November 19, 2012
Cloaking Technology Could Protect Ships From Waves
Lee Rannals for redOrbit.com — Your Universe Online
The new approach to invisibility cloaking is based on the influence of the ocean floor's topography on various "layers" of ocean water.
Reza Alam, assistant professor of mechanical engineering at the University of California, Berkeley, described at APS how the variation of density in ocean water can be used to help cloak floating objects against surface waves.
"The density of water in an ocean or sea typically isn't constant, mainly because of variations in temperature and salinity," Alam said in a statement. "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."
He said that over time, these effects add up to form a stable density stratification of two layers, with lighter fluid layers on top and the more dense fluid layers below.
Stratified waters like surface waves contain "internal waves," which are gravity waves that propagate between the two layers of water. These waves travel at a much shorter wavelength and at slower speeds than surface waves.
Both of the wave types "feel" the ocean floor's influence, which generates an energy transfer.
Alam used computer simulations to transform a surface wave into an internal wave as it approaches an object. Once the internal wave moves beyond the object, it is able to be transformed back into a surface wave.
This would be achieved by creating "corrugations" or 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," Alam said in. "In reverse, it can cause the disappearance and reappearance of surface waves in areas where sandbars or any other appreciable bottom variations exist."