Study On Hydraulic Performance Of Trapezoidal Cuttroat Flume In The Trapezoidal Channel

Accurate flow measurement is a fundamental component of water management,especially for the last channels which are closely related to farmers.Owing to their good hydraulic characteristics and easy maintenance,trapezoidal channels have been widely used in terminal water convey systems of China.Previous research indicated that the measuring flume is one of the most accepted and used structure for water discharge measurement.Increasing water head lose to improve the accuracy of flow measurement is a ubiquitous problem in irrigation districts.This research aims to solve the problem.We designed trapezoidal cutthroat flumes to measure the discharge in last trapezoidal channels.These flumes can improve flow measuring accuracy without sacrificing water head.In this context,this research probed into the applicability of the new-type trapezoidal cutthroat flumes in measuring discharge of last trapezoidal channels.Compared with the rectangular cutthroat flume,both the head loss of trapezoidal cutthroat flume when the flow was large and the measurement accuracy when the flow was small was significantly improved.Experiments on trapezoidal cutthroat flumes of different contraction angle(0°~30°)with different discharges(5~75L/s)were conducted in the trapezoidal channel under 180 flow working conditions to determine their hydraulic performances.We took water level measurements at 16 cross sections and the variation rule of water surface profiles was obtained.Based on the principle of critical flow,the relationship between discharge and its impact factors were studied,and the discharge formula was established.Moreover,we analyzed the variation rule of the Froude number,the backwater depth,the measuring precision and the head loss along the flume.The results show that under the free flow conditions,no critical flow occurs when the throat contraction angle is more than 20° and the flow rate is less than 16 L/s.When the contraction ratio range is 12°~20°,the fitted formula has a satisfactory accuracy of relative error less than 4.79%.When the contraction ratio range is 20°~30°,the backwater height is less than 3.15 cm and the average head loss is about 4% of the upstream head.Comprehensive analysis showed that the suitable throat contraction ratio range is 12°~20,and the trapezoidal cutthroat flumes is suitable for the terminal trapezoidal channel.The study provides a reference for the further application of the trapezoidal cutthroat flumes in the northern irrigation systems of China.We also researched the relationship between channel specification and the flume's parameters for its application.Based on the RNG k-? three-dimensional turbulence model along with the TruVOF technique,experiments and corresponding simulations were performed for 14 working conditions on the trapezoidal cutthroat flumes with discharges up to 75 L/s to determine its hydraulic performance.Hydraulic performance of the flume obtained from simulation analyses were later compared with observed results based on time-averaged flow field,flow pattern,and velocity distribution.The comparison yielded a solid agreement between results from two methods with the relative error below 10%.The flow in the upstream of the flume was slow flow with almost parallel flow direction,then the water surface was gradually declined along the contraction segment and the water on both sides tended to converge at the center line.Owing to the severe contraction of the throat section,the water surface near the downstream of the throat section was dramatically declined and the lowest point appeared.From the posterior part of the diffusion section,the water depth gradually increased to the downstream depth with the water depth distributing well in the horizontal section.By analyzing the variation of velocity and total head along the flume under different discharges,it was concluded that both velocity and total head loss accelerated dramatically near the throat.The turbulent dissipation was concentrated in the area near the wall and the bottom of the flume.Regression models developed for upstream depth versus discharge under different working conditions were satisfying with the relative error of 9.21%,which meets the common requirements of flow measurement in irrigation areas.Furthermore,the maximum water head loss of the trapezoidal-throated flume was less than 10% of the total head.Compared with long-throat flumes,Parshall flumes and Parabolic flumes,the head loss of trapezoidal-throated flume in trapezoidal channels was less.The three-dimensional turbulence model along with the TruVOF technique allowed one to reproduce the hydraulic characteristics of flow through trapezoidal-throated flume in trapezoidal channels.Due to the shorter time demand and lower cost of numerical simulations,compared to experimental studies in predicting the hydraulic characteristics,simulation of the trapezoidal-throated flume in trapezoidal channels flows,based on a properly validated model,provided the flow characteristics of these flumes for various flow configurations encountered in the terminal channel.Results indicated that the distribution of Froude number of trapezoidal cutthroat flume was unimodal;the critical flow appeared near the throat section and the Froude number was less than 0.5 around the upper throat,which could meet the water measurement requirement in irrigation districts;the position of maximum flow rate varied with the change of the flume's boundary.All in all,it is concluded that the trapezoidal cutthroat flume has the advantages of simple structure,low price and high accuracy,plus low head loss.This study provides a reference for the flow measurement of terminal channels in irrigation areas.