From the humble swirling of bathwater down the drain to the violent storm on Jupiter, vortices thrive by sucking the energy from smaller ones, report researchers. Such vortex cannibalism contradicts the common view that vortices grow via benevolent mergers. But it confirms a famous hypothesis and could help explain how energy moves through the atmosphere and the oceans.
In the 1970s, Robert Kraichnan proved mathematically that, in two-dimensional situations, energy should flow from smaller vortices to larger ones. But just because the math said so didn't mean anyone knew how it occurred. (Indeed, in three dimensions, energy flows from larger to smaller vortices.) The 2-D conjecture was considered extremely important because lots of weather is essentially two-dimensional; the enormous width and breadth of a hurricane makes its thickness negligible, for example. Because the 2-D case is so important to weather prediction, researchers have puzzled for 30 years over how energy actually flows between vortices in two-dimensions.
In the 3 March issue of Physical Review Letters, Gregory Eyink, a physicist at Johns Hopkins University in Baltimore, Maryland, and colleagues report that Kraichnan had it right. The group set up a pool of salt water about 1 meter square, laid it on a bed of magnets, and ran an electrical current through the water horizontally. When the magnetic field crossed the electrical field, the disturbance gave birth to hundreds of tiny vortices in the salt water. Then larger vortices took over.
The team took a step beyond Kraichnan's paper and proposed an idea of what's going on. A computer simulation showed that the larger vortices were stretching the smaller ones. Imagine a small vortex as a bunch of concentric circles. As it stretches, the path of a water molecule looping around one circle would stretch out, and the molecule would slow down, indicating that the small vortex is losing energy. This lost energy, the researchers found, is flowing to the larger vortex nearby. Energy transfer in the atmosphere should work the same way: not via mergers but rather through hostile takeovers, they conclude.
"This paper gives a very novel and plausible mechanism," for the energy flow between vortices in two dimensions, says Philip Marcus, a computational physicist at the University of California, Berkeley. Having that mechanism cleared up should be useful for improving weather models, he says.
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