Due to fluid viscosity, marine vehicles and structures immersed in fluids are inevitably affected by various fluid resistances. To obtain an effective method to eliminate drag and achieve hydrodynamic invisibility, we propose an innovative theory, called arbitrary space transformation (AST) theory, to design hydrodynamic cloaks. This AST theory provides a strategy that enables spatial transformation between different coordinate systems, compressing arbitrary geometric space into a shell-shaped space, thereby realizing the hydrodynamic concealment and excellent drag reduction effect of arbitrarily shaped target objects. The flow manipulation cloak shows outstanding performance for eliminating drag and cloaking aquatic and submerged objects under different inflow conditions. In addition, it can reduce the risk of erosion and blockage that cloaked arbitrarily shaped objects or organs may encounter by shifting the angles between the inflow and the frontal surface of structures. This investigation enables powerful means of fluid design, which will make it possible for complex geometries to be undetectable by an external observer and remain hidden in an environment filled with fluid forces.

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Water power is abundant to meet the energy needs of the world. To exploit these plentiful natural resources, a novel energy harvesting device has been designed based on the water field manipulation theory. The flow field can be redirected and the kinetic energy can be harvested flexibly and efficiently. The proposed energy concentrator exhibits promising prospects to harvest low-speed water flow energy at a highly effective level of up to 4-7 times the power of solar panels and wind turbines. While this study is at an early stage, it can prompt the exploitation of water flow energy that is not directly available due to the low flow rates.