He Xixing, Liu Yihe, Zhan Haiyang, Liu Yahua, Zhao Lei, Feng Shile
State Key Laboratory of High-Performance Precision Manufacturing, Dalian University of Technology, Dalian 116024, China.
Biomimetics (Basel). 2023 Mar 11;8(1):116. doi: 10.3390/biomimetics8010116.
Engineering marvels found throughout the exclusive structural features of biological surfaces have given rise to the progressive development of skin friction drag reduction. However, despite many previous works reporting forward drag reduction where the bio-inspired surface features are aligned with the flow direction, it is still challenging to achieve bidirectional drag reduction for non-morphable surface structures. Inspired by the flounder ctenoid scales characterized by tilted, millimeter-sized oval fins embedded with sub-millimeter spikes, we fabricate a bionic two-tier structural surface (BFTSS) that can remarkably reduce the forward skin friction drag by = 19%. Even in the backwards direction, where the flow is completely against the tilting direction of surface structures, BFTSS still exhibits a considerable drag reduction of = 4.2%. Experiments and numerical simulations reveal that this unique bidirectional drag reduction is attributed to synergistic effects of the two-tier structures of BFTSS. The array of oval fins can distort the boundary layer flow and mitigate the viscous shear, whilst the microscale spikes act to promote the flow separation to relieve the pressure gradient in the viscous sublayer. Notably, the pressure gradient relief effect of microscale spikes remains invariant to the flow direction and is responsible for the backward drag reduction as well. The bidirectional drag reduction of BFTSS can be extensively applied in minimizing the energy consumption of ships and underwater vessels, as well as in pipeline transport.
在生物表面独特的结构特征中发现的工程奇迹推动了皮肤摩擦阻力降低技术的不断发展。然而，尽管此前有许多研究报告了生物启发的表面特征与流动方向一致时的正向阻力降低情况，但对于不可变形的表面结构而言，实现双向阻力降低仍然具有挑战性。受比目鱼栉鳞启发，其特征是带有亚毫米级尖刺的倾斜毫米级椭圆形鳍片，我们制造了一种仿生双层结构表面（BFTSS），它能显著降低正向皮肤摩擦阻力，降幅达19%。即使在反向流动时，即流动方向与表面结构的倾斜方向完全相反，BFTSS仍表现出可观的阻力降低效果，降幅为4.2%。实验和数值模拟表明，这种独特的双向阻力降低归因于BFTSS双层结构的协同效应。椭圆形鳍片阵列可使边界层流动发生畸变并减轻粘性剪切，而微观尖刺则起到促进流动分离的作用，以缓解粘性亚层中的压力梯度。值得注意的是，微观尖刺的压力梯度缓解效应与流动方向无关，也是反向阻力降低的原因。BFTSS的双向阻力降低可广泛应用于降低船舶和水下航行器的能耗以及管道运输中。
