Developments to Watch
FREEZE! IT'S THE VORTEX POLICE
WHIRLPOOLS--OR, MORE properly, vortices--can be a drag. In water and air, they slow down ships, planes, and cars. In magnetic form, they limit how much current can pass through a superconductor. But vortices are difficult to study in a typical flow because there are so many of them, all interacting. So a team at the University of California at Berkeley, led by physics professor Richard E. Packard, is creating vortices one at a time in a flow of "superfluid" helium--that is, helium cooled to less than two degrees above absolute zero so that it swirls forever without loss of energy from friction. And they're creating computer graphics that will help them understand the phenomenon.
In the June 19 issue of Physical Review Letters, the Berkeley team reported that horseshoe-shaped vortices sprang to life when the superfluid coming through a slit in a tiny box reached a critical velocity. The horseshoe vortex (in red) would then pass in front of the superfluid stream (purple arrow), gaining in size and energy as it sapped energy from the stream. Packard, who is funded by the Office of Naval Research and the National Science Foundation, has proposed building a superfluid-helium gyroscope to detect tiny changes in the earth's rotation rate.EDITED BY NEIL GROSS