Computational Aeroacoustics of an axial fan with leading edge serrations
Stefan SCHODER(1) , Florian KRÖMER(1,2) , Michael WEITZ(1) , Manfred KALTENBACHER(1) , Stefan BECKER(2)(1) Institute of Mechanis and Mechatronics, TU Wien, Austria, stefan.schoder@tuwien.ac.at(2) Institute of Process Machinery and Systems Engineering, Friedrich-Alexander University Erlangen-Nürnberg, Germany

Abstract
In recent years, a trend towards modifications of the leading edge geometry in airfoils to reduce the sound emission andimprove the stall behavior has been observed. However, low-pressure axial fan design methodologies for a decreased
sound radiation rely almost exclusively on the use of fan blade skew rather than on leading edge modifications so far.
Therefore, with this study we aim to investigate the sound reduction capabilities of leading edge serrations applied to
low-pressure axial fans using the efficient perturbed convective wave equation. For this purpose, we use a reference
fan with straight leading edges and a fan with serrated leading edges that has been studied in experimental investigations before. The comparison of the CFD and CAA simulations with experimental results show excellent agreement.
Furthermore, we analyze the sound reduction mechanisms of leading edge serrations based on the CAA results. In
general, a sound reduction is obtained with leading edge serrations without influencing the fan operating point. The
study demonstrates the applicability of this aeroacoustic approach for axial fans with modified leading edges.
Keywords: Hybrid aeroacoustics, Numerical simulation, Leading edge serrations, Fan noise

INTRODUCTION
By increasing performance and efficiency, axial fans contribute to many technical systems encountered in everyday life, e.g. computers, cars, trains, ventilation systems, and powerplants. For example, to reduce the effectof increasing temperature on humans in urban regions, the demand of cooling systems increases significantly[1, 2]. Hence the number of axial fans and the sound emission rises in urban regions, which in turn also interfere with our lives. To reduce the sound emissions of axial fans, there are several approaches among which theapplication of leading-edge serrations shows promising capabilities. Therefore, we demonstrate the applicabilityof aeroacoustic simulations and present an outlook towards hybrid aeroacoustic using the perturbed convective
wave equation [17].
1.1 Sound reduction with leading-edge serrations
Under distorted flow conditions with high turbulence intensity, leading-edge serrations of an airfoil effectively
reduce airfoil noise [3, 4, 5, 6, 25, 8, 9, 10]. Recently, the investigations support that the sound-reduction
mechanisms also apply for axial-fan blades.
A reduction of the sound power level of 2.3 dB was observed with sinusoidal leading-edge serrations only on
the outer 20 % of the blade span width [11]. A similar relationship was found for ducted, unskewed lowspeed axial fans with sinusoidal leading-edge serrations under various inflow conditions [12]. Sound is reduced
mainly below 2 kHz and for serrations with the smallest wavelength and the highest amplitude, a maximum
overall reduction of 3.4 dB was identified. Krömer et al. [13] performed a study on the sound emission of aforward-skewed fan with sinusoidal leading-edge serrations of varying amplitude and wavelength under free anddistorted inflow conditions. Leading-edge serrations reduced the emitted sound for both inflow conditions. Basedon this study, a comprehensive investigation on different leading-edge modifications was carried out [14, 15, 16],
including sinusoidal, double-sine and random-amplitude serrations. The sinusoidal leading-edge serrations leadto the most effective sound reduction, namely 7 dB for distorted inflow conditions and 11 dB for free inflow
conditions compared to the reference fan with straight leading edges. As indicated by Biedermann et al. [12],the sound emission decreases with increasing serration amplitude and lower serration wavelength.

