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。     

Lorentz力对弱导电流体中圆柱受力的影响

电磁力（Lorentz力）可以改变弱导电流体边界层的结构,从而影响物体受力状况。该文将电磁激活板包覆在圆柱表面,置于弱电解质溶液中,从实验上测试并分析了Lorentz力对圆柱所受阻力的影响。为了揭示其影响过程的内在机理,对此现象进行了细致的数值研究,着重讨论了Lorentz力对圆柱阻力的各种组成部分的因素,揭示了Lorentz力主要通过改变圆柱表面涡量梯度及涡量来影响圆柱受力变化的内在过程。结果表明,在一定范围内,随着Lorentz力的增大,圆柱的表面涡量增大,流体分离点后移,表面涡量,阻力的振动幅度减小至零,圆柱所受阻力减小。     

规则波作用下刚性植物拖曳力系数实验研究

植物的消波特性能有效防止波浪冲刷所造成的岸线侵蚀。在开展根茎叶共同作用下的刚性植物消波实验的基础上,采用Kobayashi和Dalrymple经验公式计算本实验植物模型的拖曳力系数,探究了拖曳力系数变化规律。通过对比分析两种经验公式计算所得的拖曳力系数,发现采用Kobayashi消波模型对于本实验中刚性植物消波模拟效果更好。根茎叶均在不同程度上对于植物消波特性产生影响。拖曳力系数与邱卡数、雷诺数、厄塞尔数及相对水深之间的关系依赖于植物淹没度。针对包含根茎叶的刚性植物,考虑植物摆动因素,对波能耗散方程中阻力项进行修正,提出改进的消波模型。     

Effects of aquatic vegetation on flow in the Nansi Lake and its flow velocity modeling

1. INTRODUCTION China has many lakes, most of which are shallow lakes, all kinds of aquatic vegetation grow in the shallow water zone. In the recent years, scientists andengineers have paid considerable attention to use the vegetation to improve the lakes…     

Impact of a 100‐year flood on vegetation,benthic invertebrates,riparian fauna and large woody debris standing stock in an alpine floodplain

This paper investigates the impact of a 100‐year flood in May 1999 on community composition and large woody debris standing stock in an alpine floodplain (Isar, Germany). Detailed pre‐flood data sampled from 1993 to 1998 are compared with the situation directly after the flood. In those parts of the Isar floodplain mainly covered with pioneer vegetation prior to flooding, the coverage of unvegetated gravel bars increased by 22% following the flood. However, the flood did not remove larger amounts of older successional vegetation stages (willow thickets, floodplain forest). No significant changes in the benthic invertebrate fauna were recorded. The lowest densities of riparian ground beetles (Carabidae) within the study were recorded one month after the flood. Two months later, the ground beetle densities increased to the highest values ever recorded, indicating the ground beetle's high potential for recolonization. These results highlight the degree of resilience of both the aquatic and the riparian invertebrate fauna. The flood also caused a significant increase in large woody debris standing stock; in one section the number of logs increased tenfold and the volume increased by a factor of 20, leading to the assumption that most woody debris in alluvial flood‐plains is provided by catastrophic events. Copyright © 2004 John Wiley & Sons, Ltd.     

含植物明渠流动分区和特征尺度研究进展

植物是生态河流必不可少的要素之一。植物的存在,改变了水流的紊流结构。由于流动沿垂向具有较强的不均匀性,对于含植物明渠紊流统计特性的描述,至今未有公认的、统一的沿水深分布表达形式。因此,对含植物明渠流动分区和各区特征尺度的研究越来越受到重视。本文回顾了无植物明渠和含植物明渠流动分区及特征尺度研究的主要成果,重点对基于流速分布和紊流结构两种方法确定的分区和特征尺度进行了分析和总结。指出将涡结构与流动特性参数分布规律相结合进行流动分区,在各区确定合适的特征尺度,进一步探究分区界限及流动特征尺度的影响因素及具体影响,建立普适的紊流统计特性分布规律是今后含植物明渠紊流结构研究的重要内容和趋势。     

Experimental and numerical study on hydrodynamics of riparian vegetation

Recently, many channelized rivers tend to be heavily vegetated due to regime shifts in hydrological, fluvial and ecological processes. Dense vegetation in a river frequently obstructs a flood flow and reduces conveyance capacity of channels. On the other hand, river vegetation provides various ecological services such as habitats for various species and life, natural cycle of organic and inorganic substances, etc.. It is of engineering importance to understand vegetation hydrodynamics in order to preserve vegetation nature and keep a certain level of flow conveyance capacity. In view that willows tend to be densely vegetated along the shoreline of floodplains or sandbars, a field measurement, a physical model experiment and a numerical analysis were carried out for investigating hydrodynamics in an open channel with riparian vegetation. Discussion was made focusing on flow and shear layer structures developed around the vegetation canopy.     

