Soft-robotic green sea turtle (Chelonia mydas) developed to replace animal experimentation provides new insight into their propulsive strategies

Abstract

Green sea turtles (Chelonia mydas) can swim up to 50 km per day while only consuming seagrass or microalgae. How the animal accomplishes this vast journey on such low energy intake points to the effectiveness of their swimming technique and is a testament to the power of evolution. Understanding the green sea turtle's ability to accomplish these journeys requires insight into their propulsive strategies. Conducting animal testing to uncover their propulsive strategies brings significant challenges: firstly, the ethical issues of conducting experiments on an endangered animal, and secondly, the animal may not even swim with its regular routine during the experiments. In this work, we develop a new soft-robotic sea turtle that reproduces the real animal's form and function to provide biomechanical insights without the need for invasive experimentation. We found that the green sea turtle may only produce propulsion for approximately 30% of the limb beat cycle, with the remaining 70% exploiting a power-preserving low-drag glide. Due to the animal's large mass and relatively low drag coefficient, losses in swim speed are minimal during the gliding stage. These findings may lead to the creation of a new generation of robotic systems for ocean exploration that use an optimised derivative of the sea turtle propulsive strategy.

An Experimental Study of the Effects of Asymmetric Pitching Motion on the Hydrodynamic Propulsion of a Flapping Fin

“…As mentioned above, the locomotor pattern was minimized to three degrees of freedom, which was based on recent work by van der Geest et al [16,22]. The fin equations of motion (plotted in Figure 4) in flapping and sweeping are expressed using a Fourier series, with n = 8 (Equations ( 1) and ( 2)), and the pitching motion is expressed as a linear piecewise function (Equation ( 3)) with (t = 0) set at the end of the sweep stroke (SS) and the beginning of recovery stroke one (RS1):…”

“…They found that increasing the St value, while keeping the α max relatively small (less than 20 • ), led to an increase in the hydrofoil's thrust coefficient without significant changes in propulsion efficiency. Additionally, Nick et al [16] created a model based on the green sea turtle (Chelonia mydas) and developed a specialized testing rig to uncover the reason for the sea turtle's upstroke strategy, revealing that the utilization of a passive upstroke substantially reduces the animal's drag coefficient. Kumar et al [17] demonstrated that flapping fins, when moving in a compound harmonic motion, can generate thrust, and the propulsive performance is correlated with the frequency.…”

New Insights into Sea Turtle Propulsion and Their Cost of Transport Point to a Potential New Generation of High-Efficient Underwater Drones for Ocean Exploration

“…This flapping motion allows the sea turtle migration of thousands of kilometres to reach favourable breeding or feeding grounds [1][2][3][4][5]. The flapping motion has typically been described as asymmetric, with the downstroke approximately twice as fast as the upstroke [6][7][8]. In recent work by van der Geest et al [6], the flapping motion was described three-dimensionally for the Green sea turtle (Chelonia mydas), including the soft twisting of the flipper.…”

“…The authors described the flipper motion by breaking it up into five segments consisting of the Downstroke (DS), Sweep stroke (SS), Recovery stroke one (RS1), Upstroke (US) and, finally, Recovery stroke two (RS2) (Figure 1). It is understood that during the Green turtle's general flapping routine, the upstroke does not generate any thrust [6,[8][9][10]; however, during this time, the animal's drag coefficient is lowered to help reduce swim speed losses [10]. To quantify how the drag is reduced during the upstroke, van der Geest et al [10] conducted dedicated work to uncover the flow features generated by the flipper during this period.…”

“…However, though this work is insightful, it does not cover what flow features occur during the sea turtles 2 of 14 (Chelonia mydas) propulsive cycle consisting of the downstroke and sweep stroke. The propulsion cycle in sea turtles has been previously described as only occurring during the downstroke period, with peak thrust occurring at the end of the downstroke [8,9], with the most recent works taking very different approaches to uncover the propulsive cycle. David T. Booth [9] found the propulsive cycle by studying green turtle hatchlings in a flume, while van der Geest et al [8] developed a full-scale robotic sea turtle based on the green turtle that could reproduce the natural animals swimming patterns.…”