AIAA 2010-3750
16th AIAA/CEAS Aeroacoustics Conference
Aerodynamic/Aeroacoustic testing in
Anechoic Closed Test Sections of Low-speed Wind Tunnels
Takeshi ITO 1 , Hiroki URA 2 , Kazuyuki NAKAKITA 3 , Yuzuru YOKOKAWA 4
Japan Aerospace Exploration Agency (JAXA), 7-44-1, Jindaiji-higashi, Chofu, Tokyo, 182-8522, JAPAN
Wing F. NG 5 , Ricardo A. BURDISSO 6
Virginia Tech (VT), Blacksburg VA 24061, USA
and
Akihito IWASAKI 7 , Toshimi FUJITA7 Norihisa ANDO 8 , Naoki Shimada8, Kazuomi YAMAMOTO 9
JAXA, 7-44-1, Jindaiji-higashi, Chofu, Tokyo, 182-8522, JAPAN
This paper describes new anechoic closed test sections in Japan Aerospace Exploration
Agency (JAXA) and Virginia Tech (VT) not only for an aeroacoustic but also for an
aerodynamic testing ability in Low-speed wind tunnels. The anechoic closed test section
with Kevlar wall is an innovative concept and had been originally developed at VT. It was
later applied to JAXA’s 2m x 2m wind tunnel. By using a high-lift device model, the JAXA's
new test section was evaluated and validated acoustically by comparing to VT anechoic test
results. Moreover, the aerodynamic characteristics in the new test section were also
evaluated by comparing to results of the same model in JAXA's closed hard-wall test section.
New wall interference correction procedure is proposed for the Kevlar wall test section, and
it showed very good agreement with well-known corrected hard-wall results. This anechoic
test section is useful and a promising tool for both aerodynamic and aeroacoustic testing.
D
I. Introduction
URING approach and landing phase of aircraft flying near airports, its airframe noises generated by flaps, slats
and landing gears are recognized as the significant contributor to the acoustic emissions of a civil aircraft, in
particular due to recent effort in the reduction of engine noise.[1,2] To evaluate the aeroacoustic noise, measurement
techniques by using anechoic low-noise wind tunnels have been developed Recently, great attention has been paid
to microphone phased array measurement in closed test section of aerodynamic wind tunnels, which can measure
aeroacoustic data in the same condition as aerodynamic force measurement.[3-6] In JAXA, a microphone phasedarray system has been developed and used in the closed test sections of the JAXA’s 2m x 2m Low-speed Wind
Tunnel (LWT2) as well as in the 6.5m x 5.5m Low-speed Wind Tunnel (LWT1).[7,8] Using these wind tunnels,
research and development for next generation of civil transport aircraft has been started in Japan. In this project,
aerodynamic investigations were the main objectives,[9-10] but aeroacoustic performance in high-lift system and
landing gear is also one of the most important items. Its investigation for half-span high-lift device model (OTOMO)
has been carried out by using microphone phased array measurements from the sidewall of the JAXA’s LWT2
1
Manager for Low-Speed Wind Tunnel, Wind Tunnel Technology Center, Senior Member AIAA
Researcher, Wind Tunnel Technology Center, Aerospace Research Directorate, Member AIAA
3
Associate Senior Researcher, Wind Tunnel Technology Center, Member AIAA
4
Associate Senior Researcher, Civil Transportation Team, Member AIAA
5
Endowed Professor, Mechanical Engineering Department, Associate Fellow AIAA
6
Professor, Mechanical Engineering Department, member AIAA
7
Associate Senior Researcher, Wind Tunnel Technology Center
8
Engineer, Wind Tunnel Technology Center
9
Manager for Aerodynamic Design, Civil Transportation Team, Senior Member AIAA
1
American Institute of Aeronautics and Astronautics
2
Copyright © 2010 by T.Ito and W.Ng. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
tunnel. Sound pressure measurements of same model were also made in the open-jet test section of the Large-scale
Low-noise Wind Tunnel of the Railway Technical Research Institute in Japan, and successfully completed.[11-14]
Moreover, aeroacoustic research in aerodynamic wind tunnel has to be conducted through improvement to obtain
more accurate noise measurement. To reduce background noise of hard wall aerodynamic wind tunnel, a new design
concept of an anechoic test section was recently proposed and demonstrated.[15] This innovative and useful approach
was originally applied to the Stability wind tunnel at Virginia Tech. The anechoic test section concept consists of
acoustically treating the ceiling and floor of the test section while the hard side walls are replaced by stretched
Kevlar cloth that is acoustically transparent while containing the flow in the test section. In addition, anechoic
chambers are attached behind the Kevlar walls allowing aerodynamic noise measurements through the Kevlar cloth.
