The most efficient metazoan swimmer creates a 'virtual wall' to enhance performance

Fish can use coordinated fin motions to recapture their own vortex wake energy

During swimming, many fishes use pectoral fins for propulsion and, in the process, move substantial amounts of water rearward. However, the effect that this upstream wake has on the caudal fin remains largely unexplored. By coordinating motions of the caudal fin with the pectoral fins, fishes have the potential to create constructive flow interactions which may act to partially recapture the upstream energy lost in the pectoral fin wake. Using experimentally derived velocity and pressure fields for the silver mojarra (Eucinostomus argenteus), we show that pectoral-caudal fin (PCF) coordination enables the circulation and interception of pectoral fin wake vortices by the caudal fin. This acts to transfer energy to the caudal fin and enhance its hydrodynamic efficiency at swimming speeds where this behaviour occurs. We also find that mojarras commonly use PCF coordination in nature. The results offer new insights into the evolutionary drivers and behavioural plasticity of fish swimming as well as for developing more capable bioinspired underwater vehicles.

Jelly-Z: swimming performance and analysis of twisted and coiled polymer (TCP) actuated jellyfish soft robot

Monitoring, sensing, and exploration of over 70% of the Earth's surface that is covered with water is permitted through the deployment of underwater bioinspired robots without affecting the natural habitat. To create a soft robot actuated with soft polymeric actuators, this paper describes the development of a lightweight jellyfish-inspired swimming robot, which achieves a maximum vertical swimming speed of 7.3 mm/s (0.05 body length/s) and is characterized by a simple design. The robot, named Jelly-Z, utilizes a contraction-expansion mechanism for swimming similar to the motion of a Moon jellyfish. The objective of this paper is to understand the behavior of soft silicone structure actuated by novel self-coiled polymer muscles in an underwater environment by varying stimuli and investigate the associated vortex for swimming like a jellyfish. To better understand the characteristics of this motion, simplified Fluid-structure simulation, and particle image velocimetry (PIV) tests were conducted to study the wake structure from the robot's bell margin. The thrust generated by the robot was also characterized with a force sensor to ascertain the force and cost of transport (COT) at different input currents. Jelly-Z is the first robot that utilized twisted and coiled polymer fishing line (TCPFL) actuators for articulation of the bell and showed successful swimming operations. Here, a thorough investigation on swimming characteristics in an underwater setting is presented theoretically and experimentally. We found swimming metrics of the robot are comparable with other jellyfish-inspired robots that have utilized different actuation mechanisms, but the actuators used here are scalable and can be made in-house relatively easily, hence paving way for further advancements into the use of these actuators.

Free swimming of a squid-inspired axisymmetric system through jet propulsion

An axisymmetric fluid-structure interaction model based on the immersed-boundary approach is developed to study the self-propelled locomotion of a squid-inspired swimmer in relatively low Reynolds numbers (O(102)). Through cyclic deformation, the swimmer generates intermittent jet flow, which, together with the added-mass effect associated with the body deformation, provides thrust. Through a control volume analysis we are able to determine the jet-related thrust. By adding it to the added-mass-related thrust we separate the net thrust on the body from the drag effect due to forward motion, so that the propulsion efficiency in free swimming is found. This numerical algorithm and thrust-drag decomposition method are used to study the dynamics of the bio-inspired locomotion system in different conditions, whereby the performance is characterized by the aforementioned propulsion efficiency as well as the conventionally defined cost of transport.

Exploring the sensitivity in jellyfish locomotion under variations in scale, frequency, and duty cycle

Jellyfish have been called one of the most energy-efficient animals in the world due to the ease in which they move through their fluid environment, by product of their bell kinematics coupled with their morphological, muscular, material properties. We investigated jellyfish locomotion by conducting in silico comparative studies and explored swimming performance across different fluid scales (i.e., Reynolds Number), bell contraction frequencies, and contraction phase kinematics (duty cycle) for a jellyfish with a fineness ratio of 1 (ratio of bell height to bell diameter). To study these relationships, an open source implementation of the immersed boundary method was used (IB2d) to solve the fully coupled fluid-structure interaction problem of a flexible jellyfish bell in a viscous fluid. Thorough 2D parameter subspace explorations illustrated optimal parameter combinations in which give rise to enhanced swimming performance. All performance metrics indicated a higher sensitivity to bell actuation frequency than fluid scale or duty cycle, via Sobol sensitivity analysis, on a higher performance parameter subspace. Moreover, Pareto-like fronts were identified in the overall performance space involving the cost of transport and forward swimming speed. Patterns emerged within these performance spaces when highlighting different parameter regions, which complemented the global sensitivity results. Lastly, an open source computational model for jellyfish locomotion is offered to the science community that can be used as a starting place for future numerical experimentation.

Hopscotching jellyfish: combining different duty cycle kinematics can lead to enhanced swimming performance

Jellyfish (Medusozoa) have been deemed the most energy-efficient animals in the world. Their bell morphology and relatively simple nervous systems make them attractive to robotocists. Although, the science community has devoted much attention to understanding their swimming performance, there is still much to be learned about the jet propulsive locomotive gait displayed by prolate jellyfish. Traditionally, computational scientists have assumed uniform duty cycle kinematics when computationally modeling jellyfish locomotion. In this study we used fluid-structure interaction modeling to determine possible enhancements in performance from shuffling different duty cycles together across multiple Reynolds numbers and contraction frequencies. Increases in speed and reductions in cost of transport were observed as high as 80% and 50%, respectively. Generally, the net effects were greater for cases involving lower contraction frequencies. Overall, robust duty cycle combinations were determined that led to enhanced or impeded performance.

Cool your jets: biological jet propulsion in marine invertebrates

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.

The most efficient metazoan swimmer creates a 'virtual wall' to enhance performance

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.