植被作用下的复式河槽漫滩水流紊动特性

分别选取塑料吸管、鸭毛和塑料大草模拟乔木、灌木和野草，通过水槽试验，探讨了不同滩地植物作用下的浸滩水流紊动特性。试验结果表明，纵向、横向和垂向三个方向的脉动流速基本满足正态分布；时均流速与采样时间的长短有关；横向动量交换比垂向动量交换大，横向动量基本上是主槽向滩地传递；不同的滩地植物对水流的紊动强度的影响不同。滩地种植植物后，水流的紊动强度增强。纵向和垂向的紊动强度相当，都服从S型分布。     

Application of analytical solutions in trapezoidal compound channel flow

The accuracy of predictions using an analytical model for a lateral depth varying open channel flow can be affected by the friction factor and the dimensionless eddy viscosity. In this paper, different approaches to determine these two parameters are discussed by comparing analytical results with experimental data obtained from a symmetrical trapezoidal compound open channel. To reflect different hydraulic characteristics along the wetted perimeter, the trapezoidal cross‐section was divided into several sub‐sections, and the friction factor in each sub‐section was calculated with Manning's formula by adopting local hydraulic parameters. The wetted perimeters of main channel and side slope included the interface between panels to reflect the effects of the lateral momentum exchange. This approach is shown to be effective once the roughness coefficients and hydraulic radius in different sub‐regions have been determined. The results show that the friction factor is the major parameter affecting the accuracy of the analytical solution. The effect of dimensionless eddy viscosity on the precision of predictions is much smaller, which means that for practical applications, the dimensionless eddy viscosity can be determined empirically. Inclusion of a term for secondary flows in the model can improve the prediction, especially in the transition from main channel to the floodplain. A reasonable prediction of the lateral distribution of depth mean Reynolds' stress in this region is obtained. Copyright © 2010 John Wiley & Sons, Ltd.     

冲积河流滩槽定量划分方法及应用

通过理论分析与数值计算,研究了冲积河流漫滩水流水力因子的变化特性。结果表明,断面平均流速在漫滩瞬间迅速减小,然后逐渐增大;断面综合动能修正系数、动量修正系数均随水深而缓慢增加,在漫滩瞬间迅速增大,随即快速减小。根据漫滩水流水力因子沿水深的变化特性,提出采用断面横向平均流速为判别因子以定量划分复式断面的滩与槽。此外,文中将其应用于黄河下游漫滩水流的水沙数值模拟,其结果与实际情况符合较好。     

Experimental study of flow characteristics around floodplain single groyne

This study investigated the flow around river’s floodplain single groynes. Two different compound channels with one and two symmetrical floodplains having widths of 1- and 2-times of the main channel width, respectively, were used. Both impermeable and permeable groynes with three different relative lengths (relative to the floodplain width) and having three different permeability values of 40, 60, and 80% were investigated. The 3D flow velocities were measured in the horizontal plane at 0.25 and 0.5 of floodplain water depth (h_f), and in the vertical plane at the main channel’s centerline. Therefore, the flow velocities in the longitudinal, lateral, and vertical directions, and the flow water surfaces were measured and analyzed. The results showed that, as the groyne permeability increased up to 60%, a reduction of up to 30% to the maximum velocity and 22% to the tip velocity were observed. The permeable groyne length had limited influence on the flow structure. Both the groyne permeability and the length ratio had significant effects on the floodplain water depth. The scouring and the deposition activities resulting from impermeable groynes can be avoided, should the groyne length be kept below half of the floodplain width.     

复式断面明渠流能量及动量校正系数研究

在复式断面明渠流中,能量及动量校正系数对水力学计算结果的精确性有直接影响。为研究水深比（滩地水深与主槽水深之比）及滩地植被密度对复式断面明渠流能量及动量校正系数的影响,采用基于壁面建模的大涡模拟（WMLES）方法的三维数值模型,对5种水深比和3种植被密度工况进行数值模拟研究。结果表明：由于流速分布不均匀,主槽和滩地的能量及动量校正系数有明显差异,其值均大于1。随着水深比的增大,复式断面的流速分布趋向均匀,能量和动量校正系数随之减小,能量校正系数的变化范围为1.07～1.19,动量校正系数的变化范围为1.03～1.07。此外,随着滩地植被密度的增大,滩地与主槽的流速差值变大,能量和动量校正系数随之增大,能量校正系数的变化范围为1.09～1.59,动量校正系数的变化范围为1.04～1.21。最后,通过回归分析发现能量校正系数与动量校正系数存在线性关系,所得计算式可用于相同工况条件下能量及动量校正系数的预测。     

