Brief descriptions are given of the various types of large wind turbines and their sound characteristics. Candidate sources of broadband sound are identified and are rank ordered for a large upwind configuration wind turbine generator for which data are available. The rotor is noted to be the main source of broadband sound which arises from inflow turbulence and from the interactions of the turbulent boundary layer on the blade with its trailing edge. Sound is radiated about equally in all directions but the refraction effects of the wind produce an elongated contour pattern in the downwind direction

Grosveld, F. W.

Hubbard, H. H.

Shepherd, K. P.

English

NASA Technical Reports Server

BROAD RANI! SOIIFII! FROM WIND TURBINE GENERATORS 
* I  
H. E. Hubbard, K.  P .  Shepherd 
and F. W. Grosvzld 
TEE COLLEGE OF WILLLAM AND MARY 
Virginia Associated Research Campus 
Newport News, Virginia 23606 
and 
THE BIONETICS CORPORATION 
Hampton, Virginia 23666 
Grant NAG1-166 and 
Contract Nul-14970 - 
November 1981 
Nanonal Aeronaubcs and 
Space Administrahon 
Lnolay-- 
Hampton.Virginia 23665 
https://ntrs.nasa.gov/search.jsp?R=19820026311 2020-03-21T06:13:12+00:00Z
INTRODUCTION 
TABLE OF CONTENTS 
PAGE 
TYPES OF WIND TURBINE GENERA’TRS 
SOURCES OF BWMD BAN!) SOUND 
ACOUSTIC HEASURMENTS ON mD.2 MACHINE 
Test S i t e  
Example Sound Spectra 
Effects of Distance 
Far F i e l d  Corntours 
CONCLUDING REMARKS 
REFERENCES 
1 
1 
2 
3 
3 
3 
4 
4 
5 
5 
FIGURES 
i 
INTRODUCTION 
There is a growing i n t e r e s t  i n  t he  concept of  power generat ion by 
means of wind turbines. 
(of megawatt s i z e )  which may be located in s i n g l e  o r  in mult ip le  units at 
many d i f f e r e n t  places around the  uorld. 
are ava i l ab le  f o r  l a rge  wind turb ines  to  use as a g u i d e i n  t h e i r  design 
and s i t i n g  f o r  acceptable  envlronmntal impact (Refs. 1-9). 
This has l ed  t o  t h e  development of  large r c h i n e s  
To date very few acoustic data 
The purpose of t h i s  paper is to  characterize t h e  problem of eadron- 
mental noise f o r  large vind turbines .  
a r e  of i n t e r e s t  w i l l  be described b r i e f l y  along with t h e i r  sound cborac- 
t e r i s t i c s .  Sound sources  w i l l  be  i d e n t i f i e d  and w i l l  be  rank ordered 
for one machine f o r  which systematic  data are ava i lab le .  
The types of  wind tu rb ines  tht 
TYPES OF WIND TURBINE GENERATORS 
Currently operat ing wind tu rb ine  g e n e n t o r a  which cover a vide mge 
of power ra t ings  from ki lowat t s  to  aeg3vatts can be categorized aa ver- 
tical axis or hor izonta l  axis machines as indica ted  In  Figure 1. 
tical  axis machines include the  Derrieus and Cyromill types. 
cally have 2-4 blades which rotate about a v e r t i c a l  axis and they are 
nondirect ional  with respect  to  t h e  wind. 
DarrieuS -chines having a r a t i n g  of about 0.5 IW. 
V e r -  
They typi- 
The largest of  this type are 
Horizontal a x i s  machines are now opera t iona l  in t he  2 W range. 
Tney have 2 o r  3 blades, and opera t iona l  speeds in t he  range 18-30 rpm. 
They a r e  r e fe r r ed  t o  as e i t h e r  upwind o r  downwind muchines depending on 
t h e  loca t ion  of t h e  ro to r  with respect t o  t h e  supporting tower. 
operate  most e f f i c i e n t l y  when al igned with the  wind vector. 
They 
- 1 -  
A schematic i l l u s t r a t i o n  of a wind tu rb ine  sound s p e c t n n  is g i a n  
fn Figure 2. 
steady and f luctuat inf l  blade lords ,  are at  mult iples  o f  t h e  blade p888ag. 
frequency and hence occur at very low frequencies. 
colponents on the o t h e r  hand are associated with t h e  turbulent  inflow, 
t h e  blade boundary layera, and wakd turbulence. 
of d i f f e r e n t  aerudylur ic  pharomem and c o n s i s t  of  a wide range of  fre- 
quencfes from subaudible up i n t o  t h e  normal range of hearing. 
The discrete f r e q u e n 9  components asclociated with both 
The b d  band murid 
They arise fron 8 number 
The presence of  i n t a m e  d l s c r e t c  frequency corponcnts h a  beat li- 
mited t o  those b r i r o n t d  axis coa f igu ra t ioas  with downwind roto- urd 
