Research

Cloaking against sea waves

Ocean structures like ships and offshore wind turbines are constantly exposed to the forces of ocean waves, which can cause motion and drifting. Therefore, the design of these structures typically takes into account the prevailing sea conditions to ensure safe operation.

However, the idea of creating a space devoid of waves led to the research on what can be termed an "invisible cloak" against ocean waves. This concept draws inspiration from the fantasy world, where invisibility cloaks are often portrayed in works like Harry Potter, Doraemon, and Ghost in the Shell. These cloaks work by manipulating electromagnetic waves to render objects invisible, and the question arose: Could a similar technology be applied to ocean waves?

For the sake of simplicity, let's consider a cylindrical structure extending from the seabed to the ocean surface, resembling, for instance, a monopile-type foundation for offshore wind turbines. Normally, when such a cylinder is in the ocean, waves are scattered by it, as shown in the video. The scattering caused by the cylinder results in a reactive force known as the wave drift force.

Now, let's envision a thin, film-like structure enveloping this cylinder, as illustrated in the diagram. The film consists of layers with varying material properties in the circumferential direction, designed to function as an "invisible cloak."

The outcome is depicted in the following video. The waves scattered by the cylinder are canceled out by the thin film surrounding it. Beyond the film, the waves remain undisturbed. Consequently, when measuring waves outside the film, the wave pattern is identical to when there is no cylinder present. It's as if the cylinder doesn't affect the behavior of the waves. In this scenario, since the cylinder doesn't disrupt the waves, there is no drift force acting on it.

In this unique and intriguing research, our laboratory explores unconventional ideas. It's worth noting that this research is conducted in collaboration with the University of California, Berkeley.。

Reference: Iida, T., Zareei, A., & Alam, M. R. (2023). Water wave cloaking using a floating composite plate. Journal of Fluid Mechanics, 954, A4.

Contact person: Takahito Iida