Tempest in a Typhoon

According to recent numerical studies, a small, intense vortex placed inside a larger, weaker vortex (imagine a tornado inside a hurricane) may generate yet another vortex. It's a phenomenon that runs counter to the common notion that the number of vortices decreases as a turbulent system evolves over time.

Researchers at the University of California at Berkeley have experimentally confirmed the effect for the first time by simulating rotating fluids with columns of electrons trapped in strong magnetic fields. The researchers (Dan Durkin, 510-643-9702, durkin@socrates.berkeley.edu; Joel Fajans, 510- 642-3872, fajans1@socrates.berkeley.edu) found that a point vortex in a diffuse disk vortex, both spinning in the same direction, leads to a wave-like ripple on the disk's outer edge. Eventually, the ripple wraps around and closes in on itself, much as an ocean wave forms a closed tube as it races up to the shore (see movies at Physics News Graphics).

The space captured by the ripple represents a negative vorticity hole, in other words, a vortex spinning in the opposite direction to the disk and the initial point vortex. If the point vortex is sufficiently intense, it can pull the hole into the disk's interior (see figure at Physics News Graphics) where the two perform a chaotic dance. Under some conditions, a single vortex may spawn multiple negative vorticity holes.

Among other things, the effect might help explain the formation of fluid vortex crystals from random arrangements of point vortices. Negative vorticity holes allow point vortices to exchange energy with the diffuse disk, potentially permitting the vortices to settle into stable, crystal configurations.

The newly discovered vortex-in-a-vortex interaction is among the latest fluid phenomena simulated with strongly magnetized electron columns. Although the effect has not yet been observed in nature, it may be one of the factors that contribute to the dynamics of natural systems including hurricanes, ocean eddies, and Jupiter's Great Red Spot. (D. Durkin; J. Fajans, Physical Review Letters, 6 November 2000.)