On the effects of leading edge serrations on aeroacoustic
properties during stall
Yannick D. Mayer∗, B. Zang†, Mahdi Azarpeyvand‡
Faculty of Engineering, University of Bristol, United Kingdom

ABSTRACTThe effect of leading edge serrations on the surface pressure fluctuation spectra and boundary layer
quantities of a NACA 0012 airfoil has been investigated experimentally, with an emphasis on reducing
airfoil separation and stall noise. Five leading edge serration add-ons with various serration heights
and wavelengths have been tested in conjuction with a highly instrumented NACA 0012 airfoil. It isshown that all serrations cause a significant increase in surface pressure fluctuation spectra in the prestall region. However, for angles of attack in the post-stall regime, surface pressure fluctuation spectra
reductions of up to 15 dB have been observed for some serrations. These surface pressure fluctuation
spectra reductions have been linked to the changes observed in the complex flow-field near the trailingedge, namely boundary layer thickness and velocity fluctuation reductions, as a result of the counterrotating vortex pairs generated by the respective serrations.
Keywords: Leading Edge Serrations, Separation and Stall Noise, Flow Control, Surface Pressure Fluctuations, Trailing Edge Noise.

1

INTRODUCTION

As the global renewable energy production is increasing rapidly in order to combat man-made climatechange, wind turbines are playing a more prominent role. However, concerns have been raised regarding noise emission from wind turbines (1). Brooks et al. reported five different airfoil self-noise mechanisms(2), the most relevant for wind turbine noise are trailing edge noise and stall/separation noise. While trailingedge noise and its mitigation has been the subject of a variety studies (2–7), aeroacoustics of separation/stall
have received comparably less research attention. Previous experimental studies have found that stall can
result in large low-frequency noise and surface pressure spectrum increases (8–10). Stall noise is also hypothesized to significantly contribute to low frequency wind turbine noise, when rapidly changing environmental
conditions result in localized stall (11). Recently, leading edge serrations were found to reduce low-frequency
stall noise, but increase high-frequency noise (12) and previously, leading edge serrations have been utilized
in the field of aeroacoustics to reduce airfoil turbulence interaction noise (13–15). Aerodynamically, leadingedge serrations, sometimes also termed leading edge tubercles, have been found to reduce the generated
pre-stall lift, while increasing drag for a range of airfoils at moderate Reynolds numbers (16, 17). By contrast,in the post-stall regime, leading edge serration can increase the lift coefficient due to the counter rotating
vortices forming over them (16, 17).
This paper aims to experimentally examine the effect of leading edge serrations on the near-field hydrodynamics, especially during stall. Section 2 will introduce the experimental setup, as well as the utilized
airfoil and the leading edge serration add-ons, before preliminary surface pressure fluctuation and hot wire
results are presented in Section 3. Lastly, Section 4 will provide conclusions and an outlook on future work.
∗ Ph.D. Researcher Aerospace Engineering, yannick.mayer@bristol.ac.uk.
† Research Associate Aerospace Engineering, nick.zang@bristol.ac.uk.
‡ Corresponding Author, Reader in Aeroacoustics, m.azarpeyvand@bristol.ac.uk.

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2

EXPERIMENTAL SETUP

2.1 Overview
Experiments were conducted in the aeroacoustic wind tunnel facility of the University of Bristol (18). This
study used the larger nozzle with exit dimensions of 500 mm width and 775 mm height at a free stream
velocity of U∞ = 20 m s−1 . The nozzle is connected to a test section with tensioned Kevlar windows at the top
and bottom, allowing acoustic sound waves to pass through while limiting excessive flow deflection. The
angle of attack of the utilized NACA 0012 airfoil with a chord of c = 300 mm is controlled by a high torqueservo motor as detailed by Mayer et al. (19). An overview of the experimental setup and a photo of the
NACA 0012 airfoil in the test section is shown in Fig. 1.
The NACA 0012 airfoil is tripped by a zig-zag turbulator trip tape, located at approximately 13 % of thechord on both airfoil sides. The airfoil is equipped with 87 static pressure taps together with 91 embedded
microphones placed under 0.4 mm diameter pinholes, namely direct sensing FG-23629-P16 microphones andremote sensing Panasonic WM-61A microphones. All microphones were calibrated in magnitude and phasereferenced to a G.R.A.S. 40PL microphone.

