浮水植被水流水力及纵向混合特性研究

Floating vegetation is an important component of the vegetation restoration and ecological environment construction in ecological rivers and constructed wetlands. Under the combined action of floating vegetation on the upper fast moving water body by providing additional drag force, and the bed roughness on the bottom free water, the open channel flow with floating vegetation has the comprehensive feature of open channel flow, porous media flow, pipe flow and boudary layer flow, and the hydraulic and solute transport characteristics are very complex. Laboratory measurement combined with theoretical analysis will be adopted to investigate the hydraulic and mixing characteristics for open channel flow with floating vegetation. The flow structure with varied hydraulic conditions, vegetation type and distribution, and channel bed roughness will be measured to seek the interaction among them, and the mechanism of momentum and mass transmission will be investigated. The production, development and action range of the shear vortex generated around the interface of vegetation and free water, the wake in the vegetation layer will be systematically analyzed to investigate the inner physical mechanism. Based on that, a set of hydraulic parameters used to describe the flow characteristics will be deduced, and the former analytical model for the vertical distribution of streamwise velocity will be improved. Meanwhile, tracer mixing measurement will be conducted to reveal the combined influence of the vegetaion, water body and boundary roughness on the longitudinal mixing, and an analytical model for the longitudinal mixing will be presented. Finally, the characteristics of longitudinal mixing will be analyzed quantificationally and non-dimensional type of longitudinal mixing coefficient will be presented. The research findings can provide theoretical basis and technical support for the designing and operation of ecological river and constructed wetland planted with floating vegetation, and hydraulic and water quality parameters for numerical and analytical modelling.

浮水植物是生态河道、人工湿地中植被恢复与生态环境建设的重要组成部分。浮水植被以额外拖曳力的方式作用于上层快速水体，底壁粗糙直接作用于下层自由水体，水流兼具明渠流、多孔介质流、管道流和边界层流的特点，水力和混合特性十分复杂。本项目拟采用实验室实验结合理论分析的方法，通过观测不同水力条件、植被类型及分布、底壁粗糙组合下的浮水植被水流的流动特性，系统研究植被-水体交界面上剪切涡以及茎秆尺度尾涡的运动规律及作用范围，从力学机制上揭示浮水植被、水体和底壁粗糙间的交互作用机理及对动量和质量传输的影响；分析建立描述浮水植被水流特性的水力参数体系，完善纵向流速垂向分布的分析解模型；开展示踪实验研究浮水植被水流中的纵向混合特性，建立纵向混合的分析解模型，并分析建立描述纵向混合特性的准数关系。研究成果可为浮水植被型生态渠道、人工湿地的设计和优化运行提供技术参数，为此类水体的水流、水质模拟提供模型参数。

漂浮植被广泛地存在于人造浮岛、湿地、天然河道以及湖泊当中，是河道景观和生态修复工程的重要组成部分，合理的配置漂浮植被不仅能够改善水质、维护生物物种的多样性，还能改善周边居民的生活环境。较沉水、挺水植被水流而言，漂浮植被水流的流动特性非常复杂，不同的区域可表现为多孔介质流动、明渠流动、管道流动和边界层流动的特性。以往研究多采用探入水中一定深度的刚性圆柱体来模拟漂浮植被以研究其水流运动特性，而对根系不发达的浮毯型植被盖形式的漂浮植被水流运动特性研究较少。本项目采用仿真荷叶模拟不规则植被斑块或浮毯型植被盖，综合考虑植被、水体、风应力以及底壁的交互作用，采用理论推导与实验室试验相结合的方法研究漂浮植被水流的水动力学特性。.1. 针对薄层漂浮植被水流运动的时均和紊动特性进行了系统的试验研究和理论分析。从零切应力和最大切应力的位置、雷诺应力各象限主导流态、扩散通量等角度，揭示植被叶片间隙及植被斑块形态对水流结构的影响机制。..2. 系统分析了薄层漂浮植被对封闭水域静水风生流和明渠动水风生流水流结构的影响规律。从流速分布特性、紊动输移特性以及能量传输机制等方面，探究漂浮植被群和植被斑块对风生流流速切变规律的影响，并揭示漂浮植被对风生波的削减机理。..3. 基于边界层理论计算了薄层漂浮植被在明渠水流中受到的拖曳力，建立了其与纵向流速、植被区长度雷诺数和植被区高度雷诺数的关系，丰富了薄层漂浮植被水流的拖曳力系数的计算理论。..4. 将薄层漂浮植被水流分为植被主导层和床面主导层，引入双幂函数建立了薄层漂浮植被纵向流速和雷诺应力的解析解模型，相比较基于双层假定的双对数模型而言，双幂模型在最大流速处流速梯度是连续的，且函数中各项参数具有明确的物理意义。