Swimming at Higher Resonant Frequencies for Improved Thrust in Flexible Propulsors*

To swim at high speeds, biological swimmers (e.g. Tuna) oscillate their caudal fins at high frequencies, actively changing their body stiffness to efficiently amplify their tail-beat velocity. Similarly, a propulsor's higher resonant frequencies can be used to amplify tail-beat velocity. This study experimentally investigates flexible propulsors operating at their non-fundamental resonant frequencies (up to fifth natural frequency, ~50Hz). The flexible propulsors are thin aluminum beams in a clamped-free configuration in quiescent water excited by two pairs of antagonistic macro fiber composite (MFC) piezoelectric actuators. The maximum thrust of each propulsor occurs at the third or fourth resonance and corresponds with the frequency of maximum tail-beat velocity. The thrust coefficient (ratio of thrust to tail-beat velocity squared) decreases for increasing resonant frequency; this likely stems from the greater curvature in the propulsor deformation at higher mode shapes. Thus, higher resonant frequencies can produce greater thrust, but are less efficient at converting tail-beat velocity into thrust. Higher resonant frequencies could be useful as an acceleration method: temporarily gaining increased thrust but at lower efficiency, similar to acceleration in biological swimming.