分层环境中浮力射流与界面动力结构作用机制研究

Thermal stratified oceans, large lakes and reservoirs are usually receiving water bodies for pollutant discharge. Emission of pollutants can penetrate into the thermocline and induce the interfacial waves, generating a double diffusion condition of temperature and concentration, which can significantly influence the jet mixing process and entrainment features. In this project, the theories of internal waves and double diffusion problems are used to generalize the nearshore emission to buoyant jets in thermal stratified environments. Using the combined methods of physical experiments, numerical simulation, field monitoring and theoretical analysis, the mechanisms of interfacial waves and temperature-concentration double diffusion induced by buoyant jets in stratified flows will be systematically investigated. The goal is to reveal the laws of generation, transport, growing and diluting process of interfacial structures such as internal waves and salt fingers, which can contribute to develop the technology improving the dilution of pollutant. Xiangxi Bay in the Three Gorges Reservoir, a representative tributary with seasonal stratification, will be taken as a typical study area. A hydrodynamic-temperature-concentration coupled numerical model will be established and calibrated against the field monitoring data such as temperature, pollution concentration and velocity. Based on the numerical model, characteristics of pollutant dilution in stratified environment can be explored. The results can provide theoretical basis and technical support for accurate prediction of nearshore emission and improvement of the water environment by interfacial dynamics.

温度分层的海洋及大型湖库常被作为近岸排放的受纳水体。排放体携带污染物在温度跃层中激发界面波，并形成温度-浓度双流体扩散条件，显著改变排放体在环境水体中的掺混与稀释特性。本项目拟运用海洋密度分层流研究中的内波及双扩散理论，将近岸排放问题概化为分层水体中的浮力射流问题，采用物模试验、数模计算、现场监测和理论分析相结合的方法，对分层水体中浮射流数学模型的建立，射流激发界面内波的产生与破碎，淡水环境下双扩散的形成条件，温度-浓度双扩散中盐指等界面动力结构的生长及消亡过程，能量特性及对界面通量的影响等开展研究，揭示界面动力结构对污染物稀释度的影响机理。在此基础上，依托季节性分层的三峡水库香溪河库湾，建立温度-浓度-水动力耦合模型，考虑内波及双扩散效应，借助温度、浓度等监测资料，探索工程尺度分层水体中排放体掺混与输移规律。为准确预测近岸排放效果、利用界面动力机制改善水环境提供理论依据和技术支持。

密度分层界面受到各类外力作用时将形成内波、盐指等复杂的界面动力结构，可显著改变界面的物质输移特性，以及上下层水体间的掺混与稀释特性。本项目针对水-水和水-沙密度分层界面，采用数模计算、物理试验、理论分析等方法，系统研究了分层界面上流动结构形态和物质输移规律。主要研究内容、重要结果如下：.1、在水-水分层流体系统中：（1）研发了剪切流环境中双扩散数值模型，系统模拟了剪切流中盐指的产生、生长、发展过程，揭示了不同强度剪切对盐指生长过程及界面垂向通量的影响。（2）针对密度二分层与连续分层，研究了污染物排放所引发浮射流与密度跃层的相互作用规律，包括射流的上升高度、扩散范围等特性。（3）实现并改进了传统的积分模型，获得了浮射流与环境流体、密度跃层的掺混、卷吸特征，以及射流质量、动量和浮力通量的分布规律。.2、在水-沙分层流体系统中：（1）提出并实现了“统一域”数值模拟方法，实现了界面上下的“流体-多孔介质”复合域中双扩散对流系统的流场、温度和盐度强耦合求解。（2）模拟并揭示了流体层-多孔介质层界面处盐指的动力结构特征和物质输移规律。（3）数值刻画了密度分层流中内波与不规则床面障碍物的相互作用，内波环境下床面泥沙起动的判别条件与模拟方法。（4）模拟并揭示了界面内孤波对桩基的泥沙冲淤规律。.关键数据：二分层环境下，射流无量纲扩散范围关于Frp呈幂函数衰减，Frp大于7.0后无量纲突破高度趋近于1.85，径向入侵区半径和厚度则分别趋于4.5和0.4。无量纲波-坡系数小于1.55时坡波为强相互作用、介于1.55与2.07时为中等掺混作用、2.07与14.02之间为弱相互作用。波-坡系数达2.15或障碍物波-坡系数大于1.0时，内波起动坡面泥沙的概率将显著提升。.科学意义：提出了密度分层内波条件下泥沙起动的判别方法，补充了传统河流动力学关于泥沙起动的基础理论；分层流界面动力结构特征与规律成果，对完善大体积水体及复杂工程应用背景下水生态环境理论方法起到科学支撑作用。