Hansen FFSICpresentation KLH BZ

47
See discussions, stats, and author profiles for this publication at: http://www.researchgate.net/publication/282862182 INFRASOUND AND LOW-FREQUENCY NOISE FROM WIND TURBINES RESEARCH · OCTOBER 2015 DOI: 10.13140/RG.2.1.3826.5049 READS 50 3 AUTHORS: Branko Zajamsek University of New South Wales 19 PUBLICATIONS 11 CITATIONS SEE PROFILE Colin H Hansen University of Adelaide 250 PUBLICATIONS 3,624 CITATIONS SEE PROFILE Kristy Hansen Flinders University 23 PUBLICATIONS 59 CITATIONS SEE PROFILE Available from: Branko Zajamsek Retrieved on: 18 November 2015

description

Hansen FFSICpresentation KLH BZ

Transcript of Hansen FFSICpresentation KLH BZ

Page 1: Hansen FFSICpresentation KLH BZ

Seediscussions,stats,andauthorprofilesforthispublicationat:http://www.researchgate.net/publication/282862182

INFRASOUNDANDLOW-FREQUENCYNOISEFROMWINDTURBINES

RESEARCH·OCTOBER2015

DOI:10.13140/RG.2.1.3826.5049

READS

50

3AUTHORS:

BrankoZajamsek

UniversityofNewSouthWales

19PUBLICATIONS11CITATIONS

SEEPROFILE

ColinHHansen

UniversityofAdelaide

250PUBLICATIONS3,624CITATIONS

SEEPROFILE

KristyHansen

FlindersUniversity

23PUBLICATIONS59CITATIONS

SEEPROFILE

Availablefrom:BrankoZajamsek

Retrievedon:18November2015

Page 2: Hansen FFSICpresentation KLH BZ

INFRASOUND AND LOW-FREQUENCY NOISE FROM WIND TURBINES

Colin Hansen, Branko Zajamšek and Kristy Hansen

FSSIC2015, Perth, July 2015

Page 3: Hansen FFSICpresentation KLH BZ

OUTLINE● What is it about wind farm noise that distresses rural

residents?o is it imagined?o is it stress and lack of sleep related?o is it a physiological phenomenon that only affects

some people?● What weather conditions exacerbate the problem?● What mechanisms are responsible for generating the

disturbing characteristics?● Propagation effects● Experimental investigations using a model turbine

FSSIC2015, Perth, July 2015 2

Page 4: Hansen FFSICpresentation KLH BZ

Annoying wind farm noise characteristics

● Dominated by low frequencies more than 1 km from a wind farm

● Contains significant levels of periodic (multi-tonal) infrasound due to rotor noise arising from blade tower interaction

● Time varying amplitudeo Random variation (constructive/destructive interference

of blade pass harmonics – infrasound and low-frequency)

o Amplitude modulation (periodic variation)

o Beating

FSSIC2015, Perth, July 2015 3

Page 5: Hansen FFSICpresentation KLH BZ

Various forms of amplitude variation

FSSIC2015, Perth, July 2015 4

Beating

Amplitude modulation

Random amplitudevariation

Page 6: Hansen FFSICpresentation KLH BZ

Annoying wind farm noise characteristics

● Thumping noise (16 – 80 Hz)o Most likely due to part of the blade stalling

when entering a high speed air flow with incorrect angle of attack

o Worse in high wind shear conditions● Rumbling noise (16 – 80 Hz)

o Amplitude modulated gearbox noise?o Blade tower interaction?

● Swishing noise (150 to 800 Hz)o Trailing edge noise variation resulting from

directivity and possibly loading variationso Problems usually below 400 Hz

FSSIC2015, Perth, July 2015 5

Branko
Sticky Note
This image doesn't look nice.
Branko
Inserted Text
, Doppler amplification
Page 7: Hansen FFSICpresentation KLH BZ

What symptoms are experienced by some residents near wind farms?

