Effects of initial ellipsoidal geometry on droplet deformation and unsteady drag in the bag-breakup regime

The transient deformation of droplets in gas flows is strongly affected by the initial droplet geometry, whereas most previous studies have focused on initially spherical droplets. In this study, large-eddy simulation (LES) coupled with the volume-of-fluid (VOF) method was used to investigate the deformation of initially prolate spheroidal, i.e., ellipsoidal, droplets in the bag-breakup regime, with initial aspect ratios ranging from 1 to 5 and inclination angles ranging from 0° to 90°. The results show that ellipsoidal droplets generally follow a deformation sequence similar to that of spherical droplets, evolving from a disk-like structure toward a bag-like configuration, while the detailed morphology, breakup timing, and breakup mode remain strongly dependent on the initial geometry. A unified piecewise correlation is developed for the evolution of the equivalent windward-face diameter and is found to describe both spherical and ellipsoidal cases reasonably well over the present parameter range. Increasing the initial aspect ratio suppresses the growth of the equivalent windward-face diameter, whereas decreasing the inclination angle enhances it and may promote a transition from bag breakup to bag–stamen breakup. The initial geometry also alters the wake structure and pressure distribution, thereby affecting the unsteady drag response: larger aspect ratios tend to reduce the peak drag level, whereas the inclination angle mainly affects the early drag oscillation through wake asymmetry. Overall, these findings clarify the role of initial nonspherical geometry in droplet deformation and aerodynamic response under bag-breakup conditions, and provide a physical basis for future modeling of droplets with non-spherical initial shapes.