Rogue Waves
Rogue Waves
Understanding the nature of monster waves
Oldenburg physicists relate extreme events to thermodynamic laws
Oldenburg. On the trail of the monster wave phenomenon: an international team of researchers led by Oldenburg has succeeded in estimating the probability of the occurrence of a monster wave - a mountain of water up to 30 metres high - based on the waves in the surrounding area. The findings, which have been published in the current issue of the specialist journal "europhysics letters", call into question the previously prevailing view that monster waves are unpredictable phenomena that come out of nowhere and disappear again without a trace. The new work could help to better predict these extreme events one day. The main author of the publication is doctoral candidate Ali Hadjihosseini from the working group of Oldenburg turbulence researcher Prof Dr Joachim Peinke. Dr Pedro G. Lind from the University of Osnabrück and other researchers from Hamburg, the UK and Japan are also involved.
Monster waves - not to be confused with tsunamis - are individual waves on the open sea that are at least twice as large as the waves surrounding them. They develop enormous forces on impact. Smaller ships can sink, while larger ones are at risk of being unable to manoeuvre. Experts estimate that up to ten shipwrecks a year can be attributed to these wave monsters. For a long time, monster waves were dismissed as a sailor's yarn, but since satellite images proved their existence in the early 1990s, they have increasingly aroused the interest of scientists. Exactly how they arise is still a matter of debate.
To get to the bottom of the phenomenon, Hadjihosseini and his team compared measurement data from the North Sea and the Sea of Japan in the "Extreme Ocean Gravity Waves" research project funded by the VW Foundation, with the Japanese data including a monster wave. "We were able to show that monster waves do not occur randomly, but follow exact thermodynamic laws," says Hadjihosseini. The key word is entropy. In simple terms, this physical quantity describes the order or disorder of the individual particles in a physical system. How orderly the arrangement is is closely related to thermodynamic laws.
With this in mind, the researchers analysed the waves in the North Sea and the Sea of Japan. They succeeded in assigning specific entropy values to individual waves: Small waves are the result of an entropy increase, whereas the dreaded monster waves are characterised by an entropy decrease. "Despite their destructive effect, these waves can therefore be regarded as islands of order in the sea," summarises Lind. The change in entropy values thus serves the researchers as an indicator of the probability of a monster wave: the greater the entropy decrease, the more likely a monster wave is to occur in its vicinity. Another important finding of the researchers is that the increase in entropy that creates disorder and the decrease in entropy that creates order are balanced by a very precisely fulfilled thermodynamic law. Each monster wave must therefore be counterbalanced by a certain number of normal, disorderly waves.
"With these results, we have created a new approach to characterising and modelling ocean waves. Now the risks of waves in general and monster waves in particular can be better assessed and perhaps even predicted one day," says Peinke.
"Rogue Waves and entropy consumption" by Ali Hadjihosseini, Pedro G. Lind, Nobuhito Mori, Norbert P. Hoffmann and Joachim Peinke in Europhysics Letters, Volume 120, Number 3.
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Contact
Prof. Dr Joachim Peinke, Tel.: 0441/798-5050, Email: peinke@uol.de