A Review of Trailing Edge Bluntness & Tip Noise Prediction from Wind Turbine Blades

1. • The tip noise levels shift to high frequency regions of spectrum with change in tip shapes
(see Figure 7).
• The total SPL is not affected using round or sharp tips except only for blunt tip geometry at
high frequency region, f> 5kHz (see Figure 8).
• Sound pressure level exhibits dipole nature. It is low in cross wind direction compared to
upwind or downwind directions of acoustic field (see Figure 5). [4]
• Measured results of noise for 2.3MW equivalent turbine are higher at 1kHz – 5kHz by ~5dBA
in spectrum. [3]
• Hump in total sound spectrum at 1kHz is due to Bluntness noise. (see Figure 10)
• Turbulent inflow noise masks all other noise mechanisms at low frequencies, i.e. f < 200Hz
• Trailing edge noise exhibits broadband & tonal noise in certain regions of sound spectrum.
• Trailing edge bluntness noise is tonal in nature & peaks at 1kHz in total SPL (dBA) spectra.
• Tip noise is predominantly tonal at high frequency region (f>1 kHz) in sound spectrum.
• Maximum reduction of total SPL is ~ 20 dBA for change of 1 % thickness, h, scaled with
chord near 160Hz region.
• Sharp tip blade is less noisy compared with blunt and round tip geometry
• Use of serrations & porosity on airfoils offers possibility to reduce noise levels
• One of the dominant noise sources from rotating machinery equipment is from
trailing edge section of an airfoil as found in wind turbine blades.
• A computational study of noise mechanisms due to trailing edge & tip of blade for a
three bladed 2 megawatt wind turbine was performed using the semi empirical noise
model.
• The extent of reduction in total A-weighted sound power level between 4 – 22 m/s
was evaluated using two different trailing edge heights of airfoil scaled with its
chord. Free stream Mach number is ~ 0.19 & Reynolds number is ~ 7 x 106
• The effect of tip geometry on sound pressure level is predicted using same model.
The simulated results showed good agreement between total A-weighted sound
power levels & experiment values from an equivalent turbine.
• To quantify the extent of reduction in total A weighted sound power level, due to
change in trailing edge bluntness parameter of airfoil.
• Compare the sound pressure levels for the 2MW wind turbine for a receiver height
2m & located at distance of 80m from source present at height of 70m from
ground, with and without tip noise mechanism.
• Evaluate the tip noise mechanism with varying tip shape or geometry, tip pitch
angle setting & its influence on total A weighted sound power levels.
Numerical Simulation
The sound pressure level is computed using the BPM (Brookes, Pope, Marcolini)
model [1]. The blade radius 37m is discretized into 20 segments along span direction.
The blade airfoils are NACA 0012, NACA 6320, NACA 63212. The flow over airfoils
are assumed 2D in nature & sound pressure levels are calculated at every time step in
simulation for various observer positions. MATLAB software was used on 4GHz, 2GB
RAM Dell Workstation for simulation. Bluntness noise is scaled with Mach 5.5 & Tip
noise with Mach 4. The G4 & G5 functions represent shape of spectrum. G4 shows
the peak & G5 - overall shape of spectrum. The sound power level, SPL varies as
square of sound pressure level in acoustic field.
References:
[1] Thomas F Brooks, D Pope, Michael A Marcolini. Airfoil Self Noise and Prediction. NASA
publication, (1989)
[2] Wei Jun Zhu,. Modeling of noise from wind turbines, DTU Wind Energy Department.
Lyngby, Denmark, (2005)
[3] Pieter Dijkstra. Rotor noise and aero-acoustic optimization of wind turbine airfoils. TU Delft,
Netherlands, (2015)
[4] Vasishta bhargava, Dr Hima Bindu Venigalla, YD Dwivedi. Turbulent inflow noise prediction
from wind turbine blade Journal of Mechanical and Civil Engineering, IOSR, (2017)
Figure 1 Geometry of tip shapes [2]
a) Round b) Blunt c) Sharp
Separation length, l of
boundary layer at tip for
rounded, sharp & blunt tip
blades
• Angle of attack (AoA), α, computed using
BEM model, while slope angle, ψ, Average
displacement thickness using BPM model.