(a)

(b)

Figure 1. (a) Upstream view of the test section with the NACA 0012 airfoil installed and (b) Isometric rendering of the experimental setup: (A) servo motor, (B) coupling and bearing, (C) NACA 0012 airfoil and (D)tensioned Kevlar cloth.

2.2 Serration add-ons
Five different leading edge serration add-ons are investigatedin this study for varying amplitudes, h, and wavelengths, λ , as
Table 1. Serration parameters.
detailed in Table 1 and displayed in Fig. 2. The serration addNo. h/c0 λ /h Designation
ons were 3-D printed in polylactic acid material using a dual
10.05 1.0
H05Λ10extrusion Ultimaker 3 printer. All serrations were sanded to2
0.10 0.5
H10Λ05provide an aerodynamically smooth surface and since the 3-D30.10 1.0
H10Λ10
printer limited the maximum spanwise serration size, multiple
4
0.10 2.0
H10Λ203-D prints were combined. Aluminium foil tape and polyester
5
0.15 1.0
H15Λ10flash tape were used to achieve an aerodynamically smoothtransition between each individual piece as well as the airfoil
and the serrations were tripped consistently with the base airfoil. Variable sized serration pieces on the left and right allowed the serration tip and root plane to be aligned

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with the chordwise-placed in-situ microphones of the airfoil.

Figure 2. Overview of sections of all serration add-ons used in this study.
The serration add-ons overlap the existing airfoil for 60 mm with a final thickness of 0.4 mm and themean chord changes sinusoidally in the spanwise direction. Since this study focuses on serration add-ons,
the mean chord, c0 , differed for each serration amplitude and was chosen such that the root chord was equal
to 305 mm to ease manufacturing of the serration add-ons with sufficient structural integrity. In order tocreate the chordwise varying serration profiles, the baseline NACA 0012 airfoil profile was scaled along thechord direction.

2.3 Data acquisition and postprocessing
All microphone and hot-wire measurements were sampled simultaneously at a frequency of 215 Hz for 32 s
using five 16-channel NI PXIe-4499 modules mounted in a NI PXIe-1062Q chassis. All microphone signals
are zero-phase high-pass filtered at 20 Hz, to account for the low-frequency limit of both microphones. A
Dantec 55P15 boundary layer hot-wire probe was traversed using two Thorlabs LTS300 stages and operated
via a CTA91C10 module. The probe was calibrated daily using a Dantec 54H10 calibrator. The power spectral
density (PSD) of the surface pressure fluctuations, φ pp , is estimated via Welch’s method using a window sizeof 211 samples and a Hamming window with 50 % overlap, resulting in a frequency bin size of 16 Hz. All
surface pressure spectra are normalized with a reference pressure of p0 = 20 µPa.

3

RESULTS AND DISCUSSION

This section presents surface pressure spectra and boundary layer results for the tip and root plane of all
serrations in comparison with the baseline airfoil for different chordwise sensor locations and geometricangles of attack. The baseline airfoil stalls at approximately α = 14◦ angle of attack (9) and the results arepresented for α = 0◦ , 10◦ and α = 15◦ to investigate the effect of the serrations in different flow regimes.

