Although studies of hydrodynamic cloaking have attracted considerable attention, most pioneering studies have so far focused on creeping flows, in which nonlinear convective terms are neglected and cloaks are fabricated with inhomogeneous or anisotropic metamaterials. Here we discover that intangible uniform-viscosity hydrodynamic cloaks, which are capable of hiding obstacles immersed in narrow-gap Navier-Stokes flows with non-negligible convection, can be possibly designed. Via the convection-diffusion-balance method and the realizable thermally controlled tactic, the diffusion effect exerted by the obstacle surface can be balanced by the convection counterpart exerted by the manipulation of temperature-dependent viscosity inside the intangible cloak. This analysis has been validated by finite-element computational simulations that tackle cloaked-cylinder flows for Reynolds numbers in the laminar regime. The proposed study may lay the foundation for attempts to further explore hydrodynamic cloaks and possibly other metamaterial-related devices.

Although convective thermal cloaking has been advanced significantly, the majority of related researches have concentrated on creeping viscous potential flows. Here, we consider convective thermal cloaking works in non-creeping viscous potential flows, and propose a combination of the separation of variables method and the equivalent-medium integral method to analytically deduce the parameters of convective thermal cloaks with isotropic-homogeneous dynamic viscosity and thermal conductivity. Through numerical simulation, we demonstrate the cloaks can hide the object from thermo-hydrodynamic fields. Besides, by comparing the drag force cloaks bear in cloak case and the objects bear in object-existent case, we find convective thermal cloaks can considerably reduce the drag force, which appears drag-free characteristics. Finally, it is our hope that these developed methods can reduce the difficulties of metadevices fabrications, promote the development of drag reduction technology under higher Reynolds number, and shed light on the control of other multi-physics systems.

Flow control technologies play a crucial role in modern engineering and scientific research. Hydrodynamic metamaterials, as an emerging branch in the field of flow control, have attracted extensive research due to their remarkable potential in flow control, thus leading to the development of hydrodynamic metadevices. Here, we propose a meta-hydrodynamics theory for the active manipulation of fluid flows, which establishes an equivalence relation between volumetric forces and spaces and, consequently, allows for the design of active hydrodynamic metadevices. This equivalence relation can be utilized to accurately tailor the flow fields as long as the manipulation effects of the hydrodynamic metadevices on the flow fields are predetermined. By constructing a mapping relation of coordinate transformation from virtual space to physical space, we can determine the required volumetric force distributions to realize these hydrodynamic metadevices. We exemplify this theory with three different applications: hydrodynamic cloaks, concentrators, and rotators, for which we calculate the corresponding volumetric force distributions. Subsequent numerical simulations reveal the excellent manipulation performances of these hydrodynamic metadevices in both uniform and non-uniform flow fields. Finally, our research is expected to pioneer new perspectives in the development of hydrodynamic metadevices and methodologies for flow control under nonlinear flows with high Reynolds numbers.