This test section concept was also applied to the 2m x 2m Low-speed wind tunnel at JAXA (LWT2), and shakedown
tests and initial operation have been successfully completed.
The high-lift device model (OTOMO) was tested in this new anechoic tunnel configuration (LWT2-AC). In this
series of testing, aerodynamic noise data from this high-lift device were clearly observed by noise visualization
technique using microphone phased arrays. In this paper, its advantages are shown by comparison to hard-wall
closed test section data. Moreover, the Kevlar anechoic test section concept has remarkable advantages in
minimizing wall interference effects. Correction was not needed for drag coefficient (CD), and adequate correction
of lift coefficient (CL) by using static pressure in anechoic chamber was proposed. The anechoic closed test section
is very useful and a promising facility for aerodynamic and aeroacoustic evaluation of airframe aerodynamics and
noise.
II. Experimental Setup
A. JAXA 2m x 2m Low-speed Wind Tunnel and Hard-Wall closed test section (LWT2-HW)
The JAXA 2m x 2m Low-speed Wind Tunnel
(LWT2: Fig.1) is a closed circuit type and
conventional wind tunnel built in 1971. Its circuit
length is 96 m, and the LWT2 originally has HardWall test section (LWT2-HW) of 2m x 2m crosssection and length of 4 m. In addition, this wind
tunnel has characteristics of maximum wind speed
of 67 m/s and relatively low turbulent level of
0.06 %. A testing model is supported by strut from a
lower wall. 6-component forces can be measured
with pyramid type balance changing the angle of
attack and side slip angle. Models can be also
supported by sting with robot, and aerodynamic
Fig. 1: JAXA 2m x 2m Low-speed Wind Tunnel (LWT2)
forces are able to be measured using internal force
balance. Half-span model on the lower wall are
sometimes used on the external-type 4-component force
balance. Due to its characteristics of low turbulence and
Microphone array
relatively low noise and also appropriate test section size,
this tunnel has been used for a variety of tests from basic
aerodynamic research, flutter, boundary-layer control,
1m
powered-lift STOL aircraft tests, and so on. This tunnel has
also a gust-wind test cart in which added load and flight
1.35m
movement in a gust-wind force can be measured. Recently,
high-lift device (HLD) research is one of the most important
purposes in those wind tunnels to develop high-performance
HLD for civil aircraft development.
B. High-lift device model: OTOMO
1.65m
In JAXA, research for airframe noise from high-lift
device (HLD) has been started to obtain their design
approach to achieve both low noise and aerodynamic high
Fig. 2: High Lift Device Model OTOMO in
performance. To reduce airframe noise from high
hard wall test section of LWT2-HW
performance HLD, basic characteristics of aerodynamics and
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aeroacousitcs have to be understood in detail. For this research, aeroacoustic noise and aerodynamic characteristics
have to be measured simultaneously. A simplified three-element wing model OTOMO was able to be investigated
in wind tunnels using aeroacousitc and aerodynamic measurements in order to better understand relationship
between airframe noise and the phenomena of flow field around HLD.[11-13]
Figure 2 shows the model for aeroacoustic research of high-lift device (OTOMO) in LWT2. This model was
tested to clarify basic phenomena of noise generation at wing tip, flap tip, and slat including its cove, and to reduce
those sources by applying modification of the configuration. The model had 0.6 m in chord length, 1.4 m in
wingspan, no sweep-back angle, no taper, and no dihedral angle. There were full-span leading-edge slat, and 70 %span single-slotted flap at trailing-edge. The model had 189 static pressure taps. In LWT2, this model was placed on
external-type 4-component force balance, and aerodynamic force was measured. In VT-Anechoic Wind Tunnel
(AWT), static pressure and acoustic measurements with phased array were conducted.
2m x 2m Kevlar-wall
C. Anechoic Closed Test Section at JAXA (LWT2-AC)
Anechoic Closed test section
In order to measure aeroacoustic noises in wind tunnels,
acoustic access to the model in the test section is needed.
Although in-flow measurement is sometimes applied to wind
Sub Anechoic
Chamber
tunnel testing, aerodynamic noise at the microphone itself
Main Anechoic
should become problem for the measurement and affect on
Chamber Noise
aerodynamics of the model in near field measurement. Then,
the other solution is acoustic measurement from outside of
the flow at the test section. Wind tunnels for aeroacousitcs
have open-jet type test section to measure aeroacoustic
noises from outside of the test section without wall, and the
test section is located in a large anechoic chamber. Along
Front view (cross section)
this idea, the specialized wind tunnels have been facilitated.
