Vortex within a vortex

Examples of two-dimensional fluid flows abound…for example in the ocean, atmosphere, and astrophysical settings…and vortex phenomena in such flows are many and varied. Recent numerical studies by Dezhe Jin and Dan Dubin (both at the University of California, San Diego) identified a new phenomenon not yet recognized in nature, that occurs when a small, intense, pointlike vortex is within a larger, disklike vortex (imagine a tornado within a hurricane), both spinning in the same direction. Now, Dan Durkin and Joel Fajans (both at the University of California, Berkeley) have studied the dynamics of such a system experimentally. They used a strongly magnetized electron column; under the right conditions, such a column behaves two-dimension-ally, evolving by the E × B force, and is fully equivalent to a 2D fluid vortex. Among their findings was that the point vortex can induce a surface wave on the outer edge of the disk vortex. Eventually the wave “breaks,” closing in on itself and capturing some “empty” space, which then behaves like a region of negative vorticity, spinning in the opposite direction. They also found that if multiple point vortices were initially arranged symmetrically within the disk vortex, that arrangement remained stable. In addition, if the initial distribution was random, it quickly crystallized into a stable symmetric pattern, as shown above. (D. Durkin, J. Fajans, Phys. Rev. Lett. 85, 4052, 2000 http://dx.doi.org/10.1103/PhysRevLett.85.4052 .)