不同布置形态下刚性淹没植被对水流特性的影响

自然河道中植被呈现多种多样的分布状态,为探究不同布置形态下刚性淹没植被对水流特性的影响,利用室内水槽模拟含刚性淹没植被的明渠进行试验研究。结果表明:刚性淹没植被的布置形态对明渠流水流特性影响显著。在本文研究的植被布置形态下的水深、植被阻力系数与植被粗糙系数均增大,表现为交错型布置斑块型布置线性布置;流速与雷诺数均减小,表现为线性布置斑块型布置交错型布置;糙率n随着平均流速与水力半径之积VR的增大而减小。     

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.     

Numerical investigation of velocity distribution of turbulent flow through vertically double-layered vegetation

The velocity structures of flow through vertically double-layered vegetation(VDLV) as well as single-layered rigid vegetation(SLV) were investigated computationally with a three-dimensional(3D) Reynolds stress turbulence model,using the computational fluid dynamics(CFD)code FLUENT.The detailed velocity distribution was explored with a varying initial Froude number(Fr),with consideration of the steady subcritical flow conditions of an inland tsunami.In VDLV flows,the numerical model successfully captured the inflection point in the profiles of mean streamwise velocities in the mixing-layer region around the top of short submerged vegetation.An upward and downward movement of flow occurred at the positions located just behind the tall and short vegetation,respectively.Overall,higher streamwise velocities were observed in the upper vegetation layer due to high porosity,with P_r=98%(sparse vegetation,where P_r is the porosity),as compared to those in the lower vegetation layer,which had comparatively low porosity,with P_r=91%(dense vegetation).A rising trend of velocities was found as the flow passed through the vegetation region,followed by a clear sawtooth distribution,as compared to the regions just upstream and downstream of vegetation where the flow was almost uniform.In VDLV flows,a rising trend in the flow resistance was observed with the increase in the initial Froude number,i.e.,Fr=0.67,0.70,and 0.73.However,the flow resistance in the case of SLV was relatively very low.The numerical results also show the flow structures within the vicinity of short and tall vegetation,which are difficult to attain through experimental measurements.     

有植被的河道水流紊动特性模型试验研究

通过物理模型试验,研究了有植被的河道水流紊动特性.试验结果表明,在复式断面河道滩地种植柔性植被后,滩地糙率增大,水流紊动更为剧烈,河道水流紊动强度峰值由原先的滩槽交界区转移到滩地区.滩地的水流紊动强度沿程递减;滩槽交界区的水流紊动强度沿程不断增大;主槽的水流紊动强度主要与床面糙率有关,滩地植被影响了滩地水流的归槽时间,使主槽水流流速沿程增大.     

Flow structure and channel change in a sinuous grass‐lined stream within an agricultural drainage ditch: Implications for ditch stability and aquatic habitat

Many drainage ditches in the Midwest have developed a geomorphological configuration characterized by vegetated bars, or benches, on the bottom of the ditch and a stream flowing within a channel inset into these bars or benches. Past work has focused on the sedimentology of the benches and the depositional processes involved in bench development. This study investigates the three‐dimensional flow structure and short‐term channel change in a small grass‐lined stream flanked by benches at the bottom of an agricultural drainage ditch in east central Illinois, USA. In particular, it focuses on the influence of channel curvature and bank vegetation on flow through the inset channel at two different stages and explores how the structure of the flow is related to documented patterns of channel change. Results indicate that the mean flow is characterized by a submerged high‐velocity core that mainly is confined to the centre of the channel by near‐bank zones of flow stagnation/separation induced by abrupt changes in channel alignment and by strong frictional effects of grasses extending into the flow along the channel margins. Where the high‐velocity core is close to the channel margins, minor erosional adjustments of the inset channel can occur in the form of bank erosion. Patterns of turbulence kinetic energy reflect the development of shear layers near the channel margins and surrounding the submerged high‐velocity core. Locations with strong turbulence also correspond to locations of minor bank erosion. The results indicate that the inset channel is a relatively stable feature, especially where the alignment of this channel is straight, but that erosion‐control treatments may be necessary locally where the inset channel impinges on the ditch bank. Although the development of benches, a geomorphic response to ditch maintenance, is commonly viewed as a threat to drainage efficacy, preserving or constructing benches and associated inset channels in drainage ditches can enhance aquatic habitat and water quality. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

3-D hybrid LES-RANS model for simulation of open-channel T-diversion flows

The study of flow diversions in open channels plays an important practical role in the design and management of open-channel networks for irrigation or drainage.To accurately predict the mean flow and turbulence characteristics of open-channel dividing flows,a hybrid LES-RANS model,which combines the large eddy simulation (LES) model with the Reynolds-averaged Navier-Stokes (RANS) model,is proposed in the present study.The unsteady RANS model was used to simulate the upstream and downstream regions of a main channel,as well as the downstream region of a branch channel.The LES model was used to simulate the channel diversion region,where turbulent flow characteristics ate complicated.Isotropic velocity fluctuations were added at the inflow interface of the LES region to trigger the generation of resolved turbulence.A method based on the virtual body force is proposed to impose Reynolds-averaged velocity fields near the outlet of the LES region in order to take downstream flow effects computed by the RANS model into account and dissipate the excessive turbulent fluctuations.This hybrid approach saves computational effort and makes it easier to properly specify inlet and outlet boundary conditions.Comparison between computational results and experimental data indicates that this relatively new modeling approach can accurately predict open-channel T-diversion flows.     

