Noise Control and Noise Evaluation in Aircraft Engines

Noise Control and Noise Evaluation in Aircraft Engines
77
Noise Control and Noise Evaluation in Aircraft Engines
Tatsuya ISHII, Aircraft Propulsion Research Center, E-mail: [email protected]
Keywords: Noise reduction, active noise cancellation, turbo-machinery noise, source location.
1. Introduction
Aircraft engine noise emitted for example by
the jet exhaust, fan, compressor, turbine and
combustor is the predominant factor in total
aircraft noise during take-off and landing. As a
result of enormous efforts to alleviate engine
noise, noise levels have been improved by more
than 20 dB compared to the first generation of
airliners. However, the growing volume of air
transport means that further noise reduction is
still required. With this background, we decided
to concentrate on two technical aspects of
fulfilling this demand for further noise reduction.
One is active noise cancellation technology (ANC)
which has become more probable noise -proofing
treatment in the last two decades. In comparison
with passive methods, ANC has the potential to
optimize reduction performance by adapting to
the noise source. Fundamental research on ANC
started around ten years ago and demonstrated
noise reduction with a ducted fan. Application of
ANC to existing turbo-machines is now the next
theme. Another technical aspect of noise
reduction is noise measurement technology
which would provide precise information on both
acoustic modal components and source locations.
Experimental procedure based on microphone
measurement is studied as the measurement
technology. This report summarizes recent ANC
and new noise measurement research work.
2. Active Noise Cancellation
The active cancellation of spinning acoustic
modes, if appropriately combined with existing
devices, contributes to reducing the size and
weight of silencers, improving reduction
efficiency and cutting total costs. The practical
application of ANC to turbo-machinery noise is
illustrated in Fig.1. We have already completed
fundamental experiments with a ducted fan,
aimed at verifying the modal cancellation using
secondary modes. Several technical concerns
have been investigated; measurement of the
spinning modes, anti-phased acoustic mode
generation
and
piezoelectric
devices
for
secondary sources and error sensors. Our
experimental results have shown that the
anti-phased mode, which is driven by 16
loudspeakers, suppresses the primary mode by
more than 5 dB with little spillover of secondary
sound. The piezoelectric devices, used in sound
[Passive Noise Control]
Acoustic lining
[Active Noise Control]
Error sensor Anti-phased sound
source
Spinning modes
D ucted fan
(Noise Source)
Attenuated modes
Fig.1: Application of ANC to a turbo-machine.
error microphone
acoustic lining
small jet engine
loudspeaker
PZT driver
Fig.2: Experimental setup with a small jet engine.
sources and error sensors, provided as much
noise reduction as that offered by the
loudspeakers and microphones. In the next phase
of putting this noise cancellation technology into
practice, a small jet engine was employed as a
noise source with a higher noise level and
frequency. Sound pressure levels and frequency
band were investigated to design noise control
devices as shown in Fig.2. In front of the engine
inlet,
8
loudspeakers
together
with
8
piezoelectric sound sources are installed on a
duct.
3. Noise Measurement
This document is provided by JAXA.
78
Experimental technique for modal detection
and source location is a powerful tool in
forecasting precisely the characteristics of noise
and allows countermeasures to be taken with
less time and cost. In 2001, development on th e
simultaneous multiple-channel measurement
and data analysis was carried out through two
measurement programs. One measurement was
taken using a YJ69 turbojet engine. A set o f
microphones was fixed at around 3 m from the
nozzle exit. An example of this measurement is
plotted in the contour map in Fig.3. The
concerning noise is jet noise at a relatively low
frequency, and the operating condition was 50 %
- rating. Although there were certain limitations
to accuracy due to the number and position of
microphones, the noise source was roughly
determined just downstream of the nozzle exit.
Similar results were obtained under other
conditions. The other measurement is a wind
tunnel test with a scaled fuselage model coupled
to a small jet engine. A 32 -microphone section is
mounted about 1.5 m below the fuselage and
engine model as shown in Fig.4. An example of
jet noise measurement is plotted in Fig.5. The
rotation speed of the small jet engine is 110,000
rpm. The mean flow rate around the model and
the attack angle of the model are 40 m/s and 0
deg respectively. A noise source of around 3260
Hz was detected downstream of the engine
nozzle. The compressor noise of the engine, at a
higher frequency band, is also successfully
detected immediately upstream of the engine.
Fig.3: Example of measurement with the YJ69.
Microphone
Small jet engine
Fig.4: Microphones and a fuselage model combined
with a small jet engine.
Array-Type-1(G),
N=110,000rpm, V_LWT=40m/s, fc=3260Hz
0.5
References
1. T. Ishii, et. al, “Modal Technique for Active
Control of Tones Radiated from a Ducted Fan”,
Journal of Aircraft, 1998, 35-5, pp.754-760.
2. T. Ishii et al. “Active Control of Spinning
Modes Caused by a Ducted Fan”, Trans. JSME,
2000, B66-641, pp.133-140.
3. T. Ishii, K. Nagai, H. Oinuma, and K. Takeda,
“Research on Control and Evaluation of Noise
Radiated from Aircraft Engines”, NAL -SP -52,
2001, pp.59-62
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
X (m)
.
Fig.5: Example of measurement with the fuselage and
jet engine model.
This document is provided by JAXA.
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