含柔性植物明渠水流阻力特性试验研究

利用水槽试验研究了无草、淹没型、非淹没型柔性植物水流阻力特性的变化规律,分析了含植物明渠沿程水面比降和阻力系数的变化特性。得到各植物阻力系数变化与尼库拉兹曲线基本相似,紊流范围内同样可划分为紊流光滑区、紊流过渡区、紊流粗糙区;探讨了各植物明渠等效河床的问题,据此确定了各植物的粗糙度,对植物粗糙度、水流雷诺数和阻力系数之间的关系进行了分析,并通过对各植物水流阻力系数的分区回归计算,得到各植物水流阻力的分区经验表达式,该方法能较好地应用于含植物明渠水流阻力系数的推求。     

HYDRAULIC RESISTANCE OF SUBMERGED VEGETATION RELATED TO EFFECTIVE HEIGHT

Submerged vegetation has a significant impact on water flow velocity. Current investigations include the impact through adding drag resistance and increasing bottom roughness coefficient, which cannot elucidate the characters of real submerged vegetation. To evaluate the effects of submerged vegetation on water currents at different velocities, a laboratory experiment was conducted using three kinds of vegetations. The effective heights of these vegetations on varying flow velocities were evaluated. An equation describing the relationship between the normalized resistance of the submerged plants and the Reynolds number based on the plant effective height was then established and used to calculate the hydraulic resistance parameters of submerged plants in different stages of growth.     

床面均匀沙拖曳力及上举力系数的确定

床面泥沙颗粒由于其相对暴露度及外形的不同,即使在同样水流条件下,颗粒附近的水流结构也不相同,进而影响到颗粒的受力不同,主要表现在颗粒拖曳力系数和上举力系数的不同。在白玉川提出的双向相对暴露度概念及拖曳力系数、上举力系数与纵向相对暴露度之间关系的基础上,分析发现,拖曳力系数介于1.00~1.22区间的概率大于92%,平均值为1.11;拖曳力系数与上举力系数比值平均值随沙粒雷诺数变化而变化,但在颗粒雷诺数大于25 000时,拖曳力系数与上举力系数比值平均值趋于常数1.28。将拖曳力系数和上举力系数引用到泥沙起动底流速公式中,得到了散粒体泥沙起动流速公式。实测资料验证表明,起动流速公式与实则资料吻合良好,间接地验证了提出的推移力系数值及上举力系数值是正确的。     

黏性泥石流残留层的床面减阻研究综述

黏性泥石流运动理论的核心问题在于确定黏性泥石流运动阻力和流速。但由于其运动的复杂、流态的多样,导致了阻力准确计算的困难,也制约了泥石流运动流速和流量公式的建立和推广应用。关于黏性泥石流的运动阻力问题,目前多聚焦于泥石流固液间相互作用的内部阻力关系,鲜见对于其床面阻力的研究。介绍了黏性泥石流的典型运动过程—"铺床",描述了铺床过程形成的残留层导致床面减阻现象,概述了目前国内外各泥石流运动模型对泥石流阻力的研究,并总结了残留层导致减阻的可能原因和目前多采用的床面阻力计算方法。认为残留层导致的复杂减阻效应,是黏性泥石流运动尤其是高速运动研究中不可忽视的一部分,但目前各类模型和水力学经验公式都无法概化黏性泥石流运动床面的阻力变化。鉴于此,提出了未来可开展的工作:(1)认知"铺床"导致的残留层的形成和发展过程;(2)探讨残留层床面减阻机制;(3)研究残留层床面阻力计算。     

草被覆盖度下坡面流流速变化及缓流机制研究

坡面薄层水流是黄土区土壤侵蚀发生的直接动力条件,探究植被覆盖条件下坡面流水动力学特性及缓流机制,有助于揭示土壤侵蚀发生的水动力学机理。在水槽坡度为12°条件下,通过9个草被覆盖度、7个流量的组合试验,研究了不同植被覆盖度下坡面水流流速变化和缓流机制。结果表明:坡面流流速随着覆盖度的增加整体呈减小的趋势;缓流系数随覆盖度的增加呈先增大后减小的变化规律,当覆盖度大于47.66%时,缓流系数随覆盖度的增加显著减小;以覆盖度47.66%为临界值,高、低覆盖度下缓流系数随雷诺数的变化规律有所不同。最后,建立了模拟草被下的坡面流流速和缓流系数拟合公式,验证结果表明,拟合公式精度较高,可为建立土壤侵蚀预报模型提供参考。     

