Experimental topology-optimized cloak for water waves

Rendering an object invisible has fascinated humanity for centuries and become more tangible with the advent of metamaterials and transformation optics. The pioneering methodology on electromagnetic waves has quickly carried over to water waves due to the similar governing Helmholtz equations. Despite the elegant theory, necessary compromises have to be made in practice to relax the extreme requirements on anisotropic and inhomogeneous hydrodynamic metamaterials, leading to a deteriorated cloaking performance. Here we numerically design and experimentally demonstrate a topology-optimized cloak for water waves. By inversely designing the water depth distribution, the topology-optimized cloak can flexibly meet different user-defined scattering demand, without stereotyped material constrains. The underlying mechanism relies on the control of complicated scattering events inside the cloaking shell to suppress the scattering strength of a cylindrical obstacle to near zero. Our work has potential applications in protecting offshore structures and sheds a new light into the inverse design of novel hydrodynamic metamaterial. • A new method (topology optimization method) is introduced into the design of hydrodynamic metamaterial. • Varying bathymetry leads to combined refraction-diffraction interaction. • Induce refraction-diffraction interaction to cancel the scattering of the object to realize invisibility. • The effectiveness of the water wave invisibility cloak is verified by the experiment. • Arbitrariness of objective function setting makes the design method universal.