Energy Harvesting from a Symmetric Airfoil under Varying Flow Conditions

Energy harvesting from a freely responding symmetric airfoil is investigated in a two-dimensional numerical environment. Gust response with non-linear structural dynamics is evaluated. A fully passive NACA-0015 airfoil with two coupled degrees of freedom is considered. Gust model is superimposed on the free stream inlet velocity up till the critical flutter velocity. Whereas the non-linear stiffness is imposed on the heave degree of freedom as a function of the heaving oscillation amplitude. Second order transient formulation, k − ω turbulence model and dynamic meshing technique along with the 10^-6 convergence criteria is adopted. Transient response has been observed for both linear and nonlinear structural stiffness cases to find the maximum and minimum values of heave amplitude, pitch amplitude and total transient energy with their locations in the respective time history. Comparison is presented between the time histories of heaving motion (y (t)), pitching motion (θ (t)) and total energy (E (t)) for both (linear and non-linear structural stiffness) cases. The energy harvesting capacity has been determined for the range of (0.85 ≤ U/Uc ≤ 1) near the critical flutter velocity through numerical simulations. Maximum efficiency of 25.37% and 29.50% has been achieved in case of linear and nonlinear structural stiffness cases respectively with 16.29% increases in energy harvesting efficiency. Numerical evidence is provided to confirm the theoretical and experimental scenario of by-pass transition to flutter by transient growth that can cause flutter instability in non-linearly flexible airfoil at a wind velocity below the linear critical flutter velocity. As a result of structural nonlinearity, critical flutter velocity has been reduced to 17 m/s which was 18.5 m/s in case of linear structural stiffness. The effect of bypass transition of energy growth on the energy harvesting capability of the airfoil has also been evaluated.