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Technical Brief

Flutter of Variations on a 5 MW Swept Wind Turbine Blade1

[+] Author and Article Information
Scott Larwood

Assistant Professor
Mechanical Engineering Department,
University of the Pacific,
Stockton, CA 95211
e-mail: slarwood@pacific.edu

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received April 1, 2015; final manuscript received January 8, 2016; published online February 1, 2016. Assoc. Editor: Yves Gagnon.

J. Sol. Energy Eng 138(2), 024504 (Feb 01, 2016) (6 pages) Paper No: SOL-15-1083; doi: 10.1115/1.4032475 History: Received April 01, 2015; Revised January 08, 2016

Previous research has shown that as wind turbine rotor designs increase in diameter they approach flutter instabilities. Recent blade designs incorporate blade sweep to lower fatigue loads and increase rotor diameter for more energy production; however, the flutter margin may be reduced with sweep. In this work, flutter speeds are predicted and compared for turbines with straight and swept blades. For this study, a wind turbine analysis tool specifically developed for blade sweep (CurveFAST) was used, in addition to a tool developed for helicopter analysis: rotorcraft comprehensive analysis system (RCAS). The results showed that CurveFAST generally predicted a higher flutter speed and RCAS predicted a lower flutter speed compared to previous research. The difference may be attributed to the unsteady aerodynamics modeling. The results for an extended radius and swept 5 MW turbine showed that the flutter speed was most sensitive to the position of the blade center of mass axis. Using the selected design parameters, it was not possible to design a swept 5 MW blade with adequate flutter margin that would produce 5% more energy with the same loads as a baseline straight rotor. Further study of swept blades is required to determine if there is a size limit above which flutter cannot be avoided.

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Figures

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Fig. 1

Rotor speed reduction after the onset of flutter in an 8 kW wind turbine [1]

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Fig. 2

CurveFAST results for flutter instability of a 1.5 MW wind turbine in still air. Negative twist is toward feather. Nominal speed is 20.5 rpm.

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Fig. 3

Plots of frequency and damping for the Sweep5000 baseline model with frequency normalized to omega = 1.27 rad/s (12.1 rpm rated speed). Modes 1—first flapwise, 2—first edgewise, 3—second flapwise, and 4—second flap/torsion. (a) Frequency, fan lines are integer multiples of rotor speed and (b) damping, which goes positive for mode 4, indicating an instability.

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Fig. 4

Animation screen capture of unstable aeroelastic mode 4 for the Sweep5000 baseline model. Left portion of tip is the undeflected blade; right portion shows the deflection. The animation shows motion that is a combination of the second flap mode and the first torsion mode.

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Fig. 5

Swept blade parameters. Torsional stiffness is about the elastic axis.

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Fig. 6

Parametric study of the baseline Sweep5000 wind turbine with RCAS. Torsion is change in torsional stiffness at every blade station. Tip sweep is percentage of maximum chord length. Exponent is the change from the baseline exponent of 2. CM is percentage of chord from elastic axis to mass center axis; positive toward trailing edge.

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