Thrust performance of unsteady propulsors using a novel measurement system, and corresponding wake patterns.

An apparatus is described for the measurement of unsteady thrust and propulsive efficiency produced by biologically inspired oscillating hydrodynamic propulsors. Force measurement is achieved using a strain-gauge-based force transducer, augmented with a lever to amplify or attenuate the applied force and control the measurement sensitivity and natural frequency of vibration. The lever can be used to tune the system to a specific application and it is shown that, using the lever, the stiffness can be made to increase more rapidly than the measurement sensitivity decreases. Efficiency is computed from measurements of the time-averaged power imparted to the fluid. The apparatus is applied to two different propulsors, demonstrating the versatility of the system; wake visualizations are examined, which provide insight into the physical mechanisms of efficient propulsion.