Seafloor platform 'cloaks' big ocean waves

By creating specially “tuned” ripples on the ocean floor,

it may be possible to convert surface waves into internal interfacial waves to “cloak”

and shield floating objects—such as oil rigs or ships—from rough seas.

From DiscoveryNews

Floating offshore oil drilling platforms, offshore wind farms and buoys are vulnerable to waves, especially those from storm swells.
But a team of engineers has found a way to make those ocean-faring structures invisible to waves.
The technique could help protect marine structures from damage when big storms hit.
Mohammed-Reza Alam, assistant professor of mechanical engineering at the University of California, Berkeley, have designed a rippled platform that sits on the seafloor directly below an ocean-faring structure.
The ripples in the platform influence the the behavior of the water all the way up to the surface -- ultimately reducing the size of damaging surface waves.
Alam calls his technique a "cloak" because it makes the ocean structure invisible to the surrounding waves.
He presented the work November 19 at an American Physical Society conference in San Diego.

 How the cloaking works (Image: Reza Alam, UC Berkeley) : how the variation of density in ocean water can be used to cloak floating objects against incident surface waves.
Unlike most other cloaking techniques that rely on transformation optics, this one is based on the influence of the ocean floor’s topography on the various “layers” of ocean water.

The seafloor influences the beahavior of surface waves because of the unique structure of deep water.
The top is heated by the sun, making the surface less dense as the heat expands it.
Meanwhile, at the bottom, the water is cold and dense with more salt in it.
The place where the two layers meet and where the temperature and density changes is called the thermocline.

In that layer are "internal" waves.
If one could pull off the top layer of ocean, the internal waves would look a lot like ordinary surface waves, except they would probably be longer (lower frequency) and taller (higher amplitude) than the ones on the surface.
They happen because the waves on the surface are transmitting energy to the sea floor.
Just as the energy from wind makes waves on the surface, the energy is transferred to the thermocline, where it make the internal waves.

Meanwhile, the seafloor isn't perfectly smooth.
The shape of it can affect the motion of deep water as it flows over it and some of that energy can get transferred all the way back to surface waves.

Alam found that if he used a rippled sheet of material, one that had a specific set of heights and lengths, and put it on the oceanfloor, the energy from deep water would make the internal waves in the thermocline more energentic, but cancel out surface waves.
That makes for calm water on the surface.

Alam told Discovery News that if one were building a working version -- something several years away at least -- it would only need to face in one direction, since you only need to cancel the waves out when they are approaching.
Waves also tend to come from one side -- they don't often go from shore to sea, for instance.

He added that there is a lot of interest in this kind of work for another reason: underwater acoustics. Understanding how sound transmits through water is crucial for designing better sonar.
The interface between warm and cold water can affect what a sonar system "sees" and better understanding that layer could improve sonar systems.

Beyond building something to protect floating structures, this kind of work could also help engineers decide where to put them.
The "cloaking" effect sometimes occurs because the seafloor isn't level; there are areas of calm water as a result.
Putting an oil drilling platform in a place where big waves are less likely to happen at all would make the whole operation safer.

Links :

• PhysicsWorld : Invisibility cloak for water waves