In Japan, many ‘aeroacoustic testing’ has been applied to the
Kevlar Wall
field of noise research of train and car, historically.
However, the flow field of open-jet test section is
Test Section
different from the closed test section because of the shear
Microphone Array
layer around the uniform flow and flow deflection of the
flow. Actually, many wind tunnels for ‘aerodynamic testing’
have wall surface in the test section to keep uniform low3m
turbulent flow. And, many experiences of aerodynamic
1m
testing have been accumulated in the closed test section in
which many aerodynamic data of aircraft have been obtained
Sub Anechoic
Main
and aerodynamic correction method had been established.
Chamber
Flow
Chamber
A closed wind tunnel is important for aerodynamic
Plane view
measurement to obtain aerodynamic force, especially lift
coefficient of high lift devices without deflection of the flow
Fig. 3: Schematic of new Anechoic
due in the open-jet test section. Moreover, low turbulent flow
Closed test section of LWT2-AC
without shear layer and noise occurrence at the jet-catcher is
needed for aeroacoustic measurement. Problem is wall
surface of the closed test section.
If the problem is solved, the microphone array
specialized aeroacoustic testing in
the specialized facilities would
not be needed, and merits on the
cost and schedule are very large.
3m
To remain the useful effect of
the wall for aerodynamic testing,
but to be free from the blockage
of the wall acoustically, the
innovative concept was originally
Kevlar wall test section
Anechoic chamber
used in the Virginia Tech wind
tunnel, as shown later, and for
Fig. 4: Microphone array and HLD model OTOMO
JAXA-LWT2 it is also applied as
in Anechoic Closed test section of LWT2-AC
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Anechoic Closed test section (LWT2-AC), as
shown in Fig. 3. The concept consists of side walls
made by Kevlar cloths with tension which are
transparent acoustically, but plays a role like hard
walls aerodynamically. Anechoic chambers are
attached in both side of Kevlar wall of test section,
and acoustic measurement can be conducted from
the anechoic chambers. Test section also has
anechoic ceiling and floor with perforated metal
sheets covered with Kevlar sheets with acoustic
foam behind the wall for capability of anechoic
chamber. The test section has a main anechoic
chamber on the one side and a sub anechoic
chamber on the other side, but the difference
between them was acoustically very small. Here,
acoustic measurement was basically carried out
from the main anechoic chamber because of the
space for measurement setting. Acoustic
characteristics of the main chamber had been
evaluated by obtaining calibration data, and we
confirmed that it worked as anechoic chamber.
Here, this new Anechoic Closed test section in
LWT2 (LWT2-AC) was applied to OTOMO
model testing, and aerodynamic and aeroacoustic
measurements were conducted as shown in Fig. 4.
Fig. 5a: Stability wind tunnel at Virginia Tech.
183mm
183mm
Anechoic Chamber
Test section
Anechoic Chamber
Fig. 5b: Anechoic closed test section of VT-AWT
D. Virginia Tech Anechoic Wind tunnel (VTKevlar
AWT)[15]
Closed Anechoic wind tunnel of Virginia Tech
was the first facility in the world using Kevlar wall
anechoic closed test section. In this wind tunnel,
we also obtained aerodynamic and aerocoustic
data of OTOMO model to compare and evaluated
the JAXA's new wind tunnel characteristics. Here,
5m
some descriptions about the wind tunnel in
Fig.5c: Microphone array and HLD model OTOMO of
Virginia Tech are shown.
Anechoic Closed test section of VT-AWT
The Virginia Tech Stability Wind Tunnel is a
continuous, single return, subsonic wind tunnel
with a 7.3-m long removable test section, with a square cross section 1.83m on edge. Figure 5 is a schematic of the
general layout. Maximum speed of the wind tunnel in the test section is about 80 m/s for a Reynolds number per
meter up to about 5,300,000. Flow in the tunnel is driven by a fan provided by 8 Clark Y section blades and 53
stator vanes. The wind tunnel has some acoustic treatment for tunnel duct, fan, and so on as low-noise wind tunnel,
and acoustic measurement can be realized in relatively lower background noise.
