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: 1235039
• 负责人: Andres Tejada-Martinez
• 金额: $ 32.41万
• 依托单位: University of South Florida
• 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=1235039&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.

本研究的首要目标是表征小尺度朗缪尔湍流对海-气界面下的冷分子边界层（即冷皮肤）温度和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，这些短的毛细波作为广谱波的一部分与长波共存。风驱动的波和剪切流之间的相互作用导致朗缪尔湍流，其特征在于朗缪尔环流（LC）由大致沿风向排列的反向旋转的涡流组成。涡旋的典型长度范围从最初出现短毛细波时的几厘米到波谱变宽时的几十米。厘米尺度的LC在风场的阵风中非常迅速地产生（并且也非常迅速地消失），从而在间歇性和重复性阵风事件中在表面处提供非常强烈的湍流突发，其可以非常好地支配平均表面更新过程。这些更新事件对我们理解海气标量通量至关重要。到目前为止，直接数值模拟（DNS）的小尺度LC及其演变还没有进行和测量的表面更新时间尺度和其他参数的影响，这些相干结构还没有进行。因此，这项研究，基于细尺度DNS计算沿着与高分辨率的实验室实验，将捕获LC的生长阶段和过渡到朗缪尔湍流在波老化过程中。仿真验证与实验将解决以下主要问题：1。在小尺度LC发展的早期阶段，当风切变在波轨道速度上占主导地位时，小尺度LC对标量海气通量的影响是什么？2.微破波对小尺度液晶结构有什么影响？3.小型LC和微型破碎机对海面分子层的累积效应是什么？4.随着液晶和表面波光谱的加宽，液晶的结构是什么？在液晶发展的后期，液晶对分子层的影响是什么？实验将集中在小规模LC的产生和演变。他们将瞄准1。评估小尺度朗缪尔环流的结构、演化和稳定性。2.评估这些结构对冷表皮的影响，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值，用于与计算结果进行比较。预计LC将通过增强表面更新机制来影响冷皮肤温度和气体浓度分子层，该机制通常在冷皮肤和气体传输速度（跨越海-气界面的气体传输效率的测量）的参数化中调用。因此，DNS解算器配备有界面捕获技术，产生解析的表面分子层，并精确解析剧烈的自由表面运动。这些实验将包括粒子图像测速和主动红外辐射测量法，对次表层和表层运动学进行直接估计。这种数值/实验相结合的方法可能会导致新的物理见解的精细尺度过程控制的空气-海洋分子通量的热量和气体。更广泛的影响：广泛的影响，将通过加强对精细尺度物理学的基本理解，表征海洋冷皮肤和气体传输的气-海界面在LC的存在。众所周知，LC在海洋表面出现和消失很快，在全球海气标量通量中起着重要的作用。了解LC的影响将使科学家能够制定更好的全球海洋通量吸收二氧化碳等温室气体的参数。此外，所获得的结果将有助于科学家根据卫星红外测量对海洋整体温度进行估计，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，并酌情通过更受欢迎的出版渠道传播。在地方一级，通过这项研究工作获得的知识将纳入学生的教育和培训。PI将继续努力，通过实验室访问和向公众提供信息科学讲座，向更广泛的受众（K-12和公众）推广科学和研究。