Horizontal Convection at Large Rayleigh Number: Laboratory Experiments and Direct Numerical Simulation

大瑞利数水平对流：实验室实验和直接数值模拟

• 批准号: 1155558
• 负责人: Brian White
• 金额: $ 56.56万
• 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: 2012
• 资助国家: 美国
• 起止时间: 2012-06-15 至 2017-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1155558&HistoricalAwards=false
• 关键词: Horizontal; Convection; Large; Rayleigh; Number

Intellectual merit: Horizontal Convection (HC) has been used as a model to study the ocean Meridional Overturning Circulation. However, based on several influential works, the prevailing view in the oceanographic community is that HC cannot generate turbulence and is therefore unable to contribute energetically to the observed 2×10^15 W of poleward heat transport in the ocean. Based on these results, additional sources of abyssal ocean mixing (including even biological organisms) have been sought to explain the ∼2.1 TW thought to drive the MOC. However, recent results based on Available Potential Energy (APE) analysis, and this teams own Direct Numerical Simulations (DNS) are producing a surprising new picture of HC. It demonstrates fully-developed turbulence, despite an energy bound that proves energy dissipation goes to zero with viscosity (violating the first law of turbulence), and a mixing efficiency which approaches 1, much larger than the canonical value of 0.25 used to estimate the MOC energy requirement. These results suggest that HC may in fact be highly efficient at transporting heat, and leads to this fundamental study of the fluid dynamics of HC, strategically combining DNS and large-scale laboratory experiments in the UNC Interdisciplinary Fluids Lab stratified wave tank using Particle Image Velocimetry (PIV) and temperature-sensitive Laser-Induced Fluorescence (LIF). This approach will allow the detailed energetics of HC to be explored at Rayleigh numbers (forcing strength) much larger than previously possible. These tools will be used to explore (1) the connection between buoyancy forcing and mechanical energy input in maintaining HC, (2) the behavior of the mixing efficiency and whether it indeed approaches 1 at large Ra, (3) energetic pathways through which HC can be such an efficient mover of heat, (4) the nature of the seemingly paradoxical HC turbulence (with small dissipation but fully-developed), and (5) specific pathways through which mechanical mixing, driven for example by tidal flow over topography (simulated in the experiments), enters the energy budget, and affects the generation of APE. Detailed investigation of turbulent mixing and its spatial distribution will hopefully explain how HC is apparently such an efficient mechanism for heat transport.Broader impacts: The results of this study have the potential to inform our understanding of ocean circulation and to complement to ocean observations of temperature/salinity structure in the MOC. By informing our understanding of the MOC energy balance, the results may contribute to an improved understanding of the response of the ocean circulation to climate change and improved interpretation of circulation under past climatic regimes. Given its climate implications, this work is of potential interest to the general public and policymakers, and the PIs plan to share experimental demonstrations with the public, through K-12 outreach, the UNC Morehead Planetarium Summer programs, and the annual North Carolina Science Festival, showcasing science and technology in the state. They also plan to take advantage of the cyberinfrastructure resources of RENCI for data visualization and sharing, including use of the Social Computing Room, a 360 degree interactive display for HD projection, and the Teleimmersion Room, a 3D Stereoscopic room for data visualization to display and share the PIV and DNS visual data, to improve dissemination of results to the public, media outlets, and others in the scientific community. This work will support the training of one graduate student and one postdoctoral researcher. The UNC Marine Sciences-Applied Math Interdisciplinary Fluids Lab has a strong record of promoting undergraduate research, with many students regularly presenting work at national meetings, and the same is anticipated from the undergraduates funded by this project.

理论价值：水平对流（HC）已被用作研究海洋经向翻转环流的模式。然而，根据几项有影响的工作，海洋学界的普遍观点是，HC不能产生湍流，因此不能大力促进观测到的2×10^15 W的海洋向极地热输送。基于这些结果，深海混合的额外来源（甚至包括生物有机体）已经被寻求来解释被认为驱动MOC的2.1 TW。然而，最近基于有效势能（APE）分析的结果，以及该团队自己的直接数值模拟（DNS）产生了令人惊讶的HC新图像。它显示了充分发展的湍流，尽管能量界证明能量耗散随粘度趋于零（违反湍流第一定律），并且混合效率接近1，远远大于用于估计MOC能量需求的标准值0.25。这些结果表明，HC实际上可能是高效的传热，并导致了HC流体动力学的基础研究，战略地结合DNS和在UNC跨学科流体实验室分层波槽中使用粒子图像测速（PIV）和温度敏感激光诱导荧光（LIF）的大规模实验室实验。这种方法将允许在比以前可能大得多的瑞利数（强迫强度）下探索HC的详细能量学。这些工具将用于探索(1)维持HC的浮力强迫和机械能输入之间的联系，(2)混合效率的行为以及它是否确实在大Ra时接近1，(3)HC可以如此有效地移动热量的能量途径，(4)看似矛盾的HC湍流的性质（耗散小但充分发展），以及(5)机械混合的具体途径。例如，由地形上的潮汐流驱动（实验中模拟），进入能量收支，并影响APE的产生。对湍流混合及其空间分布的详细研究将有望解释为什么HC显然是一种如此有效的热传递机制。更广泛的影响：本研究的结果有可能为我们了解海洋环流提供信息，并补充MOC温度/盐度结构的海洋观测。通过对MOC能量平衡的理解，这些结果可能有助于提高对海洋环流对气候变化响应的认识，并改进对过去气候条件下海洋环流的解释。鉴于其对气候的影响，这项工作可能引起公众和政策制定者的兴趣，pi计划通过K-12外展、北卡罗来纳大学莫尔黑德天文馆夏季项目和一年一度的北卡罗莱纳州科学节，与公众分享实验演示，展示该州的科学和技术。他们还计划利用RENCI的网络基础设施资源进行数据可视化和共享，包括使用社会计算室（用于高清投影的360度交互式显示器）和远程沉浸室（用于数据可视化的3D立体室）来显示和共享PIV和DNS可视化数据，以改善结果向公众、媒体机构和科学界其他人士的传播。这项工作将支持培养1名研究生和1名博士后。北卡罗来纳大学海洋科学-应用数学跨学科流体实验室在促进本科生研究方面有着良好的记录，许多学生定期在国家会议上展示工作，预计由该项目资助的本科生也会如此。