明渠岸边横向取水口水力特性的试验研究

对明渠岸边横向直角取水口的水力特性进行了试验研究。在试验室的水槽中，通过三维超声波流速仪(ADV)测得了取水口的流速数据，试验中针对不同的分流流速比，测取了主水槽的表底层分流宽度、进水口门流速场以及紊动能k和雷诺应力的分布。试验结果显示在取水口附近的水流结构具有明显的三维特性。取水口断面的流速分布与正常的明渠流速分布不同，其断面的最大流速常常在底层。     

VELOCITY DISTRIBUTION IN TRAPEZOID-SECTION OPEN CHANNEL FLOW WITH A NEW REYNOLDS-STRESS EXPRESSION

By considering that the coherent structure is the main cause of the Reynolds stress, a new Reynolds stress expression was given. On this basis the velocity distribution in the trapezoid-section open channel flow was worked out with the pseudo-spectral method. The results were compared with experimental data and the influence of the ratio of length to width of the cross-section and the lateral inclination on the velocity distribution was analyzed. This model can be used the large flux in rivers and open channes.     

明渠均匀流流速横向分布的涡黏性模型

为了研究明渠水流的运动特性,开展了明渠水流流速横向分布数学模型的研究。本文根据量纲分析方法和实验资料,得到水流横向剪切作用下横向涡黏性系数的关系式;基于均匀流的水流运动方程,按照有限差分方法,建立流速横向分布计算模型。该模型具有计算简单、快捷和精度较高的优点,且数值计算不受横向涡黏性系数的结构形式和断面形态的影响。采用明渠水流实验资料验证数学模型,数学模型计算的流速横向分布与实验资料较一致,具有较高的理论意义和应用价值。     

FLOW STRUCTURE OF PARTLY VEGETATED OPEN-CHANNEL FLOWS WITH EELGRASS

Aquatic vegetation can influence the transport of sediment and contaminants by changing the mean velocity and turbulent flow structure in channels. It is important to understand the hydraulics of the flows over vegetation in order to manage fluvial processes. Experiments in an open-channel flume with natural vegetation were carried out to study the influence of vegetation on the flows. In a half channel with two different densities of vegetation, the flow velocity, Reynolds stresses, and turbulence intensities were measured using an Acoustic Doppler Velocimeter (ADV). We obtained velocity profiles in the lateral direction, Reynolds stresses in the vertical direction, and the flow transition between the vegetated and non-vegetated zones in different flow regimes. The results show that the streamwise velocity in the vegetated zone with higher density is almost entirely blocked. Reynolds stress distribution distinguishes with two different regions: inside and above the vegetation canopies. The turbulence intensities increase with increasing Reynolds number. The coherent vortices dominate the vertical transport of momentum and are advected clockwise between the vegetated zone and non-vegetated zone by secondary currents (a relatively minor flow superimposed on the primary flow, with significantly different speed and direction), generated by the anisotropy of the turbulence.     

明渠岸边横向取水口的三维数值计算

本文是在明渠岸边横向取水口的试验研究基础上的配套数值模拟。采用雷诺平均的N—S方程，以各向异性的雷诺应力模型(RSM)封闭方程组，用SIMPLEC算法对压力—速度场求解。计算结果与试验实测结果吻合较好，反映了此项模拟的可信性，可籍以扩大、分析一些运动规律。数值计算结果启示了横向取水口水流的复杂三维流动特性，据之对取水口附近水流各区的划分、水流的分离和反向流动、螺旋流以及床面剪应力的分布等复杂水力和泥沙输移物理现象有了进一步认识。     

复式断面河槽流速横向分布及其对滩唇形成的影响

通过对水流微小控制体进行受力分析,建立了复式断面流速横向分布模型,该模型考虑了侧向副流惯性力的影响。采用实验资料对该模型进行检验,模型计算值与实测值符合较好。运用该模型计算了不同条件下复式断面流速和挟沙力的横向分布,并定量分析了横向分布特性对滩唇形成的影响。分析表明,水流漫滩时,复式断面的横向挟沙力变幅较大,尤其是在主槽和河漫滩交界处,挟沙力减小迅速,而含沙量减小相对较小,泥沙发生淤积,容易形成滩唇。单从水流挟沙力角度考虑,水流漫滩后水深越小、滩槽的糙率差越大,越容易形成滩唇。     

