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 < 10 6

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

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: AOC 15/50 Predictions Actual Tests:

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

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

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