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。     

分层流体中冰脊拖曳力的数值模拟研究

冰脊在分层流体表面的漂移运动会在内界面激发出内波,内波又会对冰脊的运动、堆积产生重要影响。利用湍流模型对冰脊在分层流体中的运行过程进行了数值模拟研究,并与已有物理模型实验结果做了比较,冰脊拖曳力和内波界面的数值模拟结果与物理实验结果吻合良好。当流场属于跨临界区时,内波的产生使冰脊拖曳力随弗劳德数Fr先增大后减小,冰脊拖曳系数随Fr的变化规律类似。当流场属于超临界区时,拖曳力随Fr增大而增大,拖曳系数不再随弗劳德数变化。当内波波峰达到最高点的时候,冰脊拖曳力达到一个峰值。     

刚性植物对波高衰减和水流结构的影响

为了研究植物的消浪能力及对水流紊流特性的影响,开展室内水槽试验,用木棒模拟刚性植物,分析了5组植物区布置方案和4组波要素条件下的波高衰减和水流结构。结果表明:植物密度对波高衰减具有显著影响;波浪通过植物区时波高衰减的速率逐渐减小;在同等植物特征条件下,短波比长波衰减幅度更大;植物区内的拖曳力系数CD与低科勒冈-卡朋特(Keulegan-Carpenter)数KC之间存在二次函数关系,CD随着KC的增大而减小,且变化速率呈变缓的趋势;波高和植物密度的增大会引起植物区内部紊动能量ETK的增大;完全淹没植物条件下的ETK大于不完全淹没条件下的ETK;在完全淹没植物的影响下,从底面至水面ETK呈先增大后减小的规律,并在植物冠层处达到最大值。     

Influence of bending rigidity of submerged vegetation on local flow resistance

The different state of the submerged vegetation has different influences on the flow resistance. This paper explores the relationship between the state and the resistance of an individual submerged vegetation, and the relative bending rigidity of the submerged vegetation is determined by the state of the submerged vegetation. Based on the experimental observations, the state and the resistance of an individual submerged vegetation are analyzed under different inflow conditions. At the same time, the influences of the various submerged vegetations on the flow resistance are discussed under the same inflow conditions. Some interesting relationships are obtained between the flow resistance and the relative bending rigidity of the submerged vegetation, and it is shown that the flow resistance increases with the increase of the relative bending rigidity of the submerged vegetation, and they are positively correlated.     

Spectral‐ALS data fusion for different roughness parameterizations of forested floodplains

Natural river floodplains and adjacent wetlands grow typically a diverse and heterogeneous combination of herbs, shrubs and trees, which play an essential role in determining the total flow resistance. Hydrodynamic effects of trees in forested floodplains can provide the majority of flow resistance during flood events. Nevertheless, ground‐based techniques to acquire vegetation parameters are expensive and difficult to apply over long reaches. This paper presents a novel method of automated roughness parameterization of riparian woody vegetation by fusion of Quickbird multi‐spectral image with airborne laser scanning (ALS) data. The data fusion approach includes: individual tree detection and estimation of vegetation metrics from light detection and ranging (LiDAR) data, assessment of predictive models for the vegetation parameters and spatial mapping of the vegetation parameters for the forest plants in the riparian corridor. The proposed method focuses on estimation of plant density (d), crown diameters (D_C), tree height (h), stem diameter (D_S), crown base height (cbh) and leaf area index (LAI). The procedure is tested along a 14‐km reach of the Sieve River (Tuscany, Italy) characterized by high woody plant density. Due to the complex study area, the data fusion approach explains with variable reliability the local vegetation properties (R²(D_C) = 0.14, R²(h) = 0.84, R²(D_S) = 0.25, R²(cbh) = 0.66). The generated structural parameter maps represent spatially explicit data layers that can be used as inputs to hydrodynamic models used to analyse flow resistance effects in different submergence conditions of vegetation. A simple flow resistance model was applied over a test area comparing the results of the proposed method and a traditional ground‐based approach. The modelling results showed that the new method is able to provide accurate output data to describe the interaction between water levels and bio‐mechanical characteristics of vegetation. The proposed methodology provides a fast, repeatable and accurate way of obtaining floodplain roughness, which enables regular updating of vegetation characteristics. Copyright © 2010 John Wiley & Sons, Ltd.     

非淹没刚性植被影响下孤立波在岸滩上的爬高

为探究海岸刚性植被对海啸波的削弱效应,通过物理模型试验和数值模拟研究了孤立波与不同坡度岸滩上非淹没刚性植被的相互作用问题。物理模型试验在波浪槽中进行,测试了不同的入射波波高、植被密度和岸滩坡度对孤立波爬高的影响,并运用物理模型试验数据校核改进后的Boussinesq方程,得到植被的拖曳力系数。结果表明:拖曳力系数随植被密度的增大而增大,随坡度增大而减小;植被后的无量纲透射波高和无量纲岸滩爬高随着无量纲的入射波高的增大而减小,随着植被密度的增大而减小;当岸滩坡度增大时,无量纲透射波高增大而无量纲爬高并无显著差异。最后根据回归分析得出了岸滩爬高与相对入射波高、植被密度和岸滩坡度的幂函数型经验关系式。     

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.