Underwater navigation with an elastic plunging fin.

Fish can effectively achieve gradual turning and escape maneuvers using multiple fins. However, for a robotic fish it might be advantageous to use a single fin to generate thrust as well as control the fish navigation. We solved the fluid-structure interaction problem using three-dimensional computer simulations based on the lattice Boltzmann and lattice spring models to investigate turning strategies for an oscillating rectangular elastic fin actuated near its first natural frequency in a Newtonian fluid. We probe how to produce pitching and yawing moments by altering the fin plunging. The first strategy uses plunging with asymmetric stroke velocities to create pitching. The second, combines plunging with a twisting motion to create a yawing moment. In both cases, net lateral forces and moments are generated. We found that in the case of asymmetric plunging an increasing velocity ratio between strokes led to increased magnitude of force and turning moment. We identified the optimum correlation between the phase angle and the rotation amplitude maximizing the yaw moment, in which case the phase must be tuned so that the instant of the maximum force overlays with the instant of the maximum rotation.