Comment se forment les vagues (et comment les « dompter »)

Comment se forment les vagues (et comment les « dompter »)

If you choose the sea (which dances in the transparent gulfs) as your holiday destination, with its changing reflections, a privileged witness to the whims of the sky, you will surely find yourself waiting for a wave to take you, before it does. don’t come aground on the edge. We have all experienced it: nothing is more complicated than predicting how a wave will come to take us or how to get out of the water once in the middle of the impetuous waves.

At the moment of appreciating the extension of a wave, for a few seconds and in an improvised way, one becomes a physicist modeling a very complex phenomenon to predict its effects.

Waves are natural phenomena as spectacular as they are dangerous. Wave heights vary from a few centimeters to 32.3 meters for the highest recorded by an instrument, or even more than 34 meters for the highest visually observed wave.

Origin and nature of waves.

A wave is a deformation of the surface of a body of water caused most often by another fluid: the wind. At the interface, the wind creates waves in the oceans, seas, and lakes.

The Ninth Wave, painting by Hovhannes Aivazovsky (1850) – Hovhannes Aivazovsky/Wikimedia CC0

Other phenomena, much less frequent, are also a source of waves: certain earthquakes, volcanic eruptions or meteorite falls that create waves called tsunamis or tidal waves. The tide is also at the origin of very particular waves, called tidal waves, sudden rises in the water of a river or an estuary, caused by the waves of the rising tide at the mouth and the lower course of certain rivers during high tides. . Finally, ships are also sources of waves.

Waves are waves, that is, phenomena that move periodically in time (we speak of period) and in space (we speak of wavelength). They can stretch for miles before making landfall and gain strength depending on the slope of the ocean floor. To understand this phenomenon, it is necessary to look for a physical model of the wave.

A brief history of waves.

The astronomer and mathematician George Biddell Airy provided the simplest theory for regular (periodic) waves. The Airy wave has a free surface, the water surface, sinusoidal in shape. This is a very simplified view of reality, valid for waves with low “camber”, that is, the relationship between height and wavelength.

If we look closely at the waves in the sea, we see that most of them are not sinusoidal: the crests are more pointed, the hollows more flattened.

But what interests us today in Airy’s theory is that fluid particles describe nearly closed ellipses, the size of which decreases with depth. We always mistakenly think that a wave carries us: in reality, if we swim deep during the propagation of a wave, we realize that there is a back and forth movement, not necessarily intuitive from the point of view of everyday experience but well outstanding. by Airy’s theory.

A wave moves forward and the particles carried by the waves have elliptical paths
A wave moves forward and the particles carried by the waves have elliptical paths – Kraaiennest / Wikimedia CC BY-SA 4.0

In deep water, that is, for depths greater than half a wavelength, these ellipses are circles. The fact that the ellipses are not completely closed is a manifestation of “Stokes drift”. Near the free surface, the speed of a water particle is greater below a crest than the opposite speed when passing through the next trough. This results in a drift in the direction of wave propagation that can be reversed at depth.

What freedom!, painting by Ilya Repin (1903)
What freedom!, painting by Ilya Repin (1903) – Ilya Repin / Wikimedia CC0

Wave breaking approaching the beach

Let’s get back to what will occupy us this summer: facing the waves… with dignity! In fact, “owning a wave” means understanding its swell.

In fact, some waves are too steep to be stable: they break.

Approaching the shore, the depth decreases, the shape of the waves changes, first in a more or less symmetrical manner, and then generally with a progressively steeper front face as soon as the wave height is the same. order that depth. When the wave breaks, most of its energy is dissipated in eddies and air bubbles. The associated impulse contributes to accelerating the current in the direction of the break.

The shape of a wave near the shore essentially depends on the slope of the bottom. Going in the direction of rising slopes, we often distinguish three types of breakouts. Progressive or slippery breaking usually occurs on beaches with very low slopes. The waves begin to break away from the shore with a foamy-looking crest that steepens as it goes, leaving behind a layer of foam.

The sinking wave is particularly spectacular with its rolls appreciated by surfers. The wave engulfs an air pocket and then collapses creating a noticeable splash. This tends to happen more often on a steep slope or a sudden change in depth (a trap), and that’s why there are popular “spots” with surfers.

Extracting energy from waves

There is much more energy dissipated than energy reflected on the beach. The energy dissipated on the surface of the crashing wave is beneficial to the surfer who can use it as a source of kinetic energy: the wave provides him with the necessary dynamics to move and perform the figures he wants on his moving board thanks to the wave. .

The front or swell swell forms like a plunging swell, but the wave rises up the beach before the crest can rise. The rupture zone is very narrow and much of the energy is reflected towards the greater depths. When the crest goes faster than the wave itself, a thick layer of crest foam forms on top. We speak then of a slippery wave, typical of a gently sloping shore. This type of break completely disadvantages the surfer since the energy is given to the depths. The talent of the surfer as a “casual physicist” is to be able to recognize the types of surf for this purpose.

types of waves
Types of waves – SL Douglas and J. Krolak/FHWA/Wikimedia CC BY-SA 4.0

There is also an intermediate case between sinking and frontal breakage. Instead of forming a roll, the wave has a vertical face before collapsing.

Surfers are often nicknamed the “tamers” of the waves. The good conditions of the waves allow a good practice of surfing (with a lot of technique anyway!). This means good wind or swell conditions, which can vary by location.

The hollow waves, with a sinking break, are the favorite of “shortboarders”, who use short boards. Hollow waves are the most powerful waves, but not necessarily the largest. In fact, some waves that can be described as hollow are much more powerful than a smooth wave of the same size. The principle of the shortboard (short board) is to allow a more “dynamic” surf practice with multiple changes of direction on the wave to achieve different figures. This is possible with a wave whose break is better controlled.

Finally, a surfer mainly does applied research in the discipline of “good waves”.
In general, whether you play with them, contemplate them or face them, take advantage of these moments of freedom that Nature offers you. And let’s remember that there is only one planet Earth to spend beautiful summers!

This analysis was written by Waleed Mouhali, Physics Research Professor at ECE Paris.
The original article was published on the site of The conversation.

#Comment #forment #les #vagues #comment #les #dompter

Leave a Reply

Your email address will not be published.