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.     

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.     

Effect of Adverse Pressure Gradient and Different Vegetated Banks on Flow

This study examines velocity components in 3D and shear stress distributions in flows with adverse pressure gradient over a gravel‐bed channel using three types of vegetation on banks, rice, straw and Typha stems. Results of this experimental study are compared with those for flows in the bare bank channel. Results show that the flows with adverse pressure gradient are not self‐similar in vegetated and bare banks channels. The logarithmic law and parabolic law can be used to well describe flow in the inner and outer regions, respectively, but with different relative flow depths. Shear stress distributions depend on nature of bank roughness and the secondary flow effects because of small values of the aspect ratio and adverse pressure gradient. Quadrant analysis is performed to investigate the effect of different vegetation covers on the Reynolds stress distributions at the central axis of channel and near the vegetated banks. Vegetation covers influence the contributions of different quadrants to the shear stress distribution, showing that sweeps and ejections are dominant events for all vegetated banks for near bed region. However, near the water surface the contributions of outwards and inwards for banks with Typha stems are more important than those for rice and straw. Instead of using a single value for friction factor, for different vegetated banks, a range of friction factor has to be applied in river restoration projects. Vegetated banks result in an increase of more than 60% in friction factor in comparison with that for bare banks under adverse pressure gradient flows. Copyright © 2015 John Wiley & Sons, Ltd.     

A simple method for estimating bed shear stress in smooth and vegetated compound channels

Instead of a large number of measurements of the streamwise velocity, a simple method is proposed to estimate the bed shear stress in smooth and vegetated compound channels, based on the Darcy-Weisbach equation. This method contains a dimensionless parameter iA, to represent the relationship between the bed shear stress and the velocity close to the channel bed()bU, which is determined in each divided domain. This method is verified in two smooth compound channels with different geometries, and in one compound channel with emergent floodplain vegetation. The comparison of predicted and measured bed shear stresses indicates the good predictive capability of this method, particularly in the mixing region. This method is further discussed for a channel with submerged vegetation. Once the values of iA in the main channel and the floodplain are determined, this method is a practical tool for estimating the bed shear stress based on a small number of measurements of bU in compound channels.     

Hydrodynamics of longitudinally discontinuous,vertically double layered and partially covered rigid vegetation patches in open channel flow

The aim of this paper is to numerically investigate how the flow structures are affected through a longitudinally discontinuous and vertically two‐layered vegetation occupying half width of the channel, with steady flow rate and subcritical conditions. A three‐dimensional (3‐D) Reynolds stress turbulence model (RSM), incorporated by Computational Fluid Dynamics (CFD) code FLUENT, was first validated with the experimental data, and then used for simulation purpose. The results showed that the flow stream‐wise velocities within the gap regions are visibly slower than that in the vegetation patch regions. Along the cross section, the velocity in the vegetation region (VR) reduced significantly due to resistance offered by the vegetation, which affected the channel conveyance; as compared to the free (non‐vegetated) region. The flow instability in the lateral direction was triggered by the flow shear due to the presence of partly distributed vegetation, resulting in the formation of coherent vortices and exchange of momentum at the interface. The discharge percentage passing through the free region (FR) was found to be 144–525% larger than that passing through the VR. The flow resistance increased significantly with higher vegetation density, whereas it decreased when both the vegetation layers were submerged. Moreover, the flow characteristics profiles in large gaps were more stable than in small gaps. The turbulent kinetic energy (TKE) and turbulence intensity also increased significantly through the patch regions compared to that of the gap regions. The results indicated that the flow structures and the flow resistance are strongly influenced by partial and discontinuous vegetation.     

开孔促淤板周围水流紊动特性试验研究

为深入了解开孔促淤板周围的水流特性,通过室内概化水槽试验分析了开孔促淤板前后的水流紊动特性,采用声学多普勒流速仪(ADV)测量了开孔促淤板前后典型断面上的三维瞬时流速,分析了典型断面上的时均流速、紊动强度、雷诺切应力和紊动能的分布规律。试验结果表明:板后近板区出现回流区,且回流区与开孔促淤板格栅位置相对应;板后水流相对紊动强度、雷诺切应力及相对紊动能明显大于板前,开孔促淤板对板后近板区水流影响较大;随着与开孔促淤板距离的增大,回流现象消失,流速变化趋于稳定,水流相对紊动强度减小且最终趋于稳定,相对紊动能出现折减且折减率逐渐降低;靠近近板区xOy面雷诺切应力大于yOz面雷诺切应力,靠近下游xOy面雷诺切应力与yOz面雷诺切应力相差较小。     

Effects of Rigid Unsubmerged Vegetation on Flow Field Structure and Turbulent Kinetic Energy of Gradually Varied Flow

