Numerical analysis of wake structure and performance of two oscillatory mechanisms of a foil: Pure pitching and undulating

Abstract

There are various propulsion, maneuvering, and stabilization mechanisms in nature, which can provide inspiration for similar mechanisms in man-made vehicles. This study aims to elucidate and compare the propulsive vortical signature and performance of a foil in two important natural mechanisms of pure pitching and undulatory oscillations. Governing equations are solved with a pressure-based finite volume method solver, in an arbitrary Lagrangian-Eulerian framework domain containing a NACA 0012 foil moving with prescribed kinematics. The results show that in a given Reynolds number (Re), the undulating mechanism produces thrust at a higher Strouhal number (St) and with smaller growth slope, but mostly higher efficiency, versus St, than pitching mechanism. In addition, vortical structures of these mechanisms have significant differences and also vary considerably with St. One of the distinguishable features of vortical signatures is the presence of the leading-edge vortices for the pitching foil, which are not appearing in the undulating foil's vortical pattern.

Imitation of the body/caudal fin undulatory swimming of a spiny dogfish (Squalus acanthias): The kinematic equation

“…Fishes are known for their outstanding underwater agility and maneuverability as well as their efficient and noiseless swimming propulsion system. [3][4][5][6][7][8] The propulsion system and the graceful motions of the fishes offer a novel paradigm to inspire the design of man-made vehicles. Indeed, the advantages of biomimetics encourage the engineers to exploit live animal's characteristics to apply in the underwater vehicle design and control.…”