Experimental and Analytical Study on Water Quality Transfer Induced by Wind Shear Stress in Stratified Water Areas

对层状水域表面风切应力引起的水流和水质输运进行了实验和分析研究。在许多封闭水域，如湖泊、水库、内陆海湾等，都存在这种情况，密度分层在流体运动中起重要作用，风切变应力是水团运输的主要驱动力。从水圈环境的角度出发，迫切需要构建基于水动力学理论背景的环境评价工具。为了满足工程需求，本研究项目组织如下三个主题：(1)风场特性及风驱动环流形成机理的野外和数值研究；(2)风应力产生的切变层湍流结构的实验和LES模拟；(3)湾区流场的三维模拟。主要结果如下。(1)在山围大坝顶部进行了更多的风速和风向测量，以探讨其作为水库流体运动驱动力的性质。研究发现，水库周边地区的风向受当地地形影响较大，与附近平坦地区气象站观测到的风向基本无关，而该地区的风速与气象站观测到的风速有较好的相关性。由此，建议在利用风数据理论上再现水域风力流体运动之前，需要仔细检查风场。采用k-epsilon湍流模式研究了风切变驱动下的垂直环流和混合。通过分析，发现了底水上涌量、跨温跃层夹带速率和边界混合强度对密度结构和表面剪应力的依赖关系，这将是构建水质预测工具的第一步。(2)在温跃层界面发育的风致切变层是层状水体垂直水质交换的主要机制。为了建立一个理论模型，能够描述像风驱动流这样的剪切主导流中的湍流，需要找到它的动力学。采用超声摄像机和PIV图像处理方法对剪切层进行了速度测量。在那里发现了一些有组织的湍流结构的层次结构，这与在壁面湍流中观察到的非常相似。LES在再现流动结构方面表现较好；结果与实验结果几乎完全一致。研究了风驱动流场中剪切层湍流对水质转移的促进作用。(3)在大阪湾和东京湾进行了水动力试验和数值研究。特别是东京湾，由于缺氧底水上涌，造成了许多水质问题，给沿岸的渔业养殖场带来了严重的破坏。通过室内实验研究了底水上涌的机理。上升流速率对风速和密度结构的函数关系。三维模型也被参数化，以重现两个海湾的风驱动流。模型性能相当令人满意；模拟的流场与现场数据吻合较好。预期该模型可实际应用于评估在海湾及沿岸地区进行一些建筑工程后的环境变化。最后报告于1996年5月出版并送交日本水利工程机构。少

Experimental and analytical works are carried out to examine flow and water quality transport induced by wind shear stress acting on the surface of stratified water areas. Such a situation is found in many enclosed water areas such as lakes, reservoirs, land-locked bays, where density stratification plays an important roles in fluid motions and the wind shear stress is a major driving force of water mass transport. From a view point of hydrosphere environment it is urged to construct an environmental assessment tool based on a theoretical background of hydrodynamics. In order to meet engineering demands, this research project is organized so as to cover the following three themes ; (1) field and numerical works on wind field properties and formation mechanism of wind-driven circulation, (2) experimental and LES simulation on turbulent structure of shear layr generated by wind stress, (3) three dimensional modeling of flow field in bay areas. The major results are followed.(1) A filed m … More easurement on wind velocity and direction is carried out at the top of a dam surrounded by mountains to investigate its properties as a driving force of reservoir fluid motions. It is found the wind direction is profoundly affected by the local terrain around the reservoir and is almost independent on the wind direction observed at a meteorological station located in a nearby flat area, while the wind speed there has a good correlation with data from the station. From this, it is suggested one need to carefully examine wind fields before theoretically reproducing wind-driven fluid motions in water areas by using wind data. A k-epsilon turbulence modeling is performed to examine the vertical circulation and mixing driven by the wind shear. Through the analysis, it is found how amount of bottom water upwelling, entrainment rate across the thermocline and the boundary mixing intensity depend on the density structure as well on the surface shear stress, which would be a first step of construction of water quality prediction tools.(2) It is known that the wind-driven shear layr developing at the thermocline interface is a major mechanism being responsible for vertical water quality exchange in a stratified water body. For formulating a theoretical model being able to describe turbulence in a shear-dominant flow like wind-driven flows, it is required to find its dynamics. A velocity measurement of shear layr is carried out by using a ultrasonic video camera and PIV image processing method. Some hierarchy of organized turbulent structures are found there, which are quite similar to those observed in wall turbulence. LES performance in reproducing flow structure is quite well ; the result are in almost complete agreement with the experiment. It is investigated how the shear-layr turbulence promotes water quality transfer in wind-driven flow fields.(3) Experimental and numerical investigation are carried out to get hydrodynamics in Osaka and Tokyo Bays. Especially in Tokyo Bay, many water quality troubles arise due to upwelling of anoxic bottom water, which brings serious damages to fishery farms along the coast. A laboratory experiment is carried out to examine mechanism of bottom water upwelling. A functional relationship of the upwelling rate on wind velocity and density structure. A three dimensional model is also parameterized in order to reproduce wind-driven currents in the two bays. Model performance is quite satisfactory ; the simulated flow fields are in good agreement with the field data. It is expected that the model could be of practical use in assessing environmental changes after some construction works in bays and coastal areas.The final report is published and delivered to Japanese institutions of hydraulic engineering in May 1996. Less