Aspect ratio affects the equilibrium altitude of near-ground swimmers
Aspect ratio affects the equilibrium altitude of near-ground swimmers
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DOI：
10.1017/jfm.2021.307
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发表时间：
2021-04
影响因子：
3.7
通讯作者：
Q. Zhong;Tianjun Han;K. Moored;D. Quinn
中科院分区：
文献类型：
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作者：
Q. Zhong;Tianjun Han;K. Moored;D. Quinn
动物和仿生机器人可以在固体表面附近更快地游泳/飞行，效率几乎没有损失。这些好处如何随着推进器展弦比而变化尚不清楚。在这里，我们表明，降低展弦比削弱非定常地面效应，推力增强变得不那么明显，稳定的平衡高度移动较低，变得较弱，尾流不对称变得不那么明显。水槽实验和势流模拟表明，这些影响是一致的，与已知的非定常气动缩放。我们还发现了第二个平衡高度，甚至更接近墙壁（0.35弦长）。这种第二平衡是不稳定的，特别是对于高深宽比的翼型。因此，对于希望获得近地面游泳的全部益处的高纵横比游泳者，可能需要主动控制。纵横比改变近地推进力的事实表明，它可能是在海底或湖面附近游泳/飞行的动物和机器人的关键设计参数。
Abstract Animals and bio-inspired robots can swim/fly faster near solid surfaces, with little to no loss in efficiency. How these benefits change with propulsor aspect ratio is unknown. Here we show that lowering the aspect ratio weakens unsteady ground effect, thrust enhancements become less noticeable, stable equilibrium altitudes shift lower and become weaker and wake asymmetries become less pronounced. Water-channel experiments and potential flow simulations reveal that these effects are consistent with known unsteady aerodynamic scalings. We also discovered a second equilibrium altitude even closer to the wall (${<}0.35$ chord lengths). This second equilibrium is unstable, particularly for high-aspect-ratio foils. Active control may therefore be required for high-aspect-ratio swimmers hoping to get the full benefit of near-ground swimming. The fact that aspect ratio alters near-ground propulsion suggests that it may be a key design parameter for animals and robots that swim/fly near a seafloor or surface of a lake.