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/Rring⩽ 0.893, whereHis the axial separation distance andRringis 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/Rring= 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/Rring= 2.679), the rings exhibit transient merging followed by splitting and coherence recovery, accompanied by significant material exchange. For even larger offsets (H/Rring= 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.