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

知识优势：水平对流(HC)已被用作研究海洋子午线翻转环流的模式。然而，根据一些有影响力的著作，海洋学界的普遍观点是，HC不能产生湍流，因此不能对海洋中观测到的2×10^15W的极地热输送做出能量贡献。基于这些结果，深海混合(甚至包括生物有机体)的其他来源已经被寻找来解释被认为驱动MOC的2.1TW。然而，最近基于可用势能(APE)分析的结果，以及这个团队自己的直接数值模拟(DNS)，正在产生一幅令人惊讶的HC新图景。它显示了充分发展的湍流，尽管能量界限证明能量耗散随粘性为零(违反湍流第一定律)，混合效率接近1，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实际上在传递热量方面可能是高效的，并导致了对HC流体动力学的这一基础研究，战略性地结合了DNS和使用粒子图像测速仪(PIV)和温度敏感激光诱导荧光(LIF)在UNC跨学科流体实验室分层波槽中的大规模实验室实验。这种方法将允许在比以前大得多的瑞利数(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它在大Ra时是否确实接近1，(3)HC可以如此有效地传递热量的能量途径，(4)看似自相矛盾的HC湍流的性质(耗散小但完全发展)，以及(5)机械混合进入能量收支并影响APE生成的具体途径，例如由地形上的潮汐流驱动的(在实验中模拟的)。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量传输机制。广泛的影响：这项研究的结果可能有助于我们了解海洋环流，并补充海洋对MOC温度/盐度结构的观测。通过对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，并改进对过去气候制度下环流的解释。鉴于这项工作对气候的影响，公众和政策制定者可能会对这项工作感兴趣，PI计划通过K-12外联活动、北卡罗来纳州莫尔黑德天文馆夏季项目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度互动显示器)和远程沉浸室(用于数据可视化的3D立体视觉室)来显示和共享PIV和DNS可视数据，以改进向公众、媒体和科学界其他人传播结果的工作。这项工作将支持培养一名研究生和一名博士后研究员。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，受该项目资助的本科生也有望如此。

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.