It has been well documented that animals (and machines) swimming or flying near a solid boundary get a boost in performance^1-6. This ground effect is often modeled 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 wall⁷. 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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最高效的后生游泳者创造了“虚拟墙”以增强性能

有充分的文献记载，在坚固边界附近游泳或飞行的动物（和机器）的性能提高^1-6。该地面效应通常被建模为一对镜像的涡旋之间的相互作用，该镜像对由真实涡旋与壁⁷另一侧的相反符号“虚拟涡旋”表示。但是，大多数动物不会在固体表面附近游泳，因此对开放水域游泳者体内旋涡-旋涡相互作用的研究很少。在这项研究中，我们研究了迄今为止已知的能量效率最高的后生游泳者，水母Aurelia aurita阐明旋涡相互作用在游离实心边界的动物中的作用。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）以及从流速矢量中提取压力场的算法，以量化游泳性能和近身涡旋-涡旋相互作用的影响。在这里，我们显示出在先前游泳周期中在动物下方形成的涡流环（停止涡流）对于提高推进性能至关重要。这种定位良好的停止涡流在壁效性能增强期间的作用与虚拟涡流相同，其方式是帮助流体在推进表面的下方汇聚并产生明显更高的压力，从而产生更大的推力。这些发现主张，通过在两个真实的，相反的符号涡流环之间形成相当于“虚拟壁”的水母，可以在开阔水域中产生壁效应。这解释了水母相对于其他后生动物具有显着的推进优势，并且对生物工程推进系统具有重要意义。