A Review of Trailing Edge Bluntness & Tip Noise Prediction
from Wind Turbine Blades
Authors : Dr Hima Bindu Venigalla**, Vasishta Bhargava*, JV Ramakanth#
*,** – GITAM University, # – SREYAS Institute of Engineering & Technology, Hyderabad - 502329
**Department of Aerospace Engineering *, # Department of Mechanical Engineering
Email: vasishtab@gmail.com
• Trailing edge noise is produced due to result of rapid pressure fluctuations in flow
field & its interaction with surface of airfoil.
• At 0.1 % chord the trailing edge noise amplitude reduces by ~ 20dB found at
5kHz compared with 1 % chord (see Figure 4)
• The total SPL reduction is ~ 20dBA & occur 160Hz -1200Hz in spectrum ( Figure 6)
• The A weighting filter frequency range is 20Hz – 20kHz. Thresholds: 0 dB & 140 dB
Figure 4 Bluntness source, scaled for different chord lengths, Sound
Power level, dB at 8 m/s wind speed 0o observer position
3rd ISSE National Conference on
Complex Engineering Systems of
National Importance, Current Trends &
Future Perspectives
Indian School of Business, Knowledge
City, Sector 81, Mohali, Chandigarh, Punjab,
India. Oct 12-13, 2017.
Trailing Edge Noise Wind Speed
Parameter Type 4 m/s 8 m/s 15 m/s 22 m/s
𝐡
𝛅 𝐚𝐯𝐠
∗
I
113.80 113.99 114.47 114.81
Maximum value ~1kHz , SPL – dB
𝐡
𝛅 𝐚𝐯𝐠
∗
II
120.37 120.44 121.39 121.63
Maximum value ~800Hz , SPL - dB
Table 1. Simulation matrix for Trailing edge noise, dB, bluntness scaled for 1 % & 2 % blade chord & at wind speeds,
4 m/s, 8 m/s, 15 m/s, 22 m/s
Figure 6 Extent of Total SPL Reduction ,(dBA) due to change
of 1 % trailing edge bluntness at 0o observer position for
wind speed regime of 4-22 m/s
Figure 9 Trailing Edge Bluntness
Ratio (h/δ*) along blade span, height
scaled to 1 % & 2 % chord length
Tip type Maximum SPL, dB(A)
Tip
angle
Sharp 57.55 95.98 -2
Round 68.36 70.51 3
Blunt 75.94 65.38 5
Table 2. Comparison of maximum SPL for tip shapes & tip
angles
Figure 8 Total SPL (dBA) with, without tip noise at
8m/s, 0o observer position using blunt tip at 3o AoA
0
20
40
60
80
100
10 100 1000 10000 100000
Aweightedsoundpower
level[dBA]
Frequency [Hz]
without tip
With tip
Figure 5 Directivity, of sound pressure level (dB) 1/3rd octave
for different observer positions, degrees, receiver at 80, 140 &
200m from turbine [4]
0
10
20
30
40
50
0
30
60
90
120
150
180
210
240
270
300
330
80m
140m
200m
Figure 7 Influence of tip shapes on the SPL dBA
at 8 m/s wind speed 0o observer position
Acknowledgements:
The authors wish to thank GITAM University, Hyderabad, Sreyas Institute of Engineering &
Technology Hyderabad for providing computational lab facilities.
