Since the propulsion mechanism using elastic fins, such as the caudal fin and pectoral fin of fish, is effective in fluid, many studies on bioinspired elastic fins for propulsion in water and on the development of fish-type robots with elastic fins have been carried out. The optimum elasticity of the fin is not constant and varies according to the movement task and environment, such as swimming speed and oscillating frequency. However, it is very difficult to exchange fins of different stiffnesses while moving. Thus, we aimed to develop a variable-stiffness fin of which stiffness can be changed dynamically. As the one such variable-stiffness fin, we have developed a fin with a variable-effective-length spring. The effective length of a plate spring is changed by adjusting the length of the rigid plate that supports the plate spring. Apparent stiffness is changed by varying the effective length. In this paper, we have described the structure of the propulsion mechanism in fluid using a fin with a variable-effective-length spring, and the thrust force characteristics in water. Furthermore, we have discussed the optimum effective length for providing the maximum thrust force and the effect of the dynamic change of effective length on thrust force.
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.
In this study, we have developed a propulsion mechanism using a fin with variable effective length spring as variable stiffness mechanics. We measured its thrust and the lateral forces in water tank with three dimensional flow and that with quasi two dimensional environment. Additionally, three dimensional numerical analysis of the propulsion mechanism was performed. Flow field around the fin and variations of thrust and lateral forces obtained in this numerical analysis were compared with the above experimental results in the condition of rigid fin. As a result, there was a quantitative agreement between the experiments and numerical analyses. Because the computational costs of three-dimensional fluid-structure interaction analyses are very expensive, two dimensional fluid-structure interaction analyses were performed in the conditions of elastic fins. Qualitative agreement was observed between the fluid-structure interaction analyses and the above experimental results of quasi-two dimensional environment. In these conditions, an evaluated value of thrust efficiency increased with the decrease of the stiffness of the fin. The flow fields around the elastic fins were visualized to discuss the flow dynamics around the fin. From this result, it was shown that the direction of jet flow generated by vortex pair at the fin edge was closed to the rear direction of the propulsion mechanism by decreasing the stiffness of the fin.
Since the propulsion mechanism in fluid using an elastic fin, such as the caudal fin or the pectoral fin of fish, is effective. However, the optimum elasticity of the fin is not constant and changes with the movement task and environment. 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. The present paper describes the thrust force, and flow velocity corresponding to the self propelled speed of the fin in a uniform flow. And we compared the thrust efficiency in a uniform flow with the evaluating value of the thrust efficiency in a no flow. Furthermore, we developed a flow visualization system and discussed the flow-field around the fin in a uniform flow.
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