On the nonlinear dynamics of self-sustained limit-cycle oscillations in a flapping-foil energy harvester

The nonlinear dynamics of an airfoil at Reynolds number Re = 10,000 constrained by two springs and subject to a uniform oncoming flow is studied numerically. The studies are carried out using open source computational fluid dynamics toolbox OpenFOAM. Under certain conditions related to aerodynamic flutter, this two-degree-of-freedom system undergoes self-sustained limit-cycle oscillations (LCOs) with potential application as an energy harvester. When the system is given a small initial perturbation, it is seen that the response of the system decays to zero at flow velocities below the flutter velocity, or oscillates in a limit cycle at velocities greater than the flutter velocity. The flutter velocity at Re = 10,000 is shown to deviate significantly from the theoretical prediction (which is derived with an assumption of infinite Reynolds number) owing to the effect of viscosity. The LCOs at freestream velocities higher than the flutter velocity result in unsteady flows that are heavily influenced by leading-edge vortex shedding as well as trailing-edge flow separation. The influence of different system parameters on the onset of flutter and on the limit-cycle response characteristics is investigated in this research. This is done by defining a baseline case and examining the effects of varying aerodynamic parameters such as freestream velocity, and structural parameters such as the pitch-to-plunge frequency ratio and the type of spring stiffnesses. The conditions corresponding to the lowest flutter velocities (and consequently the lowest cut-in speeds for power extraction) and the parameter space that provide single-period, single-amplitude and harmonic LCOs (ideal for power extraction) are identified. Calculation of instantaneous and time-averaged power is presented by modeling the extraction of energy through a viscous damper. The highest power coefficients and efficiencies are obtained at velocities just higher than the flutter velocity. Introduction of positive cubic stiffening in the system springs is seen to make the system more stable, LCOs more harmonic and single-period, and to potentially increase power extraction efficiency of the system. (C) 2018 Elsevier Ltd. All rights reserved.