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

Experimental Study on BTI Broadband Noise Reduction with Wavy
Leading Edge for Sweep Blade
Weiyang QIAO1, Xin GUO, Wenhua DUAN, Fan TONG, and Weijie CHEN
1Northwestern Polytechnical University, China

ABSTRACT
Experimental studies are performed to investigate the Blade-Turbulence Interaction (BTI) noise reduction ofa sweep blade with wavy leading-edge (WLE) configuration. The noise radiation from the sweep blade
leading-edge is generated by the interaction of the blade and incoming anisotropic turbulence which is
produced by a rod whose wake impinges onto the downstream NACA0012 airfoil. A linear array of 31
microphones was used in this study to determine the BTI noise source. With the Clean-SC approach, clear
and quantitative sound radiation results of the leading-edge noise source of swept blade are obtained.
Parametric studies of the WLE amplitude and wavelength are conducted to understand the effect of WLE on
BTI noise reduction. It is observed that the OASPL reduction is sensitive to both the amplitude andwavelength of the WLE. The WLE with the largest amplitude and smallest wavelength can achieve the mostconsiderable OASPL reduction up to 2dB. It is found in this study that the spectra of BTI noise reduction with
Strauhal number is almost the same for same WLE under different airflow velocities.
Keywords: Broadband noise; Wavy leading edge; Sweep blade; Source identification; Microphone array

1. INTRODUCTION
The reduction of the turbulence broadband noise from the trailing-edge (TE) and leading-edge(LE)of wing or the turbomachinery blade is nowadays an important industrial need and probably one of themost challenging issues in aero-acoustics. Especially, the Blade-Turbulence Interaction noise (BTI
noise) which from the interaction between the incoming turbulence and blade leading edge is asignificant contributor to the noise of turbofan, wind turbines, ventilation systems, high -lift devices,
propellers, and so on. It has been found that the BTI noise can be the dominant source when theincoming turbulence intensity is sufficiently high[1], which is often the case at the leading edge (LE)of outlet guide vanes in modern high-bypass-ratio turbofan engines.
The wavy or serrated leading edge (Wavy Leading Edge, WLE) was originally bioinspired by owl
serrations leading-edge wings and Humpback whale flippers, has been identified as a lift -enhancing,drag-reducing and noise reducing modification[2-5]. In addition to the aerodynamic aspects, manystudies have focused on BTI noise reduction with the WLE in recent years[6-23]. Clair et al.[6]experimentally and numerically investigated the effects of wavy leading -edges on the
airfoil-turbulence interaction noise, reporting a noise reduction of 3-4 dB. Lau et al.[7] numericallyinvestigated the effects of wavy leading-edges on the airfoil-gust interaction noise(AGI), finding thatthe ratio of the wavy leading-edge peak-to-valley amplitude (A) to the longitudinal wavelength of theincident gust (λg) was an important factor in reducing AGI noise. Kim et al.[11] conducted a numericalinvestigation into the noise reduction mechanisms of wavy leading-edges, obtaining valuable results.
Mathews and Peake[12] and Lyu et al.[13] also performed theoretical analyses of noise reduction using
wavy leading-edges. Chaitanya et al.[14] conducted a detailed parametric study of the sensitivity ofnoise reduction effects to the amplitude and wavelength of the leading -edge serrations of flat plates
and a NACA-65(12)10 airfoil. The influence of the turbulence integral length scale is also studied.
Biedermann et al.[15] proposed a statistical-empirical model to predict the noise generated by wavy
leading-edge airfoils. Turner and Kim[16] numerically investigated the aeroacoustic source
mechanisms of wavy leading-edges on a flat plate, identifying a system of horseshoe-like secondary
ProjectGrant No. 51776174 supported by National Natural Science Foundation of China, and Supportedby the National Special Research Project in Aeroengine in China.
1Qiaowy@nwpu.edu.cn

