Laboratory and Numerical Experiments on Ocean-scale Turbulent Stratified Mixing

海洋尺度湍流分层混合的室内和数值实验

• 批准号: 1736989
• 负责人: Alberto Scotti
• 金额: $ 69.01万
• 依托单位: University of North Carolina at Chapel Hill
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-08-15 至 2023-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1736989&HistoricalAwards=false
• 关键词: Laboratory; Numerical; Experiments; Ocean; scale

This project will use laboratory experiments and numerical modelling techniques to identify the mechanism by which waters in the surface of the ocean are mixed with heavier deeper waters through a process that is called overturning. Water in the world oceans is arranged in nearly-horizontal layers of density increasing with depth. Because of the difference in density, transfer of material between layers (i.e., lifting of denser water and sinking of lighter water) requires an external source of energy and the whole process is called mixing. There is considerable interest in understanding mixing in stratified bodies of water. This process is slow; it is estimated that it takes about 1,000 years to completely overturn the ocean. Yet, since the deep ocean holds large stores of heat and carbon dioxide, the overturning process is important as it regulates climate on the planet over millennial and longer time scales, and thus of great societal interest.Stratified mixing requires a source of energy (stirring), and the aim of this project is to characterize the efficiency of the process. To understand the concept of efficiency, it is convenient to think by analogy to an automobile. The gas burned in the engine releases a certain amount of energy, but only a fraction of it goes into moving the drivetrain. The rest is lost as heat. Likewise, only a fraction of the energy provided by the stirring mechanism goes into doing work against gravity, the rest being dissipated as frictional heat. Direct observations of ocean mixing with increasingly high resolution using, for example, acoustic sensors and fast temperature sensors, have shown surprising structures in shear-driven turbulence that suggest mixing at ocean scales may behave differently than small-scale laboratory measurements have suggested. This project addresses these issues through the study of turbulent mixing in carefully controlled laboratory experiments at scales relevant to oceanic flows and the use of numerical simulations. The experimental component is conducted in the UNC Joint Fluid Lab tank, a 36m long facility in which stratified shear flows with high buoyancy Reynolds number can be generated. This ensures a clear separation between the Kolmogorov and Ozmidov scales. The experiments are designed to establish the behavior of the flux Richardson number (a measure of efficiency), in the limit of high buoyancy Reynolds number and finite gradient Richardson number. The data acquired in the lab are used to train, via data-assimilation, a suite of increasingly complex numerical models, with the final goal of creating tools that can be used to diagnose efficiency from ocean data and to improve the parameterization of stratified mixing in global models.

该项目将使用实验室实验和数值建模技术来确定海洋表面水中的水与更深的深水混合的机制，这一过程称为倾覆。世界上的水是在近乎度量的密度层中排列的，随着深度的增加。由于密度的差异，层之间材料的转移（即，浓水和较轻的水的下沉）需要外部能量来源，整个过程称为混合。有很大的兴趣了解分层水体中的混合。这个过程很慢；据估计，完全推翻海洋大约需要1000年的时间。然而，由于深海拥有大量的热量和二氧化碳存储，因此倾覆的过程很重要，因为它在千禧一代和更长的时间尺度上调节了地球上的气候，因此具有极大的社会兴趣。街道化的混合需要一个能量来源（搅拌），并且该项目的目的是表征该过程的效率。要了解效率的概念，可以用与汽车类似的方式思考很方便。发动机中燃烧的气体释放了一定量的能量，但其中只有一小部分用于移动传动系统。其余的是热量。同样，只有搅拌机制提供的能量的一部分才能对重力进行起作用，其余的作为摩擦热。直接观察海洋混合，使用越来越高的分辨率使用，例如声音传感器和快速温度传感器，在剪切驱动的湍流中显示出令人惊讶的结构，这些结构表明，在海洋尺度上混合的表现可能与小规模实验室测量的表现不同。该项目通过研究与海洋流量相关的尺度和使用数值模拟的尺度的湍流混合研究来解决这些问题。实验组件是在UNC关节液实验室储罐中进行的，该储罐长36m，其中可以产生具有高浮力雷诺数的分层剪切流。这样可以确保Kolmogorov和Ozmidov尺度之间的明显分离。该实验旨在建立通量Richardson数字的行为（效率的量度），在高浮力雷诺数和有限梯度理查森数字的极限下。实验室中获取的数据用于通过数据指示训练越来越复杂的数值模型的套件，其最终目标是创建工具，可用于诊断海洋数据的效率，并改善全球模型中分层混合的参数化。