Turbulent Exchanges, Energetics and Transports in Buoyant Gravity Currents

浮力重力流中的湍流交换、能量和传输

• 批准号: 1131621
• 负责人: Jonathan Nash
• 金额: $ 39.14万
• 依托单位: Oregon State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-10-01 至 2015-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1131621&HistoricalAwards=false
• 关键词: Turbulent; Exchanges; Energetics; Transports; Buoyant

Intellectual Merit: The front that bounds a tidal river plume represents a region of intense exchange between terrestrial- and oceanic-source waters. It propagates as a gravity current, with a turbulent rotor that both dissipates and radiates internal wave energy as it advects and diffuses freshwater into the coastal waters. White and Helfrich (2008) have outlined a framework in which the dissipative or radiative character of a steady front can be assessed from its density and velocity alone, contrasting the hypothesis that frontal deceleration was necessary for wave release (Nash and Moum, 2005). It is the objective of this project to detail the relationship between waves and turbulence using data from the Columbia River plume system, to test some of White and Helfrich?s (2008) ideas in the field, to connect these with laboratory and numerical experiments, and to extrapolate these to a plume-wide system,.The study focuses on 4 aspects of the plume system in which high-resolution turbulence and internal wave data have been collected but not fully analyzed:(1) Energetics and structure of the gravity current head: are details of the density/velocity structure behind the front sufficient to assess whether a front dissipates energy via turbulence or via radiated waves? And can the combination of dissipation, wave trapping and wave release account for the front?s observed evolution (growth and decay)?(2) How do these work to set mass transport and circulation patterns within the head? How/where are energy and mass transported during wave release?(3) Can mixing within the stratified shear flow behind the front be predicted a priori, and how important is it compared to that at the front, from a perspective of both energetics and cross- isopycnal transport?(4) Can (1-3) be related to the external parameters, such as river flow, tides, coastal currents?In addition to testing lab/numerics-inspired theoretical ideas about gravity current propagation, a complimentary goal is to assess which of the front or the trailing shear flow is responsible for the majority of the plume mixing. This is fundamental, as it dictates whether surface coastal waters or deeper fluid is entrained into the plume, which have important biogeochemical consequences. In addition, if energy is lost from the front via waves, that energy is not available for local mixing but may instead produce turbulence in regions remote from the plume itself.Broader Impacts: In addition to the analyses of Columbia River plume data outlined above, a second objective is to link recent fieldwork on plumes of different scale to each other, and to laboratory and numerical studies that are simultaneously being conducted. To do this, a 4-day workshop will be held with a focus on providing a flexible and adaptive forum for discussing recent results, exchanging analysis techniques, and collectively synthesizing studies and future directions. Given that there has been a significant recent interest in this area (with numerous current and recently-completed experiments), such a meeting is timely.This grant will support a PhD candidate at OSU, and provide opportunities for student exchanges among synergistic field/lab/numerical projects. Results will be broadly disseminated in scientific journals and made accessible to the public through the web and other media (newspaper articles, etc.)Finally, knowledge of the location and intensity of plume fronts and of large-amplitude waves is of significant interest to the marine community: (1) for navigation, these are a hazard due to velocity convergence and wave steepening, especially over the bar, (2) for fisherman, they represent active feeding grounds, (3) for pollution response, the front represents both a material boundary for surface contaminants, but also a conduit for exchange into deeper/coastal waters. It is hoped that these results will ultimately improve the skill of operational river-plume models.

智力优势：潮汐河流羽状流的边界代表了陆地和海洋沃茨之间强烈交换的区域。它作为重力流传播，具有湍流转子，当它将淡水平流和扩散到沿海沃茨时，既消散又辐射内部波能。白色和Helfrich（2008）已经概述了一个框架，在该框架中，可以仅从其密度和速度来评估稳定锋的耗散或辐射特性，这与锋面减速是波浪释放所必需的假设形成对比（Nash和Moum，2005）。这是本项目的目标，详细的波浪和湍流之间的关系，使用数据从哥伦比亚河羽系统，测试一些白色和Helfrich？s（2008）的观点，将其与实验室和数值实验相结合，并将其外推到一个羽状系统中，研究集中在羽状系统的4个方面，其中高分辨率的湍流和内波数据已经收集，但没有得到充分的分析：（1）重力流头的能量学和结构：锋面后面的密度/速度结构的细节是否足以评估锋面是通过湍流还是通过辐射波耗散能量？耗散、陷波和释波的结合能解释锋面吗？观察到的进化（增长和衰退）？(2)这些是如何在头部内设置质量运输和循环模式的？在波浪释放过程中，能量和质量是如何/在哪里传输的？(3)从能量学和交叉等密输运的角度来看，锋后的分层剪切流内的混合是否可以先验地预测，与锋前的混合相比，它有多重要？(4)（1-3）是否与外部参数（如河流流量、潮汐、沿岸流）有关？除了测试关于重力流传播的实验室/数值启发的理论想法外，一个补充的目标是评估哪一个前部或尾部剪切流是造成大部分羽流混合的原因。这一点至关重要，因为它决定了是否将表层沿海沃茨或更深的流体带入羽流，这会产生重要的地球化学后果。此外，如果能量是通过波从前面失去的，该能量是不可用的本地混合，而是可能会产生动荡的地区远离羽流本身。更广泛的影响：除了上述哥伦比亚河羽流数据的分析，第二个目标是连接最近的实地考察不同规模的羽流彼此，实验室和数值研究，同时进行。为此，将举行为期4天的研讨会，重点是提供一个灵活和适应性强的论坛，讨论最近的结果，交流分析技术，并集体综合研究和未来的方向。鉴于最近对这一领域的兴趣很大（有许多当前和最近完成的实验），这样的会议是及时的。这笔赠款将支持俄勒冈州立大学的博士候选人，并为协同领域/实验室/数值项目之间的学生交流提供机会。研究结果将在科学期刊上广泛传播，并通过网络和其他媒体（报纸文章等）向公众提供。最后，海洋界对羽流锋和大振幅波的位置和强度的了解非常感兴趣：（1）对于航行，由于速度收敛和波变陡，特别是在沙洲上，这些是危险的，（2）对于渔民，它们代表活跃的觅食地，（3）对于污染响应，锋代表表面污染物的物质边界，而且也是交换到更深/沿海沃茨的管道。希望这些结果将最终提高业务河流羽流模型的技能。