Department of Mechanics, Zhejiang University, Hangzhou 310027, People's Republic of China.
Bioinspir Biomim. 2022 Aug 30;17(5). doi: 10.1088/1748-3190/ac7fd4.
It is of interest to investigate how a swimming animal performs in a density-stratified fluid. This paper studies a simplified swimmer, a pitching NACA0015 airfoil, considering its locomotion in both homogeneous, or unstratified, and stratified fluid flows. A direct comparison is made between these two conditions through two-dimensional numerical simulations. Our numerical results show that the stratification modifies the dynamics of the pitching foil in both its wake structures and the drag force, or thrust, as well as its propulsive performance. We suggest that the effects of stratification on flapping performance or propulsive efficiency can be categorized according to the Froude number, or the level of stratification. First, in the range of high Froude numbers, notable modification of the flow structure can be observed, which however does not greatly affect the propulsive performance. Second, at a very low Froude number, i.e., Fr = 1, the propulsive efficiency drops markedly compared to its homogeneous counterpart, attributed to the pronounced internal waves induced by the strong stratification. Moreover, at a moderate Froude number Fr = 2, we find an increase in the propulsive efficiency, which can be explained by the unique variation in the wake structure. At= 2.50, the propulsive efficiency peaks at Fr = 2, with its efficiency 18.3% higher than its homogeneous counterpart, exhibiting a favourable influence of the stratification on a swimmer.
研究游泳动物在分层流体中的运动方式很有趣。本文研究了一种简化的游泳者，即俯仰 NACA0015 翼型，考虑了其在均质（无分层）和分层流体中的运动。通过二维数值模拟对这两种情况进行了直接比较。我们的数值结果表明，分层会改变俯仰翼型的尾流结构和阻力（或推力）以及其推进性能。我们建议，分层对扑翼性能或推进效率的影响可以根据弗劳德数（或分层程度）进行分类。首先，在高弗劳德数范围内，可以观察到流动结构的显著变化，但这并不会极大地影响推进性能。其次，在非常低的弗劳德数（即 Fr = 1）下，与均质相比，推进效率明显下降，这归因于强分层引起的显著内波。此外，在中等弗劳德数 Fr = 2 下，我们发现推进效率增加，这可以通过尾流结构的独特变化来解释。在 Fr = 2.50 时，推进效率在 Fr = 2 时达到峰值，其效率比均质对应物高 18.3%，表明分层对游泳者有有利影响。
