Flow transitions of head-on vortex ring collisions with contaminated air–water interfaces

Abstract

An experimental study was conducted on head-on collisions of Re = 2000 and 4000 vortex rings upon air–water interfaces to study the vortex dynamics and the effects of different vortex ring Reynolds numbers on the key vortex flow mechanisms. Unfiltered tap water was used where surface contaminants were present; hence, the interfacial stress levels are lower but not entirely zero like an idealized free surface. Results demonstrate that the vortex dynamics involve first, the resulting secondary and tertiary vortex rings transitioning into wavy states, before their upper loops disconnect/reconnect to the interface to form U-shaped vortex loops along the inner and outer peripheries of the primary vortex ring, respectively, in an alternating pattern. Second, tertiary vortex loops entangle around the primary vortex ring to produce counter-rotating vortex pairs that reorganize themselves along the primary vortex ring outer periphery, between the primary vortex ring and secondary vortex loops, as well as hairpin-like structures that aid ejection of primary vortex ring momentum. Third, secondary vortex loops rotate toward the collision axis before their lower segments are entrained by the primary vortex ring. A higher Reynolds number primary vortex ring would confer additional flow changes, such as a higher wave number for the secondary and tertiary vortex rings/loops, pairings of secondary vortex loops “side-arms” that reduce their instances by about half and formations of Tsai–Widnall–Moore–Saffman instabilities induced by flow perturbations. Finally, vortex flow models proposed to explain the flow mechanism at different flow stages are found to be in good agreements with the experimental visualizations.