The Effect of Flexibility and Aspect Ratios on Ribbon-Fin-Propulsion

Ribbon-fin-based propulsion has the potential to improve the maneuverability of underwater vehicles in
complex environments. In this type of propulsion a series of rays are used to send traveling waves
along an elongated fin, which is referred to as ribbon fin. In this work, in order to know the effect of
flexural rigidities and aspect ratios on undulating ribbon fin propulsion; we built a robotic ribbon fin, and
tested the physical model in a water flume. In a series of experiments we measured the propulsive
force, power consumption and the free-swimming speed of the robotic fin as a function of wave
frequency for fins with different ray stiffness and aspect ratios. The propulsive performance of the
robotic ribbon fin was based on the propulsive force generated and power consumption. A series of
kinematic experiments were performed using a high-speed camera. Based on the fin kinematics, the
natural frequencies of the ribbon fin with different stiffness were determined. We found that the flexible
rays would improve or worsen the propulsive performance compared to a rigid counterpart depending
on the actuation parameters. For the aspect ratios considered, the propulsive efficiency improves with
increase in the fin height. Our data suggest that, the ribbon fin can yield best propulsive behavior close
to its natural frequency.