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23I SVC23rd International Congress on Sound & Vibration

10-14 July 2016Athens, Greece

MODAL ANALYSIS OF AN AIRCRAFT ENGINE FAN NOISE

Natalia Gorodkova , Valeriy Chursin, Yuliy Bersenev and Ruslan Burdakov

AVIADVIGATEL joint stock company, 614990, Komsomolskiy pr. 93, Perm, Russia,

Laboratory of noise generation mechanisms and modal analisys, Perm National Research Poly-

technic University 614990, Komsomolskiy pr. 29, Perm, Russia

e-mail: chursin@avid.ru

The fan is one of the main noise sources of an aircraft engine. To reduce fan noise and provide

liner optimization in the inlet it is necessary to research modal structure of the fan noise. The

present paper contains results of acoustic tests on installation for mode generation that consists

of 34-channel generator and the inlet updated for mounting of 100 microphones, the experi-

ments were provided in new anechoic chamber of Perm National Research Polytechnic Univer-

sity, the engine with the same inlet was also tested in the open test bench conditions, and results

of the fan noise modal structure are presented. For modal structure educting, all 100 channels

were synchronously registered in a given frequency range. The measured data were analyzed

with PULSE analyzer using fast Fourier transform with a frequency resolution 8..16 Hz. Single

modes with numbers from 0 to 35 at frequencies 500; 630; 800; 1000; 1250; 1600 Hz and dif-

ferent combinations of modes at frequencies 1000, 1600, 2000, 2500 Hz were set during tests.

Modes with small enough numbers are generated well on the laboratory installation, high-

number modes generate additional modes caused by a complicated interference pattern of sound

field in the inlet. Open test bench results showed that there are also a lot of harmonic compo-

nents at frequencies lower than fan BPF. Under 0.65 of cut off there is only one distinct mode,

other modes with close and less numbers appear from 0.7 of cut off and above. At power re-

gimes 0.76 and 0.94 of cut off the highest mode also changes from positive to negative mode

number area. Numbers of the highest modes change smoothly enough with the growth of power

regime. At power regimes with Mach>1 (0.7 of cut off and above) on circumference of blade

wheel there is a well-defined noise of shock waves at rotor frequency harmonics that appears at

the range between the first rotor frequency and fan blade passing frequency (BPF). It is planned

to continue researching of sound field modal structure with acoustic measurements in near and

far field.

1. Introduction

One of the main ways of fan noise suppression is lining the channels of aircraft engine. Liners

should be tuned in a manner to provide the maximum of sound absorption while sound is propagat-

ing in a channel, especially on tonal noise frequencies.

The sound field in a channel could be described by it’s decomposition into azimuthal and radial

modes. Thus, solution of the problem of sound field modal structure allows to determine liner’s

impedance that guarantees the maximum of sound absorption. It gives the possibility to construct

liners with necessary properties for a given engine.

The current paper contains description of preparations and tests for metering modal structure of

the aircraft engine fan noise realized on installation in an anechoic chamber, development of sound

filed modal analysis techniques and the program of testing without and with flow as a part of a full-

scale engine.

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2. Noise modal structure in laboratory conditions. Development of analysis techniques of noise emitted from the inlet of aircraft en-gine to the front semisphere

For a better tuning of liners mounted in the inlet of engine on its resonance frequencies it is nec-

essary to investigate the modal structure of the fan noise. In the context of tests preparation for de-

termining noise modal structure of the full-scale next generation engine at the open test bench con-

ditions and of making research of its measurement technique, the installation constructed from 34-

channel sound generator and the inlet with a stationary circular microphone array was built-up.

Similar to such measurements were conducted earlier using a rotating microphone rake [1]. The

verification of technique was made using Matlab in an anechoic chamber.

2.1 Equipment

The inlet of aircraft engine was used as a short cylindrical duct. The full-scale inlet was used due

to the difficulty of scaling caused by conjoint sizes of microphones and dimensions of compression

driver JBL 2451H. For placing heavy inlet duct from one side and the system of sound generation

from the other side the 4-sheet veneer platform was manufactured. Location of 34 compression

drivers was chosen uniform on circumference 1820 mm in diameter. The platform has holes 35 mm

in diameter, compression drivers JBL 2451H are established under the platform in alignment with

holes. The sound sources are considered to be points with such a ratio between the hole diameter for

the output of the dynamic transducers and inlet duct diameter. The signal is given by 34 independ-

ent generators of signal analyser PULSE (Bruel & Kjaer). The circular microphone array is placed

760 mm above the platform with sound sources (see Fig. 1).

Figure 1: Scheme of installation for rotating modes generation.

1 – duct for modes propagation; 2 – platform; 3 – sound generator; 4 – circular microphone array; 5 – cali-

brating microphone; 6 – holes for the system of sound sources.

