Abstract
Flexible bell margins are characteristic components of rowing medusan morphologies and are expected to contribute towards their high propulsive efficiency. However, the mechanistic basis of thrust augmentation by flexible propulsors remained unresolved, so the impact of bell margin flexibility on medusan swimming has also remained unresolved. We used biomimetic robotic jellyfish vehicles to elucidate that propulsive thrust enhancement by flexible medusan bell margins relies upon fluid dynamic interactions between entrained flows at the inflexion point of the exumbrella and flows expelled from under the bell. Coalescence of flows from these two regions resulted in enhanced fluid circulation and, therefore, thrust augmentation for flexible margins of both medusan vehicles and living medusae. Using particle image velocimetry (PIV) data we estimated pressure fields to demonstrate a mechanistic basis of enhanced flows associated with the flexible bell margin. Performance of vehicles with flexible margins was further enhanced by vortex interactions that occur during bell expansion. Hydrodynamic and performance similarities between robotic vehicles and live animals demonstrated that the propulsive advantages of flexible margins found in nature can be emulated by human-engineered propulsors. Although medusae are simple animal models for description of this process, these results may contribute towards understanding the performance of flexible margins among other animal lineages.
Highlights
Medusae are thought to be the first metazoans to evolve muscular swimming
The utilization of robotic vehicles to examine biological propulsion has provided insight into just how important flexible propulsors are for some swimming animals
Visualizations are necessary to begin to understand the mechanistic basis of the enhancement in propulsion provided by flexible propulsors and begin applying these principles more generally
Summary
Medusae are thought to be the first metazoans to evolve muscular swimming. The simplicity yet functionality of the biomechanics of their swimming make them an ideal organismal model for studying fundamental mechanisms of animal propulsion. Many medusan species swim using rowing propulsion [1,2,3] To do so, they rhythmically contract epitheliomuscular cells which line the under, or subumbrellar, surface of their bells. They rhythmically contract epitheliomuscular cells which line the under, or subumbrellar, surface of their bells These circular muscles do not extend to the tips of the bell Pers.comm.) and so as the bell contracts and expands the highly flexible bell margin undergoes substantial flapping deformations during these cyclic movements. These motions generate large starting and stopping vortices during the contraction and expansion phases of the cycle, respectively [2]. The influence of bell margin flexibility on vortex formation and thrust production at the bell margin has not been evaluated but has emerged as a critical characteristic of rowing propulsion and for the design of robotic jellyfish vehicles
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