The influence of rigid unsubmerged vegetation on flow structure and turbulent kinetic energy of gradually varied flow are experimentally investigated in this research. Natural reed stems of different densities are employed to examine the effects of the rigid unsubmerged vegetation on the flow in rivers. The results reveal that the vegetation existence significantly changes the gradually varied flow state from type M1 to type M2 in the vegetation section. The traditional power law describing the vertical flow velocity profile is evidently invalid when the vegetation density becomes high. With the irregularity index proposed in the research, the irregularity of vertical flow velocity profile in vegetated reach can be exponentially described in relation to the vegetation density. Furthermore, the turbulent kinetic energy is found to increase and reach a maximum value near the end of the vegetation section that is a potential localized erosion area. The results of the research have significances in river ecological restoration applications utilizing aquatic vegetation. Copyright © 2014 John Wiley & Sons, Ltd.     

The inception cavitating flows over an axisymmetric body with a blunt head-form

The inception cavitating flows around a blunt body are studied based on flow visualizations and velocity field measurements. The main purpose of the present work is to study the incipient cavity evolution and the interplay between the inception cavitation and the local turbulent flows. A high-speed video camera is used to visualize the cavitating flow structures, and the particle image velocimetry(PIV) technique is used to measure the velocity field, the vorticity, and the Reynolds stresses under non-cavitating and inception cavitating flow conditions. It is found that the appearance of visible cavities is preceded by the formation of a cluster of micro-bubbles not attached to the body surface and in a hairpin-shaped vortex structure. During its evolution, the cavity moves downstream with a lower speed. The effect of the incipient cavity is significant on the local vortical structures but slight on the timeaveraged velocity distribution. The mean Reynolds stress distributions in the turbulent shear flow can be substantially altered by the incipient cavities. The presence of the incipient cavities can lead to the production of turbulent fluctuations.     

Investigating the turbulent flow characteristics in an open channel with staggered vegetation patches

In the present study, flow around circular and staggered vegetation patches was investigated numerically. For turbulence modelling, the Reynolds‐averaged Navier–Stokes technique and Reynolds stress model were adopted. The numerical model was validated with the experimental data using varying vegetation density and flow velocities. The simulated results of mean stream‐wise velocities were in close agreement with the experimental results. The results show that the mean stream‐wise velocity in the downstream regions of vegetation patches were reduced, whereas the velocity in the free stream regions were increased. The influence of neighbouring and staggered vegetation patches on the flow was observed. The vegetation patches with larger nondimensional flow blockage (aD = 2.3, where a is the frontal area per volume of patches, and D is the diameter of vegetation patches) offered more turbulence when compared to the patches with a smaller flow blockage (aD = 1.2). Larger turbulence in the form of kinetic energy and turbulent intensity was recorded within the vegetation as well as the regions directly behind the patches. Negative Reynolds stresses were observed at the top of submerged vegetation. The turbulence characteristics peaked at the top of vegetation, that is, z/h = 1.0 (where z is the flow depth, and h is the vegetation height), which may be migrated vertically as the frontal area of the vegetation patch is increased. This high frontal area also increased stream‐wise velocity above the vegetation, leading to an increased variation in turbulence around the vegetation canopy.     

鱼道紊流特性研究进展

鱼道内的紊流特性对于鱼类能否顺利上溯洄游至关重要。为了解鱼道内的紊流状态,综述了丹尼尔式、竖缝式、组合式、涵洞式等鱼道内紊流流速、紊动能、紊流强度、雷诺剪切应力以及漩涡等紊流特性的研究进展,分析不同鱼道内紊流的分布规律及特点,指出鱼道中的紊流强度、紊动能等均会对鱼类对栖息地的选择以及鱼类通行造成较大的影响。此外,目前国内对鱼道内漩涡紊流特性的研究较少,建议对其深入研究,以改善鱼道内水力条件,保证鱼道有效运行。     

水流与壁面间切应力估计方法的比较研究

精确估计水流与壁面间切应力在泥沙输移和水利工程安全方面具有重要意义。通过选取并整理文献数据,对现今七种主要的水流与壁面间切应力的估计方法在不同雷诺数的条件下进行评价和比较,发现总体上湍流动能外推法和雷诺应力外推法估计壁面切应力的可靠性和精确度最高,平均相对误差分别为3.2%和5.2%。湍流能量法、湍流动能法和雷诺应力法次之,以壁定理的精确度最低,但其所需的数据较容易获取。二阶应力法的精确度取决于阻力系数的取值。其中,壁定理和二阶应力法的估计值偏高而更加安全,湍流动能外推法的估计值较为准确,而其余方法给出的估计值偏低。所得结果对不同条件下水流与壁面间切应力估计方法的选择以及直接测量仪器的校准有一定的参考价值。     

