PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Prediction of Broadband Noise Generated from Turbulent BoundaryLayers of a Horizontal Axis Wind Turbine
Soichi SASAKI1; Zaw Moe Htet2
12

Nagasaki University, Japan

Mandalay Technological University, Myanmar

ABSTRACT
We expanded the blade element momentum theory (BEM) for the prediction of the broadband noise of ahorizontal axis wind turbine. For the prediction of the broadband noise, the acoustic radiation from theturbulent boundary layers was applied. From the results of the wind tunnel test, NACA0018 generated thehumped noise in the attached flow condition, whereas the noise spectra in the separated flow condition madethe broadband noise. In this prediction methodology, the noise level of the wind turbine could be predictedby the model size of the isolated blade and the main dimensions of the objective wind turbine. At this time,
the relative velocity and the angle of attack became the important parameters. We pointed out that the humped
noise source in the wind turbine was made from the mid­span to the blade tip on the impeller based on thismethodology.
Keywords: Wind Turbine, Blade, Aerodynamic Noise, Wind Tunnel Experiment

1. INTRODUCTION
The horizontal axis wind turbine (HAWT) is a machine that extracts kinetic energy from the windand converts it to mechanical energy. In Myanmar, the windy area such as sea side and flat land in the
middle of the country has wind speed up to 6 m/s where wind turbine can be generated well. However,in actual operating condition of the wind turbine, the aerodynamic noise becomes one of the important
technical issue. Especially, to predict the broadband noise which is generated from the airfoil is
important technical issue for the development of HAWT. The detailed review on the aerodynamics of
the wind turbine rotor is described by Herman Snel (1); the blade element momentum theory (BEM)
in the two dimensional has been explained in the article. The BEM is the methodology for the analysis
on the fluidic properties of the blade in which the annular momentum is used for the global flow
region. The BEM has been used for the analysis on the aerodynamic properties of the wind turbine,
especially, for analysis of the initial development stage. Recent year, a lot of researchers are using acommercial CFD code; however, in the present stage, a lot of CFD can guarantee the high accuracy
to solve the complicated turbulent boundary layers on the simple model, but cannot provide sufficientaccuracy for the prediction of the aerodynamic noise. Thus, in the initial design stage, to supply thesuitable methodology can enhancement the design the high performance wind turbine.
In this study, we expand the blade element momentum theory for the prediction of the aerodynamic
noise of the HWAT. First of all, the characteristics of NACA0018 isolated blade until the deep stallcondition are clarified by the wind tunnel experiment. Moreover, for the prediction of the broadband
noise, the theory of the aerodynamic noise generated by the turbulent boundary layers is applied (2).
Finally, we discuss the characteristics on the spectral distributions of the broadband noise which isgenerated by the wind turbine that of 20m diameter.

1souichi@nagasaki­u.ac.jp3831

2. EXPERIMENTAL SETUP
Fig.1 is the overview of the examined isolated blade. We employed NACA0018 for the wind tunnelexperiment. The chord length is 100mm, the span length is 400mm. In Fig. 2, the experimental setupof the wind tunnel experiment is shown. When the Reynolds number at the nozzle exit is 1.3×105, theturbulence of the main flow was less than 0.5%. A 1/2­inch microphone (Ono­Sokki, LA­4350) is set
in 90° to the main flow at 1.0 m distance from the trailing edge. The frequency response is analyzed
by the FFT analyzer (Ono­Sokki, CF­5210). The angle of attack is the angle between the center lineof the blade and the incoming flow. The nozzle size is 400mm×400mm. The lift force and drag force
are measured by the load cell which has capacity of 25 N (Tech ­ Gihan, TL2B09­25N). The lift
coefficient CL and drag coefficient CD are defined as;

CL =2L2D, CD =3rV Ar V3 A

(1)where, L is the lift force, D is the drag force, ρ is the density of the fluid, V is the main flow velocity,
A is the reference area.

Fig. 1 The examined blade (NACA0018)

Fig. 2 Experimental setup of the wind tunnel

Fig.3 is the schematic view of objective impeller of the wind turbine. The impeller is separated
into 10 segments to analyze the characteristic of the fluid force and the aerodynamic performance.
NACA0018 airfoil is used for every segments. The radius and the chord length of each segment aredescribed in Table 1. Reynold number based on the chord length at the blade tip is 4.5 × 10 5 .
Table 1 Specifications of each segment
radius