3.1 Surface pressure fluctuations
Figure 3 shows the surface pressure PSD spectra in the serration tip and root plane at x/c = 0.43 for three
angles of attack. It is discernible that the difference between the root and tip plane pressure spectra decreases
as the angle of attack is increased. At α = 0◦ , the spectra of all serrations differ by no more than 4 dB for all
frequencies displayed and for the root plane, the pressure spectra exceed the baseline spectrum by up to
10 dB, while exceeding the baseline spectrum by only up to 3 dB for the tip plane.
The difference between all serrations and the baseline is much more pronounced at α = 10◦ , as the spectraof the serrations exceed the baseline spectrum by up to 25 dB. Additionally, the serration spectra do not peakat 1000 Hz as the baseline case, but rather at frequencies below 200 Hz. Both observations indicate thatthe flow structures present in the flow have considerably increased in size, which points toward a thicker
boundary layer or even flow separation, as serrations can cause earlier stall (16). Increasing the serration
amplitude h for λ /h = 1, leads to an energy increase in frequencies greater than 200 Hz, while at lower
frequencies, a serration amplitude of h/c0 = 0.1 results in the lowest surface pressure fluctuations. On theother hand, greater serration wavelengths for an amplitude of h/c0 = 0.1 reveal that the widest serration,
H10Λ20, results in the highest surface pressure spectra across the whole frequency range. While H10Λ20
exceeds H10Λ05 by 8 dB at low frequencies, this rises to 25 dB at high frequencies. It is hypothesized thatlarger and wider serrations induce counter rotating vortex pairs (CVP) with increased vorticity explaining
this change. Lastly, it should be mentioned that for the tip plane, a hump between 100 Hz and 300 Hz occurs

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for two serrations, namely H15Λ10 and H10Λ20, which could also be caused by the CVP.
Finally, for α = 15◦ , the trends concerning varying serration wavelength and amplitude are almost identical. At low frequencies however, the serrations no longer exceed the baseline spectrum significantly, but are
rather below the baseline case, except for the shortest serration, H05Λ10. It is also notable that the shortest
serration H05Λ10 has an almost identical surface pressure spectra as the baseline case, suggesting a large
degree of similarity between their flow fields.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 3. Surface pressure PSD at x/c = 0.43: (a) α = 0◦ root, (b) α = 0◦ tip, (c) α = 10◦ root, (d) α = 10◦ tip,(e) α = 15◦ root and (f) α = 15◦ tip.
Figure 4 shows the surface pressure PSD spectra in the serration tip and root plane at x/c = 0.99 for α = 0◦ ,10◦ and 15◦ . It can be seen that the root and tip plane pressure spectra difference does not exceed 5 dB, and is

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(a)

(b)

(c)

(d)

(e)

(f)

Figure 4. PSD at x/c = 0.99: (a) α = 0◦ root, (b) α = 0◦ tip, (c) α = 10◦ root, (d) α = 10◦ tip, (e) α = 15◦ root and(f) α = 15◦ tip.
therefore smaller than at x/c = 0.43. This is indicative of a spanwise more homogeneous flow field and hencehydrodynamic pressure field. At α = 0◦ , the spectra of all serrations and the baseline case differ by no morethan 2 dB for all frequencies displayed, for both the tip and the root plane.
Again, the difference between all test cases is much more pronounced at α = 10◦ , as the spectra of theserrations exceed the baseline spectrum by up to 20 dB at low frequencies. The peak frequency of the pressure spectra has decreased further compared to x/c = 0.43, which is explained by a thicker boundary layer.
For a constant serration amplitude of h/c0 = 0.1, the sharpest serration, H10Λ05, possesses the highest low
frequency ( f < 200 Hz) energy and the second highest mid and high frequency energy in comparison to5347

H10Λ10 and H10Λ20, while H10Λ10 has the lowest energy content. Comparing the results for a constant serration sharpness of λ /h = 1, it is evident that the longest serration H15Λ10 leads to the lowest low frequency( f < 100 Hz) pressure fluctuations, but the highest fluctuations for all other frequencies.
For α = 15◦ , the trends have changed considerably. The pressure fluctuation spectra for all serrations arelower than the baseline spectrum for the complete frequency range displayed. The best performing serration
for frequencies below 100 Hz is the longest serration, H15Λ10, exhibiting a reduction of 10 dB. However,for all other frequencies serration H10Λ10 leads to the highest reduction in surface pressure fluctuations,reaching values of up to 15 dB.

