Influence of the structure of cylindrical mobile flumes on hydraulic performance characteristics in U-shaped channels

Small U-shaped channels are widely used in irrigation districts. Accurate flow measurement in U-shaped channels is an important aspect to field water management. In open channels, mobile flumes are widely used for flow measurement because of many advantages such as low cost, no sedimentation and moveable. However, how the structure of the flume influences hydraulic performance is still unclear. The principles for designing or optimizing mobile flumes are also lacking. In this study, FLOW-3D software was used to numerically simulate the flow pattern of three cylindrical mobile flumes that are most often used in U-shaped channels, including circular cylinder, elliptical cylinder and V-tailed cylinder mobile flumes. The hydraulic characteristics including backwater height, energy loss and flow measurement accuracy were analyzed. The results showed that under the same working conditions, the fluctuations of the stagnation point depth in the elliptical flume were the smallest. The backwater height and the energy loss in the V-tailed flume were the lowest. The contraction section of the cylinder could be designed as a curve with large curvature to make the upstream water flow more stable. The diffusion section could be designed as V-tail to improve flow pattern and reduce energy loss. Under a safe backwater height, the throat length could be increased to improve the submergence degree.     

Study on the applicability of four flumes in small rectangular channels

Different experiments and numerical simulation research were carried out in order to explore the hydraulic characteristics and applicability of the cutthroat flume, NACA airfoil-shaped flume, airfoil pillar-shaped flume, and optimized airfoil-shaped flume in rectangular channels. The four flumes with contraction ratios of 0.4, 0.5, and 0.6 in rectangular channels were tested under 144 free-flow conditions. Based on the Fluent software, standard k-ε three-dimensional turbulence model and volume of fluid (VOF) method were utilized for numerical simulation in each working condition. The principle of critical flow was used to analyze the correlation between the discharge, the contraction ratio, and the upstream water depth. This correlation was used to obtain the discharge calculation formulas of the cutthroat flume, NACA airfoil-shaped flume, airfoil pillar-shaped flume, and optimized airfoil-shaped flume in rectangular channels. The Froude number in front of the flume, the backwater height, the critical submergence degree, the velocity distribution, the head loss, the discharge measurement formula, and the accuracy were studied and compared. It can be concluded from the results that the cutthroat flume, NACA airfoil-shaped flume, airfoil pillar-shaped flume, and optimized airfoil-shaped flume all have good applicability in rectangular channels. Moreover, the hydraulic characteristics of all the four flumes meet the water measurement specifications in the irrigation area. The upstream Froude number of the NACA airfoil-shaped flume is the smallest, whereas the backwater height and head loss of the cutthroat flume and optimized airfoil-shaped flume are the smallest. It is also worth noting that the critical submergence degree of the optimized airfoil-shaped flume is the highest, with an average value of 0.85. The discharge measurement formulas of the four flumes have been deduced using the correlation between the discharge, the contraction ratio, and the upstream water depth. It was found that the average errors between the calculated discharge and the measured discharge of the four flumes are 3.17%, 2.17%, 1.53%, and 2.29%, respectively, which are less than 5%, and therefore meet the discharge measurement requirements. In the agricultural irrigation system and flow monitoring system, the hydraulic characteristics of the four flumes can provide a reference and basis for the selection of flumes in the rectangular channel.     

Experimental and numerical research on portable short-throat flume in the field

The use of portable short-throat flume in the field is an emerging technique developed for water discharges measurement of inlet in the field. Based on the principle of critical flow and RNG k–ε three-dimensional turbulence model along with the TruVOF technique, experiments and corresponding simulations were performed for 16 working conditions on the 76 mm width flume with discharges up to 40.01 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, Froude number and velocity distribution. Comparison yielded a solid agreement between results from two methods with relative error below ±10%. Regression models developed for upstream depth versus discharge under different working conditions were satisfying with the relative error of 9.16%, which met the common requirements of flow measurement in irrigation areas. Compared to the long-throat flume, head loss of portable short-throat flume in the field was significantly less. Further, head loss under the free flow condition was less than that under the submerged flow condition of portable short-throat flume with a flat base in the field.     

