Leading- and trailing-edge noise reduction using serrations of new geometry
Benshuai LYU∗(1) , Lorna AYTON(1) , Paruchuri CHAITANYA(2)(1) Department of Applied Mathematics and Theoretical Physics, University of Cambridge(2) Institute of Sound and Vibration, University of Southampton

Abstract
The leading-edge (LE) and trailing-edge (TE) noise due to the interaction of unsteady flow with the sharp edges
of aerofoils are common in many engineering applications. One of the most widely used reduction methods is
to use serrations, such as those of a sawtooth and a sinusoidal profile. The optimal serration geometry, however, remains largely unknown. We propose that a new ogee-shaped serration geometry can provide improved
performance over conventional sawtooth serrations for leading- and trailing-edge noise reduction. In this study
we carry out an experiment to investigate the noise reduction performance of the ogee-shaped serrations undervarious flow conditions. It is found that the new serration geometry provides an additional LE noise reduction
compared to conventional sawtooth serrations under various flow conditions by up to approximately 8 dB. The
additional noise reduction benefit starts to appear when k1 h > 3 and becomes larger as the frequency increases
until being hidden by the dominance of other noise source mechanisms. The new serrations only introduce morenoise reduction but no noise increase at any other frequencies of interest. The use of LE serrations is found to
be able to reduce the self noise. A preliminary TE noise test also sees additional noise benefit by using theogee-shaped serrations.
Keywords: leading-edge noise, trailing-edge noise, serrations

1

INTRODUCTION

Leading-edge (LE) and trailing-edge (TE) noise, as two components of aerofoil noise [1, 6], are importantnoise sources in many applications. LE noise is generated when incoming turbulence impinges on the LE of
aerofoils, leading to a strong interaction that causes the emission of LE noise. LE noise plays a crucial rule inmany applications where noise becomes an issue. This is especially true in applications that more than one rowof rotors and stators is installed, such as aero-engines, steam turbines and contra-rotating open rotors. In such
systems, the wakes from the front row blades impinge on the downstream blades, resulting in a strong fluidstructure interaction as well as efficient noise radiation. In contrast, TE noise is generated when a turbulent
boundary layer convects past the trailing edge of aerofoils and gets scattered into sound [15, 2]. TE noise isalso important in many applications, especially when a highly turbulent boundary layer is common due to large
Reynolds numbers, for example TE noise is important noise source for wind turbines.
Serrations have been proposed as one of the promising approaches to reduce LE noise [7, 13, 30, 19, 28].
Previous research on LE serrations includes experimental studies such as those by K. Hansen and Doolan [19]and Narayanan et al. [28], numerical investigations carried out by Lau et al. [21], Kim et al. [20] and Turnerand Kim [33], and analytical examinations such as those by Lyu and Azarpeyvand [23], Ayton and Chaitanya[4], etc. Similarly, TE serrations have been widely used as an effective way of reducing the TE noise. Previous
research includes a large bulk of experimental literature such as Dassen et al. [12], Chong et al. [11], Moreau
and Doolan [27], Oerlemans et al. [29], Gruber et al. [14], Chong and Vathylakis [10] and Leon et al. [22],numerical simulations such as those by Jones and Sandberg [18], Sanjose et al. [31] and Velden et al. [34], and∗ bl362@cam.ac.uk

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(a) The flat plate is placed near the exit of the wind tunnel and (b) The new ogee serration is inserted to the leading edge of flata microphone arc is suspended above the aerofoil to measure the plates. The sharpness at the tip and root can be varied by changing
the parameter b in equation 1.
far-field noise.

analytical studies such as Howe [16, 17], Azarpeyvand et al. [5], Sinayoko et al. [32], Lyu et al. [25, 24] andAyton [3].
Although experiments and numerical simulations show that using serrations can reduce both LE and TE noise,
it remains unclear how the serration geometry changes the noise reduction performance. Previous research hasstudied serrations of different geometry [19, 28, 8, 9]. These studies have shown that it is possible to achieve
greater noise reduction via changing the serration geometries, but it is not clear what the optimal serrations
geometry is. In a recent study of the authors [26], an asymptotic analysis was performed to show that in orderto achieve greater noise reduction in the intermediate- and high-frequency range, the serration profile should nothave stationary points and shaper slopes are favourable at the points where the profile function of the serrations
is not smooth. An ogee-shaped geometry was proposed and shown both analytically and experimentally to becapable of reducing more LE noise. In this paper, we perform more extensive experimental tests, including apreliminary test on TE noise, to study the acoustic superiority of the new serration geometry in reducing LE
and TE noise.
This paper is structured as follows. Section 2 briefly describes the experimental setup, while Section 3 presentsnoise spectra obtained for serrations of various sizes and geometries. The final section concludes this paper andlists some future work.

