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

Autor: Firas Siala, Alexander D. Totpal, James A. Liburdy
Rok vydání: 2018
Předmět:
Zdroj: Journal of Fluids and Structures. 82:329-342
ISSN: 0889-9746
DOI: 10.1016/j.jfluidstructs.2018.04.022
Popis: The aerodynamic performance of an oscillating heaving and pitching foil operating in the energy harvesting mode was experimentally investigated at reduced frequencies ( k = f c ∕ U ∞ ) of 0.04 to 0.08, corresponding to Reynolds numbers ( R e = 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 θ 0 = 4 5 ∘ to 75 ∘ , a phase shift between sinusoidal pitching and heaving motions of Φ = 3 0 ∘ to 120 ∘ and a heaving amplitude of 0.3 c . 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 θ 0 = 4 5 ∘ and Φ = 9 0 ∘ , whereas the optimal power extraction occurs near θ 0 = 6 0 ∘ and Φ = 6 0 ∘ , both for k = 0.08. As k is decreased (through increased freestream velocity) to 0.04, overall performance becomes relatively insensitive to θ 0 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.
Databáze: OpenAIRE
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