The view of inlet duct in an anechoic chamber and the scheme of microphones positions recom-

mended by Bruel & Kjaer engineers [2, 3] are presented on Fig. 2.

Acoustic sources JBL 2451Н allow to generate the sound source at frequencies up to 20 kHz

with sound pressure levels to a maximum of 130 dB. Signal generation is performed in a specific

software PULSE Labshop. The signal is amplified through 2-channel power amplifiers 2716 and

sets to acoustic sources. Obtained results are programmed captured via digital recorder integrated

with PULSE Labshop. Measured data then are getting off in the form of a Table to Microsoft Excel

using PULSE Reflex software, and contain signal phase and amplitude values for every measuring

channel.

1

2

3 6

4 5

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Figure 2: Installation for modes generation in an anechoic chamber and circular microphone array.

1 – experimental inlet; 2 – lining of anechoic chamber.

2.2 Modal analysis technique using a circular microphone array

Mode generation at a given frequency is realized by setting the amplitude and initial phase at

every generator. Phases and amplitudes calculation is performed by Eq. 4 and loads to all 34 gen-

erators settings using script in PULSE analyzer. Such 34-channel generator allows setting different

combinations up to 4 optional modes.

The circular 100-microphone array is used to evaluate modes amplitudes.

For calculation facility let’s take � � 0 at the center of circular array and � � �� as a radius. ��

mean the angular coordinates of microphones. Let’s expand the pressure at measuring points of a

ring in a Fourier series:

�, �, �, �� � ∑ ��, �, �, ����������

���� (1)

where ω� � 2πf�, f� ��

, Hz;

T – time realization length of a signal, s.

Let’s consider further only independent harmonic component number k and for the sake of con-

venience define P�0, r�, θ%, ω�� as Pθ%�. Frequency response between sound pressure signals at microphones №1 and №n is using to

evaluate complex coefficient Pθ%�:

� �� � '(��

�)(�

)(( (2)

where G+% – average cross spectrum of microphones №1 and №n signals,

G++ – average autospectrum of microphone №1 signal.

Expanding Pθ%� in a Fourier series of circumferential harmonics, one gets:

� �� � ∑ �,���, ���

,��� (3)

The sound field modal structure will be then a set of coefficientsP-.

To calculate them the following equation could be used [4]:

�, �(

.∑ � ���

�, �.��( (4)

where N – number of microphones,

m – mode number 0, ±1, ±2, …±100.

Acoustic characteristics where measured at fixed frequencies in a given range generating modes

with numbers from 0 to 35.

The continuous 30 seconds recording of sound pressure was registered for every sound power.

Also theoretical BPF and fixed in the spectrum where obtained for every power regime.

1

2

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In result of measurements and calculations in Matlab amplitudes and phases of modes from -100

to 100 were evaluated and the dominate mode was determined at every frequency. Then changing

the frequency one looks after changing of numbers of dominate modes.

2.3 Test results

In the process of testing the single modes from 0 to 35 were setting at frequencies 500; 630; 800;

1000; 1250; 1600 Hz and different modes combinations at frequencies 1000, 1600, 2000, 2500 Hz.

Results of modes measurements are partly presented on Fig. 3-4. Figures show that low number

modes are well generated, while when trying to generate high number modes, some additional

modes appear that could be caused by defects of modes generator manufacture and by position of

the noise generators exit ports centers that are placed 60 mm from the wall of the inlet duct, that

could produce a complicate interference pattern of the sound field in the inlet.

Figure 3: Dependence of mode amplitude on mode number. Frequency 1000 Hz, generation of mode

№ 3(left) and 9 (right).

Figure 4: Dependence of mode amplitude on mode number. Frequency 1000 Hz, generation of mode

№ 12(left) and 32 (right).

It is necessary to optimize the positions of sound sources when modernizing the installation for

testing of liners with different modal structure of sound field. In general 34-channel modes genera-

tor allows to conduct tests of fan noise modal structure measuring technique. In result of executed

work it was recommended to make experiments using full-scale engine for testing derived tech-

nique.

3. Modal analysis of the full-scale engine fan noise at the open test bench

To prepare better for the testing of a new-generation engine and defining it’s fan noise modal

structure first the usable engine with the proved in anechoic chamber inlet and 100-microphones

array was checked at the open test bench.

f, Hz

A

f, Hz

A

f, Hz

A

f, Hz

A

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3.1 Equipment and measurements technique

The noise measurements were conducted using the system of control and registration signals

formed on the base of a 39-channel signal analyzer PULSE of Bruel & Kjaer.

The noise levels, atmospheric conditions and engine parameters were registered in the process of

testing. Required conditions were:

• The high of engine position is 5 m above the surface

• No atmospheric precipitations

• Wind speed at the engine axis level – no more than 5 m/s

• Noise measurements are taken in 100 points of microphones positions on the fan circlet

simultaneously (Fig. 5).

