漂浮植被作用下的水流动力特征研究

Floating vegetations are becoming one of the optimal choices of pollutant prevention and control in city water body due to its advantages of growing more quickly, absorbing nutrient matter faster, higher efficiency for decontamination, easier to harvest as well as beautifying the environment. However, planting the floating vegetations in water body will change flow characteristics by the means of increasing the flow resistance and decreasing the exchange rate of solar and wind energy through the water surface. One of the objects of the present proposal is to study the density flow nearby the edge of vegetation mat, which is resulted from the temperature difference between the water under the mat and open area due to the shades formed by floating vegetations. Taking the water hyacinth as studying object, we will measure the water temperatures, solar radiations and flow velocities in both designed tank experiments and field observations. With the measured data, the shading coefficient will be determined and the processes of formation, development and disappearance of density flow will be visualized and investigated. Another object of the proposal is to develop a numerical model for simulating hydrodynamic processes responsible by floating vegetation on water surface, including flow resistance, wind-driven flow and density flow. With the model, the combined influence of floating vegetation on flow will be investigated and the action of hydrodynamic processes in different conditions will be compared. When finished, the research will provide strong technical supports for utilization and management of floating vegetation in water pollution prevention and control.

漂浮植被具有生长迅速、吸收营养物质快、去污效率高、易于收割且能美化环境的优点，逐渐成为城市水污染治理的首选方案之一。同时，漂浮植被的存在将增大流动阻力，减缓水面的能量传递，从而改变了水体的流动特征。由于漂浮植被的遮光作用，照射到水面的太阳辐射将大幅度减弱，引起覆盖区和敞水区间的水温差，致使覆盖区边缘附近出现密度异重流。针对该问题，本项目选择水葫芦为研究对象，通过现场观测和水槽试验，确定不同生长期的遮光系数，测定不同时间的水温差异和异重流的发生、发展和消失的过程，揭示密度异重流的产生机理和变化规律；在此基础上，结合流动阻力和风生流的研究成果，建立漂浮植被水流模拟的数值模型，分析漂浮植被对水动力过程的综合影响，探究各物理过程在不同条件下的相对作用，为水污染治理中漂浮植被的使用和管理提供技术支撑。

漂浮植被水流在自然水体中营造了各种各样的微环境，促进了于营养物质输移，是生态系统安全的重要环节。课题通过太阳辐射、气温和水温的测量，探究了植被阴影对太阳辐射的遮蔽作用，分析了气温和水温对遮蔽作用的响应规律。针对植被混合层对植被尾流的影响，建立了漂浮植被阻力分布的计算方法。由于温度差异，漂浮植被覆盖区和畅水区间存在异重流；这种流动缓慢，总体处于层流状态；但异重流头部水体紊动强烈，属于紊流。故此，选取既能处理高雷诺数，又能处理低雷诺数的RNG k-epsilon模型模拟这类流动。同时，在紊流模型控制方程中补充了源项以考虑植被的影响；与其他紊流模型不同的是，采用植被水流紊动能的测量数据，模化了紊动能耗散率的源项，使得紊流模型能很好地模拟紊流特性和时均流动。数值模拟以OpenFOAM为平台进行了开发。利用模型，研究了温差异重流的发生发展过程，并发现了植被层和植被下层间的对流现象。课题加深了漂浮植被水流水动力特性的认识，对于水环境的管理提供支持。