2

EXPERIMENTAL SETUP

The LE noise experiment is carried out in the aeroacoustic wind tunnel at the Institute of Sound and Vibration inSouthampton. The facility has an anechoic chamber that measures 8 m × 8 m × 8 m and has a cut-off frequency
around 80 Hz. A photograph of the rig is shown in figure 1a. In the experiment, the jet velocity U is varied
between 20, 40, 60 and 80 m/s. The exit of the wind tunnel has a dimension of 150 mm × 450 mm, anda number of flat plates (a mean chord length 150 mm and span 450 mm) with different serration sizes andshapes are placed at 150 mm downstream of the wind tunnel exit. To prevent tonal noise generation due tothe Tollmien-Schlichting waves convecting in the laminar boundary layer, the flow near the leading edge of theflat plate is tripped on both the pressure and suction sides to force transition to turbulence using a rough bandof tape. Details about how the flat plates are connected with serrations can be found in Lyu et al. [26] andNarayanan et al. [28].
A baseline flat plate is included in the noise tests as reference. The ogee-shaped serrations used in this study,5360

as shown in figure 1b, are described by the shape function F(η) as follows [26] (note the tan(η) function only
represents one possible form for the ogee shape),
 1
tan(bη), 0 ≤ η < 1/4,

tan b/4 1
1tan(b(−η + ), 1/4 ≤ η < 3/4,
F(η) =
tan
b/4
2




 1
3


tan(b(η − )), 3/4 ≤ η ≤ 1,
tan b/4
4(1)

where b is the parameter describing how sharp the serration is at the serration roots and tips. In the experiment,b takes the values of 0, π, 1.25π and 1.5π. When b = 0 the serration shape becomes the conventional sawtooth
profile, and the slope at the root and tip becomes increasingly larger as b increases. The serration wavelengthis denoted by λ , while h denotes the half root-to-tip amplitude. In the experiment, the value of λ varies from5 mm to 15 mm, while h takes the value of 25 mm.
A microphone arc, as shown in figure 1a, is placed above the flat plate to measure the far-field noise at different
angles in the mid-span plane. The arc consists of 11 1.27 cm condenser microphones (B&K type 4189) locatedat a radial distance of 1.2 m from the leading edge of the flat plate, spanning an observer angle from 40◦ to140◦ measured relative to the downstream jet axis. Noise measurements are carried out for 10 s at a sample
frequency of 50 kHz. The noise spectra are calculated with a window size of 1024 data points corresponding
to a frequency resolution of 48.83 Hz and a BT product of approximately 500 , which is sufficient to ensure
negligible variance in the spectral estimate at this frequency resolution. The noise spectra are presented in termsof the Sound Power Level (PWL) and Sound Pressure Level (SPL) using the procedure described by Narayanan
et al. [28].
In the LE noise experiment, upstream turbulence is generated using a bi-planar rectangular grid of 630 mm ×
690 mm. The grid is placed in the contraction section of the wind tunnel at 75 cm upstream the nozzle exit.
The energy spectrum of the grid turbulence is known to be well modelled by a number of empirical formulae,such as the one based on the Von Kármàn spectrum [23, 26]. In this experiment, the turbulence intensity isaround 2.5% and the streamwise integral length-scale is approximately 6 mm.
The TE noise experiment has a similar experimental setup as that for the LE noise measurements. Since thispaper presents mainly the LE noise spectra, we omit a detail description, which will be introduced in detail ina following study.

3

RESULTSIn this section, we show the LE noise PWL spectra for various serrations. When these spectra are shown, thefrequency is normalized as k1 h, where k1 = ω/U denoting the hydrodynamic convective wavenumber. For the
results shown in this section, the self noise is measured using the baseline flat plate by removing the upstream
turbulence grid, while the baseline spectra are measured by putting the grid back to the tunnel.
We first show the spectra for serrations of λ = 10 mm and h = 25 mm. These spectra can be found in figure 2.
Figure 2a shows the spectra when U = 20 m/s. It is clear that because of this low speed the total sound levelis quite low. It can be seen that the self noise is low for the frequencies k1 h < 10 compared to the spectrum
for the baseline LE noise. For frequencies satisfying k1 h > 10 the self noise plays an important role in the totalnoise observed in the experiment and is only approximately 5 dB lower than the baseline spectrum. When theconvectional sawtooth serration, i.e. F(η) when b = 0, is used, a significant noise reduction is achieved for
frequencies k1 h > 1. Because of this reduction, the noise at frequencies k1 h > 10 is virtually completely due
to the contribution of self noise. As we increase the value of b, we can see increasingly more noise reduction

