Analyses of the mechanisms of amplitude modulation of aero-acoustic wind turbine sound
Andreas Fischer
Helge Aagaard Madsen
Knud Abildgaard Kragh
Franck Bertagnolio
DTU Wind Energy
Technical University of Denmark
P.O. 49, DK-4000 Roskilde, Denmark
DTU Wind Energy, Technical University of Denmark
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(Normal) Amplitude Modulation (NAM) of Wind Turbine Noise [1]
• swishing sound radiated when the blade moves downwards
• Peak to trough level a few dB
• Normally only perceived close to the wind turbine (1-2D)
• Can be explained by the directivity of trailing edge noise
EWEA Conference 2014, Barcelona, Spain 2 7 July 2014
[2]
Directivity of noise emitted from an airfoil with finite chord length [3]
kc=10 kc=50
DTU Wind Energy, Technical University of Denmark
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(Other) Amplitude Modulation (OAM) of Wind Turbine Noise [1]
• Described as thumping sound
• More low frequency content and higher peak to trough level than normal AM
• Perceived at larger distance from the wind turbine
• Perceive at up and downwind locations
• Transient stall as a possible explanation
EWEA Conference 2014, Barcelona, Spain 3 7 July 2014
Directivity of noise emitted from an airfoil with finite chord length [3]
[2] kc=1 kc=5
DTU Wind Energy, Technical University of Denmark
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Objectives
• Investigate the source of trailing edge noise and stall noise (surface pressure field) on a full scale wind turbine rotor
• Relate surface pressure field to emitted far field sound
• Identify wind conditions which can lead to OAM
• Outline control strategies to alleviate OAM
EWEA Conference 2014, Barcelona, Spain 4 7 July 2014
DTU Wind Energy, Technical University of Denmark
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Outline
• Experimental noise source characterisation on a full scale rotor
(DAN-AERO MW project)
• Relation between noise source and emitted far field sound
(measurement in Virginia Tech Wind Tunnel)
• Critical atmospheric conditions to cause (Other)AM
(DAN-AERO MW project)
• Control strategies to alleviate (Other)AM
• Conclusions
EWEA Conference 2014, Barcelona, Spain 5 7 July 2014
DTU Wind Energy, Technical University of Denmark
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Outline
• Experimental noise source characterisation on a full scale rotor
(DAN-AERO MW project)
• Relation between noise source and emitted far field sound
(measurement in Virginia Tech Wind Tunnel)
• Critical atmospheric conditions to cause (Other)AM
(DAN-AERO MW project)
• Control strategies to alleviate (Other)AM
• Conclusions
EWEA Conference 2014, Barcelona, Spain 6 7 July 2014
DTU Wind Energy, Technical University of Denmark
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NEG-Micon NM80 Wind turbine with inflow sensors DANAERO MW project [4], Vestas, Siemens, LM Wind Power, DONG Energy, DTU, 2007-2010
EWEA Conference 2014, Barcelona, Spain 7 7 July 2014
Four 5 hole pitot tubes installed on a NM80 turbine
• Pressure tabs at r=13m, 19m, 30m and 37m
• Pitot tubes at r= 14.5m, 20.3m, 31m and 36m
• 60 Microphones at r=37m for high frequency surface pressure measurements
DTU Wind Energy, Technical University of Denmark
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Campaign measurements from June to September 2009 – DANAERO MW project
EWEA Conference 2014, Barcelona, Spain 8 7 July 2014
Microphone holes
Pressure holes
DTU Wind Energy, Technical University of Denmark
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NEG-Micon NM80 Wind Turbine (DANAERO MW project)
EWEA Conference 2014, Barcelona, Spain 9 7 July 2014
• Technical Data:
–Rated power 2.3MW
–Hub height 57m
–Rotor diameter 80m
–LM38.8 blades
• Unusual operational conditions:
–Constant rotational speed (16.23rpm = 1.7rad/s)
–Pitch -4.5°(towards higher AoAs, forced to stall)
–High wind speed (above 12m/s at hub)
–Yaw +/-10°
DTU Wind Energy, Technical University of Denmark
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Wind velocity profile measured at the met mast on Sept. 1, 2009 (10min average)
EWEA Conference 2014, Barcelona, Spain 10 7 July 2014
6 7 8 9 10 11 12 1310
20
30
40
50
60
70
80
90
100
110
altitude [
m]
velocity [m/s]
met mast mean wind speed
cup
sonic
HAWC2 pl
7 8 9 10 11 12 13 14 1510
20
30
40
50
60
70
80
90
100
110
altitude [
m]
velocity [m/s]
met mast mean wind speed
cup
sonic
HAWC2 pl
10:00 11:40
Shear exponent 0.3 Shear exponent 0.3
DTU Wind Energy, Technical University of Denmark
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Wind velocity profile measured at the met mast on Sept. 1, 2009 (10min average)
