On Aerodynamic Droplet Breakup

The breakup of droplets in a high-speed air stream is investigated experimentally and theoretically. This study is based on experiments conducted by exposing pendant droplets to a high-speed air jet, which allowed for imaging of the droplets at high temporal and spatial resolutions while maintaining a large field of view to capture the breakup process. Two factors of primary importance in droplet breakup are the breakup morphology and the resulting child droplet sizes. However, benchmark models have erroneous assumptions that impede the prediction of both morphology and breakup size and give non-physical geometries. The present theoretical work focuses on the 'internal flow' hypothesis, which suggests that the droplet's internal flow governs its breakup. The breakup process is subdivided into four stages: droplet deformation, rim formation, rim expansion and rim breakup. The internal flow mechanism is used to mathematically model the first two stages. The rim expansion is related to the growth of bags, while its breakup is shown to be due to Rayleigh-Plateau capillary instability. It is found that the breakup morphology is related to the division of the droplet into a disk that forms on the windward face and the undeformed droplet core. The amount of the droplet volume contained in the undeformed core decides the breakup morphology. Using this framework, it is shown that a three-step calculation can give an improved prediction of child drop sizes and morphology.