A Survey of Aeroacoustic Considerations in Wind Turbines

Robert Scott

AE 6060

Outline

I. Introduction

II. Mechanisms

III. Prediction

IV. Measurement

V. Effects

VI. Suppression

VII. Conclusions

Introduction

• Clean energy• Opposition to wind

energy development (NIMBY)– Appearance– Sound

• Cape Wind Project• Small Wind Turbines

Mechanical Noise

Sources:– Gearbox– Generator Shaft– Cooling Fans– Yaw/Blade Pitch

motors

Low Frequency Noise

• Thickness – negligible– Low local speed

• Unsteady Loading– Blade passes through

tower’s wake.

Inflow-Turbulence Noise

Blade encountering natural atmospheric turbulence causes broadband noise radiation.

Size of turbulent eddies determines frequency.

(5 Hz – 1 kHz)

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

Airfoil Self Noise

• Tip Noise– Tip vortices– Side edge– Broadband

• Separation Noise– Deep Stall– Entire chord radiates

sound– Broadband

Airfoil Self Noise

• Trailing Edge Bluntness Noise– T-E thickness causes

vortex shedding– Tonal

• Surface Imperfections– Dirt, bugs, damage– Broadband

Typical Noise Spectrum

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

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:

Measurement

Array placed upwind of wind turbine

Concentration of sourceson downward side due toDoppler amplification

0-12 dB scale

Measurement

Shift in source location corresponding to alignment angle.

Region of sources in area ofmaximum relative velocity toarray.

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

Effects

• Low frequency noise could conceivably cause windows to rattle or slight infrasound discomfort.

• Still not likely unless very close to wind turbine.

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.

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

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%

Suppression

• Dirt, bugs on blades detach flow– Noise due to

imperfections– Loss of blade

performance

• Water jets clean blades

Conclusions

• Annoyance due to large wind turbines unlikely.

• Small wind turbines actually pose bigger noise problem.

• Continuing improvements will reduce noise even further.