Bionic gradient flexible fish skin acts as a passive dynamic micro-roughness to drag reduction

Fish skin has a unique biological feature that hard fish scales are embedded in a flexible dermis layer and covered by a viscoelasticity mucus or compliant epidermis layer. Therefore, the entire fish skin system can act as a dynamic resilient energy-absorbing coating with micro-roughness to absorb external turbulent pulsations. The microscopic morphology, arrangement of fish scales, and mechanical characteristics of tuna skin inspired the extraction and fabrication of the bionic gradient flexible fish skin (BGFFS) model. First, BGFFS was fabricated by the three-dimensional (3D) printing method, spraying and casting process. Then, the 3D morphology of the bionic fish skin was characterized. The results indicated that the BGFFS exhibited good morphology of fish scales and spatial gradient mechanical properties like fish skin. Moreover, the experimental results in a circulating water tunnel showed that the BGFFS had excellent drag reduction performance in a turbulent flow. When the Reynolds (Re) number was 6.8 × 104, the maximum drag reduction ratio could reach 13.8 %. The drag reduction mechanism was analyzed by the computational fluid dynamics (CFD) method, and the energy-absorbing effect of the BGFFS was measured by the polyvinylidene fluoride (PVDF) piezoelectric film. In this experiment, the maximum voltage signal reached 0.448 V. The drag reduction mechanism of the BGFFS was that the rolling friction caused by vortexes replaced sliding friction. In the meantime, the BGFFS could absorb the micro disturbances in a turbulent flow. These findings can serve as a foundation for an in-depth analysis of the hydrodynamic performance of fish as well as a new inspiration for drag reduction and antifouling.