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

学术价值：水平对流（HC）已被用作研究海洋经向翻转环流的模式。然而，根据几项有影响力的研究，海洋学界的主流观点是HC不能产生湍流，因此不能对海洋中观测到的2×10^15 W的向极热输送做出能量贡献。基于这些结果，人们寻找了深海海洋混合（甚至包括生物有机体）的其他来源，以解释被认为驱动MOC的#8764;2.1 TW。然而，最近基于可用势能（APE）分析的结果，以及该团队自己的直接数值模拟（DNS）正在产生令人惊讶的HC新图片。它证明了充分发展的湍流，尽管能量边界证明能量耗散随粘度变为零（违反湍流第一定律），并且混合效率接近1，远大于用于估计MOC能量需求的规范值0.25。这些结果表明，HC实际上可能是非常有效的传热，并导致HC的流体动力学的基础研究，战略性地结合DNS和大规模的实验室实验，在跨学科流体实验室分层波槽使用粒子图像测速（PIV）和温度敏感的激光诱导荧光（LIF）。这种方法将允许详细的能量HC在瑞利数（强迫强度）比以前可能的大得多的探索。这些工具将被用来探索（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在大Ra时它是否确实接近1，（3）HC可以成为如此有效的热移动者的能量路径，（4）看似矛盾的HC湍流的性质（具有小的耗散但充分发展），以及（5）机械混合通过其进入能量收支并影响APE的产生的特定途径，所述机械混合例如由地形上的潮汐流驱动（在实验中模拟）。 湍流混合及其空间分布的详细调查将有望解释HC是如何显然是这样一个有效的机制热transfer. The影响更广泛：这项研究的结果有可能告知我们的海洋环流的理解，并补充海洋观测的温度/盐度结构的MOC。通过告知我们对MOC能量平衡的理解，这些结果可能有助于更好地理解海洋环流对气候变化的响应，并改善对过去气候制度下环流的解释。鉴于其对气候的影响，这项工作对公众和政策制定者具有潜在的兴趣，PI计划通过K-12外展活动，Morehead天文馆夏季计划和一年一度的北卡罗来纳州科学节与公众分享实验演示，展示该州的科学和技术。他们还计划利用国家计算机信息网络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（360度高清投影互动显示）和远程沉浸室（3D立体数据可视化室），以显示和共享PIV和DNS可视化数据，以改善向公众、媒体、和科学界的其他人。这项工作将支持一名研究生和一名博士后研究员的培训。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，许多学生定期在全国会议上展示工作，并且预计该项目资助的本科生也会如此。

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.