Silica Regeneration Processes in Nearshore Southern Lake Michigan

The seasonal depletion of dissolved silica to levels that limit diatom production is particularly critical in Lake Michigan's nearshore zone where diatom biomass is greatest, and where silica regeneration from sediments is not well-understood. In our study, intact, medium-fine sand cores, collected from an 11 m deep site in nearshore Lake Michigan during July-August 1980, released soluble reactive silica (SRS) at a mean rate of 2,707 ± 122 (SE) μg Si cm^?2 yr^?1 when incubated in darkness and at 12° C. This measured SRS release was greater than a diffusive flux (270 ± 49 (90% C.I.) μg Si cm^?2 yr^?1) estimated from SRS pore water profiles and physical sediment properties. SRS release from individual cores was not correlated with abundance of most macroinvertebrates (chironomids, pisidiid clams, or oligochaetes). However, a significant (P < 0.05) and inverse relationship between SRS release and Pontoporeia hoyi densities implied that amphipods suppressed SRS release through mixing and burial of a surficial floe layer, where most dissolution of biogenic silica occurs. Moreover, SRS release rates measured from our coarse-grained nearshore sediments were comparable to rates reported for fine-grained offshore material and further implicate dissolution of surficial biogenic silica as the source of remineralized SRS. Because nearshore areas of Lake Michigan undergo strong seasonal variations in temperature and diatom production, and because significant riverine silica inputs exist, we cannot extrapolate our results on a lakewide, or season-long basis. The data, however, strongly imply that nearshore sediments are an important participant in the Lake Michigan silica cycle.     

紊流场中固体颗粒运动轨迹的Lagrangian模型

本文用标准稳态K-ε双方程二阶湍流封闭模型模拟流场,在全面考虑固体颗粒受力情况的基础上,用跟踪颗粒运动的Lagrangian方法描述球形固体颗粒运动,数值求解了紊流场和颗粒运动轨迹。数值计算结果与实验结果相比是令人满意的。     

任意平面流场中的湍流临界准则数

作者应用涡量对流及扩散(传输)的概念和理论建立了对任意平面流场中发生湍流流动的湍流临界准则数。这个准则数不同于熟知的雷诺数。如果应用此准则数在接近直管壁和平板边界层固壁处表明流动会诱发“猝发”现象;对毛细管内的流动而言它也会指出无论管中平均流速多大、它总不会发生湍流;而对多孔介质中的流动,湍流会出现。所有这些预示颇令人感兴趣和关注。     

Ⅱ类水体悬移质泥沙含量遥感反演综述

悬移质泥沙是水环境的重要参数之一,其含量的变化对流域生态环境有着多方面的影响。传统研究方法能够实现悬移质泥含量的精确分析,但存在布点有限、时效不足等问题,而遥感技术可弥补传统研究方法的不足,实现大面积水域的同步观测。基于悬移质泥沙含量遥感反演的4类模型:物理模型、半分析模型、经验模型、人工智能算法模型,总结了Ⅱ类水体悬移质泥沙含量遥感反演的研究进展,探讨了反演研究中存在的离水辐亮度提取难度大、混合像元不易分解等问题,并针对我国内陆山区河流,提出山区Ⅱ类水体悬移质泥沙含量遥感反演发展面临的关键问题。     

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。     

环境水对混凝土静力强度影响的研究进展

对环境水对混凝土静力强度影响的一系列试验研究成果进行了综述,分析了湿态混凝土静力强度降低的物理机理;指出现有的研究大多为定性的解释,定量的研究较少;提出应研究多因素相互作用对混凝土静力强度产生的影响,并采用有限元法来模拟湿态混凝土各个界面的连接耦合。     

Role of Local Flow Conditions in River Biofilm Colonization and Early Growth

Direct numerical simulations of a turbulent boundary layer flow over a bed of hemispheres of height h are performed using an immersed boundary method for comparison with river biofilm growth experiments performed in a hydraulic flume. Flow statistics above the substrates are shown to be in agreement with measurements performed by laser Doppler velocimetry and particle image velocimetry in the experiments. Numerical simulations give access to flow components inside the roughness sublayer, and biofilm colonization patterns found in the experiments are shown to be associated with low shear stress regions on the hemisphere surface. Two bed configurations, namely staggered and aligned configurations, lead to different colonization patterns because of differences in the local flow topology. Dependence with the Reynolds number of the biofilm distribution and accrual 7 days after inoculum is shown to be associated to local flow topology change and shear stress intensity. In particular, the shear stress τ on the surface of the hemispheres is found to scale as , where Re_t = u^*h/ν, with u^* as the log law friction velocity and ν as the fluid kinematic viscosity. This scaling is due to the development of boundary layers along the hemisphere surface. Associated with a critical shear stress for colonization and early growth, it explains the increasing delay in biomass accrual for increasing flow velocities in the experiments. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

基于惯性耗散法的峡口岬角地形能耗分析

珠江河口峡口岬角地形(或者称"门")是珠江河口典型的高能耗区,研究其消能机制,对于解释珠江河口宏观动力现象,提高对河口动力过程的认识有重要意义。本文设计了峡口岬角局部突变地形的湍流能耗特性物理模型试验,采用Son Tek 16MHz ADV采集了高频流速数据,统计了时均流速及湍流特征量。利用"惯性耗散法"计算了峡口岬角突变地形与明渠水流的湍流动能耗率。峡口岬角地形的突变特征产生明显的形态阻力,本文试验工况引起的紊动强度量值是明渠的2倍到10多倍,湍流剪切应力较明渠水流大近2个数量级,湍流动能是明渠水流的40多倍,湍流动能耗散率比明渠水流湍流动能耗散率大2~3个数量级。从湍流局部平衡及能量传递理论看,峡口局部形态阻力导致时均流速的空间梯度、切应力增大及形成大量更小尺度的涡是湍流能耗率增加2~3个量级的重要原因。     

Numerical study of the effect of a V-shaped weir on turbulence characteristics and velocity in V-weir fishways

Fishways are used to allow fish to migrate around water infrastructure, whether movements are in the upstream or downstream direction. Hydrodynamic conditions within various fishways, including turbulence levels, are important for successful fish passage. A numerical hydrodynamic study was conducted for V-weir fishways, which assist species migrating upstream. The variables investigated included, fishway slopes of 4%, 7%, and 10%, relative spacing between weirs of 1.3, 2.6, and 4 (D/L, ratio of distance between weirs to pool width), and weir angles of 22.5°, 40°, and 60°. Turbulence characteristics, including turbulent kinetic energy (TKE), Reynolds shear stress (RSS), turbulent intensity (TI), turbulent dissipation (TD), as well as maximum water velocity, were examined by CFD (computational fluid dynamics) simulation using a re-normalized group or RNG turbulence model. The CFD was calibrated with flow measurements made in a physical model of the V-weir fishway in laboratory control experiments. Based on inferences from fish passage literature, the results showed that, in the range of parameters studied, a weir angle of 22.5^°, a slope of 10% and relative spacing between weirs of 1.3 and 2.6 were assessed as offering the best potential performance for several species and sizes of fish, while the latter has lower cost as it requires fewer weirs. The V-weir fishway may be adaptable for smaller fish.