A Cloak of Invisibility Against Ocean Waves
In his Physical Review Letters (084502-2012) article, ME Professor Mohammad-Reza Alam shows that the variation of density in the ocean can be utilized to cloak floating objects against incident surface waves. The density of water in an ocean or a sea is typically not constant. The variation of density is due to, mainly, variations of temperature and salinity. Solar radiation heats up the upper layer of the water, and the flow of rivers and the melting of ice lower the water density near the surface. Over time, these effects add up to form a stable density stratification with the lighter fluid on top and the denser fluid below it.
Stratified waters, besides regular surface waves, admit the so-called internal waves, which are gravity waves that propagate within the body of the water. Alam shows that that floating objects in stratified fluids can be cloaked against broadband incident waves by properly architecting the bottom corrugations.
The concept behind the presented scheme is based on nonlinear resonance of
surface and interfacial waves with the bottom topography and is obtained
due to the dispersive nature of gravity waves. Perfect cloaking against
monochromatic waves can theoretically be achieved and was further
investigated via a direct high-order spectral scheme. The presented cloak
is the alignment of bottom corrugations only, and therefore is surface
noninvasive. Cloaking in seas by bottom modifications may play a role in
protecting near shore or offshore structures (buoys) and in creating
shelter for fishermen during storms. In reverse it can result in
disappearance and appearance of surface waves in areas where sandbars (or
any other appreciable bottom variations) exist.