Effect of Tunable Stiffness on the Hydrodynamic Performance of a Tuna Tail Informed Flexible Propulsor

View Video Presentation: https://doi.org/10.2514/6.2022-0729.vid Tuna is known for its outstanding swimming performance. It is commonly believed that the flexibility of its lunate tail could improve swimming efficiency. However, the underlying mechanism for the potential efficiency enhancement remains elusive. Here we combine experimental and computational approaches to investigate the hydrodynamic performance of a tuna tail informed flexible propulsor and the potential efficiency enhancement mechanism. The kinematics and deformation of the flexible tail of a yellowfin tuna (Thunnus albacares) during steady swimming are obtained by high-speed videography. A flexible propulsor is then modeled based on the biological data. The passive deformation, hydrodynamic performance, and unsteady flow field of the flexible propulsor are calculated using a strong coupling fluid-structure interaction (FSI) solver for scenarios of uniformly and non-uniformly distributed stiffness, respectively. Experimental observations show more deformations of the tail towards the tips. Simulation results show that, for propulsor with uniform stiffness, thrust and power consumption converge as stiffness increases. A peak in propulsive efficiency is found at medium stiffness. For propulsor with non-uniform spanwise stiffness, the efficiency is increased by 25.7% at increased thrust production and reduced power consumption compared to its uniform stiffness counterpart. The flow analysis shows a stronger propulsive jet and weaker reversed jets produced by non-uniform propulsor, which is responsible for the higher efficiency. The findings are expected to bring more insights to the design of high-performance autonomous/unmanned underwater vehicles (AUV/UUV) from a fluid dynamic perspective.