PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Study on relationship between the geometry of leading and trailingedges of airfoil and their aeroacoustics parameters at low Reynolds
numberJoanna KOPANIA1
1

Lodz University of Technology, 266 Piotrkowska Street, 90-924 Lodz, Poland

ABSTRACTIn last few year’s there has been considerable interest in the use of edge modifications for the reduction of
airfoil noise. One of the most commonly used methods is sawtooth serration edge inspired by the owl's wing.
The edge serration is known to be quite effective in reducing aerodynamic drag and self noise radiation. But
the correlation between edges modifications of airfoil and noise reduction was studied also for other forms
such as wavy or M-shape. Results of an experimental study on turbulent flow over the NACA 0012 airfoil,with a flat and serrated trailing and serrated, arc and wavy leading edge are presented in this paper. Detailed
aeroacoustic measurements are presented of the noise radiated by study airfoils in a low to moderate speed
flow under acoustical free field conditions. Parametric studies of the amplitude and wavelengthcharacteristics are conducted to understand the effect of edges modifications on noise reduction. The
influences of geometric parameter applied tooth, arc and waves, free-stream velocity on the noise reduction
effect of are observed.
Keywords: noise, airfoil, NACA 0012, trailing edge, leading edge

1. INTRODUCTION
Airfoil self-noise is mainly associated with the laminar or turbulent boundary layer on the bladesurfaces. This type of noise can have tonal or broadband characteristics, and is considered to be causedby several mechanics, such as trailing edge noise, laminar boundary layer vortex shedding noise, tip
noise, separated or stalled flow noise, and blunt-trailing-edge noise. Trailing edge (TE) noise has been
an comprehensive aeroacoustic research topic for more than fifty years, both experimentally andanalytically. In literature reduction of trailing-edge noise is inspired by the silent flight of owls [1] andthe capability of trailing-edge serrations in noise attenuation [2,3,4] in theoretical, experimental andnumerical aspect [5,6]. To reduce trailing edge noise several passive methods such as serrated trailingedge [7], porous surface [8] and brushes [9] have been investigated. Experimental investigation [10]showed that the serrated trailing edge of airfoil may cause a self -noise reduction of up to 2 and 6 dB
with significant reduction at high frequencies without the loss of aerodynamic performance. The lastworks showed that further noise reduction can be achieved by the use of more complex trailing edges
such as: slitted edge, sawtooth with hole, slitted-sawtooth edge and randomly serrated trailing edges[11]. The character and level of trailing edge self-noise is highly sensitive to Reynolds number, angleof attack, airfoil geometry and trailing edge bluntness. Trailing edge noise radiated in high Reynolds
number flow has got broadband in nature but in low Reynolds number trailing edge noise has adistinctive narrow band character comprising a broadband hump superimposed with a number of highamplitude “tones” [12]. In this situation the boundary layer on the airfoil surface is mainly laminar but
potentially unstable.
The presented paper is a studies of the aeroacoustic and aerodynamic behavior of symmetrical
airfoil NACA 0012 with trailing edge serrations with additional serrated, arched and wavy plates on
the leading edge. Welocity distribution around the airfoil and 1/3 octave sound pressure level at low
Reynolds number has been investigated and presented.

