Collaborative Research: Droplet transport in the vicinity of breaking waves: Experiments and simulations

准确预测海洋表面的喷雾和气溶胶的产生对于包括天气和气候预测在内的各种应用都是必不可少的。本研究旨在表征和量化海洋破碎表面波周围湍流气流中的液滴输送。设计了一种独特的实验与数值相结合的方法。在特拉华大学的海气相互作用实验室，可控的、可重复的破碎波事件将被重新产生，不同大小的液滴浓度和速度的测量将被用来直接估计产生的通量。在圣母大学（University of Notre Dame），测量的波形和液滴产生速率被用作大型涡流模拟的输入，这些模拟被配置为重现实验室条件。液滴统计数据可以直接在实验和模拟之间进行比较，模拟将为研究动量和热力学交换率提供一种手段，并将其扩展到现场条件。数值天气和气候模式需要对海洋气溶胶源通量进行精确的参数化，只有在产生后立即了解液滴的传输才能忠实地改进这些参数化。本研究结果将用于培养2名博士研究生和从事STEM学科的本科生。研究结果还将通过利用研究材料进行教育推广工作，包括可视化，传播给广泛的受众。该项目提供了湍流波边界层中液滴动力学的前所未有的观点。几乎所有对喷雾产生率的实地观测都是根据固定高度的浓度来推断通量的，而这一过程需要对海洋边界层最低区域液滴的湍流输送作出假设。对于小液滴，许多假设都成立，但对于大液滴——那些最有可能改变空气-海洋热量和动量通量的液滴——诸如液滴惯性、破碎波后的流动分离以及首选的波相对喷射位置等因素可能违反假设。该研究计划有几个具体目标：(i)进行受控的实验室实验，为数值模拟提供信息，并对统计数据和物料平衡进行一对一的比较；（ii）在实验和模拟中直接计算大小分辨的液滴垂直通量，以揭示表面波附近的非均匀和间歇性湍流的影响；(3)利用大涡模拟确定液滴的产生/沉积速度并扩展到现实条件。关键的结果将包括修改不同大小液滴的通量分布关系，以及更好地估计喷雾引起的热量、动量和水分的通量。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。

The accurate prediction of spray and aerosol generation at the ocean surface is essential for a wide variety of applications including weather and climate predictions. This research aims at characterizing and quantifying droplet transport in the turbulent airflow around breaking surface waves in the ocean. A unique combined experimental and numerical approach is designed. At the air-sea interaction laboratory at the University of Delaware, controlled, repeatable breaking wave events will be re-produced, and measurements of droplet concentration and velocity for different droplet sizes will be used to directly estimate production fluxes. At the University of Notre Dame, measured waveforms and droplet production rates are used as inputs into large eddy simulations configured to recreate laboratory conditions. Droplet statistics are directly compared between experiments and simulations, and the simulations will provide a means for investigating momentum and thermodynamic exchange rates, as well as upscaling to field conditions. Numerical weather and climate models require accurate parameterizations of marine aerosol source fluxes, and only by understanding droplet transport immediately after production can these be faithfully improved. The results of this study will be used for training 2 PhD students and for engaging undergraduate students in STEM disciplines. The results will also be disseminated to a wide audience through the utilization of the research material for educational outreach efforts, including visualizations.The project provides an unprecedented view of droplet dynamics in the turbulent wave boundary layer. Virtually all field observations of spray production rates infer fluxes based on fixed-height concentrations, and this process requires making assumptions regarding the turbulent transport of droplets in the lowest regions of the marine boundary layer. For small droplets, many of these assumptions hold, but for large droplets - those with the largest potential for altering air-sea heat and momentum fluxes - factors such as droplet inertia, flow separation behind breaking waves, and preferred wave-relative ejection locations likely violate assumptions. The research plan is aimed at several specific objectives: (i) performing controlled laboratory experiments which inform numerical simulations, and making one-to-one comparisons of statistics and bulk balances; (ii) directly compute size-resolved vertical fluxes of droplets in both experiments and simulations, in order to reveal the influence of inhomogeneous and intermittent turbulence in the vicinity of surface waves; and (iii) determine the production/deposition velocity of droplets and expand to realistic conditions using the large eddy simulations. The key outcomes will include modified flux-profile relationships of droplets of varying size, and better-informed estimates of spray-induced fluxes of heat, momentum, and moisture.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.