a survey of aeroacoustic considerations in wind turbines robert scott ae 6060
TRANSCRIPT
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A Survey of Aeroacoustic Considerations in Wind Turbines
Robert Scott
AE 6060
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Outline
I. Introduction
II. Mechanisms
III. Prediction
IV. Measurement
V. Effects
VI. Suppression
VII. Conclusions
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Introduction
• Clean energy• Opposition to wind
energy development (NIMBY)– Appearance– Sound
• Cape Wind Project• Small Wind Turbines
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Mechanical Noise
Sources:– Gearbox– Generator Shaft– Cooling Fans– Yaw/Blade Pitch
motors
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Low Frequency Noise
• Thickness – negligible– Low local speed
• Unsteady Loading– Blade passes through
tower’s 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 < 106
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Airfoil Self Noise
• Tip Noise– Tip vortices– Side edge– Broadband
• Separation Noise– Deep Stall– Entire chord radiates
sound– Broadband
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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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PredictionRules of thumb:
50log10 10 WTWA PL
72log22 10 DLWA
4log10log50 1010 DVL TWA
(1)
(2)
(3)
Based on rated power capacity, rotor diameter, and tip speed.Tested these formulas for a wind turbine with available information:
AOC 15/50 PredictionsActual Tests:
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Measurement
Array placed upwind of wind turbine
Concentration of sourceson downward side due toDoppler amplification
0-12 dB scale
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Measurement
Shift in source location corresponding to alignment angle.
Region of sources in area ofmaximum relative velocity toarray.
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Effects
LA ,amb log10(U)
rrLL WP 210 2log10
Even at distances <1 km fromsite, wind turbine noise may be completely drowned out byambient noise due to the wind.
LW 45 dB for latest generationutility-scale turbines
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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
~1%<1%
>300%
~250% ~40%
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Suppression
• Dirt, bugs on blades detach flow– Noise due to
imperfections– Loss of blade
performance
• Water jets clean blades
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Conclusions
• Annoyance due to large wind turbines unlikely.
• Small wind turbines actually pose bigger noise problem.
• Continuing improvements will reduce noise even further.