● Sleep disruption and insomnia● Nausea● Tinnitus (ringing in ears)● Pressure in ears● Headache● Raised pulse rate● Vertigo and dizziness

FSSIC2015, Perth, July 2015 6

Page 8: Hansen FFSICpresentation KLH BZ

What causes symptoms?● Infrasound is below the audibility threshold of normal

hearing minus 2 standard deviationo However, work by Salt shows that the outer hair cells of

the ear can respond to infrasound at levels below the threshold of hearing

● If it’s not inaudible infrasound causing a physiological response, is it audible low-frequency noise?

● Well known that audible low-frequency noise annoys some people, even at low levelso Annoyance can cause a stress response which can

result in sleep disruption, which, over time, can cause symptoms

FSSIC2015, Perth, July 2015 7

Page 9: Hansen FFSICpresentation KLH BZ

Data recorded at Waterloo wind farm

FSSIC2015, Perth, July 2015 8

Measuredat 3.5 km from wind farm

Branko
Sticky Note
fix the arrow
Page 10: Hansen FFSICpresentation KLH BZ

EAR

FSSIC2015, Perth, July 2015 9

Page 11: Hansen FFSICpresentation KLH BZ

Infrasound detection

FSSIC2015, Perth, July 2015 10

Page 12: Hansen FFSICpresentation KLH BZ

What conditions correspond to greatest annoyance?

● When the contrast between background noise and wind farm noise is greatesto Occurs when there is no wind at the residence but

enough wind at turbine height to drive turbineso No wind at residence means no wind noise

In rural areas this can mean very little noise at allo Large difference between wind at turbine rotor and

residence is a result of high wind shear conditions Occurs mainly during night time in stable conditions

● In urban areas, the higher background noise (from traffic) results in turbine noise being less annoying

FSSIC2015, Perth, July 2015 11

Page 13: Hansen FFSICpresentation KLH BZ

Why is audible wind farm noise at residences dominated by low-

frequencies?

● Wind farms produce noise over the audible frequency range from infrasound to a few kHz.

● Noise detected by residents in their homes more than 1.5 km from a wind farm is mainly low frequency (usually less than 160 Hz) as a result ofo Propagation effectso House transmission effects

● Low-frequency noise is more easily detected if mid and high frequency noise is at a low level

FSSIC2015, Perth, July 2015 12

Page 14: Hansen FFSICpresentation KLH BZ

Sound propagation effects● Low-frequency and infrasonic noise domination

o No ground or atmospheric absorptiono Worse when downwind

FSSIC2015, Perth, July 2015 13

Page 15: Hansen FFSICpresentation KLH BZ

Sound propagation● Multiple reflections results in there being a point

beyond which sound propagation results in a less than 6 dB loss per doubling of distanceo Eventually becomes close to 3 dB

FSSIC2015, Perth, July 2015 14

Page 16: Hansen FFSICpresentation KLH BZ

Sound propagation● Sound may decay at less than 3 dB per doubling of

distance due to the arrangement of the wind turbines.● For example, for the Waterloo wind farm, sound decay

is much less than 6 dB per doubling of distance.

FSSIC2015, Perth, July 2015 15

Page 17: Hansen FFSICpresentation KLH BZ

House noise reduction (out – in)

FSSIC2015, Perth, July 2015 16

House 1 House 2

Noise reduction from outside to inside for wind turbine source

Page 18: Hansen FFSICpresentation KLH BZ

House noise reduction (out – in)

