Transition phase in the static thrust generation of the biomimetic fin with low oscillating frequency

Abstract

Biomimetic fin propulsion could be a promising solution for an efficient underwater propulsion mechanism. It could be designed to generate thrust for underwater locomotion efficiently. Many studies have proposed that the flexibility characteristics of the fin affect its effectiveness in thrust generation; for example, a flexible fin generates more thrust than a rigid fin. In this regard, the rigid fin may suffer a mechanical disadvantage in thrust generation. This study introduces the presence of thrust generation phases in biomimetic fins. The phases could be caused by the interaction of the fins and the surrounding fluid. To distinguish the phases clearly, the experimental setup in this study was designed for no-flow conditions. This study presents three phases of thrust generation: negative, transition, and positive. The existence of the negative and transition phases explains the mechanical disadvantages of the rigid fin. Within the range of evaluated fin frequencies, approximately 80% of the average net force of the rigid fin is in the negative and transition phases, compared to only 20% in flexible fins. In comparison to less flexible and rigid fins, a flexible fin could maximize positive thrust production three times higher at high frequency. The vector composition analysis and dye-injection flow visualization reveal the transition phase by emphasizing the balancing process between the surface friction of the fin and the inertial component of the force of the fluid and fin interaction. This study demonstrates the independence of the transition phase from the flexibility characteristics of the biomimetic fin. Because the bending characteristic of the flexible fin could direct more vectors in thrust generation, the fin could act as a thrust vectoring agent. The findings of this study could be used as a guide in designing and implementing high-performance fin propulsion in low-speed underwater locomotion.