College of Mechanical Engineering, Jiangsu Provincial Key Laboratory of Advanced Manufacturing for Marine Mechanical Equipment, Jiangsu University of Science and Technology, Zhenjiang 212013, P. R. China.
Langmuir. 2021 Oct 12;37(40):11804-11817. doi: 10.1021/acs.langmuir.1c01745. Epub 2021 Oct 1.
Pufferfish is known for its extension of tiny spine-covered skin that appears to increase skin drag and may act as turbulisors, reducing overall drag while serving a protective function. Therefore, the present study addresses a neglected aspect of how spines affect the turbulent boundary layer (TBL) for drag reduction in the pufferfish skin. Particle image velocimetry (PIV) was utilized to investigate the TBL structure on the biomimetic spine-covered protrusion samples inspired by the back skin of the pufferfish. The comparison samples of two sparse "-type" arrangements (hexagon and staggered) for three types of rough element sizes with roughness heights = 5.5-6.5 (nearly hydraulically smooth) and smooth case in bulk Reynolds numbers ( = 37,129 and 44,554) were tested. The results of turbulence statistics of these samples indicate that both the sample (type hexagon) for large rough density (λ = 0.0215) with small roughness elements and the sample (type staggered) for small rough density (λ = 0.0148) with large roughness elements have a drag reduction rate of 5-11%. These two kinds of bionic surfaces have a similar morphology to that seen in the distribution of pufferfish spines and probably serve a similar hydrodynamic function. Vortex identification shows that the spines in the front section for large density with small rough elements stabilize the TBL and generate many small-scale vortices and the dense spines with large rough elements at the back section have the effect of separating the vortices. The retrograde vortex generated by them is beneficial to increasing the driving force of the pufferfish. In addition, these two rough surfaces may effectively delay the separation of the TBL. These results will provide a preliminary research foundation for the development of a more practical prototype of the bionic drag-reducing surfaces and strengthen the theoretical investigation concerning drag reduction exploration.
河豚的皮肤可以向外伸展，布满微小的带刺鳞片，这些鳞片可以增加皮肤的阻力，同时还可以充当湍流发生器，减少整体阻力并起到保护作用。因此，本研究关注的是一个被忽视的问题，即刺如何影响河豚皮肤的湍流边界层（TBL）以减少阻力。本研究采用粒子图像测速（PIV）技术，研究了模仿河豚背部皮肤的仿生带刺突起样品上的 TBL 结构。比较了三种不同粗糙元尺寸（粗糙度高度为 5.5-6.5，接近水力光滑）的两种稀疏“-型”排列（正六边形和交错排列）的仿生样本和光滑样本，在总雷诺数（Re=37129 和 44554）下进行了测试。这些样品的湍流统计结果表明，具有大粗糙度密度（λ=0.0215）和小粗糙度元素的样品（类型正六边形）和具有小粗糙度密度（λ=0.0148）和大粗糙度元素的样品（类型交错排列）的阻力减小率都为 5-11%。这两种仿生表面的形态与河豚刺的分布相似，可能具有相似的水动力功能。涡旋识别表明，大密度、小粗糙度元素的前缘刺稳定了 TBL，并产生了许多小尺度涡旋；后缘密集的大粗糙度元素刺则起到了涡旋分离的作用。它们产生的逆行涡旋有利于增加河豚的驱动力。此外，这两种粗糙表面可能有效地延迟了 TBL 的分离。这些结果为仿生减阻表面的更实际原型的开发提供了初步的研究基础，并加强了对减阻探索的理论研究。
