Collaborative Research: Turbulence Enhanced Droplet Growth by Collision-Coalescence

地球上很大一部分降水是通过云滴的碰撞-合并形成的，但人们对自然云中这一过程的速度知之甚少。目前，相互作用的液滴的湍流碰撞-合并的定量预测能力有限，因为在自然云物理的背景下，湍流-液滴和液滴-液滴的相互作用尚未通过实验或数值完全解决。这项研究将结合新的实验和计算技术，以解决这些相互作用的规模，水滴大小为10微米。通过在实验室风洞中应用显微两相粒子图像测速(PIV)系统，将以分辨率精确到液滴的长度和时间尺度来测量气流、液滴的空间分布以及液滴的大小和速度。该PIV系统辅之以液滴尺寸分布演变和单液滴统计的相位多普勒测量，将能够确定湍流液滴碰撞核以及液滴动力学收集方程中的每一项。实验研究的结果将与最近开发的混合计算方法的模拟结果进行比较。还将结合实验结果开发一种不同的、完全分辨的模拟技术，其中在非常接近液滴表面的狭窄区域使用解析的斯托克斯流动解，以更好地处理液滴诱导的扰动流动中的短程相互作用和非线性。这些计算方法也将有助于实验系统的开发和实验结果的分析。该项目的一个成果将是更好地了解液滴在湍流、空气动力相互作用、重力和液滴惯性的综合作用下运动和碰撞的物理基础。另一个将是与降雨开始相关的尺寸范围内的液滴的经验证的湍流碰撞核。这项研究的更广泛影响将是更好地了解天然云中的降水发展，以及在数值天气预报和气候模式中更好地表示云的微物理过程。通过开发的先进方法，该项目还将影响大气科学和工程的其他领域，如对天气和气候的间接气溶胶效应、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中招募，指导和鼓励他们从事研究事业。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所机构的独特资源。

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.