Characterization and Optimization of a Fully Passive Flapping Foil in an Unsteady Environment for Power Production and Propulsion

Abstract

This thesis presents an experimental window into the duality between thrust production and energy harvesting by a flapping foil subject to unsteadiness in an oncoming flow. In particular, an airfoil is placed downstream of a circular cylinder, and allowed to interact with the vorticity shed in its wake to produce motions in both the transverse and streamwise directions. It is shown that under the right conditions, passive fluid-structure interactions arising from such a configuration can permit simultaneous extraction of energy from the flow, coupled with net thrust larger than net drag experienced by the airfoil. This observation was made previously by Beal et al. (2006), where in addition they showed that a dead fish under similar conditions appeared to swim upstream.

The contributions of the present work are threefold. Firstly, we provide measurements of the forces acting on a flapping foil in the wake of a circular cylinder and the airfoil motion that arises, for cases where the flapping motion is both active (the foil is driven through a pre-planned trajectory) and fully passive (the foil is allowed to react to the fluid forcing it experiences). These are coupled with simultaneous Particle Image Velocimetry (PIV) measurements of the flow field in the region of the airfoil. These measurements allow us to directly observe fluid-structure interactions which give rise to both thrust production and power extraction potential for the airfoil, illuminating the mechanisms driving each. It is determined that for the sinusoidal trajectories considered here, the dynamics of a fully passive flapping foil can be tuned to mirror the behaviour of a similar driven one, and the measured forces as well as fluid-structure interactions taking place are similar between the two cases.

The second focus of this work is on the optimization of the behaviour of a compliantly mounted, fully passive flapping foil for energy harvesting. A framework based on 2^ndorder linear systems theory is proposed to guide the optimization of a simplified flapping foil energy harvester, where the dynamics are determined based on spring-mass-damper characteristics of the mounting system. Tuning efforts are shown to yield significant improvements to power extraction performance relative to a naïve choice of mounting parameters, however nonlinear feedback between airfoil motion and the aerodynamic forces it experiences acts to temper the improvements seen in experiments relative to predicted power extraction performance. In addition, the effects of parasitic dynamics due to friction in the mounting mechanism are investigated, and the resulting changes to power production performance are quantified. The action of friction induces emergent behaviours for the foil not seen in the ideal case; thus, understanding these effects is key to predicting and optimizing the performance of a real engineering system.

Finally, in addition to transverse flapping, we explore the behaviour of a fully passive airfoil when it is allowed to react to the oncoming cylinder wake in the streamwise direction as well. Since the airfoil produces net thrust larger than its net drag, we observe it translating upstream, while simultaneously extracting energy from the flow. We confirm through PIV imaging that the airfoil begins translating upstream well outside of the suction region induced by the presence of the upstream cylinder, such that all thrust generated is due to its interactions with vorticity in the cylinder wake. These observations are enabled through Cyber-Physical Fluid Dynamics (CPFD), where both the transverse and streamwise behaviour of the airfoil is determined through feedback control based on measured forces acting on the foil. Such a system allows access to simulated fully passive dynamics over a range of parameter space challenging to reach using a conventional experimental setup.