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An overview of recent research on AM and OAM of wind turbine noise
Aagaard Madsen , Helge; Bertagnolio, Franck; Fischer, Andreas
Publication date:2015
Link back to DTU Orbit
Citation (APA):Aagaard Madsen , H. (Author), Bertagnolio, F. (Author), & Fischer, A. (Author). (2015). An overview of recentresearch on AM and OAM of wind turbine noise. Sound/Visual production (digital)
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An overview of recent research on AM and OAM of wind turbine noise
Helge Aagaard Madsen
Franck Bertagnolio
Andreas Fischer
DTU Wind Energy
Technical University of Denmark
P.O. 49, DK-4000 Roskilde, Denmark
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DTU Wind Energy, Technical University of Denmark
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Introduction
H Aa Madsen -- WTN 2015 Glasgow April 20-23, 2015 2
Bullmore A, Cand M, Oerlemans S, Smith MG, White P, von Hünerbein S, King A, Piper B, et al.. ReneawableUK. Wind Turbine Amplitude Modulation: Research to Improve Understanding as to its Cause and Effect. RenewableUK, United Kingdom;2013.
Madsen HAa, Fischer A, Abildgaard K. Phase 2, Mechanisms and causes of Amplitude Modulation (AM) and Other Amplitude Modulation (OAM) of aero-acoustic wind turbine noise. Report DTU Wind Energy I-0095 (EN), DTU Wind Energy August 2013. In RenewableUK 2013 report: Wind Turbine Amplitude Modulation: Research to Improve Understanding as to its Cause & Effect.
A comprehensive research project: ” Wind Turbine Amplitude Modulation:
Research to Improve Understanding as to its Cause and Effect”
coordinated and funded by Renewable UK
carried out in the period 2010-2013
final report released in December 2013
DTU involved in last part
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DTU Wind Energy, Technical University of Denmark
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DTU background for conducting the AM research in the Renewable UK study
Expertise on:
inflow measurements on a blade (since 1987)
noise source measurements with microphones
on the blade surface (since 2006)
these techniques applied in The DAN-AERO
project 2007-2010*
rotor noise modeling
modeling wind turbine aeroelasticity and control
3 H Aa Madsen -- WTN 2015 Glasgow
April 20-23, 2015
* The DANAERO project was conducted in corporation between Vestas, Siemens, LM, DONG Energy and DTU Wind Energy in 2007-2010. Funded by the Danish Ministry of Energy
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DTU Wind Energy, Technical University of Denmark
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Inflow measurements
First measurements on a 100 kW turbine in the period from 1987-1993
4 H Aa Madsen -- WTN 2015 Glasgow
April 20-23, 2015
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DTU Wind Energy, Technical University of Denmark
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Mechanism of amplitude modulation (AM) of noise – from the Ren. UK study
caused by 1p variations of the level of trailing edge
noise and change in directivity during blade rotation
variation of a few dB over one blade revolution
AM
5
Bullmore A, Cand M, Oerlemans S, Smith MG, White P, von Hünerbein S, King A, Piper B, et al.. ReneawableUK. Wind Turbine Amplitude Modulation: Research to Improve Understanding as to its Cause and Effect. RenewableUK, United Kingdom;2013.
Measured about 50 m upwind from a 93-m diameter pitch-controlled wind turbine
H Aa Madsen -- WTN 2015 Glasgow April 20-23, 2015
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DTU Wind Energy, Technical University of Denmark
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6
horizontal coordinate x [m]
altitude h
[m
]
integrated SPL [dB] for TE noise
-30 -20 -10 0 10 20 30
30
40
50
60
70
80
90
40
42
44
46
48
50
52
54
56
58
60
AM simulation - Shear exponent 0.2
horizontal coordinate x [m]
altitude h
[m
]
angel of attack [deg]
-30 -20 -10 0 10 20 30
30
40
50
60
70
80
90
6
7
8
9
10
11
12
13
14
15
horizontal coordinate x [m]
altitude h
[m
]
directivity factor for TE noise
-30 -20 -10 0 10 20 30
30
40
50
60
70
80
90
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
Directivity for TE noise AOA variation causing TE noise variation
H Aa Madsen -- WTN 2015 Glasgow April 20-23, 2015
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Characteristics of extreme AM - also called EAM or OAM
the modulation depth significantly greater than that of normal
blade swish (AM)
modulation depths up to 10 dB have been measured
increased level at frequencies below 400 Hz
the effect is generally strongest in the downwind direction
Figures from Renewable UK study on AM and OAM (EAM), December 2013
EAM
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Characteristics of extreme AM - also called EAM or OAM
8 H Aa Madsen -- WTN 2015 Glasgow
April 20-23, 2015
commonly termed ‘blade swish’
part of normal WTN
~5dB modulation at source
dominant crosswind effect
decreases away from source
dominated by mid frequencies
(400Hz to 1000Hz) ‘swish’
source mechanism understood
more impulsive ‘thump’
atypical, intermittent
>5dB (>10dB) amplitude at times?
