Cloaking technique could protect ships from waves

Mechanical engineers say they’ve found a way of adapting invisibility cloaking techniques to protect oil rigs and ships from rough waves.

The method is based on the influence of the ocean floor’s topography on the various layers of ocean water.

“The density of water in an ocean or sea typically isn’t constant, mainly because of variations in temperature and salinity,” says Reza Alam of the University of California, Berkeley.

“Solar radiation heats the upper layer of the water, and the flow of rivers and the melting of ice lowers the water density near the surface. Over time, these effects add up to form a stable density stratification of two layers – with the lighter fluid layer on top and the more dense fluid layer below it.”

Stratified waters contain ‘internal waves’ – gravity waves that propagate between the two layers of water. For the same frequency of oscillation, however, internal waves travel at a much shorter wavelength and slower speed than surface waves.

Alam used computer simulations to transform a surface wave into an internal wave as it approaches an object – meaning the wave passes beneath the object rather than crashing into it.

Once the internal wave moves beyond the object, it can be transformed back into a surface wave by creating ‘corrugations’ – wavy ripples that are tuned to a specific wavelength on the ocean floor in front of the floating object to be cloaked.

“Cloaking in seas by modifying the floor may play a role in protecting near-shore or offshore structures and in creating shelter for fishermen during storms,” says Alam.

“In reverse, it can cause the disappearance and reappearance of surface waves in areas where sandbars or any other appreciable bottom variations exist.”