Influence of self-oscillating trailing edge flaplets on turbulent boundary layer– trailing edge noise.
Edward TALBOYS(1) , Thomas F. GEYER(2) , Christoph BRÜCKER(1)(1) City, University of London, United Kingdom, edward.talboys.1@city.ac.uk / christoph.bruecker@city.ac.uk(2) Brandenburg University of Technology, Germany, thomas.geyer@b-tu.de

Abstract
The aeroacoustics of a tripped NACA0012 aerofoil with an array of self-oscillating flexible flaplets attachedto the trailing edge at low to moderate Reynolds number and at geometric angles of attack αg = 0◦ − 20◦ isinvestigated. A low frequency noise reduction occurs at a chord based Strouhal number of 2 – 3 across all
Reynolds number and angles of attack. With maximum reductions reaching up to 8 dB within the Strouhal
number range. This reduction is proposed to be due to a modification of the instabilities in the shear layer by the
self-oscillating flaplets. A slight increase in noise level is seen at higher frequencies, however once the overallsound pressure level is calculated there is still a net overall reduction.
Keywords: Passive oscillators, aeroacoustics, trailing edge flaplets, fluid structure interaction

1

INTRODUCTION

Bio-mimicking is a topic of increasing interest within the aeroacoustic community, where many different strategies have been tested and implemented in order to reduced perceived noise levels from either aircraft, windturbines or compressors; to name a few examples. Many of these strategies are inspired from the well-known‘silent’ flight of the owl [7]. Geyer et al. [6] investigated a wide range of aerofoils with different porosities,
inspired from the ‘soft downy feathers’ of the owl. They showed that even a small porosity showed already an
aeroacoustic benefit in the low to mid frequency range, of which effect increases with increasing porosity, evenin some cases reaching up to 10 dB broadband noise reduction.
Another owl-inspired technique uses trailing edge brushes or serrations, mimicking the characteristic trailingedge structure formed by the feathers of owls. Brushes have been observed to reduce noise in the highfrequency range 2–16 kHz [8], probably affecting mostly the broadband noise of the turbulent boundary layerinteracting with the trailing edge. Finez et al. [4] could show that the spanwise coherence of the shed vortices
in the wake behind the trailing edge is reduced by 25% in the presence of brushes. Serrations have been extensively researched in both the laminar boundary layer case [2] and turbulent boundary layer case [1]. Theirmechanism in noise reduction is - similar to the brushes - through the reduction of spanwise coherence in theshed vortices. Studies with a single flexible flap at the trailing edge were investigated by Schlander and Sandberg [15]. They carried out a DNS study on a flat plate with an elastic compliant trailing edge and found an
aeroacoustic benefit at low and medium frequencies with an increased noise level at the Eigen frequency of thematerial. These results were confirmed later by Das et al. [3] in an experimental investigation using a similararrangement to Schlander and Sandberg [15]. Active oscillations of a trailing edge flap were studied by Jodin
et al. [9]. Their investigation was focused on the wake structure and it was observed that the wake could be
reduced in thickness by as much as 10%.
In the present study, a novel configuration of a flexible trailing edge is used, consisting of an array of individual
elastic flaplets mimicking the tips of bird feathers aligned along the span of the wing. This type of trailing edge
modification with arrays of individual mechanical oscillators in form of elastic flaps has thus far only studied by
the authors [11, 10, 5]. Attached to the trailing edge of a NACA 0010 aerofoil, the rows of individual silicone
flaplets clearly showed a reduction in tonal noise [10]. A follow-up study on the flow modification by thistype of trailing edge was done by Talboys and Brücker [17] and demonstrated aerodynamic advantages as well.

588

mixing zone

y in mmicrophones at
z = 0.71 mcore jetnozzle
0.2
0.1
0
-0.1
-0.2
aerofoil at z = 0 m-0.53 -0.28

00.5

x in m(a) Schematic display of the measurement setup (plan view)(b) Photo of the NACA 0012 aerofoil with the flaplets adhered on the trailing edge.