audible/noticeable at large distances
downwind to >1km?
additional lower frequency content
(200 Hz to 500 Hz)? ‘whooomp’
source mechanism?
Normal AM OAM
Jeremy Moon, RES, 2014
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Causal factors raised in the Renew. UK AM and OAM study
9 H Aa Madsen -- WTN 2015 Glasgow
April 20-23, 2015
OAM due to transient stall ?
is it possible that transient stall occurs on a pitch regulated turbine in normal operation ?
characteristics of angle of attack (AoA) variations on a real full scale turbine ?
stall noise characteristics ?
the causes of variability of the occurence of OAM ?
is there a scaling effect ?
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Limits of the present overview
10 H Aa Madsen -- WTN 2015 Glasgow
April 20-23, 2015
the noise source characteristics of AM and OAM
But many other important subjects related to AM and OAM identification of all important factors influencing mechanisms behind AM and OAM
propagation effects
quantification of AM and OAM
annoyance characteristics of AM and OAM
mitigation
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Outline
the noise sources in relation to AM and OAM
the measurement techniques
inflow – (AoA and magnitude of the relative velocity)
high frequency surface pressure measurements
measurements on full scale turbines
results from two experimental data sets
scaling effect and blade leading edge degradation
conclusions
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Noise sources behind AM and OAM
12
Trailing edge (TE) noise important for AM, (OAM ?)
Stall (ST) noise
important for OAM Turbulent inflow (TI) noise
important for AM ? and OAM ?
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TE Noise Mechanism
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2y
3y
1y
1 2( )U y
( , )P k
1 2 3, ,u u u Far field sound
by scattering ( )PS
Turbulent boundary layer
Surface pressure fluctuations (SPF)
H Aa Madsen -- WTN 2015 Glasgow April 20-23, 2015 13
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Turbulent Inflow (TI) Noise
Idealized Airfoil as an Half-Plane
2y3y
1y
Far field sound
Turbulent Inflow
P2gustu
U1
Surface pressure fluctuations (SPF)
H Aa Madsen -- WTN 2015 Glasgow April 20-23, 2015 14
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Stall (ST) noise
Surface pressure fluctuations (SPF)
H Aa Madsen -- WTN 2015 Glasgow April 20-23, 2015 15
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Experimental characterization of the TE, TI and ST noise sources
and
the measurement techniques
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Experimental characterization of the source of TE, TI and ST noise
1y
H Aa Madsen -- WTN 2015 Glasgow April 20-23, 2015
Surface microphones to measure SFP
Five hole pitot tube to measure magnitude and direction (AOA) of U
AoA
17
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H Aa Madsen -- WTN 2015 Glasgow April 20-23, 2015
What are the noise sources influenced by ?
TE noise > function(AOA, U, blade roughness, transition, vortex generators)
AOA function ( inflow turbulence and shear, blade elastic torsion, blade
pitch)
TI noise > function(U, inflow turbulence and shear, leading edge geometry,
AOA ?)