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vortices developing around the wavy leading-edge. Aguilera et al.[17] investigated the interaction ofanisotropic turbulence with a wavy leading edge NACA 0012 airfoil by means of computational
aeroacoustic simulations (CAA). Reboul et al.[18] performed a CAA prediction of the broadband noise
reduction effects of a serrated OGV using synthetic turbulence. A reduction in overall sound power
level of 1.9 dB was reported[18]. More recently, a series of experimental and numerical simulation
studies on the broadband noise reduction with WLE was carried out by present authors in Northwest
Polytechnic University(NPU)[19-22].
Despite the rapid growth in this field, the understanding of the noise reduction mechanisms
associated with wavy leading edge is still underdeveloped[23]. For the above wavy leading-edges
research work, most of the studies have been focused on 2-dimensional airfoils and flat plates with
2-dimensional flow and most previous studies have used homogeneous isotropic and grid generate dturbulence. To the authors' knowledge, real fan noise reduction using wavy leading -edges has only
been reported recently by Tong et al.[24] and Reboul et al.[25](in fact, the simplified approximate
two-dimensional flow simulation method is still used in these study[24]), and the investigation ofleading-edge noise reduction with incoming anisotropic turbulence has only been reported recently
by Chen et al.[18-21] and Tong et al[22]. It is well known that the flow around blade ofturbomachinery presents a strong three-dimensional flow characteristic. It could be expected that theflow and acoustic mechanism of the wavy leading edge in turbomachinery are more sophisticated thanthat in the already existing theory with the supposing of 2-dimensional flow around airfoil.
In the current work, a swept blade which model the three-dimensional flow characteristics aroundblade is used to study the BTI noise reduction mechanism of wave leading -edge configuration.

2. EXPERIMENTAL SET-UP AND BTI NOISE IDENTIFICATION METHOD
2.1 Test facility
The experiment was carried out in the low speed open jet wind tunnel in Northwestern
Polytechnical University, and a swept blade with sweep angle of 30 degrees and constructed by
NACA0012 airfoil was experimental investigated in this study. Air is supplied by the wind tunnel at
Mach numbers ranging up to 0.35, with the Reynolds numbers based on blade chord ranging from
1.0×10 5 to 1.0×10 6. The turbulence intensity at outlet of the wind tunnel is keep below 1%.
A test swept blade with a 150 mm chord and 300 mm span is placed into the core of an open jet of
the tunnel exit which is shown in Fig. 1. The swept blade is mounted onto a plexiglass disk, whichallows tuning the angle of attack α from -5 to +5 degree as shown in figure 1(b). A circular rod with thediameter of 10mm is placed 100mm in front of the blade leading edge with the same sweep angle asthat of swept blade. The anisotropic turbulence in this circular rod wake will interact with the swept
blade and generate the leading edge noise(BTI noise).

(a)The test setup(b)The test swept blade and microphone array
Fig. 1. Test set-up and Microphone array

An unequal spacing linear microphone array with 31 microphones (as shown in Fig. 1(b)) was used
to identify the sound source around airfoil and to analysis the noise strength and spectra. The array wasplaced just underneath the test blade about 0.69 m and the center of the array was placed underneaththe center of the blade. The direction of the linear array is perpendicular to the leading edge of theswept blade. The total length of the array is 1720 mm. The maximum distance between adjacentmicrophones in the array is 150 mm and the minimum distance is 35 mm. The all microphones with its
diaphragm were mounted on a large, hard reflecting surface so that the sound pressure levels obtained
will be augmented over the whole spectrum, up to the frequency at which incident and reflected waves

3164interfere, by a factor of 2 (6 dB).
An improved sensitivity beam-forming technology to remove the spatially coherent backgroundnoise and reduce the beam width based on the deconvolution array data method, Clean -SC whichdeveloped by Sijtsma[28,29]. In order to quantify BTI noise, the sound level of BTI noise source iscalculated by superimposing and averaging the sound energy of leading -edge noise in a certain space
range.
2.2 Design of the wavy leading edge of swept blade
The swept blade with swept angle of 30 degrees and constructed by NACA0012 airfoil wasinvestigated in this study. The chord of the swept blade is of 150mm and the span is of 300mm.
In order to investigate the effect of wavy leading edges on the BTI noise, the swept blade withmodified leading edges, i.e. wavy leading edges, is designed as shown in figure 2. The wavy peak,
middle, and valley(trough) locations are also depicted in this figure. The averaged Leading Edge(LE)line of the WLE blade coincides with the LE of the Straight Leading Edge(SLE) blade so th at the mean
chord length and the wetted area of the WLE blade are maintained constant as that of SLE blade. The
WLE blade in the form of sinusoidal profile with amplitude A, wavelength W, and mean chord lengthc is shown in figure 2. It should be noted that the wave shape in figure 2 is obtained by projecting asinusoidal line with amplitude A and wavelength W perpendicular to the flow direction to the blade
span direction. Therefore, the wavelength W is perpendicular to the direction of the inflow. The
symmetrical lines of the sinusoidal profile passing through the crest and valley points of the wave areparallel to the flow direction of the airflow. The chord length of the WLE blade versus spanwise
coordinate z is of the form
§ 2S z ·Ac( z ) c   cos ¨
(1)
¸
2
© W ¹

