Energy Harvesting From Free Oscillation Of A Foil

In nature,birds and aquatic swimmers are known to exploit the flow currents to extract energy from the surrounding fluid,which makes them efficient flyers/swimmers.In recent decades,bio-inspired power extracting devices based on oscillatory motion,such as the flapping foils,have been proposed,designed and developed,which have the advantage of simple configuration,lower noise and can be deployed in the shallow water with narrow space.To study the practical engineering aspect of this bio-inspired energy extraction,we consider a passive pitching foil with thickness ratio equal to 6(c/D=6,where c is foil’s chord length,D is foil’s diameter),which is free to pitch about elastic center.The effect of damping is also considered,and the resulting Flow-Induced Vibration(FIV)problem is solved numerically to analyze the energy harvesting mechanism at low Reynolds number of 150(Re=ρDU∞/μ,where ρ is fluid density,D is foil’s diameter,U∞is free stream velocity,,μ is dynamic viscosity).This paper first investigates the foil’s pitching response at different reduced velocity Ur(Ur=U∞/fnD,where fn is the natural frequency and U∞ is free stream velocity)to find a range of stable and periodic pitching motion,where Ur is varied from 1 to 45 by changing the natural frequency fn of the system.Then the energy harvesting mechanisms are investigated in the parametric space of reduced velocity Ur and damping ratio ζ((?),where cθ is the torsional damping,I is mass moment of inertia and kθ is the torsional spring stiffness),where ζ is varied from 0.01 to 0.30.Furthermore,for a certain Ur,the pitching amplitude θ0,reduced pitching frequency f*,energy harvesting efficiency η and corresponding fluid dynamics are discussed for various damping ratios to gain insights into energy harvesting mechanism by a self-sustained foil.When the reduced velocity Ur is larger than 29,the foil begins to pitch,with further increase in Ur,the pitching amplitude increases sharply and then saturates to a constant value.For each specific Ur,the damping ratio affects the pitching amplitude and reduced pitching frequency,which are associated with energy harvesting efficiency.The maximum energy extracting efficiency is up to 15%at Ur=37 and ζ=0.130,which corresponds to the forming and shedding of well-timed leading edge vortex and trailing edge vortex.