Abstract Summary
Understanding the mechanics of fish propulsion has been an active field of research for decades and continues to inspire novel propulsion systems. Biologists and engineers have identified that a combination of kinematics, active as well as passive morphing, exceptional control and sensory perception gives aquatic swimmers their fascinating abilities [1]. Small-scale pro-pulsion solutions that resemble fishtails or flukes, such as the MIT RoboTuna [2], have been proposed for energy-efficient autonomous underwater vehicles (AUVs). At a larger-scaler biomimetic thrusters are also being studied as auxiliary ship propulsion systems and energy-saving devices that capture wave energy and reduce ship motions; see, e.g., the symbiotic ship/engine and propulsion innovation [3]. However, to mimic the high propulsive efficiency of marine mammals it is essential to investigate effects of elasticity in flapping-foil thruster designs [4]. The response of passively morphing wings is implicitly nonlinear, because deformations affects the hydrodynamic load excitation and vise-versa. Thus, fluid-structure interaction coupling is essential for accurate predictions of the wing response [5]. In the present work, a coupled boundary element/finite element method (BEM-FEM) is proposed for the hydro-elastic analysis of a flapping-foil thruster operating as the main propulsion system of an autonomous underwater vehicle (AUV). The structural dynamics are modelled using plate theory while the fluid dynamics of the lifting surface are modelled using an ideal-flow BEM solver using GPU-acceleration techniques to reduce the overall computational cost [6]. The verification of the present method is accomplished by means of comparisons against experimental da-ta and other numerical methods found in the literature. Results concerning the hydro-elastic response of the flapping-foil thruster for a realistic propulsion scenario are also presented, indicating that incorporating flexibility when tuned properly, can enhance the propulsive performance of the thruster and thus extend the operational capabilities of the AUV. The developed computational tool is able to predict the response of the passively morphing dynamic wings with acceptable accuracy, thus enabling the cost-effective optimization of such devices. References: [1] M. Sfakiotakis, D. Lane, J. Davies, Review of fish swimming modes for aquatic loco-motion, IEEE Journal of Oceanic Engineering 24 (2), 237–252, 1999. [2] D. S. Barrett, Propulsive efficiency of a flexible hull underwater vehicle, Ph.D. thesis, Department of Ocean Engineering MIT (1996). [3] D. Ntouras, G. Papadakis, K. Belibassakis, Ship bow wings with application to trim and resistance control in calm water and in waves, Ocean Engineering 10, 492, 2022. [4] Richards, A.J.; Oshkai, P. Effects of the stiffness, inertia and oscillation kinematics on the thrust and efficiency of an oscillating-foil propulsion system. J. Fluids Struct. 57, 357–374, 2015. [5] Zhu, Q. Numerical simulation of a flapping foil with chordwise and spanwise flexibility. AIAA J., 45, 2448–2457, 2007. [6] E. Filippas, K. Belibassakis, A nonlinear time-domain bem for the performance of 3d flapping-wing thrusters in directional waves, Ocean Engineering 245, 110157, 2022.
