

When you look at the sea, it’s easy to imagine that it’s just one big mass of water, but it actually has a structure consisting of layers of different temperatures and densities. They aren't mere abstractions either. If you've ever gone scuba diving you may have swum down only to hit what seems like an invisible barrier that you have to actively push your way through. That’s the point where two of these layers meet.


How the cloaking works (Image: Reza Alam, UC Berkeley)

Where these layers meet “internal waves “can travel on the interface like those on the water’s surface. The difference is that the internal waves have the same frequency of oscillation as surface waves, but a shorter wavelength and slower speed.

What both surface an internal waves have in common is that they can be distorted by the seabed in the same way that a lens can distort light. With ocean waves, there’s an energy transfer involved that Alam was able to simulate in a computer. He found that by placing a corrugated or wavy pattern tuned to particular wavelength on the seabed it would cause surface waves to temporarily become internal waves.

By placing such a pattern under an oil rig, dangerous waves would shift to the subsurface interface, travel under the rig, and then return to the surface once they've cleared the pattern. In other words, the rig becomes invisible to surface waves. The idea is similar to land-based systems proposed for diverting earthquakes using rubber barriers or plastic rings, except in this case the waves are altered in order to divert them.

“Cloaking in seas by modifying the floor may play a role in protecting nearshore or offshore structures and in creating shelter for fishermen during storms,” said Alam. “In reverse, it can cause the disappearance and reappearance of surface waves in areas where sandbars or any other appreciable bottom variations exist.”

Alam presented his scheme to the American Physical Society’s Division of Fluid Dynamics earlier this month in San Diego, California.

Source: American Physical Society (PDF)

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