Interaction of a vortex ring with a buoyant spherical particle: effects of particle size on vorticity dynamics and particle dynamics

The present work experimentally investigates the interaction of a buoyant (rigid) spherical particle with a single translating (water) vortex ring, focusing on the effects of particle-to-vortex core size ratio ( Dₚ/Dc,o ) on both the particle dynamics and ring dynamics ( Dₚ = particle diameter, Dc,o = vortex core diameter). These interactions are studied for Dₚ/Dc,o = 0.6–1.7, over ring Reynolds numbers ( Re= /ν ; = ring circulation) of 6000–67 300. As the buoyant particle comes close to the ring, it gets captured into the low-pressure vortex core, and the interaction begins. The particle within the core undergoes radial oscillation, spins and translates along the ring’s azimuthal axis. As Dₚ/Dc,o increases, the particle undergoes higher-amplitude radial oscillation and a relatively shorter azimuthal translation. The differences in the particle size and its motion within the ring lead to large differences in the ring’s dynamics. A larger particle is seen to lead to a higher ring disruption, substantially reducing the ring’s convection speed and azimuthal enstrophy, which are seen to scale as (Dₚ/Dc,o)2.3Re-0.37 and (Dₚ/Dc,o)1.3Re-0.25 , respectively. The ring disruption is significant above Dₚ/Dc,o≈ 1.0, beyond which the ring fragments, with up to 60 % drop in convection speed and 90 % drop in enstrophy, at low $Re$ , as compared with the base ring. These results for the rigid particle size effects on the vortex ring dynamics are more dramatic than for a deforming bubble. Our results could help to better understand and model buoyant particle (and bubble) interactions with coherent structures in turbulence.