Spring stiffness influence on an oscillating propulsor

Abstract We study the propulsive dynamics of a thin foil pitching about its quarter chord and allowed to passively plunge. Specifically, we focus on the effect of variations in translational spring stiffness on propulsor plunge and on the minimum oscillation frequency required to produce positive thrust. Our numerical simulation utilizes a two-dimensional hydroelasticity model of the propulsor–fluid system in a constant velocity free stream. The pitch is forced at the quarter chord by a drive shaft and the dynamics of the fluid–structure interaction coupled to the strength of a translational spring determines the plunge amplitude. We use an unsteady two-dimensional vortex lattice method to model the hydrodynamics of the propulsor producing thrust in a potential flow field. The phase relationship between the driving angle and the plunge displacement is discussed, along with the effects of changing spring stiffness on thrust and efficiency. We show that passive plunge reduces the critical frequency for positive thrust production. This allows simple one-actuator input to compete with more complicated two-actuator systems.