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A Survey of Aeroacoustic Considerations in Wind Turbines

A Survey of Aeroacoustic Considerations in Wind Turbines. Robert Scott AE 6060. Outline. Introduction Mechanisms Prediction Measurement Effects Suppression Conclusions. Introduction. Clean energy Opposition to wind energy development (NIMBY) Appearance Sound Cape Wind Project

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A Survey of Aeroacoustic Considerations in Wind Turbines

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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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