Collaborative Research: Mechanisms of Droplet Generation by Breaking Wind Waves, Experiments and Numerical Simulations

这是一项结合实验和数值的研究，在表面活性剂和盐水存在的情况下，通过破风浪产生液滴。对风浪系统中水滴的平均空间分布和直径的测量已经在波浪池和现场进行了，但是波浪破碎动力学和水滴产生之间的联系尚不清楚，水的盐度和表面活性剂对这些产生过程的影响也不清楚。通过将破碎波和液滴的实验测量与相同破碎事件的数值模拟相结合，将每种研究方法的优势协同使用，探索破碎波中液滴产生机制的物理特性。质量、动量、能量和热量的海气传递是控制天气和全球气候的动力系统的重要组成部分，破波产生的液滴对这些传递过程做出了重要贡献。本文的研究结果对系统的数值模拟有一定的参考价值。它们也适用于理解涉及自由表面和喷雾产生的工程流程，例如在化学工程流程中。最后，研究中使用的液滴直径和轨迹测量系统被认为适合作为现场测量系统使用。该项目还将为学生的教育做出贡献，在夏季期间全额资助两名研究生，一名兼职博士后研究员和一名本科生。在马里兰大学，PI将组织一个为期一周的关于非线性水波的暑期学校。他还将邀请来自一个名为ThinkStem的中学项目的学生和老师参观他的实验室。在明尼苏达大学，PI将为来自美洲原住民社区的高中生组织一个拓展项目。该项目包括一个夏令营，旨在提高学生对五大湖水面环境流体流动过程的理解。除其他数量外，实验将提供破碎波剖面的时间演变的高分辨率测量，以及直径低至100微米或更小的液滴的大小和运动的类似分辨率测量，而模拟将以最先进的空间分辨率计算空气和水流场的演变。在实验和模拟中，断路器将由高度可重复的分散聚焦波包机械产生，并由风增强。在实验和模拟中，通过在水中引入可溶性表面活性剂和盐，探讨它们对破波和液滴产生的影响。实验和模拟都将识别破裂过程中重要液滴生成事件的时间、位置和特征。液滴生成事件的实验测量将用于为数值模拟提供数据和焦点，并为小于模拟可以计算的液滴提供数据。模拟将探索产生这些液滴喷射事件的物理过程，并开发和测试生成最小液滴的子网格模型。盐度和表面活性剂对每个液滴生成过程的影响将被检查。在这些实验/数值尺度上，盐度和表面活性剂对液滴产量影响的知识将用于估计一些油田规模条件下的液滴产量。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This is a combined experimental and numerical study of droplet generation by breaking wind waves in the presence of surfactants and salt water. Measurements of the average spatial distribution and diameters of droplets in wind wave systems have been made in wave tanks and in the field, but the connection between the dynamics of wave breaking and the generation of droplets is unclear as are the effects of water salinity and surfactants on these generation processes. By combining experimental measurements of the breakers and droplets with numerical simulations of the same breaking events, the strengths of each research method will be used synergistically to explore the physics of the droplet generation mechanisms in breaking waves. Air-sea transfer of mass, momentum, energy and heat is an important component of the dynamical system controlling the weather and global climate, and droplet generation by breaking waves makes an important contribution to these transfer processes. The results of this study will be useful for numerical models of the system. They will also be applicable to understanding engineering flows in which free surface and spray generation are involved, for example in chemical engineering flows. Finally, the droplet diameter and trajectory measurement system used in the proposed study is thought to be adaptable for use as a field measurement system. This project will also contribute to the education of students by fully supporting two graduate students, a half-time postdoctoral researcher and an undergraduate student during the summer months. At the University of Maryland, the PI will organize a week-long summer school on nonlinear water waves. He will also host students and teachers from a middle school program called ThinkStem in visits to his laboratory. At the University of Minnesota, the PI will organize and outreach program for high school students from Native American communities. The program consists of a summer camp aimed at enhancing the students' understanding of the environmental fluid flow processes at the water surfaces in the Great Lakes.The experiments will provide, among other quantities, highly resolved measurements of the temporal evolution of the breaking wave profiles and similarly resolved measurements of the size and motions of droplets with diameters down to 100 microns and smaller, while the simulations will compute the evolution of the air and water flow fields with state-of-the-art spatial resolution. In both the experiments and simulations, the breakers will be generated mechanically with highly repeatable dispersively focused wave packets and augmented by wind. By introducing soluble surfactants and salt into the water in the experiments and simulations, their effects on wave breaking and droplet generation will be explored. Both the experiments and simulations will identify the time, location and characteristics of the important droplet generation events during the breaking process. The experimental measurements of the droplet generation events will be used to provide data and focus for the numerical simulations and provide data for droplets smaller than the simulations can compute. The simulations will explore the physical processes that generate these droplet ejection events and develop and test sub-grid models for the generation of the smallest droplets. The effects of salinity and surfactants on each droplet generation process will be examined. The knowledge of the effects of salinity and surfactants on the droplet production at these experimental/numerical scales will be used to estimate droplet production at some field-scale conditions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.