Making objects “invisible” in fluids

TWO research groups have designed structures that can hide objects from flows and waves in a fluid by directing their flows around the object in a manner such that they leave no wakes. In the past decade, researchers have used complex artificial materials, “metamaterials”, to build invisibility cloaks for electromagnetic waves, sound and heat that bend the waves in ways that conventional materials cannot. The second cloak is a pair of thin platforms that funnels waves around objects inside a water channel. On the other hand, for their passive hydrodynamic cloak, the Chinese team used the principle used in cloaks made with gradient index metamaterials that convert light waves into narrow “trapped modes” which avoid obstacles in optical waveguides. In tests with a broad range of wave frequencies, the team found that incoming plane waves converted to trapped modes above the platforms, leaving the middle of the tank nearly wave-free.