An experimental study on the oscillation dynamics of wind-driven droplets at the verge of shedding

An experimental investigation was conducted to explore the dynamics of droplet oscillation for wind-driven droplets. Droplet profiles and velocity fields in the symmetry plane of the droplets were measured using time-resolved particle image velocimetry (PIV) and high-speed imaging techniques. These measurements enabled analysis of the droplet oscillation dynamics. The eigenfrequencies of the droplet oscillation, velocity fluctuations in droplet wake, and the natural oscillation were measured. It was found that droplet oscillation is a self-excited oscillation rather than a vortex-induced oscillation. Due to the self-excited oscillation, the eigenfrequency of wind-driven droplets coincides with the eigenfrequency of the natural oscillation. Furthermore, the self-excited oscillation leads to periodic velocity fluctuations in the shear layer, resulting in a frequency that closely matches that of the droplet oscillation. Velocity fluctuations diminish with the cessation of droplet oscillation for high-viscosity droplets. Based on the oscillation characteristics, a dynamic model of droplet oscillation was developed to address the research gap. The oscillation characteristics of the droplet centroid predicted by the developed model are consistent with the experimental results.