Interaction of droplets with vortical structures is ubiquitous in nature ranging from raindrops to gas turbine combustors. In this work, we try to elucidate a comprehensive understanding of the mechanism of interaction of a droplet with a co-axial vortex ring. We have focussed on the droplet and vortex dynamics that evolve in Spatio-temporal fashion during different stages of the interaction. Experimental techniques such as high-speed particle image velocimetry measurement (PIV), planner laser fluorescence imaging, and high-speed shadowgraphy are used in this work. We have covered the Reynolds number range (Rec = 5135-18095) between laminar and turbulent regimes of the vortex ring. In droplet dynamics, we have identified different regimes of interaction, including deformation, stretching, engulfment and breakup. We were able to explain each of the interaction regimes using analytical models which closely matched the experimental data. In vortex dynamics, we have compared the effect of the interaction on different characteristics of vortex rings, such as pressure distribution, vorticity distribution, circulation strength, total energy, and total enstrophy. It was found that interaction (during stretching and engulfment) with the droplet leads to a decrease in convection speed, circulation strength, maximum pressure difference, total energy and total enstrophy of the vortex ring.
We believe that this study is crucial for the fundamental understanding of the vortex-droplet interaction phenomena. It can also play a vital role in designing various practical devices (injector in gas turbine and IC engines, blade in under-water drilling, wings and propeller blades in aircraft) and forecasting (shift in temperature, humidity, and aerosol concentration of clouds due to stirring and mixing of droplets caused by turbulence, loss in strength and energy of tropical cyclone, tornado or dust devil due to presence of droplets).