Influence of Initial Shape and Oscillation Dynamics on the Aero breakup of Large Droplets

This study investigates the aerodynamic breakup behavior of freely falling, large water droplets exposed impulsively to high-speed air. Unlike smaller spherical droplets with well-characterized breakup dynamics, large droplets exhibit pronounced shape oscillations that significantly affect fragmentation. Released from a custom generator, droplets oscillate in the fundamental (2,0) mode, cycling through prolate, spherical, and oblate shapes. In a series of controlled experiments, we trigger the aerodynamic breakup at specific phases of this oscillation cycle, namely, when the droplet is prolate, oblate, or in the transitional stages between these shapes. High-speed imaging and the time-resolved measurement of droplet deformation reveals that the initial geometric shape of the droplet plays a crucial role in determining the breakup regime, leading to distinctly different fragmentation patterns compared to those of static spherical droplets. A key parameter is the ratio between the quarter-period oscillation time (T/4) and the aerodynamic breakup time, governed by relative air velocity. If breakup time exceeds T/4, internal circulation and shape evolution dominate; otherwise, aerodynamic forces prevail. Also, the large droplet size promotes cascade type breakup. These insights are particularly relevant to the formation of sprays involving large, shape-oscillating droplets and to the breakup of large non-spherical droplets, such as those released from firefighting aircraft.