1

joanna.kopania@p.lodz.pl

3171

2. EXPERIMENTAL METHOD
2.1 Test model
In current work NACA 0012 airfoil has been investigated. The airfoil were manufactured by 3D
printing using PLA material. Airfoil with chord 0,15m and span 0,35m has got 4,0 mm (thickness)blunt trailing edge with a 25mm deep and 2.0 mm thick slot cut through the trailing -edge and leadingedge along the span in order to allow flat plate serration inserts to be fitted to the airfoil, as shown in
Figure 1. Chord of flat and serrated plate added to trailing edge were c = 75 mm, a span of s = 350 mm
and a thickness of d =2,0 mm. Chord of serrated plate added to leading edge were c = 30 mm. The partof each plates (25mm) was insert into the notch in the airfoil. The airfoil with straight unserrated
trailing edge and unserrated leading edge (baseline) was used as a reference model for airfoil withserrated plates. The serrated plate on the trailing edge has got height of teeth 50mm, with distance
between the tip of teeth 5mm. The serrated plates on the leading edge have got teeth, arcs and wave
with height 5mm and distance between the tip was 5mm (serrated and arc) or 10mm (waves). The flat
plate and serrated plates were used in this experiment as a additional trailing and leading edges. The
geometries serrated plates added to airfoil are presented on the Figure 2 and Table 1.

Figure 1. Studied airfoil and locations of microphones and thermo anemometer around the airfoil.

Figure 2. The geometries serrated plates added to airfoils.

3172

Table 1. The parameters of serrated plates.
Parameters of studied plates
TermBaseline

Model of airfoil
Flat plates on trailing and leading

l

2h

2h’

O

D

r

[mm]

[mm]

[mm]

[mm]

[q]

[mm]

25

-

-

-

-

-

25

50

-

-

26,6

-

25

50

5

5

26,6

-

25

50

5

5

-

-

25

50

5

10

-

2,5

edge
Serrated

Flat plate on trailing edge andserrated plate on leading edge

Serrated -Serrated plate on trailing edge andserratedserrated plate on leading edge

Serrated –Serrated plate on trailing edge andarcs

arc plate on leading edge

Serrated -Serrated plate on trailing edge andwavy

wavy plate on leading edge

2.2 Aeroacoustical measurement
The measurements were performed on the special constructed test stand with the outlet to theanechoic room. The outlet was square with parameters 150×350 mm. Airflow was induced by a fan
mounted on the inlet of the stand and regulated by the power inverter. The outlet of test stand was in theanechoic room at the Institute of Power Energy in Lodz. The anechoic test chamber is cubic,
approximately 350m3 in size and has walls that are acoustically treated with foam wedges providing areflection free environment. The airfoil was held by side plates in outlet of test duct. To measure thefar-field noise was made by SVAN 958. The two microphones was located on trailing and leading edge
(perpendicular to the direction of the flow) positioned at a distance 1200mm from the edges (in middleof airfoil). To generate turbulence structures, and obtain an elevated level of turbulence intensity in thefreestream, a rhombus grid (mesh of grid 10mm, angles of mesh 50q and 130q) was placed outside theoutlet of test duct. The experimental setup for NACA 0012 is shown in Figure 3. The microphone wascalibrated before commencing the acoustic test.

Microphone
Grid

on

air-outlet
Microphone 2

Figure 3. The photograph of NACA 0012 during the measurements
The aeroacoustical and aerodynamical parameters were studied at 0q angles of attack (zero angle
was in level at x-axis). The measurements were taken at five flow velocities in outlet of duct: 6m/s;

3173

9m/s; 12m/s; 15m/s. The velocities distribution in outlet of the stand test were measured by using a
Pitote probe and calculated by using log-Chebyshev method. For these investigations a small single
probe was used to this with a diameter of 2 mm by using data acquisition station - SAD-2, equippedwith the ADAM modules 4000+, an integrated PC with the application GeniDAQ equipped with a
Visual Basic language.
Additional, detailed flow measurements at several point along the chord line on upper surface havebeen performed using thermo anemometer measurement technique for NACA 0012 airfoil in order tobetter understand the effects of serration on the aerodynamic performance o f the airfoil. The
measurements were carried out at angle of attack, D = 0q for chord corresponding to Re=1,0×105. For
thermo anemometer the point measurement were logged at locations y=10mm, 30mm 50mm relative tothe lateral surface of airfoil. Each measurement plane was populated with 33 measurement points toaccurately capture the flow behavior at that region.