Experimental study of simple flumes with trapezoidal contraction

There are different devices available for measuring the flow discharge. Static measuring devices such as weirs and flumes (without any moving parts) play a significant role in discharge measurement in open channels. Many researchers have focused on application of flumes in irrigation networks. This investigation set out to study the flow discharge through a trapezoidal cut-throated flume (TCTF). The flume is simply constructed by placing two triangular plates on either side of a rectangular open channel to form a trapezoidal throat. The channel cross section is rectangular while throat cross section of the flume is trapezoidal. The proposed flume is simple, low-cost, easy to install and does not require high maintenance. The present study was designed to determine the effect of different variables on the flume discharge. For this, an experimental study was carried out under free outflow conditions and under upstream subcritical flow regime to formulate the flume discharge. Three models are defined for flume discharge relation based on the Buckingham's theorem of dimensional analysis, and then calibrated using the experimental data collected during this study. The first discharge model has an average error of 1.81%, while the second and third improved models have average errors of 0.96% and 1.44% respectively. To reliably estimate the flume discharge, free-flow and submerged-flow conditions should be distinguished. For this, suitable equations with an average error less than 2.23% were presented to estimate the submergence threshold. The results of this study indicate that while the downstream wall slope influences the submergence threshold, the flume discharge is not influenced by this variable. The proposed models are simple and accurate and present appropriate estimation of discharge for flows through the TCTFs. The findings of this study will be of interest for practical hydraulic engineers.     

Laboratory measurement and numerical simulation of flow and turbulence in a meandering-like flume with spurs

The paper reported herein presents the laboratory measurement of near-bed flow and turbulence induced by non-submerged spurs protruding from the bank of a meandering-like laboratory flume with smooth rigid bed. The flow property was measured using a 3D acoustic doppler velocimeter for various combinations and locations of spurs in order to assess their effect on mean flow field. Likewise, turbulent characteristics were computed from the measured data for one of the experimental cases. Furthermore, a 2D numerical model was developed for the simulation of mean flow property, turbulent intensities as well as vorticity field using cubic-interpolated pseudoparticle (CIP) numerical technique. The simulated down-stream and cross-stream mean flow property as well as turbulent intensities in shear layer was found to be in good agreement with the experimental results. The numerical simulation of vortices, generated from the tip of the spur, was seen to be reliable. In addition, the migration of small vortices was visualized in the experimental flume using a simple technique.     

Three dimensional flow simulation over a sharp-crested V-Notch weir

Thin-plate weirs are widely used to monitor the flow rate in open channels. Thereby, three dimensional (3D) modeling of the flow over a weir in an open channel can be considered as one of the main topics in hydraulic science. In this study, the flow over a sharp-crested v-notch weir (SCVW) is simulated by a 3D numerical model. Laboratory experiments were conducted to monitor and measure the behavior of the SCVW in practice. Finally, the simulated velocity distributions, water surface profiles, and hydraulic jump were compared with those of the experimental data. Due to the turbulent nature of the flow over the SCVW, a Reynolds stress model (RSM) and three types of the k–ϵ turbulence models with the fractional volume of fluid technique (VOF) were used in the analysis. In this respect, the two-phase solution method and dense mesh were used in generating the simulation domain. Results indicated that the RSM exhibited higher accuracy in defining the velocity distribution, complex flow pattern, and predicting the hydraulic jump formation downstream of the SCVW.     

Discharge prediction in flow measurement flumes with different downstream transition slopes

Experimental testing of 9 different rectangular compound cross-section flow measurement flumes with different downstream slopes was conducted to yield the coefficient of discharge and the approach velocity coefficient. The aim of the experimental research was the determination of stage–discharge relationship in compound cross-section flow measurement flumes with different downstream slopes. One empirical predictive model for each of the coefficient of discharge and the approach velocity coefficient for the 9 cross-sections have been derived using one dimensionless parameter for the coefficient of discharge and another one dimensionless parameter for the approach velocity coefficient as the single independent variable. This approach is preferred as it allows the estimation of discharge by only measuring the water depth at the head measurement section. All obtained predictive models statistics have indicated the high reliability of the derived models in estimating discharge in an open channel flume of a rectangular compound cross-section using the predicted coefficients.     

