仿生胸鳍的三维尾涡结构与参数影响分析

为研究仿胸鳍推进的机理和流体动力特性及缩小机器鱼与生物原型之间的性能差距,利用浸入边界法数值模拟了做耦合旋转运动胸鳍的非定常绕流问题。详细探讨胸鳍非定常运动的三维尾涡结构演化和推进机理,并开展胸鳍推进性能与尾涡结构的参数影响分析。结果表明:迎流面在背、腹侧边缘及鳍梢部显著涡旋结构的作用下所出现的低压力区,加之鳍表面和上游来流之间好的垂直度共同造成了在动力划水阶段的高推力;在恢复划水阶段的高升力与背侧边缘涡强度的持续增加,以及因鳍表面倾斜而引起的水动力被分解到竖直方向的比重提升有关;胸鳍尾流场被一个三维双环涡结构所支配;当前的模拟为仿胸鳍推进建立了一个最优的斯特劳哈尔数St范围(在0.55附近),在此之后平均推力仍随St的增大而增加,而推进效率则表现出一个缓慢降低的趋势;当前后拍动与纵倾运动之间的相位差为90度时,胸鳍同时取得最佳的推力和效率。

Three-dimensional wake structure and parameter effect analysis of a bio-inspired pectoral fin

To investigate the mechanism and fluid dynamic characteristics of bio-inspired pectoral fin propulsion and bridge the performance gap between robotic fishes and biological prototypes, numerical simulations are used to investigate the unsteady flow around a pectoral fin with a compound rotational motion based on an immersed boundary method. The three-dimensional wake structure evolution and propulsive mechanism during the unsteady motion of the pectoral fin are discussed in detail, and the parameter effect analysis is carried out in terms of the propulsive performance and wake structure of the pectoral fin. The results indicate that the high thrust is generated jointly by the pectoral fin in the power stroke stems from the low pressure region on the upstream surface induced by the prominent vortex structures attached to the dorsal, ventral edge and tip of the fin and the good verticality between the fin surface and upstream inflow. The high lift in the recovery stroke is associated with the increasingly strong dorsal edge vortex and the proportion increase of the hydrodynamic force reoriented in the vertical direction due to the incline of the fin surface. The wake field of the pectoral fin is dominated by a three-dimensional dual-ring vortex structure. The current simulations establish an optimal range (around 0.55) of Strouhal number St for bio-inspired pectoral fin propulsion, and an increasing mean thrust and a relatively slowly decreasing propulsive efficiency are observed as St is further raised over that range. When the phase difference between the rowing and pitching motion is 90 degree, the pectoral fin obtains the best thrust and efficiency at the same time.