Incredible crests can emerge from the sea like water eruptions

Incredible crests can emerge from the sea like water eruptions

mechanics

Tech Innovation Website Editor – 06/15/2022

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Peak waves look more like “water bursts,” which occur when small waves intersect and interact constructively.
[Imagem: University of Oxford]

Ultrasound

For centuries, roaming or traveling waves have produced stories, with sailors swearing they saw huge waves rippling across the ocean without any storms.

Only recently have these superwaves left folklore and been incorporated into scientific knowledge. Nonetheless, since they are rare events, there are few documented cases, so all we have are hypotheses about the mechanism by which they formed.

But it looks like there could be even stranger and taller waves in the ocean — a wandering wave characterized by at least twice the average wave height in a given area.

In a new laboratory experiment on ocean wave breaking, researchers show that the breaking behavior of so-called “peak waves” differs significantly from the long-established theory of traveling wave breaking.

And, because they don’t obey the models built on these theories, these peak waves can reach heights much higher than expected and occur suddenly, almost like an explosion of water.

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The profile of the peak wave (left) is quite different from the normal wave (right) and can be produced by very light cross ripples.
[Imagem: M. L. McAllister et al. – 10.1017/jfm.2021.1023]

Break the waves

A wave breaks when it becomes so steep that the crest is no longer stable. This can lead to disruption of movement and loss of energy. As a result, the height of the waves is limited by the destruction process.

Now, researchers from the UK and the Netherlands have used circular tanks to simulate how waves break up in three dimensions for the first time in ocean tanks. Until then, all experiments have been carried out in long tanks, which can only study wave-breaking processes in two dimensions.

The use of a circular tank brings another novelty: the study of axisymmetric waves – with axis symmetry – in three dimensions. These waves, better known as spike waves, are similar to the waves that form when a drop of water falls on still water and rises to a peak.

The difference is that the generation process is very different: the peak waves that researchers want to study are usually generated by wind, in which countless waves from all directions interact constructively with each other, adding up to a very high wave . bigger.

Wave trough experiments show that the failure behavior of axisymmetric waves is quite different from the wave failure associated with traveling waves.

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The formation of the water jet is almost explosive.
[Imagem: M. L. McAllister et al. – 10.1017/jfm.2021.1023]

crest break

As the waves form and converge in the tank, a huge vertical stream of water shoots out from the top of the wave, rises and quickly free-falls, hitting the water below.

Unlike traveling waves, the height of the wave crest is not limited by the onset of the interruption, but by the stability of the vertical jet that rises from the surface of the tank.

“This study sheds light on the fundamental mechanism by which crossed waves and highly directional scattered waves can become much larger than other waves, accelerating upwards much faster than gravity in a short period of time,” said Professor Mark McAllister from the University of Oxford. .

The waves produced were nearly a thousand times larger than previous experiments and have major implications for maritime safety.

Professor Ton van den Bremer from the University of Delft explained: “Peak waves are an idealized example of a behaviour that propagates so-called transoceanic, wave systems in different directions and is very dangerous for ships and structures in the open ocean. .”University of Science and Technology.

bibliography:

article: Wave Breaking and Jet Formation of Axisymmetric Surface Gravity Waves
Authors: ML McAllister, S. Draycott, T. Davey, Y. Yang, TAA Adcock, S. Liao, TS van den Bremer
Journal: Journal of Fluid Mechanics
Volume: 935, A5
DOI: 10.1017/jfm.2021.1023

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