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50

40

PWL (dB)

30

20

10

Self Noise
Baselineb=0b=b=1.25
b=1.5

45
40

PWL (dB)

Self Noise
Baselineb=0b=b=1.25
b=1.5

35
30
25
20

0

15
-10

10
100

101

100

k 1h

(a) U = 20 m/s(b) U = 40 m/s

60

70

50
45
40
35
30

Self Noise
Baselineb=0b=b=1.25
b=1.5

60
50

PWL (dB)

Self Noise
Baselineb=0b=b=1.25b=1.555

PWL (dB)

101

k 1h

40
30

25
20
20
15

10
100

101

k 1h

(c) U = 60 m/s

100

101

k 1h

(d) U = 80 m/s

Figure 2. The noise PWL spectra for the serrations with λ = 10 mm and h = 25 mm but various values of b at
various flow speeds.
for frequencies 3 < k1 h < 10. No more additional noise reduction is observed for frequencies k1 h > 10, but thisis due to the dominance of the self noise. Had the self noise been lower, we would have seen an extended
frequency range over which additional noise reduction is achieved [26]. Although the advantageous frequency
range is limited by the dominance of self noise at high frequencies, it is remarkable to see that there is virtually
no sound increase at other frequencies. We have achieved an additional noise reduction simply by changing theserration shapes.
As we increase the velocity U, LE noise is louder, as can be seen from the PWL spectra shown in figures 2b
to 2d. It seems that larger noise reduction benefit is achieved at U = 40 m/s and 60 m/s, while this advantage
starts to drop slightly as U approaches to 80 m/s. In each case, however, it is outstanding to see that no noise
increase is observed at all frequencies by using the new serrations. Note it may seem surprising that the self
noise is even larger than the total noise in figures 2c and 2d at high frequencies. This is because the self noise
spectra are measured for the baseline flat plate. Using LE serrations in fact also reduces the self noise of flat
plates, which explains the seemingly bizarre observation.

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50

40

PWL (dB)

30

20

10

Self Noise
Baselineb=0b=b=1.25
b=1.5

45
40

PWL (dB)

Self Noise
Baselineb=0b=b=1.25
b=1.5

35
30
25
20

0

15
-10

10
100

101

100

k 1h

(a) U = 20 m/s(b) U = 40 m/s

60

70

50
45
40
35
30

Self Noise
Baselineb=0b=b=1.25
b=1.5

60
50

PWL (dB)

Self Noise
Baselineb=0b=b=1.25b=1.555

PWL (dB)

101

k 1h

40
30

25
20
20
15

10
100

101

k 1h

(c) U = 60 m/s

100

101

k 1h

(d) U = 80 m/s

Figure 3. The noise PWL spectra for the serrations with λ = 5 mm and h = 25 mm but various values of b at
various flow speeds.
Figure 3 shows the PWL spectra for narrower serrations, i.e. λ = 5 mm and h = 25 mm. Examining figures 3a to
3d, we see that an even larger noise reduction benefit of up to 8 dB is achieved when a narrow serration is used,which is consistent to earlier analytical predictions [26]. The spectra in figures 3a to 3d are characteristicallysimilar to figures 2a to 2d, but we note again that there is no noise increase at any frequencies due to the useof new serrations, and the limited advantageous band is due to the dominance of self noise. Since TE noise istypically prevalent at higher frequencies compared to LE noise, it expected that the additional benefit can carryover into higher frequencies when the new serrations are used to reduce TE noise.

4

CONCLUSIONS

An experimental study is performed in this paper to investigate the noise reduction performance of ogee-shapedserrations under various flow conditions. The ogee shape is described using a tan(η) function with a parameterb specifying the sharpness at its roots and tips. Moreover, the serrations wavelength is also varied. The flow
velocity U is varied between U = 20 m/s, 40 m/s, 60 m/s and 80 m/s.

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It is found that the new serration shape provides an additional LE noise reduction compared to conventional
sawtooth serrations by up to approximately 8 dB. This additional noise reduction is slightly more pronounced
when U = 40 m/s and 60 m/s. The additional LE noise reduction benefit starts to appear when k1 h > 3 andbecomes larger as the frequency increases until being hidden by other noise source mechanisms. The newserrations introduce only more noise reduction but no noise increase at any other frequencies of interest. The
use of LE serrations can also reduce the self noise. A preliminary TE noise test also sees additional noise
benefit by using the ogee-shaped serrations. A more extensive test campaign using the new serrations is ongoing
and the results from these tests will be reported in our future work.

ACKNOWLEDGEMENTS
The authors wish to gratefully acknowledge the financial support provided by the EPSRC under the grant number EP/P015980/1.

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