ON THE ROLE OF KELVIN-HELMHOLTZ-LIKE INSTABILITY IN THE FORMATION OF TURBULENT VORTEX RINGS

It is well known that when fluid is ejected at a high Reynolds number through a nozzle, a turbulent vortex ring is formed almost immediately . To date, it remains unclear how turbulence is initiated so quickly i nto a ring. In a recent study, Glezer [Phys. Fluids A. 31 (1988) 3532] noticed that, during the formation of turbulent vortex ring, the cyli ndrical vortex sheet leaving the nozzle developed a Kelvin-Helmholtz-l ike instability. He went on to postulate that the disturbance introduc ed by the instability (henceforth referred to as secondary vortex ring s) can accelerate the onset, amplification and breakdown to turbulence of the azimuthal core. But the exact mechanism which brings about the early transition was not fully explained. In this paper, it is shown through a systematic experimental investigation that although Kelvin-H elmholtz-like instability plays an important role in initiating the tr ansition process, it is the leapfrogging phenomenon between the primar y and secondary Vortex rings which is responsible for hastening the de velopment of azimuthal bending waves. This factor, coupled with the mi salignment of the vortex rings during the leapfrogging is instrumental in producing fine-scale structures in the flow thus causing vortex ri ng to become turbulent. A model showing the process leading to the for mation of a turbulent Vortex ring is proposed.