This wind tunnel has an anechoic closed test section. JAXA's LWT2-AC was constructed by using the
innovative concept of the test section of Virginia Tech wind tunnel, that is, this basic design of the Anechoic Wind
Tunnel of Virginia Tech (VT-AWT) is similar to the LWT2-AC. The sized of cross section of the test section of
1.83m x 1.83m is a little less than LWT2-AC, but the length of 7.3 m is longer than LWT2-AC. The size of both
side anechoic chambers is the same, and acoustic measurement can be conducted from both sides. In this wind
tunnel, many kinds of tests such as wing section model, landing gear model, and OTOMO model have been
conducted.
E. Microphone Phased Array System
In LWT2, we originally put phased-array microphones of 1/4-inch on the side wall or ceiling of the hard-wall
closed test section. Noise source visualization was realized with delay-and-sum beamforming method for measured
data of phased-array microphones to reduce background noise of aerodynamic wind tunnel and reflection in the
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American Institute of Aeronautics and Astronautics
closed test section.[16-17] This microphone array was also
applied to the new anechoic test section. To this end, the
array was placed in the anechoic chamber under the
condition of less background noise and reflection.
As shown in Fig.6, our phased-array microphone
system consisted of 48 B&K microphones type 4951, 3
units of conditioning amplifier (B&K type 2694A), A/D
converter (NI PXI-4462) and PC (NI 8351). The
microphones have diameter of 7 mm, a frequency range
of 10 Hz to 20 kHz, and their dynamic range of 30 dB to
140 dB. Conditioning amplifier has frequency range of
0.1 Hz to 50 kHz and amplifier gain of -10 to 40 dB. The
A/D converters have 24 bit resolution and maximum
sampling rate of 204.8 kSamples/s. Each microphone
was calibrated by piston phone (B&K type 4228) before
airframe noise measurements were conducted. In this
research, noise measurement data were processed by
using sensitivities of microphones which were obtained
by the calibration.
In the VT-AWT, acoustic data was collected using a
63-microphone microphone phased array. The array is
shown in Figure 7. This array has a star configuration
consisting of 7 arms with 9 microphones per arm. The
array center body is provided with a laser pointer that
projects a laser dot along a line perpendicular to the
array plane passing through the array origin. This laser
pointer was used for alignment purposes.
The microphone phased array was installed in the
anechoic chamber contiguous to the test section on the
pressure side of the model; whereas the model was
mounted at the center of the test section as shown in
Figure 5.
Flow
Microphone
array
Signal Conditioner
PC
Noise
Kevlar wall
Side wall
A/D
1m
Fig. 6: Acquisition System and
Multi-spiral Microphone Array in LWT2
1.5m
III. Measurement results and discussions
Fig. 7: Microphone Array in VT-AWT
A. Acoustic measurement
Noise source visualization using the microphones
phased arrays were obtained for the high-lift device model OTOMO. This model has flap and slat, and those devices
can generate aerodynamic noises. Measured results are shown in Fig. 8. In all three facilities, OTOMO’s basic
characteristics of noise generation were adequately measured. At low frequency of 1 kHz, slat noise was remarkable,
but it was not so-called slat noise, which was 2-dimensional noise caused at slat cove, but the slat track seemed to
make strong noise. From 2 to 8 kHz, the flap tip noise was dominant, which was caused flap tip vortex. At 16 kHz,
2-dimensional slat noise was observed. Those phenomena were reasonable considering the noise characteristics of
high lift device.
Comparing the three facilities, the hard-wall test section (LWT2-HW) showed relatively stronger side-lobe and
worse S/N ratio than other two test sections. Especially, the level of side lobes at 16 kHz was almost the same as the
noise source, so it was very difficult to evaluate this data without other information. Actually, the array was the
nearest to the model, that is, the relative size of array to the model was larger than the other testing, so the
performance at high frequency was basically worse. On the other hand, the size of the noise sources at low
frequency showed smaller than the others because of the array size and proximity. For this reason, the comparison of
noise source visualization has to be considered carefully. However, basically side-lobe levels were high in all
frequencies in the hard-wall test section. For example, noise sources near the floor were found on the LWT-HW at
low frequencies that are not observed in the other two anechoic tunnels. These are likely due to reflections and high
background noise.
On the other hand, LWT-AC and VT-AWT showed very similar results. The data were much clearer than
LWT2-HW, therefore the anechoic closed test section worked very effectively. The size of the array in the VT-AWT
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American Institute of Aeronautics and Astronautics
test was larger than in the LWT2-AC one. This results in a smaller source sizes in VT-AWT tunnel, whereas, at high
frequency of 16 kHz, side-lobe increased. The basic performances of both wind tunnels were comparable for ability
of the noise source visualization.
