The Use of Flexible Biomimetic Fins in Propulsion

Abstract

This thesis documents a series of investigations exploring the role of stiffness profile in propulsion using pitching flexible fins. Stiffness profile is defined as the variation in local bending stiffness along the chord of a fin, from leading to trailing edge. An unmanned robotic submarine was created, using simple pitching flexible fins for propulsion. Its design and performance prompted a review of literature covering many aspects of oscillating fin propulsion, paying special attention to the studies of pitching flexible fins, of the type used in the submarine. In the body of previous work, fin stiffness profile was a consequence of the external shape profile of a fin; fins had not thus far been designed with stiffness profile specifically in mind. A hypothesis was proposed: “Use of a biomimetic fin stiffness profile can improve the effectiveness of a flexible oscillating fin, over that of a standard NACA designated fin shape.” Rectangular planform flexible fins of standard NACA 0012 design and 1:1 aspect ratio were tested alongside similar fins with a stiffness profile mimicking that of a pumpkinseed sunfish (Lepomis gibbosus). The fins were oscillated with a pitching-only sinusoidal motion over a range of frequencies and amplitudes, while torque, lateral force and static thrust were measured. Over the range of oscillation parameters tested, it was shown that the fin with a biomimetic stiffness profile offered a significant improvement in static thrust over a fin of similar dimensions with a standard NACA 0012 aerofoil shape, and produced thrust more consistently over each oscillation cycle. A comparison of different moulding materials showed that the improvement was due to the stiffness profile itself, and was not simply an effect of altering the overall stiffness of the fin, or changing its natural frequency. Within the range of stiffnesses and oscillation conditions tested, fins of the same stiffness profile were found to follow similar thrust-power curves, independently of their moulding material. Biomimetic fins were shown to produce between 10% and 25% more thrust per watt of mechanical input power.

Date of Award	1 Jun 2010
Original language	English
Awarding Institution	University of Bath
Supervisor	Adrian Bowyer (Supervisor) & Julian Vincent (Supervisor)