● Low-frequency noise and infrasound are much less attenuated by the building structure than mid- and high-frequency soundo At 3-4 Hz there is a resonance associated with the

mass of the walls and roof interacting with the enclosed volume in the house

o Around 8 Hz for a house with windows open, there is a resonance associated with a Helmholtz resonance effect with the window opening acting as the neck and the room as the volume

o Other noise reduction dips are harmonics of the above

FSSIC2015, Perth, July 2015 17

Page 19: Hansen FFSICpresentation KLH BZ

Overall noise reduction probability

FSSIC2015, Perth, July 2015 18

Page 20: Hansen FFSICpresentation KLH BZ

Wind turbine noise characteristics● Aerodynamic noise

o Tonal (rotor noise) Frequency range 1 Hz to 80 Hz Caused by blade tower interaction

o Broadband (trailing edge and leading edge noise) Usual frequency range 160 Hz to 1500 Hz, but mostly

interested in 160 Hz to 500 Hz Frequencies shifted lower when the angle of attack changes

due to variations in the air flow speed – can also result in stall being induced, producing the characteristic “thumping noise”.

● Gear noise radiated by blades and towero Usually amplitude modulated toneo Sometimes heard as a “rumbling noise”

FSSIC2015, Perth, July 2015 19

Page 21: Hansen FFSICpresentation KLH BZ

What produces low-frequency sound and infrasound, and their time variation?● Blade tower interaction● Thickness effect – blade pushing air out of its way● Unsteady blade loading variations due to

o Atmospheric turbulenceo Wind shearo Cross-windo Wakes from upstream turbineso Blades passing the tower

● Blade stall as a result of high wind shear conditionso Blade stalls when entering high speed air flow due

to sub-optimal pitch angle

FSSIC2015, Perth, July 2015 20

Branko
Sticky Note
Blade can stall. Some people might say that this is very unlikely noise production mechanism because modern wind turbines are pitch regulated in contrast to stall regulation which was applied back in the day. By saying this they are implying that modern wind turbine cannot stall which is not correct and it doesn't make much sense. Both control regimes can cause stall.
Page 22: Hansen FFSICpresentation KLH BZ

Broadband noise generating mechanism● Broadband noise (trailing edge, leading edge and

stall noise) originates from turbulence interaction with the wind turbine airfoilo Strongly affected

by unsteady blade loading

o Leading edge noise can extend to infrasound range when there is in-flow turbulence

FSSIC2015, Perth, July 2015 21

Page 23: Hansen FFSICpresentation KLH BZ

Trailing edge noise● Magnitude proportional to boundary layer

displacement● Can be produced in a smooth air flow due

to the turbulent boundary layer on the blade● Frequency range can extend below 150 Hz

for a large wind turbine in the presence of wind speed variations (atmospheric or wake turbulence)o Causes angles of attack that are too high,

which increases the small-scale turbulence in the flow over the blade, especially if the stall condition occurs

FSSIC2015, Perth, July 2015 22

Branko
Cross-Out
Branko
Inserted Text
Increases the angle of attack which than
Page 24: Hansen FFSICpresentation KLH BZ

Atmospheric boundary layer effects● Broadband noise enhanced by unsteady loading on the

blades● Boundary layer has a strong effect on loading unsteadiness

o Stable boundary layer occurs at night and gives rise to strong winds that increase in strength with increasing height producing unsteady loading on a wind turbine rotor

FSSIC2015, Perth, July 2015 23

Mixed layer

Mix

ed la

yer

Stable boundary

Residual layer

layer

Noon NoonSunset Sunrise

Hei

ght,

km

0.5

1

Page 25: Hansen FFSICpresentation KLH BZ

Effect of boundary layer on blade loading● Low level jets can also occur at night when a stable

boundary layer produces a high velocity gradient and low level jets around 200 m high o Cause unsteady loading and increased noise

FSSIC2015, Perth, July 2015 24

Page 26: Hansen FFSICpresentation KLH BZ

Tonal noise generating mechanisms● Rotor noise – 2 components

o Thickness noise Only apparent in the first

few harmonics of the BPFo Steady loading noise

Results from different loading on each side of the blade when angle of attack not zero

Usually much more significant than thickness noise Directivity different to thickness noise

● Blade-tower interaction (BTI) noiseo Caused by the loading excursion shown in the figureo In large turbines, can be seen up to the 60th harmonic of BPF.