Application of ISO4359 for discharge calculation in a narrow flume

Flumes with either width contractions or raised beds to force modular flow conditions (i.e. a transition from sub- to super-critical flow) are a widely used instrument for the measurement of flow rate. The conservation of mass, written as the continuity equation, and the conservation of energy are combined with the existence of a critical depth point to derive a theoretical equation for the discharge as a function of the upstream water depth. This derivation requires a number of assumptions regarding the flow both upstream and in the throat of the contraction. The international standard covering the use of such flumes, ISO4359, places restrictions on the upstream position at which the water depth should be measured, which this work shows to be unnecessary, at least for the small (100 mm throat width) flumes examined. The assumption that critical depth occurs at the end of the flume throat is also shown to be incorrect, but has negligible effect on discharge calculation using the ISO4359 method.     

Algorithm with improved accuracy for real-time measurement of flow rate in open channel systems

Most methods for flow rate measurement in open channels usually have low accuracy over a range of flow rates due to varying fluid properties, flow conditions and channel length. This paper suggests an algorithm to improve on the accuracy of flow rates computed based on hydraulic structure and slope-hydraulic radius methods. A model for determining flow rates in accelerating flows is also developed. In the proposed algorithm, the parameter used for adapting the flow rate models is obtained by comparing the measured fluid depth with the depth simulated based on the one-dimensional Saint Venant equations. The results show that an improvement from ± 2.3% to ± 0.8% accuracy in the flow rate measurement using the Venturi flume method could be achieved. In unsteady state flow in a straight-run channel, the results based on flow simulation also show possibility of achieving accurate computation over a wide range of flow rates.     

液力缓速器三维瞬态流场大涡模拟及特性计算

为深入了解液力缓速器内部复杂的三维流动,采用大涡模拟和多可动区域计算的滑动网格法,利用FLUENT软件对液力缓速器全充液工况内部三维瞬态流场进行数值模拟。对计算得到的三维流场分布特性进行深入研究,分析流动现象成因,为提高液力缓速器性能奠定理论基础。基于流场数值解对制动扭矩进行了计算,将计算结果与实验结果进行对比分析,二者误差在5%以内,说明采用的数值模拟方法是准确有效的。     

Dissipative scaling of step-pool features

This paper focuses on the dissipative similarity of step-pool units at rill, flume and stream scale. This investigation is carried out using recent advances in open channel flow resistance, applications of close-range photogrammetry to rill erosion, available published data on step-pool features in flumes and streams and a new dataset of measurements in fixed bed step-pool rills. A theoretically-based equation for calculating the Darcy-Weisbach friction factor obtained by integration of a power velocity profile is presented. The scale factor Γ of this power velocity profile, which is included in the flow resistance equation, was previously calibrated (Eq. 10) for mobile bed rills with step-pool units. At first, in this investigation an additional test of this flow resistance equation is developed using measurements carried out in fixed bed rills with step-pool features. In particular, the proposed Γ function is tested using the measurements carried out in 63 fixed bed rill reaches incised on a 24 and 26% sloped plot. This test shows that the effect of sediment transport on the flow resistance law can be considered negligible as compared to the effect of the form-induced dissipative mechanisms due to the presence of step-pools structures. Then, using measurements carried out in 61 flume experimental runs and 109 reaches of step-pool streams, this investigation demonstrates that the Γ function for flume and stream conditions can be obtained scaling that (Eq. 10) determined for step-pool rills. The Darcy-Weisbach friction factor values measured in flumes and streams are, on average, higher than those related to rills. Finally, the application of the theoretical equation for calculating the Darcy-Weisbach friction factor for flume and stream conditions requires Γ, estimated by Eq. (10), to be multiplied by a specific scale factor.     

