Mechanical Engineering and Mechanics, Lehigh University, Bethlehem, PA 18015, United States of America.
Bioinspir Biomim. 2018 May 31;13(4):045002. doi: 10.1088/1748-3190/aabf4c.
We present experiments that examine the modes of interaction, the collective performance and the role of three-dimensionality in two pitching propulsors in an in-line arrangement. Both two-dimensional foils and three-dimensional rectangular wings of AR = 2 are examined. In contrast to previous work, two interaction modes distinguished as the coherent and branched wake modes are not observed to be directly linked to the propulsive efficiency, although they are linked to peak thrust performance and minimum power consumption as previously described (Boschitsch et al 2014 Phys. Fluids 26 051901). In fact, in closely-spaced propulsors peak propulsive efficiency of the follower occurs near its minimum power and this condition reveals a branched wake mode. Alternatively, for propulsors spaced far apart peak propulsive efficiency of the follower occurs near its peak thrust and this condition reveals a coherent wake mode. By examining the collective performance, it is discovered that there is an optimal spacing between the propulsors to maximize the collective efficiency. For two-dimensional foils the optimal spacing of X = 0.75 and the synchrony of ϕ = 2π / 3 leads to a collective efficiency and thrust enhancement of 42% and 38%, respectively, as compared to two isolated foils. In comparison, for AR = 2 wings the optimal spacing of X = 0.25 and the synchrony of ϕ = 7 π / 6 leads to a collective efficiency and thrust enhancement of 25% and 15%, respectively. In addition, at the optimal conditions the collective lateral force coefficients in both the two- and three-dimensional cases are negligible, while operating off these conditions can lead to non-negligible lateral forces. Finally, the peak efficiency of the collective and the follower are shown to have opposite trends with increasing spacing in two- and three-dimensional flows. This is correlated to the breakdown of the impinging vortex on the follower wing in three-dimensions. These results can aid in the design of networked bio-inspired control elements that through integrated sensing can synchronize to three-dimensional flow interactions.
我们进行了实验，研究了两个直线排列的推进器中的相互作用模式、集体性能以及三维性的作用。实验中检查了二维翼型和三维展弦比为 2 的矩形机翼。与之前的工作不同，虽然之前曾描述过，这两种相互作用模式（相干尾迹模式和分支尾迹模式）与推进效率没有直接关系，但与峰值推力性能和最小功耗有关，但并未观察到它们与推进效率有直接关系（Boschitsch 等人，2014 年，《物理流体》，26，051901）。实际上，在紧密间隔的推进器中，随动推进器的峰值推进效率出现在其最小功率附近，这种情况揭示了分支尾迹模式。相反，对于间隔较远的推进器，随动推进器的峰值推进效率出现在其峰值推力附近，这种情况揭示了相干尾迹模式。通过检查集体性能，发现推进器之间存在最佳间距，可以最大限度地提高集体效率。对于二维翼型，最佳间距 X=0.75 和相位 ϕ=2π/3 导致集体效率和推力增强分别为 42%和 38%，与两个孤立的翼型相比。相比之下，对于展弦比为 2 的机翼，最佳间距 X=0.25 和相位 ϕ=7π/6 导致集体效率和推力增强分别为 25%和 15%。此外，在最佳条件下，两种二维和三维情况下的集体横向力系数都可以忽略不计，而在这些条件下工作可能会导致不可忽略的横向力。最后，随着二维和三维流动中间距的增加，集体和随动的峰值效率显示出相反的趋势。这与三维流中冲击涡在随动机翼上的破裂有关。这些结果可以帮助设计基于网络的仿生控制元件，这些元件可以通过集成传感来同步三维流相互作用。
