Energy harvesting of an oscillating foil at low reduced frequencies with rigid and passively deforming leading edge

Abstract

The aerodynamic performance of an oscillating heaving and pitching foil operating in the energy harvesting mode was experimentally investigated at reduced frequencies (k=fc∕U∞) of 0.04 to 0.08, corresponding to Reynolds numbers (Re=U∞c∕ν) of 24,000 to 48,000. The goal is to better understand the operational conditions that lead to improved power output when operating at off-peak efficiency conditions, in particular, those corresponding to high approach velocities for a given oscillation frequency. The operational parameters include a range of pitching amplitudes of θ0=45∘ to 75∘, a phase shift between sinusoidal pitching and heaving motions of Φ=30∘ to 120∘ and a heaving amplitude of 0.3c. Aerodynamic force measurements are used to determine the energy extraction performance and flow field measurements using phase-locked particle image velocimetry are used to identify flow features that contribute to energy extraction. In addition, inertia-induced chord-wise deformation at the leading edge (LE) of the foil is investigated to assess its feasibility to enhance power output. Results indicate that for the range of k values studied the optimal efficiency occurs near θ0=45∘ and Φ=90∘, whereas the optimal power extraction occurs near θ0=60∘ and Φ=60∘, both for k = 0.08. As k is decreased (through increased freestream velocity) to 0.04, overall performance becomes relatively insensitive to θ0 and Φ. This is supported by the PIV measurements, which show at lower k a premature leading edge vortex (LEV) formation and detachment from the foil surface. Efficiency and power coefficient results are shown versus the feathering parameter, χ, indicating a rapid decline in performance as χ increases due to increasing pitching amplitude or with decreasing k. The relative strength of the LEV is found to increase with decreasing k, however the timing of the LEV formation and detachment is such that there is reduced power production. Study of the inertia-induced leading edge deformation shows that only during a small portion of the cycle is the lift force enhanced. This is a consequence of the inertial response of the leading edge and an enhancement of the LEV growth rate. These results indicate that use of a deforming surface to improve overall performance requires proper tuning relative to the heaving motion.