Alexander Matta

Scombrid fish lunate caudal fins are characterized by a wide range of sweep angles. Scombrid that have small sweep-angle caudal fins move at higher swimming speeds, suggesting that smaller angles produce more thrust. Furthermore, scombrids occasionally use high angles of attack (AoA) suggesting this also has some thrust benefit. This work examined the hypothesis that a smaller sweep angle and higher AoA improved thrust in swimmers by experimentally analyzing a robophysical model. The robophysical model was tested in a water tunnel at speeds between 0.35 and 0.7 body lengths per second. Three swept caudal fins were analyzed at three different AoA, three different freestream velocities, and four different Strouhal numbers, for a total of 108 cases. Results demonstrated that the fin with the largest sweep angle of 50° resulted in lower thrust production than the 40° and 30° fins, especially at higher Strouhal numbers. Larger AoA up to 25° increased thrust production at the higher Strouhal numbers, but at lower Strouhal numbers, produced less thrust. Differences in thrust production due to fin sweep angle and AoA were attributed to the variation in spanwise flow and leading edge vortex dynamics.

Thunniform swimmers are known to travel at high speeds for long periods of time and at high hydrodynamic efficiency. Thus, there is a great deal of interest in their swimming physics. In order to better understand these physics, a newly designed robotic tuna was constructed that allows for interchangeable caudal fins. This robot was put in a water tunnel and tested at tail beat frequencies ranging from 0.5 to 1.0 Hz and at freestreams of 0, 0.2, and 0.4 m/s. A lever assembly was used to transmit thrust force to a load cell, and power was calculated using data from current sensors. Preliminary results suggest that swept caudal fins produce more thrust and are more efficient than trapezoidal fins at higher freestreams while the opposite is true at lower freestreams. However, several induction factors need to be resolved before more confident assertions can be made.