Collaborative Research: DNS and high resolution measurements of scalar transfer across an air-water interface during inception and growth of Langmuir circulation

合作研究：朗缪尔环流起始和增长过程中空气-水界面标量传递的 DNS 和高分辨率测量

• 批准号: 1233808
• 负责人: Fabrice Veron
• 金额: $ 35.4万
• 依托单位: University of Delaware
• 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: 2012-09-15 至 2017-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1233808&HistoricalAwards=false
• 关键词: Collaborative; Research; DNS; resolution; measurements

The overarching goal of this study is to characterize the effect of small scale Langmuir turbulence on the temperature of the commonly occurring cool thermal molecular boundary layer (i.e. the cool skin) beneath the air-sea interface and on slightly soluble gas exchange rate across the interface. When wind blows over an initially quiescent air-sea interface, it first generates short capillary waves which in time coexist with longer waves as part of a broad spectrum of waves. The interaction between the wind-driven waves and shear current leads to Langmuir turbulence characterized by Langmuir circulation (LC) consisting of counter rotating vortices roughly aligned in the direction of the wind. The typical length scale of the vortices ranges from several centimeters when short capillary waves first appear up to tens of meters when the spectrum of waves broadens. The centimeter-scale LC are generated very quickly with the gustiness in the wind field (and disappear very quickly as well), thereby providing, over intermittent and repeated gust events, very intense turbulent bursts at the surface which may very well dominate the average surface renewal processes. These renewal events are critical to our understanding of air-sea scalar fluxes. To date, direct numerical simulations (DNS) of small scale LC and its evolution has not been made and measurements of surface renewal time scales and other parameters influenced by these coherent structures have not been performed. Accordingly, this study, based on fine-scale DNS computations along with high resolution laboratory experiments, will capture the growth stages of LC and transition to Langmuir turbulence during the wave aging process. Simulations validated with the experiments will address the following major questions: 1. What is the influence of small scale LC on scalar air-sea fluxes during the early stages of LC development when wind shear is dominant over wave orbital velocities? 2. What is the impact of micro-breaking waves on the structure of small scale LC? 3. What is the cumulative effect of small scale LC and micro-breakers on sea surface molecular layers? 4. What is the structure of LC and the effect of LC on molecular layers during later stages of LC development as LC and surface wave spectra broaden? The experiments will focus on the generation and evolution of small scale LC. They will aim at 1. Assessing the structure, evolution, and stability of small scale Langmuir circulations. 2. Evaluating the impact of these structures on the cool skin with potential influence on the air-sea heat and gas flux. 3. Collecting high quality dynamics measurements within the water column for comparison with the computations. It is anticipated that the LC will impact the cool skin temperature and gas concentration molecular layer by enhancing the surface renewal mechanism, often invoked in parameterizations of the cool skin and gas transfer velocity (a measure of gas transfer efficiency across the air-sea interface). Accordingly, the DNS solver to be used is equipped with an interface capturing technique yielding resolved surface molecular layers, and accurate resolution of violent free-surface motions. The experiments will include PIV and active infrared radiometry yielding direct estimates of sub-surface and surface kinematics. This combined numerical/experimental approach will likely lead to new physical insights into the fine scale processes which control the air-sea molecular fluxes of heat and gas. Broader Impacts: Broad impacts will be made through enhanced fundamental understanding of the fine-scale physics characterizing the ocean cool skin and gas transfer across the air-sea interface in the presence of LC. LC is known to appear and disappear quickly at the ocean surface and plays an important role in global air-sea scalar fluxes. Understanding the influence of LC would allow scientists to develop improved parameterizations of global ocean flux uptake of greenhouse gases such as CO2. Furthermore, results obtained would benefit scientists making estimates of bulk ocean temperatures based on satellite infrared measurements while having to account for the cool skin. Results from this research will be disseminated in journal publications and conferences, and, where appropriate, more popular avenues of publication. At a local level, knowledge gained through this research effort will be incorporated into the education and training of students. The PIs will continue their efforts to promote science and research to a broader audience (K-12 and public) through laboratory visits and informational scientific talks to the public.

本研究的总体目标是表征小规模朗缪尔湍流对气海界面下方常见的冷热分子边界层（即冷表层）温度以及界面上微溶气体交换率的影响。当风吹过最初静止的海气界面时，它首先产生短的毛细波，这些波及时与较长的波共存，作为广谱波的一部分。风驱动波和剪切流之间的相互作用导致朗缪尔湍流，其特征是朗缪尔环流（LC），由大致与风向对齐的反向旋转涡流组成。涡旋的典型长度范围从短毛细波首次出现时的几厘米到波谱变宽时的数十米。厘米级的 LC 随风场中的阵风快速产生（并且也很快消失），从而在间歇性和重复性阵风事件中，在表面提供非常强烈的湍流爆发，这很可能主导平均表面更新过程。这些更新事件对于我们理解海气标量通量至关重要。迄今为止，尚未对小尺度液晶及其演化进行直接数值模拟（DNS），也尚未对表面更新时间尺度和受这些相干结构影响的其他参数进行测量。因此，这项研究基于精细尺度 DNS 计算和高分辨率实验室实验，将捕获 LC 的生长阶段以及波老化过程中向 Langmuir 湍流的转变。通过实验验证的模拟将解决以下主要问题： 1. 在 LC 发展的早期阶段，当风切变对波轨道速度占主导地位时，小规模 LC 对标量海气通量有何影响？ 2. 微破碎波对小尺度液晶结构有何影响？ 3. 小型LC和微破碎机对海面分子层的累积效应是什么？ 4. 液晶的结构是什么，以及在液晶发展的后期，随着液晶和表面波光谱的拓宽，液晶对分子层的影响是什么？实验将集中于小规模液晶的产生和演变。他们的目标是 1. 评估小规模朗缪尔环流的结构、演化和稳定性。 2. 评估这些结构对冷表层的影响以及对海气热量和气体通量的潜在影响。 3. 收集水柱内的高质量动态测量结果，以便与计算进行比较。预计 LC 将通过增强表面更新机制来影响冷表层温度和气体浓度分子层，表面更新机制通常在冷表层和气体传输速度（跨气海界面的气体传输效率的测量）的参数化中调用。因此，所使用的 DNS 解算器配备了界面捕获技术，可产生解析的表面分子层，并精确解析剧烈的自由表面运动。实验将包括 PIV 和主动红外辐射测量，从而直接估计地下和表面运动学。这种数值/实验相结合的方法可能会对控制热和气体的气海分子通量的精细尺度过程产生新的物理见解。更广泛的影响：通过增强对精细尺度物理的基本理解，将产生更广泛的影响，这些精细尺度物理表征了在液晶存在的情况下海洋冷表层和气体在海气界面上的传输。众所周知，LC 在海洋表面快速出现和消失，并且在全球海气标量通量中发挥着重要作用。了解LC的影响将使科学家能够改进全球海洋通量吸收二氧化碳等温室气体的参数化。此外，获得的结果将有利于科学家根据卫星红外测量来估计总体海洋温度，同时必须考虑到凉爽的皮肤。这项研究的结果将在期刊出版物和会议上传播，并在适当情况下通过更受欢迎的出版途径传播。在地方层面，通过这项研究工作获得的知识将被纳入学生的教育和培训中。 PI 将继续努力通过实验室参观和向公众进行信息科学讲座，向更广泛的受众（K-12 和公众）推广科学研究。