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Aeroacoustic Considerations in Wind Turbines: Mechanisms, Effects, and Suppression

Explore a comprehensive survey of aeroacoustic considerations in wind turbines, covering mechanisms, prediction, measurement, and the effects of noise. Discover strategies for noise suppression and how clean airfoil design impacts noise levels.

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Aeroacoustic Considerations in Wind Turbines: Mechanisms, Effects, and Suppression

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  1. A Survey of Aeroacoustic Considerations in Wind Turbines Robert Scott AE 6060

  2. Outline • Introduction • Mechanisms • Prediction • Measurement • Effects • Suppression • Conclusions

  3. Introduction • Clean energy • Opposition to wind energy development (NIMBY) • Appearance • Sound • Cape Wind Project • Small Wind Turbines

  4. Mechanical Noise Sources: • Gearbox • Generator Shaft • Cooling Fans • Yaw/Blade Pitch motors

  5. Low Frequency Noise • Thickness – negligible • Low local speed • Unsteady Loading • Blade passes through tower’s wake.

  6. Inflow-Turbulence Noise Blade encountering natural atmospheric turbulence causes broadband noise radiation. Size of turbulent eddies determines frequency. (5 Hz – 1 kHz)

  7. Airfoil Self Noise • Trailing Edge Noise • Turbulent eddies enhanced by trailing edge • Broadband • Vortex Shedding • Trailing edge noise creates B-L instabilities • Tonal • Re < 106

  8. Airfoil Self Noise • Tip Noise • Tip vortices • Side edge • Broadband • Separation Noise • Deep Stall • Entire chord radiates sound • Broadband

  9. Airfoil Self Noise • Trailing Edge Bluntness Noise • T-E thickness causes vortex shedding • Tonal • Surface Imperfections • Dirt, bugs, damage • Broadband

  10. Typical Noise Spectrum

  11. Prediction • Codes can predict low frequency noise based on FW-H Eqn. • Empirical methods, approximations to flat plates used to predict noise due to turbulence. 3-blade downwind, 60 rpm NACA 0012, S822 predicted T-E noise

  12. Prediction Rules of thumb: (1) (2) (3) Based on rated power capacity, rotor diameter, and tip speed. Tested these formulas for a wind turbine with available information: Actual Tests: Predictions AOC 15/50

  13. Measurement Array placed upwind of wind turbine Concentration of sources on downward side due to Doppler amplification 0-12 dB scale

  14. Measurement Shift in source location corresponding to alignment angle. Region of sources in area of maximum relative velocity to array.

  15. Effects for latest generation utility-scale turbines Even at distances <1 km from site, wind turbine noise may be completely drowned out by ambient noise due to the wind.

  16. Effects • Low frequency noise could conceivably cause windows to rattle or slight infrasound discomfort. • Still not likely unless very close to wind turbine.

  17. Suppression • Mechanical Noise • Early wind turbines • Exposed machinery, large contribution of mechanical noise • New wind turbines • Nacelle covering with acoustic treatment on inside nearly eliminates mechanical noise.

  18. Suppression • Aerodynamic Noise • Operation • Lower tip speed • Decrease blade pitch • Both options not ideal • Design • Configurations • Upwind less sensitive to inflow turbulence • Blade Design • Airfoils • Tip Shapes

  19. Suppression • Clean airfoil with low T-E thickness will have low tonal noise due to less vortex shedding. • Rounded, serrated, and porous trailing edges can reduce acoustic efficiency of trailing edge noise. ref >300% ~40% ~250% ~1% <1%

  20. Suppression • Dirt, bugs on blades detach flow • Noise due to imperfections • Loss of blade performance • Water jets clean blades

  21. Conclusions • Annoyance due to large wind turbines unlikely. • Small wind turbines actually pose bigger noise problem. • Continuing improvements will reduce noise even further.

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