Figure 1. Experimental set-up
Table 1. Flaplet Dimension and Material Properties
Length (L)Width (s)

20 mm

5 mm

Inter
spacing (d)1 mmThickness

Density

180 µm

1440 kg/m3

Young’s
Modulus3.12 GPaEigenfrequency
107 HzDetailed High-Speed PIV measurements, coupled with simultaneous motion recordings of the flap tips, provea stabilisation mechanism of the flaps on the boundary layer on the suction side. A lock-in was triggered bytuning the fundamental frequency of the structural bending mode of the oscillator to match with the fundamental
frequency of the shear-layer on the suction side, forming regular vortex rollers in the boundary layer. This lockin delays the growth of non-linear instabilities such as the merging of the rollers, beneficially affecting also theoverall aerodynamic performance. A detailed aeroacoustic study was then carried out by Talboys et al. [18],where an untripped NACA0012 aerofoil was tested with the flaplets placed on the pressure and suction sidesof the aerofoil separately, across a large range of Reynolds number and angles of attack. It was observed thatwhen the flaplets were placed on the pressure side of the aerofoil, the laminar separation bubble was seemingly
modified, due to the severe reduction in the tonal noise components of the acoustic spectra. When the flaplets
were on the suction side of the aerofoil, a reduction was still seen.

2

EXPERIMENTAL ARRANGEMENT

The aerofoil used for the present study was a NACA 0012, with a chord (c) of 0.2 m and a span of 0.28 m.
The model was 3D printed giving the aerofoil a trailing edge bluntness of 0.5 mm with a solid angle of16◦ . The aerofoil had a boundary layer trip placed at 0.1c, such that the boundary layer was turbulent. The
flexible trailing edge flaplets were manufactured, using a laser cutter, from a thin polyester film (see table 1 fordimensions and material properties). The flaplets were attached to the aerofoil using a thin strip of double sided
tape, and placed such that the free ends were orientated downstream at 1.1c, allowing them to freely oscillate
at their Eigen frequency in the flow field. The Eigen frequency was determined to be 107 Hz in a previous
study [17], using cantilever beam theory.
Acoustic measurements took place in the small aeroacoustic open jet wind tunnel [14] at the Brandenburg
University of Technology in Cottbus, with a setup similar to that used in [6]. The wind tunnel was equipped589

with a circular nozzle with a contraction ratio of 16 and an exit diameter b of 0.2 m. With this nozzle, themaximum flow speed is in the order of 90 m/s and at 50 m/s, the turbulence intensity in front of the nozzle
is below 0.1 %. For the present study the chord based Reynolds number was varied from 94,000 – 384,000
and the geometric angle of attack, αg , was varied from αg = 0◦ to 20◦ . During measurements, the wind tunnel
test section is surrounded by a chamber with absorbing walls on three sides, which lead to a quasi anechoic
environment for frequencies above 125 Hz.
The acoustic measurements were performed using a planar microphone array, consisting of 56 1/4th inch microphone capsules flush mounted into an aluminium plate with dimensions of 1.5 m × 1.5 m (see [12]). The
microphone layout is included in Fig. 1a. The aperture of the array is 1.3 m. The array was positioned out of
the flow, in a distance of 0.71 m above the aerofoil.
Data from the 56 microphones were recorded with a sampling frequency of 51.2 kHz and a duration of 60 s
using a National Instruments 24 Bit multichannel measurement system. To account for the refraction of soundat the wind tunnel shear layer, a correction method was applied that is based on ray tracing [13]. The resulting
microphone auto spectra and cross spectra were averaged to yield the cross spectral matrix. This matrix was
further processed using the CLEAN-SC deconvolution beamforming algorithm proposed by Sijtsma [16], which
was applied to a two-dimensional focus grid parallel to the array and aligned with the aerofoil.
The resulting sound pressures were then converted to sound pressure levels L p re 20 µPa and 6 dB weresubtracted to account for the reflection at the rigid microphone array plate.