1000
500

1000

1000

1000

1000

1000

1000

1000

1000

10000

Fig. 3 Schematic virew of the objective impeller

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1000

chord

10.50.319.5

0.38

8.5

0.44

7.5

0.50

6.5

0.56

5.5

0.63

4.5

0.69

3.5

0.75

2.5

0.81

1.5

0.88

Blade shape

NACA0018

3. EXPANSION OF BLADE ELEMENT MOMENTUM THEORY
The blade element momentum theory (BEM) is used to calculate the local forces on a propeller orwind turbine blade. The velocity component of the relative velocity W is given by induced factors " a" and "a ́ " as shown in Fig. 4. The two factors are given as;

a=s Cy4 sin f + s C y2, a' =s Cx
Bc, s =
4 sin f cos f - s C x
2p r

(1)

Here,

tan f =rw
1 1- a, l (r ) =l (r ) 1 + a'V

where, σ is the local solidity at the position r, c is the chord length, B is the number of blades. The
angle of attack α can be calculated by the yaw angle φ and the pitch angle θ as;

a = f -q(2)

Then, the relative velocity is expressed as Eq. (3).

W=(r W ( 1 + a' ))2 + ( V ( 1 - a ) )2(3)When we take into account of the output power of the wind turbine, the power can be estimated by
the integral from hub side to tip side as;

P=1
1C p r V 3p R 2 , C p =C p × 2 p r dr , C p = 4a' (1 - a) l (r ) 22 ò2p R
drdLdD

Vr Ω ( 1 + a’ )

r

Ω

(4)

zyα

ω

θ

φ V ( 1 –a )WxFig. 4 Schematic view of the blade element method
According to reference (2), the sound power radiation by a number of flat plate or airfoils, eachhaving a surface pressure distribution caused by turbulent boundary layers, may be written as;

dEN bW3=φpp (w )d w 6 p w r a03(5)where, E is the sound power, N is the number of blades, b is the blade span, W is the relative velocity,
φpp is the wall pressure spectrum density. The statistical wall pressure spectrum density is given as (2);

φ pp = 10-3 r 2 d W 3 , δ = 0.093 c Re

-0 .2

(6)

where, δ is the boundary layer displacement thickness. When the frequency is normalized by the chord
length c and the relative velocity W, the equation (7) is established.

wo cm Wo=wm c0 Wm(7)

The ratio on the sound pressure p2 of the measured value of the model in the wind tunnel test to the3833

target value of the blade element in the actual wind turbine is given by;
32po 2 N o bo æ Wo ö æ wm ö æ rm ö φpp (w )o=ç÷ ç÷ç ÷pm 2 N m bm è Wm ø è wo ø è ro ø φpp (w ) m,

E 4 p r2 2=p2
r a0(8)where, subscript m is the model value, o is the objective value of the blade element for the target windturbine. In the case of the noise level, since decibel notation is common, let me express equation (13)as logarithmæN öæb öæW öL p o = L p m + 10 log ç o ÷ + 10 log ç o ÷ + 30 log ç o ÷
è Nm ø
è bm ø
è Wm ø(9)æ φ (w ) o öæw öær ö+ 10 log ç m ÷ + 20 log ç m ÷ + 10 log ç ppç÷÷
è wo ø
è ro ø
èφpp (w ) m øIn the experimental analysis of the broadband noise using the scaling law, the target noise level of thewind turbine can be predicted by the model size of the isolated blade and the scale of the actual windturbine.

4. RESULTS AND DISCUSSIONIn Fig. 5, the lift coefficient and drag coefficient of NACA0018 are shown. The main flow velocity
is 20.4 m/s. The Reynolds number based on chord length is 1.3 ×105 . The solid line is the lift
coefficient calculated by Xfoil (3). The circle symbol is measured values of the lift coefficient by windtunnel experiment. The lift coefficient of the measured value is almost same as the calculated valuewithin the attached flow domain. The stall point of the blade is approximately 16 °. Although measured
drag coefficient is small at the attached flow domain, the measured drag coefficient agrees well withthe calculated value within the attached flow domain. When the angle of attack exceeds the stall point,
the drag coefficient starts to increase.

2.0

NACA0018

2.0

Stall point

1.0
Measurment
Cal. (Xfoil)

0

­1.0
­2.0

CDCL

1.0

Attached flow0

Separated flow50
α , deg.

(a) lift coefficient

C = 100 mm
V = 20.4 m/s 5Re = 1.3×10100

Attached flow

Separated flow

NACA00180

­1.0
­2.0

Stall point

Measurment
Cal. (Xfoil)050
α , deg.