矩形明渠三维紊流的数值模拟

在明渠三维流动中，紊流产生的二次流起关键作用，它对主流流场分布，壁面应力及污染物的扩散有很大影响，有时还影响床面的稳定，Ｋ－ε紊流模型及类似的双方程模型不能计算这种二次流动。本文将非线性Ｋ－ε紊流模型用于明渠三维紊流的计算，模型用张量形式推出，比代数应力模型更易推广到复杂边界流动，应用这一模型及另两种代数应力模型对矩形明渠流动进行计算，所得结果，尤其是表述紊流特征的量，与实测资料进行了详细比较，并     

明渠柱体上游马蹄涡的运动学特征研究

马蹄涡是水下柱体结构周围床面冲刷的主要动力。为掌握马蹄涡的运动学特征,引入了基于旋转强度的马蹄涡识别方法,提出了基于椭圆涡拟合的马蹄涡特征提取方法,其中,椭圆涡由Oseen涡和倾斜单向剪切流叠加而成。基于上述方法,通过开展明渠圆柱绕流PIV实验,对柱体上游对称面时均流场中马蹄涡的运动学特征进行了研究。结果表明,在柱体雷诺数大于104的紊流柱体绕流中,马蹄涡上游的流动分离点随柱体雷诺数的增大而缓慢向下游移动,但马蹄涡的位置保持不变,涡中心距柱体中轴约0.67D(D为柱体直径或宽度),距床面约0.06D;随着柱体雷诺数的增大,马蹄涡的半径不变而强度增加,使得马蹄涡作用于床面的剪切应力增大。研究结果建立了客观提取马蹄涡运动学特征的数学方法,得到了流动分离点、马蹄涡位置等运动学特征随水流条件的变化规律,揭示了明渠柱体绕流与其它流动中柱体绕流的差异,初步构建了马蹄涡的动力作用概念模型。     

Entropy generation in bypass transitional boundary layer flows

The primary objective of this study is to evaluate the accuracy of using computational fluid dynamics(CFD) turbulence models to predict entropy generation rates in bypass transitional boundary layers flows under zero and adverse pressure gradients. Entropy generation rates in such flows are evaluated employing the commercial CFD software, ANSYS FLUENT. Various turbulence and transitional models are assessed by comparing their results with the direct numerical simulation(DNS) data and two recent CFD studies. A solution verification study is conducted on three systematically refined meshes. The factor of safety method is used to estimate the numerical error and grid uncertainties. Monotonic convergence is achieved for all simulations. The Reynolds number based on momentum thickness, Re_θ, skin-friction coefficient, fC, approximate entropy generation rates, S'', dissipation coefficient, dC, and the intermittency, γ, are calculated for bypass transition simulations. All Reynolds averaged Navier-Stokes(RANS) turbulence and transitional models show improvement over previous CFD results in predicting onset of transition. The transition SST k-ω 4 equation model shows closest agreement with DNS data for all flow conditions in this study due to a much finer grid and more accurate inlet boundary conditions. The other RANS models predict an early onset of transition and higher boundary layer entropy generation rates than the DNS shows.     

EXPERIMENTAL INVESTIGATION OF HYDRODYNAMIC CHARACTERISTICS OF OVERLAND FLOW WITH GEOCELL

The hydrodynamic characteristics of the overland flow with a geocell slope are different from those of traditional flows because of its special structure. In this paper,a hydraulic flume with different slope gradients is used to study the hydrodynamic characteristics of the overland flow with geocell. The differences of flow characteristics between the overland flow with the geocell slope and the traditional flows are studied, and the hydrodynamic characteristics are obtained, including the flow pattern, the flow velocity and the hydraulic friction factor for the slope flow with geocell under different flow rates and slope gradients. The results show that there is a positive power function relationship between the rill depth of the slope surface (h) and the drag coefficient of the Darcy Weisbach (f).There is a positive logarithmic function relationship between the drag coefficient f and the Reynolds number Red,and there is a negative power function relationship between the drag coefficient f and the Froude number Fr.     