EWEA Conference 2014, Barcelona, Spain 11 7 July 2014
6 7 8 9 10 11 12 1310
20
30
40
50
60
70
80
90
100
110
altitude [
m]
velocity [m/s]
met mast mean wind speed
cup
sonic
HAWC2 pl
7 8 9 10 11 12 13 14 1510
20
30
40
50
60
70
80
90
100
110
altitude [
m]
velocity [m/s]
met mast mean wind speed
cup
sonic
HAWC2 pl
10:00 11:40
Shear exponent 0.3 Shear exponent 0.3
DTU Wind Energy, Technical University of Denmark
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Surface pressure level on suction side at x/c=0.84, Sept. 1, 2009 (evaluated every 0.5sec)
EWEA Conference 2014, Barcelona, Spain 12 7 July 2014
10:05 11:48
time t [s]
cente
r fr
equency f
c [
Hz]
Surface Pressure Level [dB1/12
]
0 1 2 3 4 5 6 7 8 9 10
200
400
600
800
1000
0 1 2 3 4 5 6 7 8 9 106
7
8
9
10
11
12
time t [s]
angle
of
att
ack
[deg]
102
104
106
108
110
112
10
20
30
40
50
60
time t [s]
cente
r fr
equency f
c [
Hz]
Surface Pressure Level [dB1/12
]
0 1 2 3 4 5 6 7 8 9 10
200
400
600
800
1000
0 1 2 3 4 5 6 7 8 9 108
9
10
11
12
13
14
time t [s]
angle
of
att
ack
[deg]
100
105
110
115
10
20
30
40
50
60
DTU Wind Energy, Technical University of Denmark
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Aerofoil Pressure distribution Sept 1, 2009, 11:48
EWEA Conference 2014, Barcelona, Spain 13 7 July 2014
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
x/c [-]
-p [
psi]
t=1.24s, =8.7o
t=4.74s, =9.1o
t=8.74s, =9.3o
t=3.74s, =11.9o
t=2.74s, =12.9o
t=6.24s, =12.9o
DTU Wind Energy, Technical University of Denmark
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Electrical Power and Angle of Attack, Sept 1, 2009, 11:48
EWEA Conference 2014, Barcelona, Spain 14 7 July 2014
0 1 2 3 4 5 6 7 8 9 101420
1440
1460
1480
1500
1520
1540
time t [s]
ele
c.
pow
er
[kW
]
0 1 2 3 4 5 6 7 8 9 108
9
10
11
12
13
14
time t [s]
angle
of
att
ack
[deg]
DTU Wind Energy, Technical University of Denmark
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Outline
• Experimental noise source characterisation on a full scale rotor
(DAN-AERO MW project)
• Relation between noise source and emitted far field sound
(measurement in Virginia Tech Wind Tunnel)
• Critical atmospheric conditions to cause (Other)AM
(DAN-AERO MW project)
• Control strategies to alleviate (Other)AM
• Conclusions
EWEA Conference 2014, Barcelona, Spain 15 7 July 2014
DTU Wind Energy, Technical University of Denmark
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Virginia Tech Stability Wind Tunnel
EWEA Conference 2014, Barcelona, Spain 16 7 July 2014
Air exchange tower
Control room
0.5MW Drive and Fan
Flow
Acoustic test section
Anechoic chambers 3m
DTU Wind Energy, Technical University of Denmark
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Virginia Tech Stability Wind Tunnel
EWEA Conference 2014, Barcelona, Spain 17 7 July 2014
Air exchange tower
Control room
0.5MW Drive and Fan
Flow
Acoustic test section
Anechoic chambers 3m
DTU Wind Energy, Technical University of Denmark
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Virginia Tech Stability Wind Tunnel
EWEA Conference 2014, Barcelona, Spain 18 7 July 2014
Air exchange tower
Control room
0.5MW Drive and Fan
Flow
Acoustic test section
Anechoic chambers 3m
DTU Wind Energy, Technical University of Denmark
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Prediction of far field sound pressure with measured surface pressure
102
103
104
20
25
30
35
40
45
50
55
60
center frequency f [Hz]
far
field
sound s
pectr
um
Spp [
dB
1/1
2]
NACA64-618, Re=1.5M
E=-0.88o
E=0.93o
E=2.77o
E=6.52o
mic. array
EWEA Conference 2014, Barcelona, Spain 19 7 July 2014
102
103
104
25
30
35
40
45
50
55
60
65
center frequency f [Hz]
far
field
sound s
pectr
um
Spp [
dB
1/1
2]
NACA64-618, Re=1.9M
E=6.27o (pre stall)
E=8.23o (post stall)
E=12.21o (deep stall)
102
103
104
70
75
80
85
90
95
100
105
110
115
120
center frequency f [Hz]
surf
ace p
ressure
spectr
um
Spp [
dB
1/1
2]
NACA64-618, Re=1.9M
Trailing edge noise [5] :
The effect of stall on noise emission:
DTU Wind Energy, Technical University of Denmark
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Outline
• Experimental noise source characterisation on a full scale rotor
(DAN-AERO MW project)
• Relation between noise source and emitted far field sound
(measurement in Virginia Tech Wind Tunnel)
• Critical atmospheric conditions to cause (Other)AM
(DAN-AERO MW project)
• Control strategies to alleviate (Other)AM
• Conclusions
EWEA Conference 2014, Barcelona, Spain 20 7 July 2014
DTU Wind Energy, Technical University of Denmark
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DANAERO MW Project 2009 Vestas, Siemens, LM Wind Power, DONG Energy, DTU
EWEA Conference 2014, Barcelona, Spain 21 7 July 2014