Figure 5: Engine with the inlet equipped by 100 microphones at the open test bench.

Signal synchronized registration was made at all 100 microphones at frequency range from 20 to

12800Hz. 3 thrust curves were obtained during measurements. Signals were measured during 40 s

twice for every power regime, the intermediate regimes were also been registered.

3.2 Test results

Sound pressure spectrum measured by the 1 microphone at maximum engine power is shown at

Fig. 6. One can see that there are many harmonic components under the BPF.

At power regimes with Mach number M>1 at the periphery of blade wheel there is a well-

observed shock wave noise appeared on harmonics of rotor frequency that could be seen at fre-

quency range between the first rotor frequency and fan BPF. Shock wave noise has a high sound

power level due to the used lemniscate inlet duct without lining and waisting.

It is recommended to use the inlet with waisting and adapter further to decrease shock wave

noise appearing during measurements.

Sound pressure spectra for the maximum engine power with time averaging 30 and 3 seconds are

shown at Fig. 6-7.

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Figure 6: Sound pressure spectrum. Maximum engine power. Time averaging 30 s.

Increasing of time averaging leads to increasing of fan BPF and fan revolutions harmonics peaks

breadth caused by the fan engine revolutions instability due to wind speed changes at the open test

bench.

Figure 7: Sound pressure spectrum. Maximum engine power. Time averaging 3 s.

Modal structure on fan BPF at 0,65; 0,7 regimes of cut off and at maximum power is presented

on Fig. 8-9. At power regimes below 0,65 of cut off the one single mode is dominate. After this,

from 0,7 of cut off and higher accompany modes with close but less numbers appear. At higher

power regimes there is a change of mode’s number sign from negative to positive.

5 dB

400 Hz

L, dB

f, Hz

Fan BPF

5 dB

400 Hz

L, dB

f, Hz

Fan BPF

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Figure8: Modal structure at BPF at engine powers 0,65 and 0,7 nominal.

Figure 9: Modal structure at BPF at maximum engine power

Fig. 10 demonstrates the dependence of the highest mode number on fan BPF from the fan BPF

while changing the power.

Figure 10: Dependence of maximum amplitude mode number on fan BPF.

It should be noted that the sign of highest mode number changeover from negative to positive

happens at different engine powers for different thrust curves, from 0,76 to 0,94 of cut off. The

changing of highest modes numbers goes smooth enough with changing the power.

Numbers of circular modes in general don’t correspond to the numbers of modes appeared from

rotor-stator interaction due to the Tyler-Sofrin theory and, probable, are determined by input flow

distortion and fan interaction.

N

A

N

A

N

A

f, Hz

N

100 Hz

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4. Conclusions

The installation constructed from 34-channel sound generator and the inlet with a stationary cir-

cular microphone array was built-up for a current research. By means of it:

• Inlet duct sound field is measured in an anechoic chamber and the modal structure calcula-

tion technique is tested in Matlab.

• It is found out that such technique generates low-number modes very well. At high mode

numbers additional modes generates due to complicated interference pattern of sound field

in the inlet.

• Based on the results of measurements it is recommended to optimize the sound sources posi-

tions in modernization the installation of modes generation to test liners with different modal

structured sound source.

Fan noise measurements as a part of the full-scale engine were conducted at the open test bench:

• Required data were obtained for fan noise modal analysis.

• It is found that at power 0,65 of cut off regime and less there is only one dominated mode.

Accompany modes with close but less numbers appear above the 0,7 of cut off. There is a

changeover of the sign of the highest mode number from negative to positive at different en-

gine powers for different thrust curves, from 0,76 to 0,94 of cut off.

• Obtained results shows that for the future testing of a new-generation engine it is necessary

to use the inlet duct with waisting and adapter with ICD to decrease shock wave noise ap-

pearing during measurements and smoothing the input flow.

Presented technique for modal analysis of a fan noise will be used in the new-generation aircraft

engine testing and in the researches of liners improvements used in the engine inlet.

It is planned to continue investigation of sound field axis and radial modes structure with acous-

tic measurements in near and far field.

REFERENCES

1 D.L. Sutliff, Turbofan duct mode measurements using a continuously rotating microphone rake, Inter-

national Journal of Aeroacoustics 6, 147-170, (2007).

2 Jorgen Hald, Ring array design, operating and developing manual, Bruel & Kjaer, (2014).

3 Robert P. Dougherty, Jeff M. Mendoza, “Nacelle In-duct Beamforming using Modal Steering Vectors”,

AIAA Paper 2008-2812, (2008)

4 P. Sijtsma, J. Zillmann, “In-Duct and Far-Field Mode Detection Techniques for Engine Exhaust Noise

Measurements”, AIAA Paper 2007-3439, (2007).