19 April 2012

A rogue wave sinks a Lego pirate ship

For the first time, physicists have created a rogue wave in a laboratory tank. Although the 3-centimeter-tall wave would topple only a tiny model ocean liner, the observation lends credence to the idea that a simplified theory of water waves can explain freak waves, which have been blamed for sinking real ships...

For a deckhand on a fishing boat, a rogue wave is a behemoth that appears out of nowhere, potentially capsizing the ship. For a physicist or an engineer, such a wave has a more precise definition: A rogue or freak wave is one that rises more than 2.2 times as high, from trough to crest, as the average of the largest one-third of nearby waves. So in seas with 3-meter-high waves, a rogue wave would measure 7 meters or more. Scientists and engineers once debated whether tales of such waves were merely myths. However, in recent years, wave measurements and satellite observations have proved that not only do they exist, but they are too common to be produced by chance when smaller waves overlap and simply add their heights. Instead, rogue waves must somehow amplify themselves through some sort of "nonlinear" feedback.

That equation has several weird solutions, including one with the basic properties of a rogue wave. Discovered in 1983, the so-called Peregrine solution consists of a single peak that suddenly emerges out of a smoothly varying wave train (a so-called sine wave) by sucking energy out of it, zipping along for a while, and then disappearing back into the sine wave. In October 2010, experimenters produced an optical version of that wave with light.

Now, mathematician Amin Chabchoub and physicist Norbert Hoffmann at the Hamburg University of Technology in Germany and physicist Nail Akhmediev of Australian National University in Canberra have produced a Peregrine rogue wave in a water tank 15 meters long, 1.6 meters wide, and filled to a depth of 1 meter. Using a computer-controlled paddle, the researchers generated a sine wave with an amplitude of 1 centimeter, which they measured with an electric depth gauge, as described in a paper in press at Physical Review Letters.
Text from Science, via Fuck Yeah Fluid Dynamics.

1 comment:

  1. That is very cool. It's interesting how the sine wave flattens out right before the rogue wave, which is obviously where the energy is coming from - I can see how that would be completely invisible on the ocean so the rogue really 'comes out of nowhere'.


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