Vortices get in a spin
Nov 7, 2000
Physicists have seen for the first time a vortex phenomenon that may shed light on the complex dynamics of many natural systems including hurricanes, ocean eddies and Jupiter's Great Red Spot. Dan Durkin and Joel Fajans, physicists at the University of California at Berkeley in the US, placed a small vortex inside a larger vortex. They noticed that the vortices create a hole that acts like a vortex spinning in the opposite direction (D Durkin and J Fajans 2000 Physical Review Letters 85 4052).
Ordinary experimental fluids are subject to effects like viscosity and can be difficult to manipulate. Durkin and Fajans therefore used a cylindrical column of magnetically confined electrons to simulate a perfect fluid. Electron density is equivalent to fluid vorticity, and this strongly magnetized electron column behaved as a vortex. The researchers used a photocathode to produce a region of high electron density - or vortex - in the cylinder of electrons.
The intense point-like vortex traced a circular clockwise path, spinning in the same direction as the larger vortex. As the small vortex circulates, it causes a wave to form on the perimeter of the larger vortex. The wave grows and produces a trailing filament. After several revolutions of the large disc, the filament rejoins the main body of the vortex, enclosing a "vorticity hole" which becomes incorporated into the weaker disc. The hole acts like a vortex spinning in the opposite direction. As it is pulled further into the disc by the point-like vortex, the motion of the whole system becomes chaotic.
Durkin and Fajans also experimented with other configurations. The pair introduced seven randomly placed intense vortices into a weaker vortex. The same phenomenon took place, but surprisingly, the vortices arranged themselves into the shape of a regular hexagon with one central vortex - a process known as crystallization.
"What still amazes me is how well three-dimensional electron columns model two-dimensional fluids", Durkin told PhysicsWeb. "The columns are essentially analogue computers that model 2D flows, and our ultimate goal is to simulate a planet's atmosphere."