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1.2 Aeroacoustic model
For a deeper understanding of the physical processes causing the sound reduction, numerical simulations of theflow and sound field are performed. Based on a highly resolved CFD simulation of the fan, radiated sound is
propagated by Ffowcs Williams-Hawkings analogy (FW-H) [18]. We used the implementation of Star-CCM+(Siemens PLM software, Plano, TX/USA).
Besides FW-H, one can use a special hybrid aeroacoustic workflow for low Mach number applications. Startingfrom the acoustic/viscous splitting technique [19], many different linear and non linear wave equations havebeen derived [20, 21, 22, 23]. All these methods have in common that the flow field quantities are split into
compressible and incompressible partspv=

p̄ + pic + pc = p̄ + pic + pa
= v̄v + v ic + v c = v̄v + v ic + v a(1)ρ= ρ̄ + ρ1 + ρ a .

(3)

(2)

First, the field variables are decomposed into mean ( p̄, v̄v, ρ̄) and fluctuating parts. Additionally, the fluctuatingparts are further split into acoustic (pa , va , ρ a ) and flow components (pic , vic ). Finally, a density correction ρ1
is built according to (3). An ALE (Arbitrary Lagrangian-Eulerian) description for the operators∂D=+ v − vr · ∇ ,
Dt
∂t

(4)

where v r is the relative velocity of the grid, yields the perturbed convective wave equation (PCWE) for rotating
systems (see [17])1 D2 ψ a
1 Dpic
a
−
∆ψ
=
−
.
(5)
c2 Dt 2ρ̄c2 Dt
This scalar convective wave equation describes aeroacoustic sources generated by incompressible flow structures
and wave propagation through moving media, as it occurs in the axial fan configuration. Compared to otheraconvective wave equations, the PCWE reduces the number of unknowns (acoustic pressure pa = ρ̄ Dψ
Dt andparticle velocity v a = −∇ψ a ) to just one scalar unknown, the acoustic velocity potential ψ a . All computationsof the PCWE are exectued in CFS++ [24] and the modeling strategies for rotating system can be found in [17].
1.3 Motivation and outline
In this study, we aim to investigate the sound-reduction capabilities of leading-edge serrations by aeroacoustic
simulations. In Sec. 2, the fan design and the leading-edge parameters are outlined and the most important
facts of the experimental investigation are described. A comparison of the simulated noise reduction and themeasurement data is given Sec. 3. Final conclusions are drawn in Sec. 4.

EXPERIMENTAL INVESTIGATIONThe experimental investigation used for the validation of the simulation results have been previously publishedand are described in [15]. Upon that, we briefly describe the most important facts of these measurements.
2.1 Fan design
The investigated reference fan has straight leading edges and unskewed fan blades (see Fig. 1). Nine fan blades
are mounted on the hub (hub diameter dhub = 247.5 mm) and the axial fan is operated at a constant rotational
speed n = 1486 rpm. The outer diameter of the axial fan is dfan = 497 mm. During the investigations, the fanswere installed in a duct with a diameter of dduct = 500 mm, resulting in a tip gap stip = 1.5 mm. As illustrated
in Fig. 1, the turbulent structures are controlled by a turbulence grid that is mounted inside the duct. Further

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y
z
x

γ

Figure 1. Simplified axial fan with straight leading edges.

fans were investigated. One fan blade of each fan blade set is shown in Fig. 3. Thereby, the mean chord lengt
details on the fan design
canisbeidentical.
found in [14]. The laser-cut blades are made of t = 6 mm thick aluminum
bladesplates and have a constant stagger angle γ = 20◦ along the fan blade span without any camber or twist. The
stacking line of the blades connects the fan-blade leading edges that are centric towards
λLEthe axis of rotation.
2.2 Leading edge parameteraLE
Leading-edge modifications affect the aerodynamic and the acoustic properties of lifting sections. We use themost common form of such modifications, that is, a sinusoidal leading edge. This sinusoidal modification canbe described by two leading edge parameters, the wavelength λLE and the amplitude aLE as a percentage of themean chord length l c = 69.6 mm (see Fig.Fig.
2). 2Figure
2 shows theusedleading-edge
serrated afan
bladewavelength
used in λLE .
Leading-edge
sineparameters:
amplitude
LE andthese investigations with the parameters being λLE = 16.7 %l c and aLE = 6.7 %l c .

aLEλLE(a) USK(b) USK_A133λ67(c) USK_A167λ67(d) USK_A133λ100

Figure 2. Serration wavelength λLE and amplitude aLE for a sinusoidal leading edge (right) and a leading-edge
serrated fan blade A167λ 67.