Figure 10 Comparison of total SPL (dBA) for all noise
mechanisms at 8m/s, 0o observer position , with
measured results [3] of an equivalent turbine , 2.3MW ,
40m blade
𝐥
𝐜
= 𝟎. 𝟎𝟎𝟖 ∙ 𝛂 𝐓𝐢𝐩
𝐥
𝐜
= 𝟎. 𝟎𝟐𝟑𝟎 + 𝟎. 𝟎𝟏𝟔𝟗 ∙ 𝛂 𝐓𝐢𝐩
′
𝐟𝐨𝐫 𝟎 𝟎
≤ 𝛂 𝐓𝐢𝐩
′
≤ 𝟐 𝟎
𝟎. 𝟎𝟑𝟕𝟖 + 𝟎. 𝟎𝟎𝟗𝟓 ∙ 𝛂 𝐓𝐢𝐩
′
𝐟𝐨𝐫 𝛂 𝐓𝐢𝐩
′
≥ 𝟐 𝟎
𝛅 𝐚𝐯𝐠
∗
=
𝛅 𝐩
∗
+ 𝛅 𝐬
∗
𝟐
𝐒𝐭′′′
=
𝐟𝐡
𝐔
𝐒𝐭 𝐩𝐞𝐚𝐤
′′′
=
𝟎. 𝟐𝟏𝟐 − 𝟎. 𝟎𝟎𝟒𝟓 ∙ 𝛗
𝟏 + 𝟎. 𝟐𝟑𝟓
𝐡
𝛅 𝐚𝐯𝐠
∗
−𝟏
− 𝟎. 𝟎𝟏𝟑𝟐
𝐡
𝛅 𝐚𝐯𝐠
∗
−𝟐
𝐌 =
𝐔
𝐜
𝐒𝐏𝐋 𝐓𝐨𝐭𝐚𝐥 𝐝𝐁𝐀 = 𝟏𝟎. 𝐥𝐨𝐠𝟏𝟎 𝟏𝟎
𝐒𝐏𝐋 𝐓𝐁𝐋−𝐓𝐄
𝟏𝟎 + 𝟏𝟎
𝐒𝐏𝐋 𝐓𝐈
𝟏𝟎 + 𝟏𝟎
𝐒𝐏𝐋 𝐓𝐄 𝐁𝐥𝐮𝐧𝐭
𝟏𝟎 + 𝟏𝟎
𝐒𝐏𝐋 𝐓𝐢𝐩
𝟏𝟎
𝐒𝐏𝐋 𝐓𝐢𝐩 = 𝟏𝟎. 𝐥𝐨𝐠𝟏𝟎
𝐌 𝐦𝐚𝐱
𝟑 𝐌 𝟐 𝐥 𝟐 𝐃 𝐡
𝐫𝐞
𝟐
− 𝟑𝟎. 𝟓 𝐥𝐨𝐠𝐒𝐭′′ + 𝟎. 𝟑 𝟐 + 𝟏𝟐𝟔
𝐒𝐏𝐋 𝐓𝐄 𝐁𝐥𝐮𝐧𝐭 = 𝟏𝟎. 𝐥𝐨𝐠𝟏𝟎
𝐡𝐌 𝟓.𝟓
𝐋𝐃 𝐡
𝐫𝐞
𝟐
+ 𝐆 𝟒
𝐡
𝛅 𝐚𝐯𝐠
∗ , 𝛗 + 𝐆 𝟓
𝐡
𝛅 𝐚𝐯𝐠
∗ , 𝛗,
𝐒𝐭′′′
𝐒𝐭 𝐩𝐞𝐚𝐤
′′′
(1)
(2)
(3)
Figure 2 Schematic of airfoil bluntness [1]
Chord
Figure 3 Structural properties of 37m blade
0
2
4
6
8
10
12
14
0 0.2 0.4 0.6 0.8 1
Referencechord[m]&twist
[deg]
Non dimensional blade radius [-]
Ref chord, c
Twist
0
20
40
60
80
100
120
140
10 100 1000 10000 100000
SoundPowerLevel[dB]
Frequency [Hz]
0.1 % chord
1 % chord
2 % chord
0
10
20
30
40
50
60
70
80
10 100 1000 10000 100000
SoundPowerLevel[dBA]
Frequency [Hz]
Sharp tip
Blunt tip
Round tip
Results & Discussion:
0
10
20
30
40
50
60
70
80
90
100
10 100 1000 10000 100000
Soundpowerlevel(dBA)
Center frequency [Hz]
Measured ( P. Djikstra)
TBL-TE
Inflow
Total (dBA)
TEB-VS
Tip
Observer Positions
Observer
Distances
0
1
2
3
4
5
6
7
0.03 0.12 0.33 0.59 0.81 0.95
TEbluntnessratio[h/δ*)
Non dimensional blade radius [-]
2% chord 1 % chord
Objectives:
Methodology:
Conclusions & Future work:
Introduction :