Propulsive Performance of a Two Degree-of-Freedom Fish Platform*

A two degree-of-freedom fish platform was used to investigate the relationship between simplified fish kinematics and propulsive performance. Its design, construction, and actuation provide control of maximum trailing-edge excursion; heave-to-pitch ratio; and phase offset between the tail and caudal fin while the oscillation frequency is fixed. Thrust and power input were measured to evaluate the performance of each parameter set while the flow field was measured using stereo particle image velocimetry. A parametric sweep of all parameters, excluding frequency, was used to evaluate the performance and flow field. This space included and extended beyond the known biological space. It was found that within the biological range of 0.20 < St < 0.40, the efficiency was maximized for a heave-to-pitch ratio of 0.75 and a phase offset of 115 degrees. For a given heave-to-pitch ratio, the thrust was maximized for a phase offset between 93 and 115 degrees and the power input was minimized for a phase offset between 115 and 135 degrees depending on the amplitude of the trailing-edge excursion. This work provides insight into the relationship between near-field vortex dynamics (finlet vortex, leading-edge vortex, and forming trailing-edge vortex) and propulsive performance.