液压集成块湍流模型修正及内流特性分析

基于粒子图像测速技术（PIV）建立了带有刀尖角容腔的直角转弯流道流场的数值计算模型,并进行三维流场仿真。通过将数值计算得到的典型涡系结构与实验结果进行对比,考察了工程上常用的7种湍流模型对带有刀尖角容腔直角转弯流场的预测性能。通过定义权重误差K,筛选出S-A模型作为基础湍流模型并对其进行了参数修正。结果表明,当S-A模型Cb1取值从默认值0.1355修正为0.17时,出流方向正对刀尖角容腔模型权重误差值上升25.0％,入流方向正对刀尖角容腔模型权重误差值下降34.7％,修正后的S-A湍流模型对两种直角转弯流场的综合预测精度有所提高。运用筛选修正后的S-A湍流模型分析了4种典型直角转弯流道的内流特性,结果表明圆弧过渡直角转弯流道相比于带有刀尖角容腔的转弯流道具有更小的压力损失。     

Numerical simulation of flow structures over dunes to develop an empirical relationship for separation zone dimensions

Rivers have non-uniform beds, which are called bedforms. Depending on the hydraulic conditions, these shapes have different types that cause resistance to the flow. Despite a great number of research works and experiments on bedforms, no accurate and comprehensive equation has been proposed to estimate separation zone dimensions formed on the lee side of the dunes. The present study is mainly focused on simulating the flow motion numerically on dunes in open channels to evaluate the effect of dune geometry on the flow separation zone. Twenty-nine simulations were conducted to study the effect of the geometry of five types of dunes with different angles and heights under different hydraulic conditions and bed roughnesses. RANS and DES turbulence models were used to simulate small and large-scale dunes, respectively. The results of the numerical model were compared with the experimental results of previous researchers to validate the work, indicating the appropriate accuracy of the numerical model. Bursting events analysis were carried out to understand dominant events phenomena, it was observed that all the events have high fluctuations near the bed and none of them is dominant. However, this could decrease and diverge by getting closer to the water surface, so that all the events would tend to converge in the vicinity of the water surface. Empirical equations were derived to estimate flow separation dimensions in terms of bed characteristics and flow conditions. The equations showed that flow separations zone dimensions are highly related to lee side angle of the dunes.     

Experimental modeling of flumes with two semi-cylinder contractions (free and submerged flows)

In this study, the flow through a simple low-cost flume constructed using two semi-cylinder contractions on either side of a rectangular channel was experimentally investigated. In practice, these flumes may operate under free- or submerged-flow conditions depending on the downstream flow depth. To reliably estimate the flume discharge, free- and submerged-flow conditions should be accurately distinguished. At present, there is no equation to estimate the submergence threshold. This study investigates the possibilities of using these flumes under submerged-flow conditions. The discharge of the submerged flume may be calculated on the basis of the free-flow discharge equation. Thus, two new free-flow discharge equations were deduced: a general equation by considering the contraction ratio with an average error of 2.25%, and another one by neglecting it with an average error of 4.88%. Compared with the existing discharge equations, the proposed free-flow equations are the best ones. Under submerged-flow conditions, the submergence ratio affects the flow discharge. Using the measurements carried out in this study, the submergence threshold ratio and submerged-flow discharge were formulated and calibrated with the average errors of 2.96% and 3.80%, respectively. The equation to distinguish the free- and submerged-flow conditions was also deduced by equating the submerged- and free-flow discharge equations yielding a novel real solution.     