(a)

(b)
Fig.2 The wavy leading edge for swept blade

Five wavy blade modifications comprising all combinations of the sinusoidal serrations of different
amplitude (A/c) of 5%, 10% ,15% and wavelength (W/c) of 5%, 10%, 20% are designed in this study.
All wavy blades are labeled by the combinations of the non-dimensional percentage of the amplitude
and wavelength. The sweep angle of the swept blade is labeled using S30, that means the sweep angle
is 30 degree. The ranges of the non-dimensional wavy amplitude-to-wavelength ratio (A/W) is from
0.5 to 2.0. In order to facilitate the replacement of wave leading edge blades and control the cost ofexperiment, a demountable blade test piece is adopted in this test. When rep lacing the experimental
blade, only the wavy leading edge was replaced. The wavy leading edge is made of organic resin andis formed by 3D printing as shown in fig 2(b).

3. TEST RESULTS
The BTI noise for the swept blade with and without wavy leading edge were tested with the flow
speeds of U=30m/s, 40m/s, 60m/s and 70m/s, and the corresponding Reynolds numbers based on the
airfoil chord are respectively of 30,0000 ~ 70,0000.
3.1 The spectra of the BTI noise of the swept blade
The spectra of BTI noise at different airflow velocities is presented in figure 3. It can be clearly

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seen that there is same obvious tone noise at low frequencies with St=0.153 at four different airflow
velocities. According to Berland's theory[31], when the rod is far away from blade(L/d is larger than
3.5), the flow pattern behand circular rod is "wake mode". In this test, the ratio of the distance between
rod and the blade leading edge to the rod diameter L/d=10, the flow behand of the rod is "wake mode"
flow pattern according to Berland's theory. The tone noise is caused by periodic interaction between
the Karman Vortex Street and the leading edge of the blade, and the broadband noise is generated by
the interaction between wake turbulence and blade leading edge. As we all know, th e dimensionlessfrequency of Karman Vortex Street is 0.19(Strauhal number) for air flow passing through a cylinder
vertically. In this study, the airflow flows passing through the cylinder at an inclination angle of 30
degrees, so the Strauhal number of the Karman Vortex Street will be obviously smaller than that of thecylinder with perpendicular inflow. The Strauhal number of the tone of BTI noise in this study is of
0.153, which is consistent with the results of previous studies on inclined cylinders flo w [32].

Fig. 3 Spectra of the BTI noise of the swept blade with straight leading edge
3.2 Effect of wavy amplitude on the BTI noise reduction
Figure 4 shows the BTI noise source identification results for the straight leading edge blade andthe wavy leading edge blades with the same wavelength and with increasing amplitude at the airflow
velocity of 70 m/s. It can be seen that the flow noise around a cylinder does not change at the same air
velocity. However, BTI noise is changed with the using of wavy leading edge. It could be seen thatwith the small amplitude wave leading edge, the BTI noise may be increased in the low frequency
range relative to the straight leading-edge blade. With the increase of wave amplitude, the noise in low
frequency region will be reduced gradually.
In order to quantitatively describe the influence of wavy leading edge on BTI noise, the spectra of
BTI noise of swept blade at different airflow velocities with different leading edges were compared.
Comparing with the spectral results of straight leading edge blades, the spectrum of BTI noise
reduction ∆SPL with the using of wavy leading edge can be obtained,'SPL SPLBaseline  SPLWavy(a) BaselineS30

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(5)

(b) A5W10S30

(c) A10W10S30(d) A15W10S30
Figure 4. The identification of rod-blade interaction noise sources (at 70m/s of inflow speed)
Figure 5 shows the experimental results of BTI noise reduction at different flow velocities and withdifferent wave amplitude at same wavelength of 0.1c. It can be clearly seen from this figure that thenoise reduction of blades with different wave amplitudes tends to decrease with the increase of
frequency at various airflow velocities. When the frequency is greater than a certain value, the noise
will increase.