2.3 Experimental setup
All experimental investigations were carried out in a standardized inlet test chamber [14]. The fans were casedby a short duct with a diffuser on the pressure side and an inlet bellmouth on the suction side. The fan motor
and a torque meter were installed outside the duct. Improving the measurement conditions, the measurement
FSK(g) FSK_A133λ67(h) theFSK_A167λ67(i) FSK_A133λ100
chamber is built as an anechoic(f)chamberwith absorbers
on the walls,
floor and the ceiling.
To avoid
distinctive noise from rotor-stator interactions, four non-centric struts were used as fan support.

(e) USK_A(j) FSK_A1

2.4 Sound field
Seven 1/2 inch free-field microphones, type 4189-L-001 (Brüel & Kjær), connected to microphone conditioners,
type NEXUS 2690-A (Brüel & Kjær), arranged in a semicircle aligned with the axis of rotation (Fig. 3), were(k) BSK

Fig. 3(l) BSK_A133λ67(m) BSK_A167λ67(n) BSK_A133λ100(o) BSK_A

Fan blades with straight and sinusoidal leading edges: USK (a - e), FSK (f - j) and BSK
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Table 1

Leading-edge serration parameters.

5.2 Ffowcs Williams-Hawkings Methodeused to measure the sound field. A PXIe-1075 front-end with 24-bit PXIe-4496/PXIe-4492 data-acquisition
modules (National Instruments) was used for data acquisition. The measurement time was tm = 30 s with adirekt vor dem Ventilator befindet, sollen die Ergebnisse an dieser Position betrachtet und dis◦
M1: +90In
kutiert werden.
Abbildung 5.4 werden die ermittelten Oberflächenquellen am gesamten Lüfter
◦M2: +60
für beide Modelle dargestellt.
30 ◦M3: +30◦
M4: 0◦

1000

M5: −30◦
M6: −60◦

M7: −90◦

Figure 3. Microphone positions, schematic (left) and photograph (right). Dimensions in mm.
sampling frequency fs = 48 kHz. Further details on the measurement procedure can be found in [14].

SELECTED RESULTS AND DISCUSSION

Abbildung
5.4: Quelltermformulierung
FW-H
Methodeam kompletten
Lüfterrad zum
Thissection visualizes
aeroacoustic quantities nachbasedderon anaccurate
CFD simulation
and an aeroacoustic
propagation simulation. Zeitpunkt
First, the soundof theflow simulationMic4.
are determined and thereafter the soundt=0,48sourcess für den
Mikrofonpunkt
emission is computed using FW-H analogy and the PCWE. The analysis of the aeroacoustic sources on the fan
blades reveals the sound reduction mechanism. Due to unexpected issues, we show only results of the FW-H
analogy.
Anhand von Abbildung 5.4 lässt sich bereits erkennen, dass das Referenzmodell deutlich mehr
Quellterme mit höheren Werten aufweist, als der Ventilator A167λ67. Für eine genauere An-

3.1 Impact of the leading-edge serrations on the aeroacoustic sourcesin Abbildung
5.5 ein
Lüfterblattundasanhand
dessenweitereInsichtorderwird
to propagate
sound using
the FW-H
analogy,vergrößert
we defined dargestellt
the fan surface
propagation
originand the
Untersuchungendurchgeführt.
regionwhere the main
sources are located. Sources in the fluid region are neglected. Figure 4, comparing the

Abbildung
5.5: Quelltermformulierungnachanalogy
der FW-H
am einzelnen
Lüfterblattzum
Figure4. Aeroacoustic
source terms of the FW-Hof theMethodereference axial-fan(left) andthe leading-edge
serrated axial fan(right),
withrespect
to
M4
(see
Fig.
3).
Zeitpunkt t=0,48 s für den Mikrofonpunkt Mic4.