3.2 Hot wire results
This section concerns the hot wire measurements conducted near the trailing edge, at x/c = 0.98. Figure 50shows the mean velocity, U, and the velocity fluctuations root-mean-square, Urms , boundary layer profiles◦◦◦
for α = 0 , 10 and 15 , normalized with the velocity at the edge of the boundary layer, Ue . For α = 0◦ , allserrations lead to an increased boundary layer thickness and velocity deficit for both the root and the tip
plane. Additionally, the outer region of the boundary layer shows an increase in velocity fluctuations for all
serrations. These changes confirm the observations from the pressure fluctuation spectra which showed asmall increase between the serrations and the baseline case.
However, at α = 10◦ , significant differences are visible in the boundary layer profiles. The boundary layer
thicknesses for all serrations exceed the baseline boundary layer thickness by up to 300 % and the boundary
layers of the serrations H05Λ10 and H10Λ05 are on the verge of separating. This is a possible explanation for
the fact that these two serrations possess the largest low frequency ( f < 100 Hz) energy in their surface pressure spectra. Similarly, the maximum velocity fluctuations have almost doubled in magnitude for H05Λ10
and H10Λ05, while the other three serrations also display considerable increases, in particular near the airfoil surface. These elevated velocity fluctuations near the surface of the serrations H15Λ10 and H10Λ20,
i.e. the widest and longest serrations, could offer an explanation for the surface pressure increases at frequencies in excess of 100 Hz for those two serrations. It is assumed that these two serrations both createstronger counter-rotating vortex pairs resulting in a reduced velocity deficit, but greater associated velocity
fluctuations.
Finally, for α = 15◦ , the two serrations H05Λ10 and H10Λ05 reveal similarly separated boundary layer
velocity profiles as the baseline case, albeit with larger vertical boundary layer separation extents, possibly
due to an increased mean chord length. On the other hand, the remaining three serrations have resulted
in velocity profiles with comparable boundary layer thicknesses and reduced velocity deficits, which are
likely related to stronger CVPs for these larger and wider serrations. Interestingly, the serration with thesmallest velocity deficit, H10Λ20, shows the largest near wall velocity fluctuations of all cases up to y/c = 0.2,
including the baseline case. These velocity fluctuations might explain why this serration has the highest
surface pressure spectra of all serrations, except for frequencies below 60 Hz. Lastly, the serrations H10Λ10and H15Λ10 possess the lowest velocity fluctuations throughout the boundary layer in combination with areduced boundary layer thickness compared to the baseline case. This ultimately yields the lowest surfacepressure fluctuation spectra for these two serrations.

4

FINAL REMARKS

The effect of five leading edge serration add-ons, with various serration heights and wavelengths, on thesurface pressure fluctuations and velocity field has been investigated. It is concluded that serrations causesignificant increases in surface pressure spectra mid chord, both pre- and post-stall. Similar increases wererecorded near the trailing edge, but limited to the pre-stall regime. Post-stall, surface pressure spectra reductions of up to 15 dB have been observed for the serrations H10Λ10 and H15Λ10 and the flow-field results
near the trailing edge are consistent with this observation. The serration add-ons are found to cause significant boundary layer thickness increases, prior to reaching the stall angle of attack of the baseline airfoil.
Post-stall however, the serrations H10Λ10 and H15Λ10 reduce the boundary layer thickness in combination
with smaller velocity fluctuations, compared to the baseline airfoil. The authors therefore hypothesize that
serrations require a certain minimum length to be effective in reducing surface pressure fluctuations, and in

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(a)

(b)

(c)

(d)

(e)

(f)

Figure 5. Hot wire measurements at x/c = 0.98: (a) α = 0◦ root, (b) α = 0◦ tip, (c) α = 10◦ root, (d) α = 10◦ tip,(e) α = 15◦ root and (f) α = 15◦ tip.
addition, a wavelength-to-serration amplitude ratio near unity appears to reduce surface pressure fluctuations in the post-stall regime.
Most certainly, the complex flow fields caused by different serrations and their effect on the unsteady5349

surface pressure generation begs further research. Future investigations will take further advantage of thehighly instrumented airfoil by investigating the static pressure coefficient, spanwise coherence spectra of
the pressure fluctuations and far-field noise measurements. Additionally, surface pressure-velocity fieldcoherence studies will shed further light on the pressure generation regions of the flow field.

ACKNOWLEDGEMENTS
The first author gratefully acknowledges the financial support of the ICA-ASA-DEGA Young Scientists Grant
and all authors acknowledge the support of EPSRC (Grant No. EP/R010846/1). Thanks to Pete Coddington
for his 3D-printing expertise and to James Filbin and Ben Clark for their help.

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