A hydrodynamic investigation of brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) redd selection at the riffle scale

High‐resolution velocity profile measurements were taken over a series of riffles on a gravel‐bed stream using a Pulse Coherent Acoustic Doppler Profiler (PCADP) to quantify the fluid structure of riffles and nests (redds) where brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) spawned. Velocity profiles were obtained on a highly discretized planometric scale ranging between 20 and 40 cm grid spacings, with vertical observations occurring every 1.6 cm. From the velocity profiles, between 15 000 and 38 000 velocity measurements were obtained over each riffle on any given day of measurement. Velocity profiles were converted to streamwise velocity magnitude, flow depth, Reynolds number, Froude number, shear stress, vertical velocity components and turbulent kinetic energy per unit area to evaluate the spatial structure of the riffles and the spatial structure of redds (pits and tailspills) relative to the surrounding riffle structure. Semi‐variograms were employed to evaluate the persistence of the fluid structure based upon the metrics evaluated. Results showed that discrete velocity observations poorly described the spatial structure of the flow system and poorly correlated with redd locations. Reynolds number analysis identified a relatively consistent fluid property for distances typically 2–3 times the longitudinal length of redds. Turbulent kinetic energy per unit area consistently identified common regions on all riffles studied that corresponded with the location selections for redds where flow was identified as essentially uni‐directional. Froude number was found to be insensitive in predicting the fluid spatial structure in wadeable flow depths and relating it to the fluid structure of redds. Results indicated that a series of metrics at varying spatial scales of turbulence may be necessary to understand the spatial complexity of redd selection. Copyright © 2011 John Wiley & Sons, Ltd.     

Turbulent flow structure at a 90-degree open channel confluence: Accounting for the distortion of the shear layer

City channels often have a smaller width-to-depth ratio in comparison to natural rivers due to the limited land availability. The penetration of the tributary into the main channel can cause the distortion of the shear layer. The main purpose of the present study is to investigate the mean and turbulent flow structure in the distorted shear layer in a discharge-adjustable plexiglass circulating flume. Three-dimensional velocities were collected and hydrodynamics and turbulence characteristics such as mean velocity field, turbulent kinetic energy, Reynolds shear stress, turbulence spectrum, and occurrence probabilities of quadrant events were analyzed. The results showed that a stronger helical cell was formed and extended for a longer distance downstream when the tributary channel had a higher flow rate than the main channel. The maximum Reynolds shear stress and the ejection and sweep events were mainly distributed at the middle zone of the water depth, rather than near the water surface, which were coincident with the shear layer as indicated by the turbulence kinetic energy. No obvious energy concentration was observed, and the power law relations for individual velocity components all had an exponent slightly larger than −5/3 in the flow frequency. The distortion of the shear layer resulted in an increase in occurrence probabilities of ejection and sweep events within the shear layer, which were related to the turbulence presenting vortices induced by wall. If the discharge ratio remained unchanged, an increase in the discharge of both channels resulted in an increase in some parameters, such as velocity, turbulent kinetic energy, and the absolute values of Reynolds shear stress, while the shear layer was distorted to a larger extent as the discharge of each channel decreased. All these results suggested that sediment transport, bed morphology and contaminant transport in the distorted shear layer at city channel confluences may differ significantly from that at natural river confluences.     

二维自由剪切湍流的实验结果及其与理论计算的比较

本文用热线风速仪和数据采样技术,对二维湍流混合层、湍尾流和湍射流自模拟区内的平均速度、湍流强度、雷诺应力和脉动速度的高阶关联进行了测量,所得结果与不可压缩流体剪切湍流理论的计算结果进行了比较,两者吻合得较好。     

黄河什四份子弯道冰期水流及冰塞特征研究

覆冰条件下,弯道水流固有的三维特性更为复杂,对河流弯道冰下水流特征及冰塞的研究有助于推进防凌减灾工作。依据2020年1月稳封期的原型观测数据,对黄河什四份子弯道冰下水流及冰塞特征进行了详细分析。结果表明,弯顶节点工程引起的回流是阻冰的重要条件,上游下潜的冰花因水动力不足及河势等因素影响发生堆积,冰塞在弯内形成并沿凹岸主槽逐渐向上游发展,同时,弯顶卡冰封河与下游较大的水流流速,促进了清沟的形成;入弯至弯顶区间,冰下水流流速垂线分布基本服从双对数分布规律;受凹岸冰塞及回流影响,纵向上,水流流速呈减小趋势,在弯顶附近开始恢复;径向上,河道主流被冰塞压迫并逐渐趋于凸岸,致使弯道主流易位,凸岸水流流速大于凹岸,清沟向凸岸偏移;弯道水流湍动能与雷诺应力主要在近底处存在较大的变幅,表明近底水流运动活跃,且二者大小与流速大小在径向与纵向分布上具有较好的一致性,受冰盖影响,水流湍动能沿水深近似呈"S"形分布,雷诺应力分布无明显规律。     

