the diffusion section to reduce the head loss,and the length of the throat can be appropriately increased to improve the critical flooding degree.