The characteristics of secondary flows in compound channels with vegetated floodplains

The experiments were conducted in compound channels with vegetated floodplains for investigating the influence of vegetation types on the characteristics of secondary flows. In terms of the streamwise and transverse velocities and the depth-averaged velocity, the secondary flow coefficient, M , is proposed, with good physical meanings, and it may characterize the rotational direction and the intensity of the secondary currents. The experimental results show that, for the cases without vegetation and with grass, the rotational directions of the secondary flows are all anticlockwise while for the cases with shrubs, they are all changed to the clockwise direction in the whole cross-section. However, when trees are planted, the secondary flows rotate in the anticlockwise direction in the main channel and in the clockwise direction on the floodplain. In addition, for all cases, the intensities of the secondary currents on the floodplain are stronger than those in the main channel.     

漫滩水流特征综述

水流漫滩后,主槽流速减小,滩地流速增大,但主槽流速大,滩地流速小,滩槽之间流速的差别使滩槽交界面上产生剪切力,两部分水流在交界面上发生动量交换,滩槽交界面产生大量次生流、旋涡,并伴随着大量的能量耗散。滩槽的边壁切应力分布不均,尤其是滩槽交界面处。流速和切应力不均匀分布导致了主槽水流含沙量降低,滩地水流含沙量升高。从五方面对漫滩水流特征的研究成果作简要概述。     

Bulk drag of a regular array of emergent blade-type vegetation stems under gradually varied flow

The drag induced by flow through vegetation is affected by the velocity, shape of vegetation stems and wake interference among stems. To account for the interference effects, previous works generally related the bulk drag coefficient of vegetation to the solid volume fraction ϕ of the vegetated zone, and the trends of the results were found to be inconsistent. In this work, a systematic laboratory study has been carried out to investigate the effect of the distribution pattern of vegetation stems on the hydrodynamics of gradually varied flow through emergent blade-type vegetation. The finite artificial vegetation patches of solid volume fractions ranging from 0.005 to 0.121 have been used and the stem Reynolds number ranges from 500–2600. The longitudinal water surface profiles have been measured, and the effect of increasing roughness density with respect to varying longitudinal and lateral spacing under the flow conditions is examined. The momentum equation that relates the vegetation resistant force and water surface profile has been used to obtain the mean bulk drag coefficient C_d of the canopy. The results show that C_d decreases with increasing stem Reynolds number, decreases with increasing ϕ at fixed lateral spacing due to sheltering effect, and increases with ϕ at fixed longitudinal spacing due to channeling effect. An empirical equation has been obtained relating C_d to the lateral and longitudinal spacing instead of ϕ.     

Numerical investigation of flow through vegetated multi-stage compound channel

This paper addresses the problem of the renormalization group k turbulence modeling of a vegetated multi-stage compound channel. Results from Micro acoustic Doppler velocimeter（ADV） tests are used with time and spatial averaging（doubleaveraging method） in the analysis of the flow field and the characterization. Comparisons of the mean velocity, the Reynolds stress, and the turbulent energy distribution show the validity of the computational method. The mean velocity profile sees an obvious deceleration in the terraces because of vegetation. Secondary flow exists mainly at the junction of the main channel and the vegetation region on the first terrace. The bed shear stress in the main channel is much greater than that in the terraces. The difference of the bed shear stress between two terraces is insignificant, and the presence of vegetation can effectively reduce the bed shear stress.     

Simulation of flow in compound open-channel using a discontinuous Galerkin finite-element method with Smagorinsky turbulence closure

The small-scale spatial variability of eddy viscosity which is characteristic for the turbulent shear stress in compound open-channel flows was studied and investigated in this paper. Different options including a constant value, zero-equation, one-equation, two-equation, and Smagorinsky turbulence models for parameterizing the eddy viscosity were developed in the framework of the discontinuous Galerkin finite-element SLIM model and applied for presenting the complex velocity profile in two different experimental data sets of laboratory flumes. A very good qualitative agreement was achieved between numerical results and measurement data for both velocity and flow depth of all experimental data sets in general. In addition, the calculation results showed that the turbulent Smagorinsky empiricism allowed a better presentation of non-uniform velocity in the floodplain and transition regions between plain and main channels than the others in all calculated cases. This empiricism predicted a very close variation of eddy viscosity in comparison with the results calculated by the depth-averaged Reynolds' stress and the lateral gradient of longitudinal velocity. The eddy viscosity varies significantly in the channel section; in particular the small values often occurred around the middle location of floodplains and the central location of the main channel while the large values appeared in the transition regions, presenting different minimum and maximum values of eddy viscosity in each flow region. The effects of eddy viscosity variation on lateral distribution of velocity profile were also investigated and discussed.