Collaborative Research: A new understanding of droplet breakup: hydrodynamic instability under complex acceleration

The breakup of droplets is important in many engineering applications such as agricultural sprays, combustion of liquid fuels, and the impact of rain droplets on aircraft. The breakup of droplets in a gas occurs due to a difference in the gas and droplet velocities, acting on the droplet surface like waves on the ocean. At low speeds, droplet breakup occurs under relatively constant conditions. However, at high speeds droplet breakup is more complex, involving sudden changes in the gas velocity. In many high-speed applications, such as the impact of rain droplets on hypersonic aircraft, droplets will first experience a sudden increase in gas velocity driving rapid breakup, followed by rapid decrease in velocity, slowing and modifying the breakup process. In order to design advanced hypersonic vehicles, accurate and efficient models for these complex breakup processes are needed. This project will use a unique combination of experiments and simulations to examine droplet breakup under these complex conditions and develop a simple model for use in computer simulations. The project will also engage both graduate and undergraduate students in research and experimental design, training them in skills vital to our national security. The overall goal of this project is to significantly improve our understanding of droplet breakup under complex, variable accelerations by developing a general hydrodynamic instability-based model for droplet breakup. The project will test the central hypothesis: droplet breakup is dominated by a variable acceleration, three-layer Rayleigh Taylor instability. Simulations will fully resolve interfacial dynamics and study the growth rate of instabilities under varying acceleration histories. Experiments will image droplet breakup and child droplet sizes with high-speed fluorescence techniques under different acceleration histories. These measurements will be used to test a new breakup model incorporating variable acceleration hydrodynamic instability models, large-scale droplet deformation, and dynamic gas-droplet conditions. The breakup model will be useful in a predictive capacity in a wide range of applications such as blast mitigation in industrial processes, hypersonic weather impacts, and advanced detonation-based propulsion cycles. The model will make it possible to perform system-level simulations in these applications, enabling accurate and efficient modeling of droplet events. The broader impact of this work lies in enabling efficient simulation of high-speed multiphase problems of particular importance to energy efficiency and national defense and in engaging students of diverse backgrounds in multiphase studies and research.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.