Numerical optimization of a fully-passive flapping-airfoil turbine

This paper deals with an aeroelastic problem that consists into self-sustained, pitch-heave oscillations of an elastically-mounted airfoil. Such oscillations of an airfoil could be used in order to develop a novel, fully-passive hydrokinetic energy flow harvester that is relatively simple from a mechanical point of view. Indeed, the motion of such an airfoil emerges as a result of the fluid-structure interaction between the flow, the airfoil and its elastic supports, and is sustained through a net transfer of energy from the flow to the structure. In this numerical study, the OpenFOAM-2.1.x CFD toolbox is used for solving the aeroelastic problem. Through unsteady two-dimensional viscous simulations at a Reynolds number of 500,000, the fully-passive turbine is optimized and investigated to develop a better understanding of the physics at play. Following a gradient-like optimization of the turbine, two-dimensional efficiencies as high as 34% have been obtained, and two fundamental mechanisms have been found to be very beneficial for enhancing the performances of the turbine: the adequate synchronization between both degrees-of-freedom, and the nonsinusoidal shape of the pitching motion.