Collaborative Research: Turbulence Enhanced Droplet Growth by Collision-Coalescence

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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.