January 25, 2019
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Scientists recreate 'freak wave' that can reach 100 ft high for first time
Researchers have managed to recreate a 'freak wave' in the lab for the first time - and found it perfectly mirrors a famous Japanese painting from 1800.
Freak waves, officially known as Draupner waves, are unexpectedly large in comparison to surrounding waves. They are difficult to predict, often appearing suddenly without warning, and are commonly attributed as probable causes for maritime catastrophes such as the sinking of large ships.
Most famously, they were captured in 'The Great Wave off Kanagawa' - also known as 'The Great Wave' - a woodblock print published in the early 1800s by the Japanese artist Katsushika Hokusai.
The Draupner wave was one of the first confirmed observations of a 'freak wave' in the ocean; it was observed on the 1st of January 1995 in the North Sea by measurements made on the Draupner Oil Platform.
To recreate them in the lab, the team at the University of Oxford recreated the wave using two smaller wave groups and varying the crossing angle - the angle at which the two groups travel.
'The measurement of the Draupner wave in 1995 was a seminal observation initiating many years of research into the physics of freak waves and shifting their standing from mere folklore to a credible real-world phenomenon. By recreating the Draupner wave in the lab we have moved one step closer to understanding the potential mechanisms of this phenomenon,' said Dr Mark McAllister at the University of Oxford's Department of Engineering Science.
To the researchers' amazement, the wave they created bore an uncanny resemblance to 'The Great Wave off Kanagawa' - also known as 'The Great Wave' - a woodblock print published in the early 1800s by the Japanese artist Katsushika Hokusai. Hokusai's image depicts an enormous wave threatening three fishing boats and towers over Mount Fuji which appears in the background.
Hokusai's wave is believed to depict a freak, or 'rogue', wave. It was the crossing angle between the two smaller groups that proved critical to the successful reconstruction.
The researchers found it was only possible to reproduce the freak wave when the crossing angle between the two groups was approximately 120 degrees. When waves are not crossing, wave breaking limits the height that a wave can achieve. However, when waves cross at large angles, wave breaking behaviour changes and no longer limits the height a wave can achieve in the same manner.
Prof Ton van den Bremer at the University of Oxford said: 'Not only does this laboratory observation shed light on how the famous Draupner wave may have occurred, it also highlights the nature and significance of wave breaking in crossing sea conditions. The latter of these two findings has broad implications, illustrating previously unobserved wave breaking behaviour, which differs significantly from current state-of-the-art understanding of ocean wave breaking.'
The laboratory-created freak wave also bears strong resemblances with photographs of freak waves in the ocean. Previous experiments carried out in an ocean research facility showed that when waves intersect at an angle greater than approximately 60 degrees, they cause the surface level of the ocean to rise. This adds to the overall height of the resulting combined wave.
Dr Ton van den Bremer, who led the study while at the University of Edinburgh, said: 'This improves understanding of rogue waves, decades after this aspect of their behaviour was suggested. The more we know about this dangerous phenomenon, the better equipped we will be to design offshore structures and to navigate the oceans.'
Experiments were carried out in the FloWave Ocean Energy Research Facility at the University of Edinburgh. The testing tank is able to simulate ocean currents and waves of any type, which are monitored using overhead sensors. Researchers used the 25-metre circular tank to study the complex interactions that occur when waves cross in open water. The study was carried out in collaboration with the University of Oxford and supported by the UK's Engineering and Physical Sciences Research Council.
Dr Mark McAllister, who took part in the research while at the University of Edinburgh, said: 'These experiments provide new insight into how a heightened, or set-up, wave actually forms. They revealed that this behaves like a partial standing wave, which forms underneath waves as they cross. This insight allowed us to create a simple theory to predict when such waves might occur.'
Rogue waves happen spontaneously and cause huge damage as well as risk to human life. The unpredictable nature of the waves is a particular problem for oil-rigs and other off-shore structures and can have devastating consequences. The rogue waves are referred to by scientists as 'extreme storm waves' and are steep sided, have deep troughs and can become twice the size of surrounding waves.
The paper was published in the Journal of Fluid Mechanics.
