On the dynamics of unconfined and confined vortex rings in dense suspensions

An experimental study of particle-vortex interactions has been undertaken in suspensions with volume fractions up to. Time-resolved particle image velocimetry measurements using a refractive index matching technique were performed to characterize the formation and evolution of vortex rings in both unconfined and confined configurations. It is shown that vortex rings in dense suspensions are more diffuse, which results in larger vortex cores and lower maximum vorticity. Furthermore, these vortex rings remain stable during their evolution, whereby the primary vortex breakdown and the formation of secondary vortices are inhibited. Although similar to vortex rings generated at lower Reynolds numbers in pure water, further results demonstrate that the vortex-ring circulation and non-dimensional vortex-core radius in dense suspensions remain higher than those in pure water at the same equivalent Reynolds number. Thus, the modification of vortex-ring behaviour in dense suspensions cannot be described solely through a variation in the effective viscosity. Finally, unlike in pure water, the confinement does not impact the non-dimensional vortex-core radius, vortex-ring circulation and maximum vorticity in dense suspensions. This unusual result demonstrates that the dynamics of vortex rings in dense suspensions are strongly insensitive to the effect of confinement.