带水平剪切的双扩散对流流动结构及输运特性

Double diffusive convection refers to the buoyancy-driven convection turbulence which is driven by two different scalar components simultaneously. Double diffusive convection is ubiquitous in many natural and engineering flow environments. In the Ocean, seawater density depends mainly on temperature and salinity, and double diffusive convection plays critical roles in the vertical mixing and transport. In the real oceanic environment, the vertical double diffusive convection usually affected by background large scalar shear which is induced by other mechanisms, such as internal waves. Thus, it is of great interests for science and engineering applications to study the double diffusive convection with external shear, especially on the critical flow structures and turbulent transport produced by them. In this project on double diffusive convection with shear, we will develop highly efficient numerical methods suitable for scalar field with large Schmidt number and shear turbulence and conduct systematical direct numerical simulations. Based on the dataset so obtained, we will identify the key flow structures in different flow regimes and their dynamic evolution. Theoretical model will also be developed to provide a parameterization of the turbulent transport properties. The project will focus on the dependences of the flow structures and turbulent transport on the control parameters, including the strength of the temperature and salinity gradients, and that of the external shear.

双扩散对流是指由两种不同组分共同驱动的浮力对流湍流，该流动现象广泛存在于众多的自然和工程流动中。由于海水密度主要由温度和盐度决定，双扩散对流是影响海洋中垂直混合及输运特性的关键机制之一。真实海洋环境中，垂直方向的双扩散对流往往受到其他因素导致的大尺度背景剪切流动的影响，如重力内波等等。对带剪切双扩散对流的研究，尤其是其中关键流动结构以及相应湍流输运特性的研究，具有重要的科学意义和应用价值。本项目将以带背景剪切的双扩散对流湍流为研究对象，发展针对大施密特数标量场和剪切湍流的高效并行数值计算程序，开展系统性的直接数值模拟研究。在数值模拟结果的基础上，明确不同流动状态下的关键流动结构及其演化行为，发展描述湍流输运特性的参数化模型。本项目将重点考察流动结构和湍流输运随温度、盐度梯度强度和背景剪切强度的变化规律。

温盐驱动双扩散湍流是海洋中小尺度湍流的重要物理机理，是影响海洋小尺度湍流混合输运的关键因素。同时双扩散过程不可避免地受到更大尺度上海洋流动导致的速度剪切等外加效应的影响。本项目主要针对海洋温盐驱动双扩散湍流开展直接数值模拟和理论分析，所得结果对理解和参数化描述海洋小尺度湍流混合输运规律有重要的科学意义和应用价值。项目取得的主要成果有：（1）发现低纬度海域盐指型双扩散温盐台阶在高流动参数区间的多态行为，揭示盐指界面的温盐传输由温盐梯度和洁面后动共同决定，提出描述盐指界面传输规律的幂律模型。（2）发现弱剪切使盐指结构的水平分布规则化，中等强度剪切则抑制热量和盐度的传输。（3）通过理论分析揭示均匀剪切作用下，高纬度海域扩散型双扩散湍流的非线性不稳定性，通过直接数值模拟揭示温盐台阶的演化和传输规律。（4）发现水平温盐梯度可在相邻深度水体中诱导水平分层流动现象，阐明水平温盐混合的一种新途径。（5）提出一种流动模型在较低瑞利数区间证明浮力湍流终极态理论的核心物理图景。在本项目的支持下发表SCI论文7篇，支持博士后1名，博士生4名，指导硕士毕业论文2篇。