Siemens 3.6 MW Turbine
Pitot tube mounted at radial position r=36m
DTU Wind Energy, Technical University of Denmark
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DANAERO MW Project 2009 Vestas, Siemens, LM Wind Power, DONG Energy, DTU
EWEA Conference 2014, Barcelona, Spain 22 7 July 2014
Høvsøre Test Site
DTU Wind Energy, Technical University of Denmark
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Correlation of wind shear to variations in angle of attack
EWEA Conference 2014, Barcelona, Spain 23 7 July 2014
March 28, 2007
DTU Wind Energy, Technical University of Denmark
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Correlation of wind shear to variations in angle of attack
EWEA Conference 2014, Barcelona, Spain 24 7 July 2014
March 28, 2007
DTU Wind Energy, Technical University of Denmark
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Correlation of wind shear to variations in angle of attack
EWEA Conference 2014, Barcelona, Spain 25 7 July 2014
Difference between inflow angle (IA) and angle of attack (AOA)
DTU Wind Energy, Technical University of Denmark
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Outline
• Experimental noise source characterisation on a full scale rotor
(DAN-AERO MW project)
• Relation between noise source and emitted far field sound
(measurement in Virginia Tech Wind Tunnel)
• Critical atmospheric conditions to cause (Other)AM
(DAN-AERO MW project)
• Control strategies to alleviate (Other)AM
• Conclusions
EWEA Conference 2014, Barcelona, Spain 26 7 July 2014
DTU Wind Energy, Technical University of Denmark
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Mitigation - Decreasing mean angle of attack
EWEA Conference 2014, Barcelona, Spain 27 7 July 2014
• HawcStab2 simulations with varying min pitch angle
DTU Wind Energy, Technical University of Denmark
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Mitigation - Decreasing angle of attack variations
EWEA Conference 2014, Barcelona, Spain 28 7 July 2014
• HAWC2 simulations with individual pitch control, sheared inflow exp=0.5, no turbulence
DTU Wind Energy, Technical University of Denmark
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Mitigation - Decreasing angle of attack variations
EWEA Conference 2014, Barcelona, Spain 29 7 July 2014
• Yaw misalignment
DTU Wind Energy, Technical University of Denmark
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Outline
• Experimental noise source characterisation on a full scale rotor
(DAN-AERO MW project)
• Relation between noise source and emitted far field sound
(measurement in Virginia Tech Wind Tunnel)
• Critical atmospheric conditions to cause (Other)AM
(DAN-AERO MW project)
• Control strategies to alleviate (Other)AM
• Conclusions
EWEA Conference 2014, Barcelona, Spain 30 7 July 2014
DTU Wind Energy, Technical University of Denmark
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Conclusions
• Variation of the angle of attack during a revolution causes changes in the spectral energy of the noise sources on the blade
• Under normal conditions the variations of spectral energy are too small to lead to amplitude modulation far away from the turbine (NAM)
• If the blade undergoes transient stall the spectral energy in the low frequency range is significantly increased and it can lead to OAM
• Wind conditions leading to transient stall: high shear in combination with a mean wind speed close to rated wind speed
• Control strategies to mitigate OAM:
– reducing the mean angle of attack (collective pitch)
– reducing the angle of attack variations (individual pitch or yaw control)
EWEA Conference 2014, Barcelona, Spain 31 7 July 2014
DTU Wind Energy, Technical University of Denmark
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References
1) Oerlemans S. An explanation for enhanced amplitude modulation of wind turbine noise. In: Wind Turbine Amplitude Modulation: Research to Improve Understanding as to its Cause and Effect. RenewableUK, Dec. 2013.
2) Brooks TF, Pope DS, Marcolini MA. Airfoil Self-Noise and Prediction. NASA Reference Publication 1218, 1989.
3) S. Moreau and M. Roger. Back-scattering correction and further extensions of Amiet’s trailing-edge noise model. Part II: Application. J. of Sound and Vib. 323 (2009) 397–425
4) H. A. Madsen et al. The DAN-AERO MW Experiments: Final report. Tech. Rep. Risoe-R-1726(EN), Risoe-DTU, Roskilde, Denmark, September 2010.
5) Roger M, Moreau S. Back-scattering correction and further extensions of Amiet’s trailing-edge noise model. Part 1: theory. J. Sound Vib. 2005; 286:477–506.
EWEA Conference 2014, Barcelona, Spain 32 7 July 2014
Thank you!
7 July 2014