Biomimetic and Live Medusae Reveal the Mechanistic Advantages of a Flexible Bell Margin

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.

How the bending kinematics of swimming lampreys build negative pressure fields for suction thrust

“…This suggests that the role that bending kinematics play in generating thrust is similar among swimming and flying animals. Studies have shown that the magnitude of bending of flexible propulsors affects the amount of thrust produced and that thrust increases with increasing flexibility of propulsors up to an optimal level, after which increasing flexibility diminishes thrust (Alben, 2008;Colin et al, 2012;Lucas et al, 2015;Mountcastle and Daniel, 2010;Tytell et al, 2010). However, we have much less of an understanding of the mechanisms through which bending kinematics interact with adjacent fluids to enhance thrust.…”

“…However, we have much less of an understanding of the mechanisms through which bending kinematics interact with adjacent fluids to enhance thrust. One effect of bending is that it enhances negative pressure regions of the fluid adjacent to the inflexion point of the bending appendage (Colin et al, 2012;Li et al, 2012;Nakata and Liu, 2012). This serves to accelerate more fluid than fluid around rigid propulsors, which enhances the momentum of the surrounding fluid.…”

Passive energy recapture in jellyfish contributes to propulsive advantage over other metazoans

“…Thus, studies investigating the propulsion of jellyfish have primarily focused on the contraction phase (1)(2)(3)(4). Potential advantages in swimming efficiency of gelatinous zooplankton locomotion have been previously overlooked because efficiency of swimming is commonly estimated using the Froude number (E f ) (5-7), a metric originally designed to quantify the propulsive performance of ships.…”

Bending rules for animal propulsion

“…Evaluation of these results has been complicated by a lack of common bending criteria to guide experimental design. Consequently, bending has been projected to occur both evenly over the full length 17,20 and only over a limited portion 21,22 of the propulsor. Most frequently, flexibility has been evaluated in terms of material properties such as elastic modulus 10,17 or flexural stiffness 19,23,24 of the propulsor rather than actual bending patterns.…”