A Novel Aquatic Propulsor Inspired by Mobuliform Swimming

We present a new type of flexible propulsor for autonomous underwater vehicles (AUVs), whose design was inspired by biological observation but also includes elements from classical marine hydrodynamics. The interest in bioinspired AUVs with compliant thrusters that can take advantage of fluid-structure interaction phenomena has significantly grown in recent years because many swimming animals still outmatch their human-made counterparts in terms of agility, efficiency, controlled maneuverability, and even in some cases, speed. Most researchers working on bioinspired fluid locomotion believe that systems with flexible thrusters can potentially outperform rigid platforms. The design of the thruster proposed here draws from mobuliform swimming, characterized by the flapping motion of wing-like pectoral fins, but also from traditional hydrodynamic apparatuses that use rigid cambered surfaces to augment thrust production in comparison with flat and symmetric surfaces. Specifically, the fin of the proposed thruster was conceived to oscillate at similar frequencies as those reported for some large batoid fishes (ranging from 0.4 Hz to 1.0 Hz) and have a large planform area relative to the size of the AUVs to be propelled. Additionally, the fin, with a rectangular area and the very low aspect ratio of 0.7, is allowed to pitch using a passive rotation mechanism, and its flexible membrane is allowed to undulate due to the reactive hydrodynamic forces acting on its structure. By leveraging its ability to undulate and constraining two of its borders, the flexible membrane of the fin is forced to adopt cambered shapes, which are favorable for thrust production, during different phases of a flapping cycle. Experimental results obtained using a grounded setup demonstrate the suitability of the presented approach. Further-more, a preliminary analysis of the experimental data indicates that, for experimentally-identified optimal flapping conditions, the proposed propulsor is endowed with superior dynamic characteristics in terms of the output-thrust-to-input-power ratio relative to that of a rigid benchmark thruster.