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。     

Vertical mass and momentum transport in open-channel flows with submerged vegetations

The importance of flow and turbulence to the ecology of aquatic benthic organism has been widely reported. Aquatic vegetation is one of environmental variables that influence turbulence and the ecological condition of rivers. Aquatic canopies have the potential to greatly improve water quality through the removal of nutrients and trace metals. To describe the efficiency of this removal, we must be able to quantify the rate of vertical mixing. In fully submerged vegetation flows, coherent turbulent motions are generated near the vegetation edge and these large-scale eddies control the vertical exchange of mass and momentum. It is therefore important for river management to investigate hydrodynamic characteristics and coherent eddies in open-channel flows with vegetation canopies. Turbulence structure and transport mechanism of momentum in vegetated flows have been investigated intensively in the past decade.However, the effect of the submerged vegetation on the vertical mass transport and turbulent diffusion has not been fully investigated. Therefore, in the present study, continuous dye injection experiments were conducted to evaluate the mass transport structure in open-channel flow with rigid vegetation models by changing the vegetation density. A combination technique between PIV and planar laser-induced fluorescence (LIF) was developed by using two sets of CCD cameras, to measure the instantaneous velocity and concentration field simultaneously. The technique is capable of determining the turbulent scalar flux as well as the Reynolds stress, mean and fluctuating velocity and concentration fields. Consequently, the effects of coherent vortices on the vertical turbulent diffusivity were examined in detail.     

CURVED OPEN CHANNEL FLOW ON VEGETATION ROUGHENED INNER BANK

A RNG numerical model together with a laboratory measurement with Micro ADV are adopted to investigate the flow through a 180o curved open channel(a 4 m straight inflow section,a 180o curved section,and a 4m straight outflow section)partially covered with rigid vegetations on its inner bank.Under the combined action of the vegetation and the bend flow,the flow structure is complex.The stream-wise velocities in the vegetation region are much smaller than those in the non-vegetation region due to the retardation caused by the vegetation.For the same reason,no clear circulation is found in the vegetated region,while in the non-vegetation region,a slight counter-rotating circulation is found near the outer bank at both 90o and downstream curved cross-sections.A comparison between the numerical prediction and the laboratory measurement shows that the RNG model can well predict the flow structure of the bend flow with vegetation.Furthermore,the shear stress is analyzed based on the numerical prediction.The much smaller value in the inner vegetated region indicates that the vegetation can effectively protect the river bank from scouring and erosion,in other words,the sediment is more likely to be deposited in the vegetation region.     

Prediction of Bank Erosion in a Reach of the Sacramento River and its Mitigation with Groynes

The paper reports on the prediction of flow in a reach of the Sacramento River with focus on a part of the river’s bank where serious erosion has occurred. The simulations were obtained using a three-dimensional Navier–Stokes solver which utilized body-fitted coordinates to represent the complex river bathymetry. Comparative predictions were obtained using a two-dimensional, depth-averaged formulation. Local (nested) mesh refinement was employed to provide the necessary resolution of the bank geometry in the region of interest. The study focuses on the assessment of the effectiveness of a particular arrangement of groynes which was found in physical model studies to significantly reduce the rate of erosion in the region under consideration. To validate the computational models, predictions were first obtained for the case of turbulent flow in a straight rectangular channel with one groyne. Measurements of velocity and boundary shear stress were used for model validation. For the reach of the Sacramento River under consideration, velocity measurements obtained in the large-scale physical model were also used to check the computational model prior to its use for prediction of the river flow with groynes. Here, too, both depth-averaged and three-dimensional computations were performed with the objective being to determine the influence of the groynes on the flow velocity. The bank erosion rate was estimated by coupling the ‘excess shear stress’ method to the computed mean velocity field. The results show that for the groynes configuration that was found optimal in the physical-model studies, and that was actually implemented in the Sacramento River, the groynes are effective in reducing the bank erosion of the affected zone but at the cost of transferring a far less severe problem further downstream.     

三维明渠流动数值模型

本文对经典的SIMPLE方法进行了改进,在模拟具有自由表面的湍流问题时,摒弃了对自由表面常作的“刚盖”假定。通过求解k-ε模式闭合的三维雷诺方程及自由表面运动方程,得到了渠道槽沟区水流速度、涡粘性系数、湍流切应力、湍动能的分布规律和自由水面位置的变化规律,计算结果与实测值十分吻合。