On the effect of axial separation in axis-offset collisions of vortex rings

This study examines the dynamics of axis-offset collisions between two vortex rings using particle image velocimetry (PIV) measurements.At a fixed Reynolds number, the influence of axial separation on the flow evolution is systematically investigated.Finite-time Lyapunov exponent (FTLE) fields and combined vorticity-velocity analyses reveal three distinct interaction regimes.(1) In the collision and radial outflow regime ($H/R_{\mathrm{ring}}\leq~0.893$, where $H$ is the axial separation distance and $R_{\mathrm{ring}}$ is the ring radius), vortex rings undergo axial compression and generate strong radial outflows, with negligible material exchange between oppositely signed vortices.(2) In the collision and rebound regime ($H/R_{\mathrm{ring}}=~1.786$), the rings—bearing the same sign—experience a near-elastic rebound, leading to disintegration without reconnection.(3) In the merging-splitting and recovery regime ($H/R_{\mathrm{ring}}=~2.679$), the rings exhibit transient merging followed by splitting and coherence recovery, accompanied by significant material exchange.For even larger offsets ($H/R_{\mathrm{ring}}=~3.572$), interactions are negligible.These findings categorize axis-offset vortex ring interactions into three regimes based on flow behavior and yield insights relevant to enhanced mixing and heat dissipation in engineering applications.