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Es wird ersichtlich, dass die Quellen besonders an den Blattoberflächen deutliche Unterschiede
aufweisen. Betrachtet man das Referenzmodell, so erkennt man, dass die auftretenden Oberflächenterme über die gesamte Fläche im oberen Bereich der Schaufel auftreten. Im Vergleich dazu
treten beim Modell A167λ67 kaum dominante Quellterme auf der Saugseite auf. Ein Grund hierfür könnte sein, dass durch die veränderte Vorderkantengeometrie die auftretenden Quellterme sobeeinflusst werden, dass diese sich gegenseitig auslöschen und somit über die Oberfläche zerfallenKapitel 5 Numerische Untersuchung der Akustik

aeroacoustic source of the two different blade types, shows a significant reduction in the source terms on the
und keinen größeren Einfluss mehr für die Schallabstrahlung darstellen. Diesen dekorrelierensuction
sideof the fan
blade.konnte
As bereits
indicatedpreviousden Effekt durch
die Verwendungvon serrations
durch Lauinet al.
[8] bewiesen studies [25], the reduction is a result of the decorrelation
werden. Betragsmäßig kleinere Quellen können dennoch am Grund der serrations festgestellt
effectoftheaeroacoustic
sources.
However,smallsources are visible at the base line of the serrations and thewerden. Das Lüftermodell A167λ67 weist den Schwerpunkt der dominanten Schallquellen am
mostdominantsources
at theof the serrated blade.
äußeren Bereichder Schaufeln
auf, welcheoccur
höher ausfallen
als dietip
des Referenzmodells.
Die Darstellung der Oberflächenquellterme an den anderen Mikrofonpositionen ist der Vollständigkeit halber im Anhang B für das Referenzmodell und in Anhang C für das Lüftermodell
A167λ67 zu finden.

3.2 Impact of the leading-edge serrations on the sound emission
5.2.2order
Vergleich
Schallemission
ValidierungIn
todercompare
theundmeasurement
results and the simulation results, the sound field is exported at the seven
Um die in der Simulation
erhaltenen Ergebnisse
zu können, werden die
Datensätze
microphone
positions
duringauswerten
the simulation.
The
data is then Fourier-transformed and energetically averaged.
der sieben Mikrofonpunkte exportiert und in Matlab über eine Fast-Fourier-Transformation vom
Figure5shows
theaveraged
spectrumofthereference
fan (left) and the leading-edge serrated fan (right). The
Zeitbereich in den Frequenzbereich übertragen. Im Anschluss werden die Frequenzdaten für diesechs Umdrehungenenergetisch richtig
gemittelt.
redgraph denotes
the measurements
and the blue graph the simulated results, respectively. From the simulation
In Abbildung 5.6 sind die gemittelten Spektren für das Referenzmodell REF dargestellt. Dabei
results
ofrotethefan
we undseein derthatmaximum
frequency of 5 kHz is resolved, where the broadband noise
entspricht die
Liniereferencedem Spektrum der
MessungFarbeaBlau
ist das simulierte
Ergebnis aufgetragen. Die maximal auflösbare Frequenz beträgt circa 5000 Hz. Man erkennt,
isgenerally
underestimated
since
the
volumecontributions
to FW-H are neglected.5.2 The
first blade passing
Ffowcs Williams-Hawkings Methodedass gemessene und simulierte Ergebnisse gut übereinstimmen, wobei die Simulation den Breitfrequency
atgering
223vorhersagt.
Hz is Die
capturedwell bybei theOverall, the trend of the sound emission is resolved.
bandschall leicht zu
erste Blattfolgefrequenz
223 Hzsimulations.
kann durch die
FW-H Methode gut abgebildet werden.

