Making objects “invisible” in fluids

Fluid flow in the space with the obstacle. The obstacle is subject to hydrodynamic force.

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. One is a Chinese team from Zhejiang University and Xiamen University led by Siyuan Zou and the other is a South Korean team from Seoul National University led by Juhyuk Park. Both the works have been published in a recent issue of “Physical Review Letters”.

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. Stealth aircraft, for example, use electromagnetic cloaks of this kind. For fluids, previous attempts at a “hydrodynamical cloak” have required active elements, such as micropumps.

In the combined design the first cloak involves a ring of small pillars—a passive cloak that does not require any input energy—that deflects incoming fluid in such a way that there is zero drag in the middle of the ring. The second cloak is a pair of thin platforms that funnels waves around objects inside a water channel.

The Korean researchers calculated the type of metamaterial needed to cancel the drag on a small obstacle sitting in a sheet of slow-moving fluid. The resulting design is a circular “maze” consisting of 523 pillars that direct flow away from the central region where the obstacle is located. It was found that when an object, say a cylinder, is placed inside the cloak, the flowing water was deflected around the object without generating any wake. With improvement, such a device could reduce the drag on ships or submarines.

On the other hand, for their passive hydrodynamic cloak, the Chinese team used the principle used in cloaks made with gradient index metamaterials (GIMs) that convert light waves into narrow “trapped modes” which avoid obstacles in optical waveguides. The researchers installed two thin platforms along the sidewalls of a 60 metre-long wave tank. The platforms acted like GIMs by creating shallow regions where waves travel more slowly. 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.