1.0kHz
2.0kHz
4.0kHz
8.0kHz
16.0kHz
LWT2-HW
LWT2-AC
VT-AWT
Fig.8: Noise source visualization for OTOMO model.
(U=50m/s, AoA=10deg, Landing, with wing tip fence and Droop)
95
90
Sound pressure level
Not only noise source visualization, but also noise
source level is another topic of the aeroacoutic research.
Especially, using phased array system noise level of
each noise source can be evaluated, and it should be a
valuable measurement tool. The measurement results in
VT-AWT using phased array had been validated by
comparisons to open-jet conventional anechoic wind
tunnel results. Here, LWT2-AC was compared with
VT-AWT, and its ability and performance of the noise
level is evaluated. The sound pressure level from noise
source visualization results was shown in Fig. 9.
Originally, the level of the noise source of VT-AWT
and LWT2-AC were very different by about 6dB,
which is due to the microphone array setting. The
microphones in the phased array used in LWT2-AC are
flush-mounted on a rigid plate as shown in Fig. 6. On
the other hand, the VT-AWT has no plate, e.g. the
sound just propagates through the array. To account for
85
80
75
5dB
70
65
5dB
60
LWT2-HW
LWT2-AC
55
VT-AWT
50
500
1000
2000
4000
8000
16000
Frequency (Hz)
Fig.9: Comparison of noise source level
from noise visualization map.
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the doubling of the pressure in the LWT2-AC array, the output should be divided by 2 which is the same as a 6dB
reduction. Moreover, the loss of 4dB at Kevlar sheet was also considered to compare with LWT2-HW. This value is
a little larger than measurement results of Kevlar sheet characteristics, so it might include other effects of difference
between hard wall and anechoic test section and data processing method. With these corrections, the noise levels
between the three facilities agree very well. On the other hand, it had been confirmed that the noise source level of
OTOMO measured in the VT-AWT was reasonable, and LWT2-AC showed reasonable noise source level by
considering 6dB correction for the array in this case. Although Kevlar sheet correction of 4dB need to be discussed
more, LWT2-AC can be used effectively to measure the noise source level data when we apply the normal array
system in the anechoic chamber.
(a)
Closed Anechoic
6.0 deg
-6
7.0 deg
8.0 deg
9.0 deg
LWT2_0808_cw0203_09.0deg
Hard-wall uncorrected 6.0 deg
0704HW_SN037
Closed
Anechoic(AoA=6.0deg)
(VT) 8.0 deg
LWT2_0808_cw0203_07.0deg
-5
LWT2_0808_cw0203_08.0deg
-4
VT_SN102 (AoA=8.0deg)
Cp
-3
-2
-1
0
1
2
25%
1
1.5
2
X/C
2.5
(b)
-6
LWT2_0808_cw0203_14.0
d eg14.0 deg
Closed
Anechoic
LWT2_0808_cw0203_15.0deg15.0 deg
LWT2_0808_cw0203_16deg 16.0 deg
LWT2_0808_cw0203_18deg 18.0 deg
Hard-wall
uncorrected
14.0 deg
0704HW_SN037
(AoA=14.0deg)
Closed
Anechoic
(VT) 16.0 deg
VT_SN106
(AoA=16.0deg)
-5
-4
-3
Cp
-2
-1
0
1
2
1
25%
1.5
2
X/C
2.5
Fig. 10: Surface pressure distribution on the OTOMO
model in three types of test sections
0.8
0.6
CD
B. Aerodynamic measurement
Surface pressure measurement of the OTOMO
model was conducted in the three test sections to
compare the aerodynamic flow field around the
model. The measured pressure distributions on the
model are shown in Fig. 10. Here, pressure data at
various angle of attack in the LWT2-AC tunnel
are shown, and one of these angles can be
compared with the data from LWT2-HW and VTAWT tunnels. For example, Fig. 10b showed
larger Cp on the suction side of the model at 14
deg in LWT2-AC than in LWT2-HW, and also
larger Cp in VT-AWT than in LWT2-AC at 16
deg. Larger Cp on the suction side means smaller
lift. Note that the lift coefficient measured in the
closed test section was larger than the free-stream
(actual) lift coefficient due to the flow limited by
down-wash by the wall. Therefore, it means that
smaller lift coefficient was less affected by the
wall interference. On the other hand, lift
coefficient of open-jet test section was smaller
than free-stream results, and the Kevlar wall
results was between the hard-wall closed test
section and open-jet test section as described in
reference 15. Moreover, wall interference of
LWT2-AC was larger than VT-AWT although
both test section has Kevlar wall. One of the
reasons was that amount of Kevlar wall deflection
of LWT2-AC can be smaller than VT-AWT
because the length of test section of VT-AWT is
longer than LWT2-AC. Both of the closed test
section and open-jet test section need wall
interference correction, and many types of method
of the correction have been presented and
established. However, this Kevlar wall
interference needs to be corrected using new
method and it will be shown that the correction is
very small.
