It has been well documented that animals (and machines) swimming or flying near a solid boundary get a boost in performance. This ground effect is often modelled as an interaction between a mirrored pair of vortices represented by a true vortex and an opposite sign ‘virtual vortex’ on the other side of the wall. However, most animals do not swim near solid surfaces and thus near body vortex–vortex interactions in open-water swimmers have been poorly investigated. In this study, we examine the most energetically efficient metazoan swimmer known to date, the jellyfish Aurelia aurita, to elucidate the role that vortex interactions can play in animals that swim away from solid boundaries. We used high-speed video tracking, laser-based digital particle image velocimetry (dPIV) and an algorithm for extracting pressure fields from flow velocity vectors to quantify swimming performance and the effect of near body vortex–vortex interactions. Here, we show that a vortex ring (stopping vortex), created underneath the animal during the previous swim cycle, is critical for increasing propulsive performance. This well-positioned stopping vortex acts in the same way as a virtual vortex during wall-effect performance enhancement, by helping converge fluid at the underside of the propulsive surface and generating significantly higher pressures which result in greater thrust. These findings advocate that jellyfish can generate a wall-effect boost in open water by creating what amounts to a ‘virtual wall’ between two real, opposite sign vortex rings. This explains the significant propulsive advantage jellyfish possess over other metazoans and represents important implications for bio-engineered propulsion systems.

中文翻译：

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最高效的后生动物游泳者创造了一个“虚拟墙”以提高表现

有充分证据表明，动物（和机器）在固体边界附近游泳或飞行会提高性能。这种地面效应通常被建模为一对镜像涡流之间的相互作用，该涡流由真涡流和墙另一侧的相反符号“虚拟涡流”表示。然而，大多数动物不在固体表面附近游泳，因此在开放水域游泳者的身体涡-涡相互作用附近的研究很少。在这项研究中，我们检查了迄今为止已知的能量最高的后生动物游泳者 Aurelia aurita，以阐明涡流相互作用在远离固体边界的动物中可以发挥的作用。我们使用了高速视频跟踪，基于激光的数字粒子图像测速 (dPIV) 和一种从流速向量中提取压力场的算法，以量化游泳性能和近体涡旋相互作用的影响。在这里，我们展示了在前一个游泳周期中在动物下方产生的涡环（停止涡）对于提高推进性能至关重要。这种定位良好的止动涡流与壁效应性能增强过程中的虚拟涡流的作用相同，通过帮助推进表面下侧的流体会聚并产生显着更高的压力，从而产生更大的推力。这些发现表明，水母可以通过在两个真正的、符号相反的涡环之间创造相当于“虚拟墙”的东西，在开阔水域产生壁效应增强。