| Summary: | Tunas, lamnid sharks and whales are some of the fastest sustained swimming animals. To propel themselves these animals use the thunniform propulsion mode, and are physically characterized by having streamlined bodies with narrow necking of the caudal peduncle and a high aspect ratio lunate tail generating lift-based thrust. For these reasons, thunniform propulsion has received considerable attention from biologists and bio-inspired engineers. Thunniform propulsion is assumed to have the highest propulsive performance of all swimming modes, meaning high propulsive efficiency at fast swimming speeds. However, there is no direct empirical evidence to support this common idea, due to the difficulty of obtaining force measurements for these animals. Therefore, indirect approaches are used, such as theoretical and experimental studies. But these experiments oversimplify the animal (motion, shape or material property) and/or the flow condition. Our goal was to assess the propulsive performance of the Atlantic bluefin tuna, Thunnus thynnus, which is our case study for thunniform propulsion, by an experimental approach of the highest bio-fidelity currently performed. A computed tomography scanner and a polyjetTM 3-Dimensional printer were used to make three tail models: two with materials of similar properties to the caudal fin, and one of uniform stiffness. Each model was actuated in a water tunnel by a computer controlled, motorized system to follow motion paths typical for a tuna. Propulsive efficiencies and thrust coefficients were calculated from force and torque measurements. Flow structures were visualized by means of particle image velocimetry (PIV). For the 30 motion regimes the mean thrust over a tail-beat was positive. About half of those generated sufficient thrust to counter the whole body drag estimates (CT ≥0.19). Propulsive performance trends and values were similar for all our tail models and to previous experiments investigating a similar parametric space, where the peak propulsive performance was observed for all tail models and hydrofoils at Sttip =0.35 and α_max =20º. The average peak propulsive performance for the tail models was ηp =0.43 and CT =0.3. As with recent studies, we conclude propulsive performance is more sensitive to kinematics rather than the shape and bending behavior of the caudal fin.
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