柔性植物阻流特性研究

河流中的植物不仅影响河流的泄洪能力而且在生态的修复中起重要作用,因此对含植物明渠水流的阻力特性及紊流特性的研究显得非常重要。将河道中含柔性植物的水流视为充分发展了的紊流,在实验室中用特定材料模拟柔性植物,通过水槽试验用三维ADV对淹没柔性植物段紊流特性进行了测量研究,分析了含淹没柔性植物明渠水流的水力学特性及阻力特性并得出其曼宁糙率n的变化规律。对试验结果分析表明,含淹没柔性植物的垂向流速分布不再符合指数分布;其紊流具有明显的各向异性特性;植物冠层交界处,水流紊动交换强烈,该处紊流强度及雷诺应力出现最大值。     

有植被河道的水力特性研究

利用三维超声波多普勒流速仪(ADV)量测有植被时水槽不同测点位置的瞬时流场。分析了测点位置和水深与植被高度比对时均流速分布的影响。试验结果表明植被层上下水流结构复杂,柔性植被的存在使垂线流速降低,所测得的流速分布在植被层顶部附近呈现“<”形状,在植被区域内部流速分布近似为“3”形。分析了植被区水流紊动强度和雷诺应力的垂线分布情况。水流紊动强度和雷诺应力在植被区域的上层相对较大,说明在该处发生着剧烈的质量和动量交换以及很强的剪切作用。     

Characteristics of Flow Resistance in Open Channels With Non-Submerged Rigid Vegetation

The flow resistance factors of non-submerged rigid vegetation in open channels were analyzed. The formulas of drag coefficient CD and equivalent Manning＇s roughness coefficient na were derived by analyzing the force of the flow of non-submerged rigid vegetation in open channel. The flow characteristics and mechanism of non-submerged rigid vegetation in open channel were studied through flume experiments.     

含植物水流动力特性研究进展

鉴于植物与水动力之间复杂的相互作用过程,从植物对水流的阻力作用、植物引起的紊动作用以及植物对波浪的消减作用三方面,综述了国内外含植物水流动力特性研究进展,总结了不同水动力条件下含植物河道Manning系数、拖曳力系数与植物特性之间的定量关系,讨论了植物引起的纵向流速垂向分布和紊动强度变化以及植物在海岸带波浪消减中起到的作用。针对目前含植物水流动力特性研究中存在的不足,指出植物拖曳力系数、含植物水流紊动特征、植物-水流-波浪耦合作用机理为今后研究的重点。     

Numerical Model for Flow Motion with Vegetation

A set of governing equations for turbulent flows in vegetated area were derived with the assumption that vegetation is of straight and rigid cylinder. The effect of vegetation on flow motion was represented by additional inertial and drag forces. The new model was validated by available experimental data for open channel flows passing through vegetated areas with different vegetation size, density and distribution. Numerical results are in good agreement with the experimental data. Finally, the flow around a supposed isolated vegetated pile was simulated and the effects of vegetation density on the wake flow were discussed. It is found that the presence of vegetation, even at a very low density, has the pronounced influence on the dissipation of flow energy, both inside the vegetation domain and outside it in the wake flow region.     

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.     

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.     

Hydrology and aquatic habitat characteristics of a riverine swamp: I. Influence of flow on water temperature and chemistry

