Collaborative Research: Turbulence Enhanced Droplet Growth by Collision-Coalescence

地球上相当一部分的降水是通过云滴的碰撞合并而形成的，但人们对自然云中这一过程的速率知之甚少。预测相互作用的液滴的湍流碰撞-聚结的定量能力目前是有限的，因为在自然云物理学的背景下的湍流-液滴和液滴-液滴相互作用还没有完全解决实验或数值。这项研究将结合联合收割机新的实验和计算技术，以解决这些相互作用的规模下降到10微米的液滴大小。气流，液滴的空间分布，液滴的大小和速度将测量分辨率下降到液滴的长度和时间尺度，通过应用显微镜，两相粒子图像测速（PIV）系统在实验室风洞。该PIV系统，辅以相位多普勒测量液滴尺寸分布和单液滴统计的演变，将允许确定湍流液滴碰撞内核，以及液滴动力学收集方程中的每个项。从实验研究的结果将进行比较，从最近开发的混合计算方法的模拟。一个不同的，完全解决的模拟技术，其中分析斯托克斯流的解决方案是在一个狭窄的区域非常接近液滴表面，也将开发结合实验结果，以更好地解决短程相互作用和液滴引起的扰动流的非线性。这些计算方法也将有助于实验系统的开发和支持实验结果的分析。 该项目的成果之一将是更好地理解湍流、空气动力学相互作用、重力和液滴惯性综合作用下液滴运动和碰撞的物理基础。另一个将是一个有效的湍流碰撞内核的水滴的大小范围内有关的雨开始。这项研究的更广泛的影响将是更好地了解自然云中的降水发展，并在数值天气预报和气候模式中更好地代表云微物理过程。该项目通过开发的先进方法，还将影响大气科学和工程的其他领域，如气溶胶对天气和气候的间接影响，喷雾燃烧，粉末生产和工业排放。研究生和本科生将被招募，特别是从代表性不足的群体，指导和鼓励追求研究事业。他们将享受独特的教育体验，并将充分利用参与这项研究的三个机构的独特资源。

A significant fraction of precipitation on Earth develops through the collision-coalescence of cloud droplets, yet the rate of this process in natural clouds is poorly understood. Quantitative capabilities for prediction of turbulent collision-coalescence of interacting droplets are currently limited, as turbulence-droplet and droplet-droplet interactions in the context of natural cloud physics have not been fully resolved experimentally or numerically. This research will combine novel experimental and computational techniques in order to resolve these interactions down to the scale of droplet sizes of 10 micrometers. Airflow, droplet spatial distribution, and the sizes and velocities of droplets will be measured with resolution down to the droplet's length and time scales, by applying a microscopic, two-phase particle image velocimetry (PIV) system in a laboratory wind tunnel. This PIV system, supplemented by Phase Doppler measurements of the evolution of droplet size distribution and single-droplet statistics, will allow determination of the turbulent droplet collision kernel, as well as each term in the droplet kinetic collection equation. Results from the experimental investigation will be compared to simulations from a recently developed hybrid computational method. A different, fully resolved simulation technique, in which an analytical Stokes flow solution is used in a narrow region very close to the droplet surface, will also be developed in combination with the experimental results, to better address short-range interactions and nonlinearities in droplet-induced disturbance flows. These computational methods also will aid in the development of the experimental system and support the analysis of the experimental results. One outcome of the project will be a better understanding of the physics underlying the motion and collisions of droplets under the combined effects of turbulence, aerodynamic interaction, gravity, and droplet inertia. Another will be a validated turbulent collision kernel for droplets in the size range relevant to rain initiation. The broader impacts of this research will be better understanding of precipitation development in natural clouds, and better representation of cloud microphysical processes in numerical weather prediction and climate models. This project, through the advanced methods developed, will also impact other areas of atmospheric science and engineering such as indirect aerosol effects on weather and climate, spray combustion, powder production, and industrial emissions. Graduate and undergraduate students will be recruited, particularly from underrepresented groups, mentored and encouraged to pursue research careers. They will enjoy a unique educational experience and will take full advantage of unique resources at the three institutions involved in this study.