FSSIC2015, Perth, July 2015 25

Page 27: Hansen FFSICpresentation KLH BZ

Rotor noise generation

FSSIC2015, Perth, July 2015 26

● A small element in the rotor plane generates sound as a blade passes through it.

● Repeats every blade pass for every element.

● Each element acts as a simple oscillator radiating noise at blade pass frequency and harmonics.

Page 28: Hansen FFSICpresentation KLH BZ

Effect of support tower● Change in air flow speed in front of the tower

o Changes effective angle of attacko Shifts trailing edge noise to lower frequencies

● Resulting change of blade loading produces tonal emissions at BPF and harmonics (up to 60th harmonic)

FSSIC2015, Perth, July 2015 27

Page 29: Hansen FFSICpresentation KLH BZ

Effect of tower wake velocity defect shape on noise spectrum shape

● Very small differences in the wake shape can have a very large effect on the noise spectrum shape

FSSIC2015, Perth, July 2015 28

Large turbine measurement1.3 km from nearest turbineCross-wind

From Thresher, R. W. (1981). Wind turbine dynamics. NASA Technical report.

Page 30: Hansen FFSICpresentation KLH BZ

Experimental work● Aims

o To investigate noise source distribution in the rotor plane, using a microphone array

o To investigate broadband and tonal noise directivity characteristics using point microphones

o To investigate the effect of the support tower on broadband and tonal noise

● Blades – NACA 0012, 70 mm chord, 450 mm length

● Model turbine run as a propellero Similar noise producing

mechanisms as a large turbine

FSSIC2015, Perth, July 2015 29

1. Blade2. Slip ring3.Torque sensor4. Motor5. Support tower

Page 31: Hansen FFSICpresentation KLH BZ

Model vs full size frequency ranges● Different frequency ranges but same noise producing mechanisms● Rotor noise extends to 60th harmonic of blade pass frequency in a full

size turbine (3 MW)● Larger turbines produce lower frequency TE noise due to larger chord● Frequency range between TE and rotor noise dominated by leading

edge noise and stall noise.

FSSIC2015, Perth, July 2015 30

Page 32: Hansen FFSICpresentation KLH BZ

Measurement set-up● Support tower – rotor spacing 70 mm for model represents

approximate large wind turbine spacing (6-7 m)● Microphone array distance from rotor plane

o 100 mm for statistically optimised nearfield acoustic holography (SONAH)

o 1.5 m for beam forming

FSSIC2015, Perth, July 2015 31

Page 33: Hansen FFSICpresentation KLH BZ

Microphone array● Used for acoustic holography and beam forming● 64 mics and 1.5 m diameter

FSSIC2015, Perth, July 2015 32

Page 34: Hansen FFSICpresentation KLH BZ

Beamforming● Used to locate and quantify broadband noise source

in the rotor plane● Sound sources are identified by steering the beam

using the standard delay and sum approach

FSSIC2015, Perth, July 2015 33

Page 35: Hansen FFSICpresentation KLH BZ

Beamforming (and SONAH) processing methods

● Time averaged● Phase averaged

FSSIC2015, Perth, July 2015 34

Page 36: Hansen FFSICpresentation KLH BZ

Time averaged beamforming

FSSIC2015, Perth, July 2015 35

● 0º angle of attack and 3.15 kHz 1/3 octave band (scaled from 150 Hz for full size turbine)

● Figure shows reflection from support tower and uniform blade noise generation independent of blade location

Page 37: Hansen FFSICpresentation KLH BZ

Phase averaged beamforming

FSSIC2015, Perth, July 2015 36

P2

P1

● 4 kHz 1/3 octave band, 10º angle of attack

● SPL greater at P1 than at P2● reduction of SPL at P2 is due to a

combination of acoustic destructive interference (between direct and tower-reflected waves) and scattering

Leading edge

Branko
Sticky Note
Similar results for 5 AOA. 5 AOA is closer to wind turbine normal operational angle of attack which is estimated to be on average 6 degrees. 10 degrees is close to stall regime.
Page 38: Hansen FFSICpresentation KLH BZ

