An investigation on the performance of an oscillating flat plate fin with compliant joint for underwater robotic actuation

Fishes are very good swimmers possessing impeccable maneuverability, efficiency, and stealth. Understanding the fish propulsion and mimicking the same is important in obtaining improved performance by non-conventional types of underwater robot/vehicle actuators. The role of the caudal fin in fish locomotion is predominant at higher swimming speeds producing most of the thrust force. Generally the fin motions are oscillatory in nature. In oscillatory trajectories, where reversal of motion is required, a normal actuator has to spend energy to accelerate as well as to decelerate(braking). During braking phase, a normal actuator, without any energy storage mechanism, dissipates energy. Ubiquitous presence of compliance in biological actuation system can store some energy which otherwise get dissipated. Fishes are evolved to tune their compliant joint stiffness according to the swimming conditions. The elastic joint(contributed by compliant muscle) can store energy in part of a cycle during fin oscillation and interaction with surrounding fluid in the form of potential energy and can release in next part of the cycle. This article investigates the role of compliance in the fin joint on propulsion performance through simulations. The analysis considers a rigid body model of trapezoidal flat fin with elastic joint and presents hydrodynamic force modeling for interacting fluid in Newton-Euler rigid body framework using blade element technique.