基于改进的低雷诺数湍流模型的软性磨粒流加工仿真与实验

针对模具结构化表面软性磨粒流精密加工中壁面特性求解困难的问题,基于改进的低雷诺数湍流模型提出了一种对软性磨粒流壁面特性进行分析的方法。该模型仿真的湍流可以从充分发展的高雷诺数湍流到低雷诺数湍流,所以,对于壁面区域的求解精度更高。以机械加工中常见的U形沟槽为仿真对象,数值模拟了其内部流场,尤其是壁面区的流场特征参数。分析结果表明:当初始压力p0在2.5～4.5MPa之间时,流道的内部流动是不完全发展的湍流,详细分析了p0为4.5MPa时渐变截面流道的内部流动,压力与速度分布均是中心高、两侧低,而且越靠近U形底部,压力值和速度值越高;磨粒流速度一直增大,磨粒受到的驱动力增大,使得单位时间内颗粒与壁面的碰撞增加。通过碰撞观测实验发现磨粒对壁面的碰撞速度与仿真结果相符,加工实验进一步验证了仿真结果。     

基于神经网络的液压缸内泄漏在线测量研究

内泄漏是液压缸最常见故障，严重影响液压缸的正常工作，因此对其在线测量显得尤为重要。提出内泄漏在线测量的工作原理，包括在线测量系统、应变片的固定方式和流量-应变信号转换的数学模型，并搭建实验系统采集内泄漏和应变数据并进行数据处理。分别采用BP神经网络和卷积神经网络对液压缸内泄漏进行预测，结果表明，卷积神经网络准确度高、效率高，为其他液压元件微小流量的在线测量提供一种新的思路。     

弯管流动的非均匀性及其整流

利用RSM湍流模式和贴体无结构网格,对三维弯管的流动进行了数值模拟,并利用三维激光测速仪(LDV)对流动进行了测量,数值模拟的结果同试验吻合较好。针对流动的非均匀性提出了双环结构的整流装置,并对该整流装置进行了数值模拟和试验,表明该装置能对气流进行很好的整流,使流动在弯管下游很快变得均匀和稳定。     

Application of video imagery techniques for low cost measurement of water surface velocity in open channels

Developments in digital video recording technology make the video imagery tools more popular for velocity measurement in water flows. This has especially been of large interest due to its inherent advantage of non-contact nature which is quite handy in extreme flow conditions. Particle Image Velocimetry (PIV), Particle Tracking Velocimetry (PTV) and Large Scale Particle Tracking Velocimetry (LSPTV) are applied to free surface channel flow for water surface velocity measurement. Experiments are conducted to measure either a single point velocity applying PTV or velocity profiles across the channel width applying PIV on the water surface in a rectang typical velocities of nearly 1 andular tilting flume for various flow conditions. Technical issues regarding tracer particle size and type, travel distance, lighting, recording speed, camera position, image distortion and state of flow are discussed. Measured data is compared to computational results obtained from a numerical model involving a non-linear turbulence model capable of predicting turbulence driven secondary flows. Confirmation of reasonable match between computational and experimental results whereby applying mutual collaboration of them for discharge measurement has been attested. In addition to discharge, boundary roughness has also been predicted as an outcome of the numerical solution.     

双流道转轮水力透平机的全流道数值模拟

依据给定的参数,对水力透平机进行模型设计。采用FLUENT软件对所设计的水力透平机进行了全流道三维定常湍流数值模拟,计算以雷诺时均N-S方程为基础,基于贴体坐标和交错网格划分,对控制方程进行变换和离散。采用SIMPLEC算法实现速度、压力变量的分离求解。定常湍流计算采用了标准k-ε湍流模型。模拟计算获得了水力透平各过流部件内部的流动机理,该数值计算方法和结果对水力透平机的水力优化设计具有重要意义。     

Flow measurement structures

Weirs, flumes and gates have been used for centuries to determine discharges in open channels. In this paper the distinct flow types, modular flow and non-modular flow, are described and applied to derive the head-discharge relations of weirs with a horizontal crest. The flow measurement structures are classified according to international rules. For a number of decades, the ability of flow measurement structures to allow fish migration has also been judged. On the other hand, some well-known fishways have been considered to act as an alternative flow measurement structure. The presentation of the Venturi gate, which combines the functions of flow measurement structure and fishway, concludes the paper.