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1. The most efficient metazoan swimmer creates a ‘virtual wall’ to enhance performance

   https://doi.org/10.1098/rspb.2020.2494  

   Gemmell, Brad J. ; Du Clos, Kevin T. ; Colin, Sean P. ; Sutherland, Kelly R. ; Costello, John H. ( January 2021 , Proceedings of the Royal Society B: Biological Sciences)

   null (Ed.)

   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. 

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2. Jellyfish and Fish Solve the Challenges of Turning Dynamics Similarly to Achieve High Maneuverability

   https://doi.org/10.3390/fluids5030106  

   Dabiri, John O. ; Colin, Sean P. ; Gemmell, Brad J. ; Lucas, Kelsey N. ; Leftwich, Megan C. ; Costello, John H. ( September 2020 , Fluids)

   Turning maneuvers by aquatic animals are essential for fundamental life functions such as finding food or mates while avoiding predation. However, turning requires resolution of a fundamental dilemma based in rotational mechanics: the force powering a turn (torque) is favored by an expanded body configuration that maximizes lever arm length, yet minimizing the resistance to a turn (the moment of inertia) is favored by a contracted body configuration. How do animals balance these opposing demands? Here, we directly measure instantaneous forces along the bodies of two animal models—the radially symmetric Aurelia aurita jellyfish, and the bilaterally symmetric Danio rerio zebrafish—to evaluate their turning dynamics. Both began turns with a small, rapid shift in body kinematics that preceded major axial rotation. Although small in absolute magnitude, the high fluid accelerations achieved by these initial motions generated powerful pressure gradients that maximized torque at the start of a turn. This pattern allows these animals to initially maximize torque production before major body curvature changes. Both animals then subsequently minimized the moment of inertia, and hence resistance to axial rotation, by body bending. This sequential solution provides insight into the advantages of re-arranging mass by bending during routine swimming turns. 

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3. Cool your jets: biological jet propulsion in marine invertebrates

   https://doi.org/10.1242/jeb.222083  

   Gemmell, Brad J. ; Dabiri, John O. ; Colin, Sean P. ; Costello, John H. ; Townsend, James P. ; Sutherland, Kelly R. ( June 2021 , Journal of Experimental Biology)

   null (Ed.)

   ABSTRACT Pulsatile jet propulsion is a common swimming mode used by a diverse array of aquatic taxa from chordates to cnidarians. This mode of locomotion has interested both biologists and engineers for over a century. A central issue to understanding the important features of jet-propelling animals is to determine how the animal interacts with the surrounding fluid. Much of our knowledge of aquatic jet propulsion has come from simple theoretical approximations of both propulsive and resistive forces. Although these models and basic kinematic measurements have contributed greatly, they alone cannot provide the detailed information needed for a comprehensive, mechanistic overview of how jet propulsion functions across multiple taxa, size scales and through development. However, more recently, novel experimental tools such as high-speed 2D and 3D particle image velocimetry have permitted detailed quantification of the fluid dynamics of aquatic jet propulsion. Here, we provide a comparative analysis of a variety of parameters such as efficiency, kinematics and jet parameters, and review how they can aid our understanding of the principles of aquatic jet propulsion. Research on disparate taxa allows comparison of the similarities and differences between them and contributes to a more robust understanding of aquatic jet propulsion. 

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4. Amphibious Bioinspired Robots for Ocean Objects Identification

   https://doi.org/10.2514/6.2021-2781  

   Fill, Dominik ; Strauss, Emily ; Dunning, Cole ; Western, Anastasia ; Hassanalian, Mostafa ( July 2021 , AIAA SCITECH 2022 Forum)

   Agility, robustness, endurance, and sustainability are the main challenges of the current distributed systems for ocean objects identification. Nowadays, developing a novel marine observation network to help identify threats and to provide both an early warning and data for forecasting models is a priority of marine missions. Autonomous systems, such as underwater robots and drones, can provide worthwhile information from the ocean environment; still, they have challenges associated with endurance, performance, and recovery. Skimming drones cannot be used to perform underwater missions, need a significant amount of energy to take off, and have stability problems due to the constant ocean wave motion. As for underwater swimming robots, they are generally slow and use a significant amount of energy. To this end, there is a need to design some novel bioinspired amphibious concepts that can overcome these challenges. In this paper, a network of distributed hybrid-amphibious robots with energy harvesting capabilities will be presented. This is accomplished through novel robot systems. The Lizard-Spider Octopus-Jellyfish-Rolling Robot (LSOJRR) is one of these novel ideas, which imitates the characteristics of a Golden wheel spider with rolling, jumping, and folding capabilities over the water, a Green Basilisk lizard with running capability over the water, and an octopus with unique underwater propulsion mechanism. The LSOJRR also has applications beyond Earth, and alternative designs of this robot are explored, particularly those involving the dispersal of swarms of smaller robots that also derive their design from biology. All of the designs presented in this paper draw inspiration from nature, and strive to achieve the goal of furthering the development for marine exploration. 

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5. Flexibility is a hidden axis of biomechanical diversity in fishes

   https://doi.org/10.1242/jeb.245308  

   Jimenez, Yordano E. ; Lucas, Kelsey N. ; Long, John H. ; Tytell, Eric D. ( April 2023 , Journal of Experimental Biology)

   ABSTRACT Nearly all fish have flexible bodies that bend as a result of internal muscular forces and external fluid forces that are dynamically coupled with the mechanical properties of the body. Swimming is therefore strongly influenced by the body's flexibility, yet we do not know how fish species vary in their flexibility and in their ability to modulate flexibility with muscle activity. A more fundamental problem is our lack of knowledge about how any of these differences in flexibility translate into swimming performance. Thus, flexibility represents a hidden axis of diversity among fishes that may have substantial impacts on swimming performance. Although engineers have made substantial progress in understanding these fluid–structure interactions using physical and computational models, the last biological review of these interactions and how they give rise to fish swimming was carried out more than 20 years ago. In this Review, we summarize work on passive and active body mechanics in fish, physical models of fish and bioinspired robots. We also revisit some of the first studies to explore flexural stiffness and discuss their relevance in the context of more recent work. Finally, we pose questions and suggest future directions that may help reveal important links between flexibility and swimming performance. 

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