3. EXPERIMENTAL RESULTS
3.1 Acoustical parameters
The knowledge about aerodynamic and acoustic parameters of airfoil in low Reynolds number isimportant for learning its physical nature. Most of the flight animals (insects, birds) fly at Reynolds
number of 103-105 due to their low speed and small length scales. Current trend "inspiration on
nature" to improved the aerodynamical of flight and reduce flight's noise parameters requires
knowledge in this aspect. In this work the acoustic measurements of NACA 0012 airfoil as the baseline
and serrated configurations were carried out for 0q angle of attack and velocities: 6m/s; 9m/s; 12m/s;
15m/s which correspond to chord-based Reynolds Re = 8×104 to 2,0×105. The main goals of this work
was to determine the impact of plates steel, cut as flat, serrated, arc and wavy and added to trailing andleading edges of two studied airfoils, to emitted noise.
The spectra of the acoustic pressure level in the 1/3 octave bands were determined for tested air foilwith different configuration. As we seen from Fig. 4 and 5 there are differences in spectrum of SPL
between baseline and serrated models and for the rest of studied models. Tonal character of SPL
spectrum of studied models of airfoils is observed.
For baseline and serrated models three peaks are observed at lower Reynolds number: in 400Hz(observed also for the rest models), 1600Hz and 4000Hz (Fig. 4). For the rest of studied models serrated-serrated, serrated-arc and serrated-wavy - between frequencies 1250 – 7000 Hz significantly
lower SPL values are observed than for the baseline and serrated models. So, its suggest then airfoil
with modified trailing and leading edge could reduce noise of airfoil. From Fig. 4 is seen that there are
not so significantly differences between the spectrum from microphone 1 and 2 - probably the distance
between the microphones was too small, what can influence on the spectrum. One difference isobserved - the small peak around 1000Hz - for serrated-serrated, serrated-arc and serrated-wavy
models. For serrated-serrated and serrated-arc models small inflexion point at 2500Hz is observed
The some differences in spectrum of SPL between baseline and serrated models and for the rest of
studied models are observed in higher Reynolds number (Fig. 5) . Tonal character of SPL spectrum ofstudied models of airfoils is also observed. For baseline and serrated models peaks are observed in
400Hz, 1600Hz, 4000Hz and 6300Hz. For serrated model small inflexion point at 1000Hz is observed.
For serrated-serrated, serrated-arc and serrated-wavy models, between frequencies 3500 - 10000Hz
significantly lower SPL values are observed compared to baseline and serrated models. For
serrated-serrated, serrated-arc and serrated-wavy models three peaks, at 400Hz, 800Hz, 2500Hz and
3150Hz are observed. The peak at 400Hz is not related to spectrum of studied airfoil models. This peakis observed also in spectrum of test duct, without of models. So, the test duct must be modified, if thestudied will carried out in lower frequencies (below 500Hz). But in these studies, the comparison of
1/3 octave SPL spectrum between the model with flat plates added to leading and trailing edges to themodels airfoil with serrated edges is important. So, it does not matter the SPL spectrum of background(test duct). Based on the results, it could be concluded that combination of modified edges of airfoil
could reduce of its noise.

3174

Figure 4. The 1/3 spectrum of SPL from microphone 1 (on the left from leading edge) and microphone 2 (onthe right from trailing edge) for tested models of NACA 0012 with flat plate and serrated added to trailing andleading edge, at D=0q and Re = 8×104.

