Flow Energy Harvesting of an Oscillating Foil With Rigid and Passive Surface Flexibility

The aerodynamic performance of an oscillating pitching and plunging foil operating in the energy harvesting mode is experimentally investigated. Experiments are conducted in a closed-loop recirculating wind tunnel at Re of 24,000 to 48,000, and reduced frequencies (k) of 0.04 to 0.08. Foil kinematics are varied through the following parameter space: heaving amplitude of 0.3c, pitching amplitudes of θ0 = 45° to 75°, as well as phase lag between sinusoidal pitching and heaving motions of Φ = 30° to 120°. Aerodynamic force measurements are collected to show the energy extraction performance (power coefficient and efficiency) of the foil. Coupled with the force measurements, flow fields are collected using particle image velocimetry. The flow field characteristics are used to supplement the force results, shedding light into flow features that contribute to increased energy extraction at these k values. In addition, inertia-induced passive chord-wise flexibility at the leading edge (LE) of the foil is investigated in order to assess its feasibility in this application. Results indicate that favorable performance occurs near θ0 = 45°, Φ = 90° and k = 0.08. When k is decreased (through increased U∞) to 0.04, overall extraction performance becomes insensitive to θ0 and Φ. This is supported by the flow field measurements, which show premature leading edge vortex (LEV) evolution and detachment from the foil surface. Although overall performance was reduced with the passive LE flexibility, these results indicate that a proper tuning of the LE may result in a delay of the LEV detachment time, yielding increased energy harvesting at this otherwise inefficient operating parameter space.