An active torque model for regulating tuna finlets

Finlets, characterized by their series of small triangular fins, represent a notable adaptation observed in tuna and other scombrid fish renowned for their remarkable high-speed swimming abilities. This study focuses on elucidating the control mechanism underlying the pitching motion of finlets by establishing and numerically analyzing both passive and active finlet models through coupled simulations. While the passive model demonstrates qualitative similarities with experimental data, discernible disparities highlight the necessity for active control mechanisms to fully understand and replicate the intricate dynamics of finlet motion. An active torque model is introduced, derived from the correlation between the necessary torques and the observed experimental pitching motions. The results of this active model demonstrate good alignment with the prescribed model, confirming its effectiveness in reproducing responses similar to those of the muscles attached to the finlet root. These findings provide quantitative validation for the active control mechanism that regulates tuna finlets, thus elucidating the underlying mechanics of the fluid. Moreover, they offer valuable insights for advancing research in active flow control, not only in tuna but also in other biomimetic propulsion systems.