A Survey of Aeroacoustic Considerations in Wind Turbines

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 towers wake.

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

• Trailing Edge Noise
• Turbulent eddies enhanced by trailing edge
• Broadband
• Vortex Shedding
• Trailing edge noise creates B-L instabilities
• Tonal
• Re lt 106

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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

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Typical Noise Spectrum
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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
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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
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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 lt1 km from site, wind turbine
noise may be completely drowned out by ambient
noise due to the wind.
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Effects

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

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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.

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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

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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
gt300
40
250
1
lt1
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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.