An experimental model with passively variable stiffness to investigate the effect of body stiffness on the fish fast-start maneuver

We present an experimental model to investigate the influence of body flexibility on the fast-start response. The model is fabricated with a variable stiffness, characterized by the direction of applied force. When the fluid force is applied in one direction, the structure acts flexibly and bends, and when the force is applied in the opposite direction, it remains rigid. The structure is rotated from its initial straight position in its flexible direction to a given angle (Stage 1), and then it is rotated back in its rigid direction to an angle in the opposite direction (Stage 2). The wake patterns are studied with bubble image velocimetry (BIV) showing that the main propulsive jet, previously observed in the live fish fast-start, can be produced using this model. Based on our results, we determine that flexibility during Stage 1 and increased rigidity during Stage 2 predict velocity profiles and escape angles consistent with those observed in live fish. A parametric study on the effect of angular velocity and total swept angle on the produced power reveals that the magnitude of swept angle is critical to formation of the propulsive jet. The peak power transfer is found to be more sensitive to increasing the angular velocity for larger swept angles, where a complete jet is already formed. Graphic abstract