(a) 30 m/s(b) 40 m/s(c) 60 m/s(d) 70 m/s
Fig. 5 Spectra of the BTI noise reduction with wavy leading edge at same wavelength
When the airflow velocity is 30 m/s, in the frequency range which the Strauhal number is less than
0.306, the wavy leading edge with the smallest amplitude does not reduce but i ncrease thelow-frequency noise. When the airflow velocity is greater than 30 m/s, the wavy leading edge with3167

small amplitude can achieve minimal noise reduction in the low frequency range. Noise reduction
increases with the increase of wave amplitude. In the frequency range of Strauhal number less than
0.306, all wavy leading edge can achieve noise reduction for the inflow speed larger than 30m/s. In thefrequency range where the Strauhal number is greater than 0.306, the BTI noise will increase using thewavy leading edge with the amplitude smaller than 0.15c.
It is noteworthy that when the Strauhal number is 2 times the Karman vortex street frequency(St=0.306), the BTI noise reduction using wavy leading edge configurations with different wave
amplitudes at different airflow velocities is all close to zero. This shows that the noise reduction in the
low frequency range is independent of the wave amplitude, but only related to the wavelength.
3.3 Effect of wavelength on the BTI noise reduction
In Figure 6(b) shows the OASPL reduction of BTI noise using WLE at different flow velocities andat same wave amplitude of 0.1c but with different wavelength. The OASPL in fig. 6(b) is computedfrom St=0.2 to St=1.5. As can be seen from the figure, when the wavelength is l arge, W/c=0.2, theWLE configuration can’t effectively reduce the OASPL of BTI noise in the experimental range of
airflow velocity, and with the WLE at W/c=0.2, the BTI noise will increase with the using WLE
configuration. This shows that noise reduction can’t be achieved using WLE configuration with thelarger wavelength. At the same airflow velocity, the smaller the wavelength, the larger the OASPL
reduction of BTI noise. It is worth noting that with the same wavelength, the higher the airflow
velocity, the greater the noise reduction.

(a) with same wavelength(b) with same wave amplitudeFig. 6 The reduction of the OASPL of the BTI noise with wavy leading edge
3.4 Effect of inflow speed on the BTI noise reduction
In fig. 7 shows the OASPL reduction of BTI noise with different amplitudes and wavelengths at
different airflow velocity. It can be seen from fig. 7 that in the experimental range of airflow velocity,when the airflow velocity is of 30m/s, the OASPL reduction is small with WLE configuration. Whenthe airflow velocity is of 40m/s, the OASPL reduction is obviously increased. However, with thefurther increasing of airflow velocity, the OASPL reduction of BTI noise does not change much. It
could be also obviously seen from fig. 15 that WLE configuration of swept blade will not reduce the
BTI noise if the ratio of amplitude to wavelength of the wavy leading edge is too small(0.5).

4. CONCLUSIONS(1) Wavy leading edge will reduce the BTI noise of swept blade in low-frequency range, however,
WLE will increase BTI noise in high-frequency range. Within the range of experimental parameters,
not all wave leading edges can reduce BTI noise of the swept blades. If the wave amplitude is too smallor the wave length is too large, the OASPL of the BTI noise will be increased.
(2)The amplitude and wavelength of wavy leading edge significantly affect the noise reduction
spectrum. The wavelength of WLE will also affect the frequency range of noise reduction. The larger
the wave amplitude of WLE, the larger the noise reduction in the low frequency range. The smaller the
wavelength of WLE, the larger the frequency range of BTI noise reduction.
(3) The larger of the wave amplitude of wavy leading edge, the larger of the OASPL reduction of
BTI noise. The smaller of the wavelength of WLE, the larger of the OASPL reduction of BTI noise.

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However, in the case of fixed wave amplitude, there will be a minimum wavelength limit, when the
wavelength is less than this value, further reducing the wavelength will have no effect on noise
reduction.
(4) The spectra of BTI noise reduction with Strauhal number is almost the same for same WLE
under different airflow velocities. For the WLE with the same wavelength of 0.1c, the dimensionless
Strauhal number at that the BTI noise reduction is zero will keep as a constant of 0.306 under different
airflow velocity. However, for the WLE with the same wave amplitude, the dimensionless Strauhal
number at that the BTI noise reduction is zero will be changed with the chang ing of the wave length.
The smaller the wavelength is, the larger the Strauhal number with zero noise reduction is, and it
gradually increases from 0.216 to 0.508 when wave length increase from 5mm to 20mm.

(a) WLE with same wavelength(b) WLE with same wave amplitudeFig. 7 The reduction of the OASPL of the BTI noise with wavy leading edge at different inflow speed

ACKNOWLEDGEMENTS
The present work is supported by the National Natural Science Foundation of China (Grant No. 51776174),
the National Special Research Project in Aeroengine in China.

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