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Siemens Wind Power

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EWEC 2006

Noise Optimization of a Multi-Megawatt Wind Turbine

Aero-acoustic noise measurements of an SWT-2.3-93

Aero-acoustic noise calculations of an SWT-2.3-93 and comparison with measurements

Posibilities for low-noise power production

Conclusions

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Aero-acoustic noise meassurements of an SWT-2.3-93

Aerial view of Høvsøre National Test-site for large prototype wind turbines

SWT-2.3-93

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Aero-acoustic noise meassurements of an SWT-2.3-93

Turbine

Turbine rated power: 2300kW

Blade length: 45m

Control system: Variable speed, pitch control

Tower height: 80m

Acoustic noise recording and processing

Hardware: Brüel & Kjær

Software: Brüel & Kjær (Pulse)

Measurement location: On ground 100m downwind of rotor

Temporal resolution of averages: 10s bins

Frequency resolution of averages: 1/12 octave spectra

Turbine data logging (pow, pitch, rpm, wind etc): Full inclusion in noise recording

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Aero-acoustic noise meassurements of an SWT-2.3-93

The acoustic data was recorded during 2 consecutive days in may 2006.

Approximately 11hrs of data

3D data matrix to populate with recordings

1st dimension: Wind: 4 to 12 m/s

2nd dimension: Pitch: -4 to 12 degrees

3rd dimension: Rotor speed: 9-18rpm

Post-processing details of 10s binned 1/12 octave spectra

Background noise subtraction

High frequency bird noise identification and subtraction

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Aero-acoustic noise meassurements of an SWT-2.3-93

4 6 8 10 12 14 16 1870

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rpm

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[dB

]

Measured Soundpower levels for all 10s binned spectra

-3 to -1 deg pitch, 4-8 m/s wind-3 to -1 deg pitch, 8-12 m/s wind

-1 to 1 deg pitch, 4-8 m/s wind

-1 to 1 deg pitch, 8-12 m/s wind

1 to 3 deg pitch, 0-4 m/s wind1 to 3 deg pitch, 4-8 m/s wind

3 to 5 deg pitch, 0-4 m/s wind

3 to 5 deg pitch, 4-8 m/s wind

3 to 5 deg pitch, 8-12 m/s wind5 to 7 deg pitch, 0-4 m/s wind

5 to 7 deg pitch, 4-8 m/s wind

7 to 9 deg pitch, 0-4 m/s wind7 to 9 deg pitch, 4-8 m/s wind

9 to 11 deg pitch, 0-4 m/s wind

9 to 11 deg pitch, 4-8 m/s wind

11 to 13 deg pitch, 0-4 m/s wind11 to 13 deg pitch, 4-8 m/s wind

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Aero-acoustic noise meassurements of an SWT-2.3-93

Note: High rotor-speed sensitivity, less pitch sensitivity and very litle wind sensitivity on acoustics

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Measured (red) Soundpower contours [dBa]

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Aero-acoustic noise calculations of an SWT-2.3-93

The aero-acoustic source modelThe aero-acoustic source model

5 types of noise:

• TE bluntness vortex shedding (BPM model)

• Laminar boundary layer TE vortex shedding (BPM model)

• Turbulent boundary layer TE (BPM model)

• Turbulent boundary layer separation (BPM model)

• Turbulent inflow (Amiet model with simplified Guidati)

Model implementation: NAFNoise (Moriarty, NREL)

Boundary layer inputs: XFoil (Drela, MIT).

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Aero-acoustic noise calculations of an SWT-2.3-93

The aero-acoustic propagation modelThe aero-acoustic propagation model

Modifications to simple radial propagation from a point source:

• Rotor distributed sources

• Directivity (blade acts an acoustic dipole)

• Air absorption

• Atmospheric shear correction

• Doppler shift

Absent modifications:

• Non-flat terrain

• Multiple sound ray reflections due to shear

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Aero-acoustic noise calculations of an SWT-2.3-93

Superposition of calculated soundpower contours at 8m/s

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Measured (red) and calculated (black) Soundpower contours [dBa]

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Aero-acoustic noise calculations of an SWT-2.3-93

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1/3 octave frequencies [Hz]

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(a)]

wind = 5.2 m/s. pitch = 7.3 deg. rpm = 9.85 Measured: 89.6 dB(a). Calculated: 91.4 dB(a)

turbulentinflow

suction sideTBL

total

pressure sideTBL

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turbulentinflow

separationTBL

total

Spectral comparisons at low rotor-speed:

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Aero-acoustic noise calculations of an SWT-2.3-93

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So

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dB

(a)]

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suction sideTBL

total

pressure sideTBL

turbulentinflow

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turbulentinflow

suction sideTBL

total

pressure sideTBL

separationTBL

Spectral comparisons at high rotor-speed:

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Posibilities for low-noise power production

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Posibilities for low-noise power production

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Wind speed at hub height [m/s]

Sou

ndpo

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[dB

(a)

Calculated noise curve for the SWT-2.3-93

Aero-acoustic low-noise analysisAero-acoustic low-noise analysis

Max. sound emission at 11m/s, just before rated power is reached.

Low-noise power production is aimed at the operation point at 11m/s hub height wind.

Quick ‘n dirty gradient analysis

Pitch variation:

-1.05 dB/deg, -0.2 %AEP/dB, 0.0 %flapload/dB

maxRPM variation:

0.72 dB/rpm, -0.5 %AEP/dB, -1.5 %flapload/dB

Chord variation:

-0.03 dB/(%chord) 2.5 %AEP/dB, 28 %flapload/dB

Blade thickness variation:

-0.02 dB/(%thick), -8.4 %AEP/dB, 4.5 %flapload/dB

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Conclusions and future work

Acoustic model validation

Turbulent boundary layer separation noise is qualitatively well reproduced by model, but is overpredicted. Rotational 3D-effect that postpones stall might be part of the explanation.

TE bluntness model overpredicts measurements by 5+ dBs, hence excluded. Turbulent boundary layer TE noise model fits measurements well. Turbulent inflow noise model generally fits measured low frequencies well.

Low-noise turbine operation

Positive pitching (away from stall) is the primary handle according to model – however, measurements indicate much less pitch sensitivity. Reduced RPM also reduces noise at a low cost according to both model and measurements.

Chord- and thickness-variations do not show significant impact on acoustics, and AEP- and/or load-cost is significant.

Every dB-favorable change has a cost, either on AEP or loads.

Overall the model can deliver accurate predictions, once the deficiencies (bluntness) and weaknesses (separation noise) are identified. It will assist future blade design.

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Thank you for your attention