Computational modeling of swimming in marine invertebrates with implications for soft swimming robots

Marine flatworms (polyclads) employ a wide variety of body kinematics for swimming. In the current study, we employ computational fluid dynamics to study the hydrodynamics and swimming performance of a large variety of swimmers inspired directly from flatworms as well as two other marine invertebrates: Aplysia and Spanish dancers. The free-swimming performance is evaluated via two metrics: Froude efficiency and terminal swimming speed. The study examines the effect of the flapping of the lateral margins of the body as well as body undulation, which are used in various combinations by these animals to achieve swimming. The simulations suggest that a spanwise compact wake with distinct vortex ring structures is well correlated with a high swimming performance. We find that the addition of even a small magnitude of body undulation to lateral flapping results in significant changes in the wake patterns and noticeable improvements in the swimming performance compared to swimmers that employ only lateral flapping. Periodic body-bending synchronized with lateral flapping, as employed by the Spanish dancer, is found to be a very effective swimming gait. Some gaits that employ body undulations but no lateral flapping are found to generate high swimming speeds but with limited swimming efficiencies. Taken together, this study provides insights that could inform the design of swimming robots.