Fluid Mechanics: Hostile takeover

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Fluid Mechanics: Hostile takeover
Fluid Mechanics: Hostile takeover

Hostile Takeover

Sometimes the world is flatter than it really is - for example with vortices.


It's almost like in business: The big ones swallow the small ones. There can be no talk of a cooperative partnership. What applies to Daimler and Chrysler or to Vodafon and Mannesmann finds its equivalent in nature. What is meant here is less the behavior of predatory animals, which usually only dare to approach the old, small and weak. No! The same applies to inanimate nature.

For water vortices for example. Gregory Eyink, a physicist at the American Johns Hopkins University in B altimore, together with his research team, has now discovered in an experiment that large vortices withdraw energy from vampires just like smaller ones and ultimately absorb it.

Robert Kraichnan – also from Johns Hopkins University – suspected that this was the case back in the 1970s when he was doing calculations on the dynamics of vortices. But his considerations applied exclusively in two dimensions. As is well known, our world is (at least) three-dimensional. And for three spatial dimensions, mathematicians were able to show that the energy should actually flow from the larger objects to the smaller ones.

Since the results of calculations of the dynamics of such turbulent flows are often extremely sensitive to the tiniest changes in the boundary conditions and behave in a highly chaotic manner, an experiment had to be carried out - mathematics or not. Only the real test proves the correct application of the formulas.

Therefore, Eyink and his team constructed a shallow basin one meter wide, which they filled with electrically conductive s alt water and placed on magnets. Then they sent an electric current through the solution. At the numerous places where the electric field crossed the field lines of the magnets, turbulence was created. The experimenters observed that the large whirlpools gradually swallowed up the small ones. So Kraichnan was right – although the observed effect was slightly weaker than he had predicted.

The experimenters therefore proposed their own explanation, which they supported with the help of computer simulations. As a result, the large vortices deform the smaller, usually circular, mini-tornadoes, causing the water molecules zipping around on the very outside to slow down. As a result, the weaker water funnel loses energy, which its larger counterpart absorbs.

These findings are extremely useful for building severe weather forecast models, according to Philip Marcus. The American physicist from the University of California at Berkeley works intensively with computer simulations. Even if weather fronts or even hurricanes are basically three-dimensional, the scientists agree, their height is usually negligible compared to their extent. Therefore, the math of the levels obviously leads to better predictions.

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