for which the re  is t h e  p o a a i b i l i t p  o f  s t r o n g  rotorctower rrrke interac- 
t ions.  
In t e rac t ions  of vertical axis nuchinas. Broadband somd,  howemr, is of 
concern f o r  a11 types of machines. 
focus on t h e  broadband components. 
Discrete frequency souuds a l s o  arise f m s  t h e  torque tub.-bl.de 
The r e m i n d e r  o f  t h i s  paper w i l l  
SOURCES 0 )  BRDAD BAND SOUND 
There are a n u d e r  of  posaible sources of  broadband sound due to 
t h e  i n t e rac t lons  of r o t a t i n g  blades wfth t h e  surrounding air. 
these which could be important f o r  t h e  rangas of geometry m d  operat ing 
conditiona of wind tu rb ine  generators are l i s t e d  In Figure 3. 
they are categorized a8 being associated with the turbulent  inflow, t h e  
a i r f o i l  i t s e l f  o r  t he  vakes. 
Soar o f  
Note t h a t  
Included are such phenomena as d i r e c t  r ad ia t ion  from t h e  .rod-c 
w8kea of t he  blades and the turbulent boundary l aye r s  on t h e i r  surfaces ,  
vortex shedding, separated flows due t o  local ized s t a l l i n g  and the  ia- 
t c r ac t ions  of the aerodynamic flow with surface roughness, protuberancea, 
- 2 -  
c a v i t i e s  and slots. As vi11 be i l l u s t r a t e d  subsequently, rusuremmts 
on a largc hor izonta l  u i s  upwind machine suggested that its hro r i n  
noise  w u r c s s  are inflow turbulence and turbulen t  boundary layer inter- 
ac t ions  vita the  blade t r a i l i n g  edges. 
to  be *rtmt.  
None of  t h e  o t h e r  sources  - 
ACOUSTIC HEAS- ON MOD-2 IWXIINE 
T e s t  S i t e  --
The o p p r t u n i t y  vas taken to r k e  8 s y s r m t i c  series of m m d  ma- 
s u r m t s  on t he  DOE-NASA-BPA EJDD-2 -chine shown in the inset photograph 
of P igure  4. It is a tu0 blade, horizontal axis, upwind chine which 
has a 300 foot  diawter r o t o r  and an rpm of 18. 
Coodnoe B i l l s  region on t h e  north rim of t h e  Colubia Uver Gorge in t h e  
S t a t e  of Washington. Examples of acous t i c  daLa from Ref. 1 are shoun in 
Figures 5, 6 and 7. 
It is located in the 
-le Sound Spectra  
Heasurements In both t h e  near and f a r  acous t i c  f i e l d s  Indicated 
characteristic s p e c t n m  shapes such as those in the  e x a p l e  p lo t  of 
Figure 5 .  
sult from d i f f e r e n t  aerodynamic f l o w  p h . n o r n a  on t he  blades, ms indicated 
by the  a i r f o i l  sketch. 
Note that two broadband sound peaks are present  rad  these  re- 
Available twldence suggests that t h e  low frequency peak is asweb- 
t ed  with inflow turbulence. 
angle  of a t t a c h  changes which in turn result in unsteady l i f t  and d r r g  
loads and ass -c ia ted  sound radiat ion.  These sound components are m- 
surable  and W i l l  propagate w i t h  very l i t t l e  atmospheric a t tenuat ion ,  but  
are not r ead i ly  observable above the  background noise .  
Random ve loc i ty  f luc tua t ions  muse e f f e c t i v e  
- 3 -  
(In t he  othes  hsitd, t he  higher  frequency peak is bel ieved t o  result 
from the  in t e rac t ions  of the turbulen t  boundary layers on t h e  8Irfol l  sur- 
faces  v i t h  the  a i r f o i l  t r a i l i n g  edge. 
doritrate t he  spec t ra  near t h e  machine, and are i n  a frequency range a r e  
the  ear is s e n s i t i v e  and t h e  background noise  level is low. 
d i s t a a t e s  a t  the  opera t iona l  site where t he  background A-scale noise level 
was about 30 dB were about 1400 m (4600 f t )  upwind and i n  excess o f  2100 m 
(6900 f t )  downwind. 
These sound components seam to  
Detection 
Effect8 - Of CiStaxkCe 
Figure 6 contains  a p l o t  of a a s u r e d  sound pressure  levels UJ fmrc- 
t i on  of distance f o r  c o g a t i s o n  with predict ions.  
example instantaneous pressure t h e  h i s t o r y  for one of the measuring 
polnts Is shown In the  i n s e t  a t  the  bottom of the  f igure .  It was a s h  
f o r  t h e  pred ic t ions  that t h e  obsamer  was at ground level in f r o n t  of t h e  
machine and t h a t  t he  sound source could be represeated by a concantr8ted 
d ipole  a t  hub height. 'hro results CUI be seen. 
sured values  seem t o  be i n  excellent agreement except f o r  t h e  clou-in 
s t a t ions .  
