Dual-coupling drag reduction inspired by tuna skin: Fan-shaped imbricated fish scale composited with flexible coating

The excellent hydrodynamic properties of fishes enable them to respond rapidly to exterior excitation in water. In this study, three types of bionic surfaces were designed and manufactured based on the construction and mechanical properties of the tuna skin. The surface and coating features of these bionic surfaces were analyzed, and their drag reduction performance was investigated in a circulating water tunnel. The results revealed that the drag reduction effect was proportional to the flexible coating thickness, and a maximum drag reduction of 7.22% was achieved for the dual-structure coupling surface with a flexible coating thickness of 140 µm. The simulation results indicated that the “vortex stretching” effect formed near the fish scale wall further decreased the high-velocity regions, leading to low- and high-velocity streaks inside the boundary layer close to the bionic fish scale surface along the streamwise direction. As the flexible coatings could absorb turbulent fluctuations, they formed larger low-pressure areas near the surface, enabling a better drag reduction effect. The drag reduction mechanism for the dual-structure coupling surfaces arose from the combined actions of the flexible coatings and fan-shaped imbricated fish scales. This study may provide an ideal alternative for drag reduction and antifouling properties in underwater vehicle planning.