Bag film rupture mechanisms in droplet fragmentation under uniform airflows

The fragmentation of a droplet under aerodynamic forces induced by the surrounding flow is a fundamental phenomenon crucial to various spray applications. Bag breakup represents a fragmentation regime characterized by the formation of a forward-facing, hollow “bag” or thin film around the droplet, which ultimately ruptures, resulting in the generation of multiple smaller fragments. We perform three-dimensional numerical simulations to investigate the rupture of the liquid film, referred to as the “bag film,” which forms during the bag breakup of a droplet. Three different combinations of Reynolds and Weber numbers are used to examine the impact of the wake gas flow structures on bag film breakup. The temporal evolution of the bag's length and diameter agrees very well with previous results in the literature. The shape of the wake flow structures is found to vary with Reynolds numbers. The gas enstrophy on the leeward side of the bag film indicates that film rupture is associated with a rise in enstrophy. Just prior to breakup, an attached vortical structure is observed at the film interface at the rupture location. This structure induces localized pressure fluctuations within the film, which subsequently lead to the formation of weak spots that precede hole formation. Using a critical thickness, independent of grid size, to artificially perforate the bag film allows for an accurate representation of the formed holes and the subsequent holes merging. The droplet size distribution, following the complete fragmentation of the bag film, exhibits a lognormal distribution, which closely aligns with experimental results.