Collisions between elliptic vortex rings and inclined walls at key axis-switching stages

Collisions between Re=4000 elliptic vortex rings (EVRs) of aspect ratios AR=2 and 4 with 30° inclined walls have been studied experimentally using planar laser-induced fluorescence visualizations and time-resolved particle image velocimetry measurements. The inclined wall was located at key axis-switching locations of freely translating EVRs and effects of vortex ring orientation were studied as well. Similar to circular vortex rings colliding with inclined walls, the primary vortex cores collide with the inclined wall first (near-end primary vortex cores) and those collide with the wall later (far-end primary vortex cores) shrink and bulge in their physical sizes, respectively. Near-end secondary vortex cores entangle around the primary vortex cores in most cases, while the far-end secondary vortex cores show varied behaviors, such as formation and collision of secondary vortex loops. Changing the orientation of vortex ring mainly alters the secondary vortex core behaviors at the far-end and increasing the aspect ratio from 2 to 4 does not change the vortex dynamics fundamentally. Vortex core trajectories reveal that similar trajectories can be achieved by altering the collision locations and vortex ring orientations at the same time. Particularly, net vortex-stretching/compression effects on primary vortex cores result from combined effects arising from both axis-switching and wall inclination. The additional vortex-compression effects caused by the wall inclination at the far-end are varied at different collision locations. In contrast, the near-end primary vortex cores almost all experience additional stretching. Finally, hydrodynamic impulse resulted by the collisions is estimated and discussed as well.