Since the propulsion mechanism in fluid using an elastic fin, such as the caudal fin or the pectoral fin of fish, is effective, a number of studies have examined the use of elastic fins for propulsion in water and the development of fish robots using elastic fins. However, the optimum elasticity of the fin is not constant and changes with the movement task and environment, such as the swimming speed and the oscillating frequency. It is very difficult to exchange fins of different stiffness while the robot is swimming. Thus, we attempt to develop a variable-stiffness fin with a variable-effective-length spring. The apparent stiffness of this spring can be changed dynamically. We constructed a water tunnel to investigate the characteristics of the fin in a uniform flow. The present paper describes the thrust force, thrust efficiency, and flow velocity corresponding to the self propelled speed of the fin in a uniform flow. Furthermore, we developed a flow visualization system and discussed the flow-field around the fin in a uniform flow.
2009 by The Japan Society of Mechanical Engineers