Directivity, 4 kHz 1/3 octave band

FSSIC2015, Perth, July 2015 37

● 10º angle of attack● Decrease of 3 dB

compared to the BPM model at 90°, is most likely due to destructive interference between direct and tower-reflected waveso Less for bottom mics

● Possible contributor to enhanced amplitude modulation

Mics at rotor centre height

Mics at rotor bottom height

Page 39: Hansen FFSICpresentation KLH BZ

Effect of blade-tower spacing

FSSIC2015, Perth, July 2015 38

● Tonal amplitude is a function of blade-tower spacing● Relative amplitudes similar to full size turbine

o Corresponds to 70 mm spacing in model

Page 40: Hansen FFSICpresentation KLH BZ

Measured infrasound● 2 locations, Houses 1 and 2● Blade-tower spacing 6 – 7 m

o Corresponds to 70 mm in model

FSSIC2015, Perth, July 2015 39

Page 41: Hansen FFSICpresentation KLH BZ

Near field holography

FSSIC2015, Perth, July 2015 40

● Sound visualisation and sound localisation technique● Capture of an evanescent wave is essential, so

hologram plane must be close to source plane (100 mm in our case)

Page 42: Hansen FFSICpresentation KLH BZ

Time averaged holography● Tower has a significant effect on

blade pass harmonics● Higher SPL occurs to the left of the

tower as it takes a finite time for the noise to develop and radiate

FSSIC2015, Perth, July 2015 41

Page 43: Hansen FFSICpresentation KLH BZ

Phase averaged holography

● 4th blade pass frequency (180 Hz)

FSSIC2015, Perth, July 2015 42

Page 44: Hansen FFSICpresentation KLH BZ

Directivity for blade pass harmonics● Sound pressure level for 180º, 1.5 m in front of rotor plane

– near field for 1st and 3rd harmonics (45 Hz and 135 Hz)o In far field, lower order harmonics become more like the

dipole shape of the 10th harmonic (450 Hz)o First harmonic of industrial wind turbine has a wavelength

of 400 m

FSSIC2015, Perth, July 2015 43

Page 45: Hansen FFSICpresentation KLH BZ

Control of noise● BTI produces infrasound and low-frequency tones that

can be detected by sensitive people at large distances from a wind farm

● Possible means to reduce BTI noiseo Blade phase desynchronisation and randomisation to vary

times when blades from different turbines pass the tower Will shift around the areas of max. sound reinforcement

o Blade pitch change as it passes the tower o Use of swept back blades to control higher harmonics by

causing the sound generated along the blade span to be out of phase

o Adding strakes to the tower to change the wake characteristics

o Active noise cancellation system in residences

FSSIC2015, Perth, July 2015 44

Branko
Sticky Note
That is right. We want to have smooth wake in order to reduce higher BPF harmonics.
Page 46: Hansen FFSICpresentation KLH BZ

Control of noise

● Thumping noise – most likely caused by fluctuating loads on the blade could be controlled byo continuously varying the blade pitch to prevent stallo Fixing the blade pitch sufficiently below the stall angle

for the mean wind speed so that turbulence or wind shear does not cause stall

o Use of random blade pitch variation

● Infrasound and low-frequency noise caused by aero-acoustic excitation of bladeso Use more efficient blade designs

FSSIC2015, Perth, July 2015 45

Page 47: Hansen FFSICpresentation KLH BZ

Conclusions

● Wind farm generated infrasound and low frequency noise is a problem for some nearby residents

● Some people have been adversely affected at distances of 5 km from the nearest turbine in a large wind farm

● BTI noise is an important source in the infrasonic and low-frequency range

● It is possible to reduce turbine low-frequency noise and infrasound but guidelines for acceptable noise levels need to be revised if all residents are to be protected from adverse effects

FSSIC2015, Perth, July 2015 46