Naoki Izutsu

Cross-linking of two vortex rings

The mechanism for cross‐linking two vortex rings has been investigated experimentally. Phase‐locked velocity measurements using x‐type hot wires were carried out point by point over the entire flow field, and time‐dependent vorticity fields were educed. It is found that a new pair of vortex tubes is created, which connects the interacting vortex rings and grows gradually, while the vorticity intensity of the main rings decreases. This is considered to be a type of bridging phenomenon of the vortex tubes, as proposed by Kida and Takaoka [Phys. Fluids 30, 2911 (1987)] in their numerical simulation.

Bifurcation of an elliptic vortex ring

Time-dependent vorticity fields of elliptic vortex rings of aspect ratios 2, 3 and 4 were measured by means of hot-wire anemometry. The time evolution of their vorticity fields was analyzed and the processes of vortex ring formation, advection, interaction and decay, and the mechanism of vortex bifurcation are studied. The following crosslinking model is proposed: A thick vortical region composed of many equivalent vortex filaments with distributed cores is initially formed at the orifice and they behave as inviscid filaments. The elliptic ring deforms and the end parts of its major axis get closer. Then, the vortex filaments interact at the touching point and the ring partially bifurcates. Almost simultaneously, turbulent spot appears at this point, and propagates around the ring cross section, thus preventing further bifurcation. And it becomes a turbulent blob. This model is also supported by numerical simulation by a high-order vortex method and the Navier-Stokes solution.

Bridging phenomenon in crosslinking of vortex rings.

Bridging phenomenon in crosslinking two vortex rings has been studied. Phase locked measurements of velocity using X-type hot wire were carried out over the whole flow field trasversing point by point, and time dependent vorticity fields were educed. Creation of new pair of vortex tubes was found in the interacting rtex rings.