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The effects of passive, inertia-induced surface deformation at the leading and trailing edges of an oscillating airfoil energy harvester are investigated experimentally at reduced frequencies of k = f c=U¥ = 0.10, 0.14 and 0.18. Wind tunnel experiments are conducted using phase-resolved, two-component particle image velocimetry to understand the underlying flow physics, as well as to obtain force and pitching moment estimates using the vortex-impulse theory. Results are obtained for leading and trailing edge deformation separately. It is shown that both forms of deformation may alter the leading edge vortex inception and detachment time scales, as well as the growth rate of the circulation. In addition, surface deformation may also trigger the generation of secondary vortical structures, and suppress the formation of trailing edge vortices. The total energy harvesting efficiency is decomposed into contributions of heaving and pitching motions. Relative to the rigid airfoil, the deforming leading and trailing edge segments are shown to increase the energy harvesting efficiency by approximately 17% and 25%, respectively. However, both the deforming leading and trailing edge airfoils operate most efficiently at k = 0:18, whereas the peak efficiency of the rigid airfoil occurs at k = 0:14. It is shown that the deforming leading and trailing edge airfoils enhance the heaving contribution to the total efficiency at k = 0:18 and the negative contribution of the pitching motion at high reduced frequencies can be alleviated by using a deforming trailing edge.
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Energy Harvesting Performance of Thick Oscillating Airfoils Using a Discrete Vortex Model
Abstract The energy harvesting performance of thick oscillating airfoils is predicted using an inviscid discrete vortex model (DVM). NACA airfoils with different leading-edge geometries are modeled that undergo sinusoidal heaving and pitching with reduced frequencies, k = f c/U∞, in the range 0.06–0.14, where f is the heaving frequency of the foil, c the chord length, and U the freestream velocity. The airfoil pitches about the mid-chord with heaving and pitching amplitudes of h0 = 0.5c and θ0 = 70°, respectively, known to be in the range of peak energy harvesting efficiencies. A vortex shedding initiation criteria is proposed based on the transient local wall stress distribution determined from computational fluid dynamics (CFD) simulations and incorporates both timing and location of leading-edge separation. The scaled shedding times are shown to be predicted over the range of reduced frequencies using a timescale based on the leading-edge shear velocity and radius of curvature. The convection velocity of the shed vortices is also modeled based on the reduced frequency to better capture the dynamics of the leading-edge vortex. An empirical trailing-edge separation correction is applied to the transient force results using the effective angle of attack modified to include the pitching component. Impulse theory is applied to the DVM to calculate the transient lift force and compares well with the CFD simulations. Results show that the power output increases with increasing airfoil thickness and is most notable at higher reduced frequencies where the power output efficiency is highest.
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- Award ID(s):
- 1804964
- PAR ID:
- 10383002
- Date Published:
- Journal Name:
- Journal of Fluids Engineering
- ISSN:
- 0098-2202
- Page Range / eLocation ID:
- 1 to 33
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The vortex dynamics and lift force generated by a sinusoidally heaving and pitching airfoil during dynamic stall are experimentally investigated for reduced frequencies of k = fc=U1 = 0:060:16, pitching amplitude of 0 = 75 and heaving amplitude of h0=c = 0:6. The lift force is calculated from the velocity fi elds using the nite-domain impulse theory. The concept of moment arm dilemma associated with the impulse equation is revisited to shed-light on its physical impact on the calculated forces. It is shown that by selecting an objectively de ned origin of the moment-arm, the impulse force equation can be greatly simpli ed to two terms that have a clear physical meaning: (i) the time rate of change of impulse of vortical structures within the control volume and (ii) Lamb vector that indirectly captures the contribution of vortical structures outside of the control volume. The results show that the trend of the lift force is dependent on the formation of the leading edge vortex, as well as its time rate of change of circulation and chord-wise advection relative to the airfoil. Additionally, the trailing edge vortex, which is observed to only form for k 0:10, is shown to have lift-diminishing e ects that intensi es with increasing reduced frequency. Lastly, the concept of optimal vortex formation is investigated. The leading edge vortex is shown to attain the optimal formation number of approximately 4 for k 0:1, when the scaling is based on the leading edge shear velocity. For larger values of k the vortex growth is delayed to later in the cycle and doesn't reach its optimal value. The result is that the peak lift force occurs later in the cycle. This has consequences on power production which relies on correlation of the relative timing of lift force and heaving velocity.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1115/1.4056339
