柔性拍动翼的水动力与推进效率研究

Study on hydrodynamic force and propulsive efficiency of flexible flapping foils

摘要:

为研究柔性拍动翼的水动力性能及指导仿生推进器的设计，应用自主开发的浸入边界法CFD程序数值模拟了三维柔性翼的非定常绕流问题.在固定的笛卡尔网格上通过基于虚拟网格的边界条件重建算法来施加复杂移动变形边界对流动的影响，以能量的视角提出一种对刚性和柔性拍动翼都适用的推进效率数值计算方法，并系统地研究了各参数对三维柔性翼推进性能的影响.结果表明：当变形运动控制参数ε处于2.0~3.5，且弦向变形系数δ为0.1时，柔性拍动翼的推进效率将超过刚性翼；柔性翼性能随变形运动控制参数的变化取决于水动力产生表象之下的涡动力学，包括翼尾缘变形对漩涡脱落的控制和展向涡量输运机制；柔性拍动翼的平均推力随无因次频率k的增大而单调增加，与实验中依靠胸鳍推进的鱼通过增加胸鳍的拍动频率来增加推力的现象是一致的；存在一个使柔性翼的推进效率最高的最优k，即实验中依靠尾鳍推进的鱼存在一个最优斯特劳哈尔数，使效率最佳的特性在模拟中得以体现.

Abstract:

To investigate the hydrodynamic performance of flexible flapping foils and provide guidance for the design of bio-inspired propulsors, numerical simulations are used to investigate the unsteady flow around a three-dimensional (3D) flexible foil based on an in-house developed immersed boundary method CFD code. The effect of complex moving deformation boundary on the flow on a fixed Cartesian gird is imposed by a boundary condition reconstruction algorithm based on ghost-cells. A numerical method calculating propulsive efficiency which can apply to both rigid and flexible flapping foils is proposed in views of energy. Moreover, the effect of various parameters on the propulsive performance of the 3D flexible foil has been systematically investigated. The results indicate that the propulsive efficiency of the flexible flapping foil is higher than its rigid counterpart when deformation parameter ε lies between 2.0 and 3.5 together with chordwise deformation coefficient δ of 0.1. The variation of the flexible foil performance with deformation parameter depends on the vortex dynamics underlying the force production, including the control of the trailing-edge deformation on the vortex shedding and the mechanism of spanwise transport of vorticity. The mean thrust of the flexible flapping foil increases monotonically with the dimensionless frequency k, which is consistent with the experimental phenomenon that fish propelled by pectoral fins enhance the thrust through the increase of the flapping frequency of pectoral fins. There exists an optimal k to get maximum propulsive efficiency of the flexible foil, and the simulation reproduces the characteristic of fish propelled by caudal fin in experiments, i.e., they cruise at a Strouhal number tuned for optimal efficiency.