C = 100 mm
V = 20.4 m/s 5
Re = 1.3×10100(b) drag coefficientFig. 5 Aerodynamic characteristics of the NACA0018
The spectral distributions of aerodynamic noise generated from NACA0018 with different angleof attack are compared in Fig. 6. The broken line is the background noise level of the wind tunnel. We
can notice that the noise level in the high frequency domain than that of the 2000Hz cannot bemeasured because the noise level is not different from the background noise. In the angle of attack 0°,
the humped noise spectra become large in where the frequency is around 1000 Hz. This noise may
generate by the wake vortices in the wake of the blade. In overall noise level, the maximum noiselevel was at the angle of attack 70 °. The noise spectra in such a deep stall condition becomes large inthe low frequency than that of the other frequency domain. This results indicated that the low
frequency noise becomes dominant for the overall noise level in the deep stall condition.
In Fig. 7, the output power of the wind turbine analyzed by the BEM is presented. The blade
element in each segments is NACA0018 with pitch angle 30 °. The diameter of the impeller is 21 m;
the rotation speed is 20 rpm. The common wind velocity is within 5 to 8 m/s, so that the design3834

condition is defined as 6 m/s, whereas the wind velocity 12 m/s is defined as the off­design condition.
The prediction indicates that the design condition of the pitch angle 30 ° can generate more output
power than the other conditions in the off­design condition. The wind turbine can produce themaximum output power 25 kW at the wind velocity 12 m/s.

30
α=0 deg. (attached flow)
α=24 deg. (separated flow)
α=70 deg. (separated flow)60

12 deg.
18 deg.
24 deg.
30 deg.
36 deg.

20

L , kW

Lp , dB

80

4010BGN

20C = 100 mm
V = 20.4 m/s 5Re = 1.3×100102

N = 20 rpm
D = 21 m

2000 Hz103
f , Hz00104

Fig. 6 Spectra of the aerodynamic noise (NACA0018)

10V , m/s

20Fig.7 Output power of the wind turbine

In Fig. 8, the relationship between the radius from the impeller of the wind turbine and the angleof attack in each blade element is shown. The circle symbol is the design condition (6 m/s); therectangular symbol is the off­design condition (12 m/s). In the design condition, most of the blade
elements become the attached flow. In the off­design condition, the flow regime becomes the separated
flow condition from the mid­span to the hub of the impeller. The relative velocity distributions in each
operation condition are compared in Fig. 9. The relative velocity in the off­design condition becomes
fast then that of the design condition. The both velocity distributions increases from hub side to theblade tip.

6030

α , deg.

4020

N = 20 rpm
D = 20 m
θ = 30 deg.
separated flow

0W , m/s

V =6 m/s
V =12 m/s

20

V =12 m/s
V = 6m/s

N = 20 rpm
D = 20 m
θ = 30 deg.

10attached flow

­200tip

hub5r,m10Fig. 8 The radius and the attack angle

00tip

hub5.0r,m10.0Fig. 9 The radius and the relative velocity

In Fig. 10, the predicted noise spectra generated from the wind turbine in the different operation
condition are compared. The thin black line is the design condition; the thick red line is the off­design
condition. In the both operation conditions, the noise level becomes large in the vicinity of 4000 Hz.
This humped noise may the broadband noise which is generated by the Karman Vortex shedding.
The noise spectra in each span position analyzed by the BEM is shown in Fig. 11. The distance
between the sound source and the microphone assumed 100 m. The predicted noise spectra indicatedthat the noise spectra become broadband noise at the hub side, and the noise level becomes sufficient
low; whereas the blade elements from the tip side to mid span generate the hump noise at highfrequency domain.

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40
N = 20 rpm ; D = 20 m
Z = 3 ; r = 100 m

20

V = 12 m/s (58.8 dB)

40
20

102

103
f , Hz0­20

V = 6 m/s (56.4 dB)

0tip side ( r = 9.5 m )mid span ( r = 6.5 m )hub side ( r = 2.5m )NACA0018

Lp , dB

Lp , dB

60NACA0018

N = 20 rpm ; D = 20 m
Z = 3 ; r = 100 m

104

Fig. 10 Comparison on the noise spectra in the­40102103
f , Hz104

Fig. 11 Noise spectra in each span position

design and off­design operation condition

5. SUMMARY
We proposed the prediction methodology for the broadband noise generated by the wind turbine
based on the blade element momentum theory. From the results of the wind tunnel test, NACA0018
generated the humped noise in the attached flow condition, whereas the blade generated the broadband
noise in the separated flow condition. Moreover, in the deep stall condition, the broadband noise in
the low frequency domain increased. The broadband noise in the off­design condition became large
than that of the design condition. We clarified that the humped noise source in the wind turbine wasmade from the mid­span to the blade tip on the impeller.

REFERENCES
1. H. Snel, Review of the Present Status of Rotor Aerodynamics. Wind Energy. 1998; 1: 46­69.
2. B. D. Mugridge, Acoustic radiation from aerofoils with turbulent boundary layers. Journal Sound Vibration.
1971; 16 (4): 593­614.
3. NACA4 digit airfoil generator, http://www.airfoiltools.com/airfoil/naca4digit, accessed 31. May. 2019.

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