wind noise  c o a t a d r u t i o n  of some memured data. 
agreement f o r  t h e  predicted and muarumd fa l l  o f f  rates with l n c r u r l n g  
dis tance suggests  that the  -chine can be represented a d e q u t t l y  u a 
ctmcentrated d ipole  source fo r  f a r  f i e l d  sound predic t ion  p u r p r e s .  
For i n f o r m t i o n  an 
The predic ted  and w- 
This good agreement m y  be f o r t u i t o u s  bccauae of t h s  porrlbl~  
The apparent ly  v d  
Far Field Contours -- 
Baaed oil the  data  of Figure 6 p l u s  o the r  eaeasurements and obserm- 
Shown t l o a s  the polar diagram p o t s  of Figure 7 have beea constructed. 
on Figure 7 a r e  estimated A-level contour l i n e s  f o r  6 5 ,  5 5 ,  45 and 35 dB 
- 4 -  
values. Shown :-1so is t h e  de t ec t ion  l i m i t  d i s t ance  of  about 1400 II for 
t h e  southvest quadrant, for which the  A-level backffround noise u86 about 
30 dB. 
tance of about 2100 m, thus confirming t h e  ex i s t ence  o f  an elongation of 
the  r ad ia t ion  pa t t e rn  in t h e  donwind d i r ec t ion .  This e l o n g 8 t ~ a n  i 8  be- 
l ieved due to  t h e  r e f r a c t i o n  e f f e c t 6  of t h e  wind r a t h e r  than t o  any p n -  
f e r r e d  d i r e c t i o n a l  p rope r t i e s  of t h e  aource. 
In the downwind d i r ec t ion ,  t h e  sound was clearly aud ib le  at  a dis-  
It was general ly  o b a e m d  that t h e  upvind (wst d i r e c t i o n )  propa- 
00 the  o t h e r  gated sound signals were r e l a t i v a l p  r teady in amplitude. 
hand i n  both the  south d i r e c t i o n  (crosswind) and t h e  eaat direction (down- 
wind) the  sound signal had a pe rcep t ib l e  m p l i t u d e  lodu lo t ion  at t h e  
blade passage frequency. 
d i s t ances  ( p a r t i c u l a r l y  dowavlnd), t h e  sound w y  be de tec t ab le  from only 
t h e  topmost portion of t h e  rotor disk. 
It has thua been suggasted that at t h e  l a w r  
CONCLUDING REMARKS 
Measurements on a l a r g e  uparind configurat ion wind t u r b k - s  suggart 
that t h e  main source of  sound Ls t h e  r o t o r  r a t h e r  than t h e  gears, shafts 
and electrical generation equipment. 
sound spectrum and they are noted t o  arise fmo inflow turbulence and 
from turbulent  boundary l a y e r - t r a i l i n g  edge in t e rac t ions .  
diated about equally in all d i r e c t i o n s  but t he  r e f r a c t i o n  effect6 of t h e  
wind produce an elongated contour p a t t e r n  ia t h e  downwind d i r ec t ion .  
Broadband components dominate t h e  
Sound i r  ra- 
REFERENCES 
- 1. Hubbard, H. H.; Shepherd, K. P.; and Ctoeveld, F. W.: Sound Measure- 
merits of the b e i n g  HOD-2 Wind Turbine Generator. 
July 1981. 
NASA-CR-165752, 
- 5 -  
2. - 
3. - 
4. - 
5 .  - 
6. - 
7. - 
8. - 
9. - 
Kelley, N. D.: Noise Generated by Large Wind Turbines. Presented at 
Wind EDergy Technology Conf., Kansas City,  MO, March 16-17, 1981. 
An Exploratory Suroey of Noise Levels Associated Balombin, J. 3.: 
v i t h  a 100 kW Wind Turbine. NASA-TM-81486, 1980. 
Stephens, D, G.; Shepherd, K. P.; and Groswld, F.: Wind Turbine 
Acoustic Standards (Proceedings of t h e  Second DOE/NASA Wind Turbine 
Conf., Cleveland, OH, Feb. 24-26, 1981). 
Greene, G. C.; and Hubbard, H. E.: Some Calculated Effect8 of Non 
Uniform Inflow on t he  Radiated Noise of a Large Wind Turbine. 
NASA-IM-81813, 1980. 
Vlterna, L. A,: The NASA-LeRC Wind Turbine Sound P red ic t ion  Code. 
(Proceedings of t h e  Second DOE/NASA Wind Turbine Conf., ClbPBlaLld, 
OH, Feb. 24-26, 1981). 
Kelley, N. D.: Acoustic Noise Generation by WE/NlrsA MOD-1 Wind Tur- 
bine. 
Cleveland, OH, Feb. 24-26, 1981). 
(Proceedings of t h e  Second DOE/NASA Wind Turbine Conf., 
Martinez, R.; Widnall, S. E.; and Harris, W. L.: Predict ions of Low- 
Frequency Sound from t h e  M O P 1  Wind Turbine. 
MIT, 1980. 
FDL Report No. 80-5, 
Thompson, D. W.: Analyt ical  Studies  and Field Measurements of Infra- 
sound Propagation at  Howard's Knob, NC. 
Penn S t a t e  Univ,, Final R e p o r t  18, Sept. 1980. 
Dept. of Metemrology, 
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Broad band sound from wind turbine generators

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