70

Averaged sound pressure level Lp in dB

Averaged sound pressure level Lp in dB

70
60
50
40
30
20
10
10

60
50
40
30
20
10

2

10

3

Frequency in Hz

Abbildung 5.6: Schallspektrum der Messung (rot) und des Lüftermodells REF (blau).

10

4

10

2

10

3

Frequency in Hz

10

4

Abbildung 5.7: Schallspektrum der Messung (rot) und des Lüftermodells A167λ67 (blau).

Figure 5. Comparison of simulation (blue) and measurements (red) of the averaged sound pressure level of the
Abbildung 5.7 sind die gemittelten Spektren für das Lüftermodell A167λ67 dargestellt. Dabei
reference axial-fan (left) and the leading-edge serrated Inentspricht
axial
fan(right).
die rote Linie dem Spektrum der Messung und in der Farbe Blau ist das simulierte
52

Ergebnis aufgetragen. Die maximal auflösbare Frequenz beträgt ebenfalls circa 5000 Hz. Des
Weiteren erkennt man, dass gemessene und simulierte Ergebnisse ebenso gut übereinstimmen,

Moving on to the discussion of the simulation results wobeiof theleading-edge
serrated
axial
fan,Es thegridresolves
die Simulationden Breitbandschallauch zu geringvorhersagt.
fällt auf,dass dieerstensieben Blattfolgefrequenzen korrekt abgebildet worden sind aber der Pegel zu gering vorhergea maximum frequency of 5 kHz. In general the trendsagt
ofwordentheist.soundemissioniscaptured
well
and
again
the
Im Vergleich zum Referenzmodell weist das Lüftermodell A167λ67 höhere tonale
broadband components are underestimated. The tonal components
well represented
in theresults
Komponenten an derare
Blattfolgefrequenz
und deren harmonischen
auf. simulation
Dieser Effekt könnte
daerklärt werden, dass durch die serrations die effektive Länge der Vorderkante größer ist
and compared to the reference configuration the tonal durch
components
dominate the sound radiation. This increase
als beim Referenzmodell und somit auch eine größere Angriffsfläche für die auftreffende Strömungincreasing
bietet, was akustische
Auswirkungenin the tonal components may be partly addressed to thecontourlinezeigt.
of the leading-edge. However,
Zusammenfassend kann gesagt werden, dass vor allem der breitbandige Anteil der Schallemisthe broadband sound emission as well as the overall soundemissionisdecreased
byvontheleading-edgesion des Lüftermodells A167λ67 durch den Einsatz
serrations
reduziert werden serrated
konnte.
Betrachtet man beide Frequenzspektren, so kann geschlussfolgert werden, dass die Schallvoraxial fan.

CONCLUSIONS

hersagemethode nach Ffowcs Williams-Hawkings die Blattfolgefrequenzen gut abbildet, jedochbei beiden Modellen einen leicht unterschätzten breitbandigen Anteil des Schalls aufweist. Dies
könnte daran liegen, dass für die FW-H Methode nur die Lüfterblätter und die Nabe als schallabstrahlende Oberflächen angegeben wurden. Betrachtet man den realen Systemaufbau mit
welchem die Messungen durchgeführt worden sind, so wird klar, dass auch die anderen Komponenten des Systems, wie zum Beispiel das Turbulenzgitter, an den erzeugten Schallemissionen
beteiligt sind.

According to the theory, we see a sound reduction of the broadband components in both the measurements
and the acoustic simulations. A comparison of the aeroacoustic sources on the fan blade shows a significant
53
reduction in the source terms on the suction side of a fan blade with leading-edge serrations. For a leadingedge serrated axial fan, the tonal components become more dominant since the broadband sound emission issubstantially decreased. We attribute the slight underestimation of the broadband components to the neglectedvolume terms in the FW-H formulation. The tonal components are well represented in the FW-H simulation
results. In the future, we will use the PCWE to compute sound emissions of leading-edge serrated fans and wewill compare the findings with the present study.

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