Enhancing bio-locomotion of elastic propulsors by combining internal and external actuation

We investigate using three-dimensional computer simulations the hydrodynamics of a bio-inspired elastic propulsor that undergoes plunging motion in a Newtonian fluid. To enhance the hydrodynamic performance of the propulsor, in addition to the heaving motion at the base, the propulsor is actuated by a periodic internal bending moment imposed along the propulsor length. We vary the phase difference between the two actuation modes to probe their synergetic effect on the propulsor hydrodynamics. The simulations reveal that by tuning the phase difference, the propulsor thrust and free-swimming velocity can be varied in a wide range while maintaining high hydrodynamic efficiency. Furthermore, the propulsor with the hybrid actuation significantly outperforms the propulsors with either of the single actuation modes. We relate the improved performance to reduced center of mass displacement with increased trailing edge displacement of the propulsor achieved at specific values of the phase difference. The results of our study are useful for designing highly efficient robotic swimmers utilizing propulsors made of smart materials such as piezoelectric macro-fiber composites.