Hurricanes, other vortices seize energy via 'hostile takeovers'

For decades, scientists who study hurricanes, whirlpools and other large fluid vortices have puzzled over precisely how these vast swirling masses of gas or liquid sustain themselves. How do they acquire the energy to keep moving? The most common theory sounded like it was lifted from Wall Street: The large vortices collect power as smaller vortices merge and combine their assets, in the same way that small companies join forces to create a mega-corporation.

But researchers from The Johns Hopkins University and Los Alamos National Laboratory now believe the better model is a much different business tactic: the hostile takeover. Working with theoretical analysis, computer simulations and lab experiments, the team has concluded that large fluid vortices raid their smaller neighbors in an energy grab and then leave their depleted victims either to wither away or to renew their resources by draining still smaller vortices.

The findings were published in the March 3 issue of the journal Physical Review Letters. "This discovery is important because it could lead to a better understanding of how hurricanes and large ocean eddies form," said Shiyi Chen, an author of the paper. "It should also help us to create better computer models to make more accurate predictions about these conditions."

Chen is a professor in the Department of Mechanical Engineering at Johns Hopkins, where he occupies the Alonzo G. Decker Jr. Chair in Engineering and Science. He supervised the computer simulations in this two-and-a-half-year research project.

The team looked at large energetic vortex structures that form in irregular or turbulent two-dimensional flows of gas or liquid. Common examples are the Red Spot on Jupiter and hurricanes or typhoons on Earth. The researchers wanted to figure out how energy is transferred from smaller vortices to these large-scale circulation patterns. The basic phenomenon, called "inverse energy cascade," was predicted almost 40 years ago by pioneering turbulence theorist Robert H. Kraichnan. However, the dynamical mechanism underlying the inverse cascade has remained obscure. Does it occur, as some scientists suggested, through a merger of small vortices to form a new larger one?

"We went into this with an open mind, but we found that the popular idea of mergers was not correct," said Gregory Eyink, a Johns Hopkins professor of applied mathematics and statistics and currently the 2006 Ulam Scholar at Los Alamos Laboratory's Center for Nonlinear Studies. He served as the primary theorist in the project and was an author of the journal article. "We found that such mergers are very rare."

He said the energy transfer actually occurs through a process described as a "thinning mechanism."

"You have a large vortex spinning around, with a smaller one inside," Eyink said. "The large vortex has a shearing effect on the smaller one, like cake batter being stirred. The large-scale vortex acts like a giant mixer, stretching and thinning out the smaller one, transferring its energy into the larger vortex. The large-scale vortex actually acts like a vampire, sucking the energy out of the smaller one."

This phenomenon sustains a steady-state inverse energy cascade. "We end up with a group of large predator vortices preying on smaller ones, which in turn prey on smaller ones still, forming a food-chain of vortices," Eyink said.

Through computer modeling at Johns Hopkins and laboratory experiments at Los Alamos on thin salt-water layers, the scientists were able to observe the physical processes and measure the energy transfer. This confirmed their theory that an energy transfer by stretching of small-scale vortices is what sustains large-scale vortices.

"This is the first time a quantitative connection has been made between the process of vortex-thinning and inverse energy cascade," said Robert Ecke, director of the Center for Nonlinear Studies at Los Alamos, an author of the journal article and supervisor of the lab experiments.

Source: Johns Hopkins University

Related stories:

Taming 900 vortices gives plasma energy insight
ANU researchers have come closer to understanding how energy is retained in turbulent systems that self-organise - such as the atmosphere, the universe and plasma - after designing a simple experiment in their laboratory which creates 900 vortices in electrolytic fluid.
'Vortex lattices' may help explain material defects
What do you get when you superimpose a rotating pattern of intersecting laser beams on a spinning cloud of ultracold atoms in a thin gas? Pretty pictures, for one thing--but also a new method that could be used to simulate why and how defects arise in superconductors, important materials that are difficult to study directly.
Magnetic needles turn somersaults
For about ten years now, tiny magnetic structures measuring a few millionths of a millimetre have met with growing interest from the worlds of science and technology, particularly on account of their potential applications in magnetic storage. A fascinating quantum mechanical phenomenon occurs in these structures: the vortex core, which has been predicted in theory for forty years, but which experiments revealed only four years ago.
Study of 'Solitons' Adds Insight into Nanomagnet Behavior
Scientists are a long way off from a complete understanding of the interactions and behaviors that govern nanomagnets — magnets on the scale of a billionth of a meter. However, a groundbreaking new study helps answer some basic questions about nanomagnets, which could one day revolutionize (and drastically miniaturize) computer data-storage systems, telecommunications devices, and other technologies that rely on magnets.
Nano-finding points to new computer technologies based on magnetic spin
An unusual pool of scientific talent at the U.S. Department of Energy's Argonne National Laboratory, combined with new nanofabrication and nanocharacterization instruments, is helping to open a new frontier in electronics, to be made up of very small and very fast devices.
First vortex 'chains' observed in engineered superconductor
They look like tiny swirling dust devils on the surface of the superconductor: "vortices" that appear where magnetic fields interact with the material. Unlike harmless dust devils, however, vortices can sap a superconductor's ability to transmit current without resistance.
Finding Superconductors That Can Take the Heat
By studying how superconductors interact with magnetic fields, Pitt researchers advance quest for higher-temperature superconducting materials.

Superconductors are materials with no electrical resistance that are used to make strong magnets and must be kept extremely cold-otherwise, they lose their superconducting abilities. Even the "high-temperature" superconductors discovered in the 1980s must be kept at around -300°F.
From ‘macro’ to ‘micro’ – turbulence seen by Cluster
Thanks to measurements by ESA’s Cluster mission, a team of European scientists have identified ‘micro’-vortices in Earth’s magnetosphere.

News discussion:

Physics news