Figure 5. The 1/3 spectrum of SPL from microphone 1 (on the left from leading edge) and microphone 2 (onthe right from trailing edge) for tested models of NACA 0012 with flat plate and serrated added to trailing andleading edge, at D=0q and Re = 2,0×105.
In this work, it was important to understanding, how different serrated added to leading edges oftested airfoils influence on the noise of these airfoils in regard with the airfoils with straight edges. The
differences in the sound pressure level between airfoils with straigh t and serrated edges were
determined (“differential SPL”). That allowed to define the bands in which the added plates reduce thenoise. The difference of measured SPL between serrated and straight airfoils depending to velocities
for NACA 0012 airfoil are presented in Figure 6 and Figure 7 as a colour-maps. As is seen from the
Figure 6 and Figure 7 the maximum noise reduction value, depending on velocity and frequency, andit could exceed 6dB (red fields on the maps) for all studied models with serrated edg es.
Impact of leading edge noise at low frequency becomes more prominent from 11m/s onwards(pictures b), c) and d)). In each case (Figure 6) noise reduction is observed, e.g. for serrated model at
about 7m/s above 2000Hz and 9m/s in whole range of frequencies (approximately 2dB). After add theserrated plates to leading edge of airfoil the noise reduction increase, e.g. for serrated-serrated model
above 11m/s in whole range of frequencies noise reduction is around 5 -7dB.
The noise contributions associated with the leading edge and trailing edge are well separated in thefrequency domain and velocity. In lower velocity there are not influence of modified (serrated) plates
added to leading edge of airfoil, what suggest that in these range of Reynolds number and in these
conditions will not reduction of noise.
Generally the noise reduction of studied airfoils have got a resonance character, what mean that theincrease and decrease of differential SPL depended on frequencies is observed – especially around
10m/s, where are minus values of differences SPL what mean that in this region and by this velocity
there are not noise reduction.

3175

Figure 6. Colour-maps of relationship difference sound pressure level between baseline airfoil and serrated
airfoils depending on velocities for microphone 1 (from leading edge), dB (ref 20 PPa), for NACA 0012 forthe case of D= 0q (differences between baseline and serrated -a); differences between baseline andserrated-serrated b); differences between baseline and serrated-arc c); differences between baseline andserrated-wavy d)).
Colour-maps of differential SPL are very similar for signals obtained from microphone 2. As isseen from the Figure 7 the maximum noise reduction value also exceed 6dB (red fields on the maps)
for all studied models with serrated edges. Impact of leading edge noise at low frequency becomes
more prominent from 11m/s onwards (pictures b), c) and d)). In each case noise reduction is observed,
as a red fields for serrated-serrated, serrated-arc and serrated-wavy models and as green fields forserrated model. No differences in differences SPL colour-map (from microphone 1 and 2) indicate anyneed of modification of test duct and its retesting.

Figure 7. Colour-maps of relationship difference sound pressure level between baseline airfoil and serrated
airfoils depending on velocities for microphone 2 (from trailing edge), dB (ref 20 PPa), for NACA 0012 forthe case of D= 0q (differences between baseline and serrated -a); differences between baseline andserrated-serrated b); differences between baseline and serrated-arc c); differences between baseline andserrated-wavy d)).

3176

3.2 Aerodynamical parameters
Flow measurements at several point along the chord line on upper surface have been performed
using thermo anemometer measurement technique, to illustrate the laminar flow separations.

Figure 8. Colour-maps distribution of velocity along the chord on upper side of airfoil NACA 0012 for thecase of D= 0q (baseline -a); serrated b); serrated-serrated c); serrated-arc d); serrated-wavy-e)).
The laminar separation bubble is formed when the previously attached laminar boundary layer
encounters an adverse pressure gradient of sufficient magnitude to cause the flow to separate. In thefirst region of airfoil, near the leading edge, relatively slow flow is observed, which is responsible forforming the bubble. In the next region of flow consists of the free shear layer which undergoes
transition due to disturbance amplification occurring in the unstable laminar layer. Momentum transfer
due to turbulent mixing eventually eliminates the reverse flow near the wall and the flow reattaches tothe surface of airfoil (in point "a"). For studied models point “a” slightly move to leading edge forthree models: serrated-serrated, serrated-arc and serrated-wavy (this point is near x/c=0,7) incompared to baseline model, where point “a” is x/c=0,9-1,0. Process of separation, transition andreattachment results in a laminar separation bubble that has a predominate effect on the entire airfoil
flow field and its decrease can improve of lift force of airfoil.