We monitored water temperature, air temperature, hydrologic characteristics, and chemical variables for 25 months in a 254‐km² portion of the Atchafalaya Basin, Louisiana, USA, a distributary swamp of the Mississippi River. We related observed trends in thermal and chemical variables to characteristics of the flood pulse, available channel types, and paths of water flow within the study area. Dissolved oxygen (range 0.05–13.05 mg L⁻¹), percent oxygen saturation (range 0.6–142.4%), and pH (range 6.31–8.52) increased as river stages decreased, and water temperatures (range 4.75–32.25°C), specific conductance (range 207.8–931.5 μmhos), and the top–bottom difference in dissolved oxygen (−7.5–12.05 mg L⁻¹) increased as air temperatures rose. The maximum variance observed for all chemical and thermal habitat characteristics occurred during periods when air temperatures exceeded, and river stages were less than, their respective medians observed during the study. At any point in time, thermal and chemical variables varied most among stations in water courses with no visible water turbulence (low‐energy channels), and these channels consistently had lower dissolved oxygen, percent oxygen saturation, and pH than turbulent water courses (high‐energy channels) or lakes contained. Water was directed from the river into one region of the study area from two opposing directions, thereby impeding water flow through the region. Low‐energy channels in this region consistently exhibited lower dissolved oxygen concentrations, percent oxygen saturation, and pH than was observed in low‐energy channels of regions where water flow was unimpeded. The quantity and quality of aquatic habitat characteristics in riverine swamps can be manipulated by controlling the timing and magnitude of the flood pulse, relative availability of different water courses, and internal paths of water flow through the swamp. Copyright © 1999 John Wiley & Sons, Ltd.     

水生植物对河道形态影响的三维湍流模型

植被是河床和河岸稳定性指标中的一个重要参数,它主要通过影响水流结构进而影响河道的形态。本文建立曲线坐标系下的三维湍流模型研究水生植物对河道形态的影响,在水流控制方程中加入植物阻力项和植物密度项来描述水生植物对水动力特性的贡献,通过泥沙质量守恒方程求解河道变形,在适体网格中求解模型的控制方程。应用该三维模型数值模拟了矩形水槽内两侧交替布置的非淹没刚性植物对水流结构的影响以及梯形河道内两侧交替布置的非淹没刚性植物对河道形态的影响,数值计算结果与实测结果吻合良好。     

Comparison of blockage factors in modelling the resistance of channels containing submerged macrophytes

The proportion of a river channel containing vegetation (termed the blockage factor) has historically been determined in three ways: cross‐sectional, surface area and volumetric. The first two versions are two‐dimensional measures. Meanwhile, the three‐dimensional volumetric version is biased towards deeper sections of a reach. A fourth version of the blockage factor is proposed that does not have such limitations: the multi‐cross‐sectional blockage factor. Between five and nine cross‐sections were sampled to determine the four blockage‐factor versions for 35 river sites containing the clumped submergent macrophyte, Ranunculus subgenus Batrachium (water‐crowfoot). The ability of these four measures to act as predictors of vegetative channel resistance was then assessed. The vegetated proportion of individual cross‐sections was poorly related to the vegetation resistance of a channel reach, primarily due to the high spatial variability of patch‐forming macrophytes. The weighted median of all cross‐sectional blockage factors measured at each site produced the strongest relationship with vegetation resistance, though this was not significantly better than the volumetric or surface area versions. The resistance model using the surface‐area blockage factor gave a very high residual between predicted and calculated resistance for a mat‐forming macrophyte species, and this model is unlikely to hold for conditions other than baseflow. Likewise, the volumetric version is not expected to hold for sites that have more depth variability than those measured in this study. Copyright © 2005 John Wiley & Sons, Ltd.     

THREE-DIMENSIONAL LARGE EDDY SIMULATION OF FREE SURFACE TURBULENT FLOW IN OPEN CHANNEL WITHIN SUBMERGED VEGETATION DOMAIN

1 .　INTRODUCTIONInactualrivers ,theriverbediscommonlycoveredbyvegetation .Thevegetationinriversplaysanimportantroleontheenvironmentalfunc tionoftherivers,thatis,theamenitytohumanlife ,livingspaceformanylives ,purificationofwaterqualityandsoon .However ,thevegetationinriversalsoplaysakeyroleonthecarryingca pacityofachannelwithfloodplains .Therefore ,thehydraulicsoftheflowwithvegetationshouldbemoreimprovedorrefined .Logicalexplanationandaccurateevaluationaboutturbulentflowinopenchannelwith…     

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。     

Flow characterization of an attraction channel as entrance to fishways

The flow field inside and downstream of an open channel placed near the surface of a free flow (such as the tail water of a turbine) is characterized in detail. The channel cross‐section is U‐shaped and in the downstream end is placed a ramp on the bottom which accelerates the flow passing through the channel. This flow is intended to catch the attention of fish and improve their entrance to fishways, which has also been successfully demonstrated in field tests. The flow through the channel is subcritical and the ramp thus blocks some water from passing through. To find the optimum vertical position of the channel, a down‐scaled model channel is placed in a water flume and flow fields in vertical planes directed along the flow are visualized using particle image velocimetry. Results show that increasing the depth over the ramp has little effect on the maximum velocity while it makes the accelerated water more perceptible downstream the channel. This will most likely improve the channel's ability to attract fish. It is also shown that a recirculation zone is formed in the channel for the small depths tested. Finally, it is shown that a modest tilt of the channel will not affect the flow field in any significant way. Copyright © 2010 John Wiley & Sons, Ltd.