ST noise > function(AOA, U, blade roughness, transition, vortex generators)
AOA function ( inflow turbulence and shear, blade elastic torsion, blade
pitch)
18
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Measurement of SPF with microphones mounted on the blade
H Aa Madsen -- WTN 2015 Glasgow April 20-23, 2015 19
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Measurement on a full scale rotor blade, 80m rotor, 2MW turbine - - DANAERO MW project 2009
surface pressure and
inflow with five hole pitot
tubes measured at 4 radial
stations
60 flush mounted
microphones for high
frequency surface
pressure measurements
measurement
campaigns from June to
September 2009
H Aa Madsen -- WTN 2015 Glasgow April 20-23, 2015 20
Madsen HAa et. al. , The DANAERO MW Experiments, paper AIAA 2010-645 presented at 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, 4 - 7 January 2010, Orlando, Florida
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Installation of the 38.8m instrumented blade in May 2009
Microphone holes
H Aa Madsen -- WTN 2015 Glasgow April 20-23, 2015 21
A new test blade was manufactured and instrumented
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Measurement of inflow (AOA) with a five hole pitot
4 pitot tubes on the 80m, 2MW, NM80 turbine
One pitot tube on the Siemens 3.6 MW turbine
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April 20-23, 2015
The DANAERO MW experiment 2009
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Experimental data sets
H Aa Madsen -- WTN 2015 Glasgow April 20-23, 2015 23
1. High frequency surface pressure data correlated with inflow
(AoA) data measured on an 80m diameter wind turbine rotor – the DANAERO experiment in 2009
provides insight into change of noise source characteristics at transition from attached flow to separated flow (stall)
2. Measurement of inflow (AoA) on the rotating blade over three weeks on the same turbine in 2003 provides statistics on the occurrence of high angle of attack (stall)
on a wind turbine in normal operation – (2014 study)
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1. Measurement of SPF on a full scale rotor blade, 80m diameter rotor, 2MW
SPF spectra derived at each red dot
Wind shear measured in the met mast
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TE spectra measured during free inflow at 9-11m/s -- amplitude modulation
Each spectrum is based on 0.5sec
25 H Aa Madsen -- WTN 2015 Glasgow
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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
Surface pressure level on suction side at x/c=0.84, Sept 1, 2009, 10:05
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Below stall
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Surface pressure level on suction side at x/c=0.84, Sept 1, 2009, 10:05 binned on AoA
102
103
104
80
85
90
95
100
105
110
115
center frequency fc [Hz]
Spp [
dB
1/1
2]
=7o
=8o
=9o
=10o
=11o
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Below stall
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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
DANAERO – NM80 turbine - forced operation at high angle of attack (AOA) by negative pitch and constant rpm – transient stall
High 1p modu-lation (15 dB) at the low frequencies. Microphone close to the trailing edge
H Aa Madsen -- WTN 2015 Glasgow April 20-23, 2015 28
Strong wind shear causing variations of AOA reaching stall
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DANAERO – NM80 turbine - forced operation at high angle of attack by negative pitch and constant rpm
H Aa Madsen -- WTN 2015 Glasgow April 20-23, 2015 29
Stall
[Hz]
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2. Measurement of inflow (AOA) on the rotating blade over three weeks
on the same turbine in 2003 provides statistics on the occurrence of high angle of attack
(stall) on a wind turbine in normal operation
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2. Inflow measurements with one five hole pitot tube on same turbine – 2003 - 3 weeks with normal operation – basis for statistics of inflow (AOA)
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Turbine situated in a small wind farm with 8 turbines
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Inflow measurements – 2003 - about 2000 of 10 min. time series
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Inflow measurements – 2003 - analysis of about 2000 10 min. series
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AOA amplitude is max to min
Free inflow Wake operation
Madsen H Aa, Bertagnolio F, Fischer A, Bak C, Correlation of amplitude modulation to inflow characteristics. Proceedings of INTERNOISE 2014, Melbourne, Australia, November 16-19, 2014.
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Inflow measurements - 2003 - 10 min. statistics of AOA variations - the risk of transient stall
5 degrees between grid lines
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An example of high AOA
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Mx is flapwise blade root moment
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A scale effect due to rotational sampling of turbulence
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Spectrum of the wind seen from the tip of the rotating blade
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A scale effect due to rotational sampling of atmospheric turbulence
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1p
Increase in 1p content of AoA when upscaling rotors
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Blade leading edge roughness
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Leading edge rough-ness will decrease Clmax and increase risk for transient stall
http://www.nacleanenergy.com/articles/17870/reliability-centered-maintenance-for-wind-turbine-blades NM80 turbine – DANAERO project
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Conclusions
strong 1p modulation seen in measured SPF spectra on
rotating 38m blade
a shift in SPF spectra to lower frequencies when transient stall occurs
very likely that trailing edge separation causing stall noise is a major mechanism of OAM
wake and shear effects increase AoA variations
transient stall seems closely linked to the variable speed operation of the turbine
the intermittent character of the AoA variations corresponds well to observed characteristics of OAM
upscaling of rotors will increase 1p AoA variations
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Thank you for your attention
40 H Aa Madsen -- WTN 2015 Glasgow
April 20-23, 2015