4. CONCLUSIONSIn this work the experimental results show that the serrated trailing and leading edges of airfoil canchange the aerodynamic performances and have got influence on SPL spectrum, reduce the noise insome frequencies. The level of these change depends on the type of airfoil, and the geometri cal
characteristics of the serration. The colour-map results show that the serrations significantly affect onnoise reduction compared to the baseline but its dependent on velocity and frequencies. The noise
spectra usually comprise two or three peak dependent on geometries of added plates. Generally thenoise reduction of studied airfoils with modified edges have got a resonance character, what mean that3177

the increase and decrease of differential SPL depended on frequencies is observed. This is very
important when considering noise generated from example by fan flow interaction with inlet
turbulence plays a major role in airfoil self noise and further works in this aspect can allow for
construction of "silence fan".
A physical phenomena of laminar flow separations and suppress the burst of the laminar separation
bubbles for better aerodynamic performances of low-Reynolds-number airfoils is very important. Thisrequires a detailed knowledge about transient behavior of the separated laminar boundary layers andthe evolution of laminar separation bubbles. In low Reynolds number in which appearance of laminar
separation (formation of a separation bubble on the airfoil) is observed, flows around the airfoil has got
unsteady nature, it makes the flow field complicate and influence oh noise of airfoil. This is also major
factor of deterioration of lift force of airfoil by due to boundary layer separation on the airfoil.

REFERENCES
1. G.M. Lilley, “A Study of the Silent Flight of the Owl”, AIAA Paper 1998–2340, 1998.
2. M.S. Howe, Noise produced by a sawtooth trailing edge, J. Acoust. Soc. Am. 90 (1) (1991)
482–487.
3. T.P. Chong, A. Vathylakis, F.J. Joseph, M. Gruber, Self-noise produced by an airfoil with nonflat
plate trailing-edge serrations, AIAA J. 51 (11) (2013) 2665–2677.
4. B. Lyu, M. Azarpeyvand, S. Sinayoko, Prediction of noise from serrated trailing edges, J. Fluid
Mech. 793 (2016) 556–588.
5. F. Avallone, S. Pröbsting, D. Ragni, Three-dimensional flow field over a trailing-edge serration andimplications on broadband noise, Phys. Fluids 28 (2016) 117101.
6. L.E. Jones, R.D. Sandberg, Acoustic and hydrodynamic analysis of the flow around an aerofoilwith trailing-edge serrations, J. Fluid Mech. 706 (2012) 295–322.
7. Gruber, M., Azarpeyvand, M., and Joseph, P., Airfoil trailing edge noise reduction by the
introduction of sawtooth and slitted trailing edge geometries, Proceedings of 20th International
Congress on Acoustics, 2010, pp. 1–9.
8. Moreau, D., Brooks, L., and Doolan, C., On the noise reduction mechanism of a flat plate serrated
trailing edge at low-to moderate Reynolds number, 18th AIAA/CEAS Aeroacoustics Conference(33rd AIAA Aeroacoustics Conference), 2012, pp. 1–20.
9. Geyer T., Sarradj E., Fritzsche C., Porous airfoils: noise reduction and boundary layer effect,
International journal of aeroacoustics, v. 9, nr 6, 2010, 787-34.
10. T. P., Chong, Joseph P. F., An experimental study of airfoil instability tonal noise with edge
serrations, Journal of Sound and Vibration, 332, 2013, 6335-6358.
11. Shyy, W., Lian, Y., Tang, J., Viieru, D. and Liu, H., Aerodynamics of Low Reynolds Number
Flyers, New York: Cambridge Univ. Press, 2008.
12. Kurtulus D. F. , On the Unsteady Behavior of the Flow Around NACA 0012 Airfoil with SteadyExternal Conditions at Re=1000, International Journal of Micro Air Vehicles, Volume 7 Number 3
2015, 301-326

3178