3

RESULTS

Figure 2 shows the far field acoustic spectra, in third octave bands, for all of the test cases. For clarity,each Reynolds number test is spaced with a 20 dB increment from the previous test case. The corresponding
Reynolds number can be seen adjacent to the spectra on the secondary axis. At αg = 0◦ , Fig. 2a, aninteresting noise reduction can be seen in the lower frequency range (∼0.1 – 0.4 kHz). The frequency at which
this reduction occurs at, increases with Reynolds number (the scaling of which will be discussed with Figures
3 and 4). Another observation is that a gain noise level at high frequency is observed for all Rec . Whenthe angle of attack (αg ) is increased the low frequency benefits are still present but are reduced. The gain in
the high frequency noise is also present and increases with αg . These two features of the acoustic spectra,
low frequency noise reduction and high frequency gain, has also been seen in a previous study; however inthat study the boundary layer was laminar (not tripped) [18]. The findings here give further evidence to theconclusion of Talboys et al. [18] where they concluded that these differences were due to a modification of
the shear layer instability on the suction side of the aerofoil by the flaplets. It should be noted here that at
Reynolds numbers < 121,000 at αg = 10◦ , Fig. 2b, a peak can be observed at ∼ 300 Hz. This is believed tobe due to the relaminarisation of the boundary layer which in turn leads to the formation of laminar boundary
layer tonal noise. This in itself is an interesting result as it could further confirm that the flaplets stabilise theboundary layer, as was seen in [17], as such promoting relaminarisation by locking into with the fundamental
instabilities within the boundary layer and dampening out the non-linear instabilities.
A common scaling factor that is used when investigating trailing edge noise is the fifth power of the Mach number (Eqn. 1) and was first proposed in the theoretical study of Ffowcs Williams and Hall [19]. Figure 3 shows
all of the test cases once they have been scaled and plotted against their respective chord based Strouhal number. The scaling works well across all Reynolds number and angles of attack. Between the Strouhal numbersof 1 and 5, the low frequency reduction can be clearly seen and at Strouhal number >20, the high frequency
increase is also observed.

590

L∗p = L p − 10 log10 (M 5 )(1)∆L∗p = L∗p, ref. − L∗p, flaplets

(2)

200

150

243k
100

172k
150k

50

Rec
384k

150

300k

300k
243k

Lp 1/3 [dB]

Lp 1/3 [dB]

200

Rec
384k

100

172k
150k

50

121k

121k
108k

0
Flaplets
Reference
0.1

0.2

0.5
1
fc [kHz]

108k

0
Flaplets
Reference

94k
2

4

0.1

0.2

(a) αg = 0◦0.5
1fc [kHz]94k2(b) αg = 10◦

200

200

Rec
384k

150

Rec
384k

150

300k

300k
243k

100

172k
150k

50

Lp 1/3 [dB]

Lp 1/3 [dB]

243k
100

172k

50

121k

121k
108k

0
Flaplets
Reference
0.1

0.2

0.5
1
fc [kHz](c) αg = 15◦

4

108k

0
Flaplets
Reference

94k
2

4

0.1

0.2

0.5
1
fc [kHz]94k24(d) αg = 20◦

Figure 2. Comparison of the third octave sound pressure level (Lp-1/3 ) between the baseline aerofoil and when
the flaplets are attached. Each chord based Reynolds number has had an offset of 20 dB to the previous test
case for clarity. The Reynolds number of the test case can be seen on the second axis, adjacent to the spectra.
591

120
Lp 10 log10(M 5) [dB]

110
100
90
g=0
g = 10g = 15g = 20

80
70
60

1

2

5

f c/U

10

20

50

100

20

20

10

10

Lp* [dB]

Lp* [dB]

Figure 3. Third octave sound pressure levels, scaled with the fifth power of the local Mach number against thechord based Strouhal number. Points which are shaded in red indicate the cases with the flaplets and black are
the reference case.

0
10
20

10
1

2

20

20

50 100

1

2

10
0
10

5 10
f c/U

20

50 10020

50 100(b) αg = 10◦

20
Lp* [dB]

Lp* [dB]

5 10
f c/U
(a) αg = 0◦

20

20

0

10
0
10

1

2

5 10
f c/U
(c) αg = 15◦20

50 100

20

1

2

5 10
f c/U
(d) αg = 20◦

Figure 4. Difference in scaled Lp, between flaplets and reference case. The transparency of the points indicates
the Reynolds number, a darker point represents a higher Reynolds number and a lighter is a lower one.
When subtracting the two scaled cases from each other, Eqn. 2, the differences become much more obvious(Figure 4). A negative value of ∆L∗p indicates a reduction in noise level, conversely a positive value means an

592

60

5

60

5

50

0

0
Flaplets
Reference
OSPL

40
30

100

150

200

250

Rec

(a) αg

10

350

40
30

5
100

150

= 0◦

200

250

Rec

300

10

350

(b) αg = 10◦

60

5

60

5

50

0

50

0

40
30

5
100

150

200

250

Rec

(c) αg

= 15◦

300

350

10

10

OSPL [dB]70

OSPL [dB]10

OSPL [dB]

70

300

5

40
30

OSPL [dB]50

OSPL [dB]10

OSPL [dB]70

OSPL [dB]10

OSPL [dB]

70

5
100

150

200

250

Rec

300

350

10

(d) αg = 20◦

Figure 5. Overall sound pressure levels for the reference cases and with the attached flaplets. ∆OSPL has beenplotted on the second axis to yield a clear indication of the difference at each Reynolds number. The zero lineon the ∆OSPL axis is shown as ( )increase. At αg = 0◦ , the test data collapse well on top of each other, very clearly showing the reduction andincrease in noise levels at certain frequency bands, as previously discussed. The average maximum reduction
is approximately 6 – 8 dB which occurs between a Strouhal number range of 2 – 3. Therefore, using thisparameter, the frequency range at which the noise reduction occurs at can be calculated for a given Reynolds
number. When the angle is increased the points start to become less correlated from one another, especiallyonce the Strouhal number increases past 5. The noise reduction does still stay within the Strouhal number
range of 2 – 4, albeit reduced, as seen in Fig.2.
To quantify the overall effect of the flaplets the overall sound pressure level has been calculated for the reference
and flaplet case at each of the increments in Reynolds number. An indication of the magnitude in difference
between the two aerofoils can be seen from the second axis (∆OSPL). Figure 5a, shows that for all tested
cases there is a noise reduction despite the higher frequency increase seen in Fig.2a and 4a. From the lowest
Reynolds number (98,000) till Rec = 172,000 there is an almost constant net reduction which is of the order
2 dB. As Rec increases further the benefit approaches zero. This is thought to be due to the Eigen frequencyof the flaplets being too low for the expected shedding frequency of the vortices, hence their benefit starts tobecome negligible. At αg = 10◦ (Fig.5b), the relaminarisation of the boundary layer can once again be seen
to have a negative effect on the acoustics. This can be seen as a sharp increase from Rec =108,000 – 121,000.
However after this point the flaplets show a slight benefit, before again converging to the same OSPL as thereference case. Increasing the angle further (Fig.5c and 5d), more or less yields an overall negligible impact on
the OSPL.

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4

CONCLUSIONSThis study builds off from the recent study by Talboys et al. [18] on the acoustic effect of the trailing edge
flaplets when the aerofoil is subjected to a laminar boundary layer. The same low frequency benefits and highfrequency increase is seen herein, and gives more indication that this effect is due to the shear layer stabilisation.
At αg = 10◦ and at low Reynolds number, the flaplets seemingly cause the boundary layer to relaminarise bydampening out the non-linear instabilities in the boundary layer (as seen by Talboys and Brücker [17]). A chordbased Strouhal number range of 2 – 3 has been identified to be the range in which the maximum reduction
occurs. This shows that for the current flaplet arrangement, this band of Strouhal numbers can be specificallytargeted. Further investigations in order to fully quantify this effect with different flaplet properties is currently
be carried out by the authors.

ACKNOWLEDGEMENTS
The position of Professor Christoph Brücker is co-funded by BAE SYSTEMS and the Royal Academy ofEngineering (Research Chair no. RCSRF1617\4\11) and travel funding for Mr. E. Talboys was provided by The
Worshipful Company of Scientific Instrument Makers (WCSIM), both of which are gratefully acknowledged.

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