Aerodynamic force measurement results are
shown in Fig. 11. Here, hard-wall results,
corrected hard-wall results, and LWT2-AC results
are shown.[18] (We were not able to obtain
aerodynamic force in VT-AWT because of the
limitation of experimental setup.) Corrected hardwall estimation showed almost free-stream results
LWT2_0808_cw0203_06.0deg
0.4
0.2
Hard wall
Hard wall Corrected
Kevlar wall
0.0
-5
0
5
10
15
AoA[deg]
20
25
30
(A) CD-Alpha
Fig. 11: Aerodynamic force measurement of
OTOMO model in JAXA wind tunnel
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3.5
3.0
CL
2.5
2.0
Hard wall
Hard wall Corrected
1.5
Kevlar wall
1.0
-5
0
5
10
15
AoA[deg]
20
25
30
(B) CL-Alpha
-0.3
Hard wall
Hard wall Corrected
Kevlar wall
-0.4
Cm
because the correction method in hard-wall closed test
section was historically established through many
experiments. Therefore, this corrected value should be
a target in order to be obtained as true values. CD of
Kevlar wall data in LWT2-AC was almost same as the
corrected hard-wall data. It means that the CD in
anechoic closed test section does not need any
aerodynamic correction. CD correction mainly comes
from blockage factor of the model, but the each
Kevlar wall deflected at upstream region toward each
of the anechoic chamber here, as shown in the
following section, and it seemed to be automatically
corrected. On the other hand, CL and pitching-moment
coefficient (Cm) did not show the target value in this
case, although it was shown between the hard-wall
and corrected value. It means the wall interference
was less than the hard wall, but the Kevlar deflection
was not enough to neglect correction. The value to be
corrected has to be different value from hard-wall
which was established, so we need to propose other
method for anechoic closed test section with Kevlar
wall to achieve.
C. Kevlar wall deflection
-0.5
Kevlar wall of this anechoic closed test section can
-5
0
5
10
15
20
25
30
be deflected by the down wash caused by this high-lift
AoA[deg]
model. Flow field should be different from fixed-wall
(C) Cm-Alpha
test section at this condition. To evaluate this effect,
Fig. 11: Aerodynamic force measurement of OTOMO
measurements of deflection of Kevlar wall were
model in JAXA wind tunnel (Continued)
carried out by using infrared
Kevlar Wall
distance measuring sensors, as
shown in Fig. 12. These sensors
Test Section
Traverse
were calibrated before this test. The
range of this calibration was from
Measurement of
distance
18cm to 30cm. In this range
sufficient accuracy about 2 - 3 mm
14 IR-sensors
was obtained. The measurement
results are shown in Fig. 13. Kevlar
wall at pressure side of the model
Sub Anechoic
Main Anechoic
Chamber
Flow
was largely deflected toward the
Chamber
anechoic chamber. This deflection
Fig. 12: Kevlar wall deflection measurement using IR sensors
was caused by the down wash by
=0deg.
=8deg.
Flow
Flow
Main chamber side Sub Chamber side
Main
=16deg.
Sub
Main
Fig. 13: Kevlar wall deflection measurement using IR sensors
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Sub
the high lift devices. On the other hand, the other Kevlar wall at suction
side was deflected into the test section near the model because of the lower
pressure caused by the suction effect at the upper surface of the model.
However, small deflection toward the anechoic chamber happened at
upstrearm region. It was caused by the pressure increase by blockage of the
Sub Anechoic
model. The sketch of deflection is shown in Fig.14. This deflection can
Main Anechoic
Chamber
Chamber
affect and have relationship to aerodynamic force. It will be discussed in
the following section. This effect can be compared with VT’s closed
anechoic test results. The basic phenomena were the same and reasonable,
as shown in Fig. 15 of deflection of VT-AWT. Moreover, the maximum
value of the deflection of LWT2-AC was about 4cm at 16deg of AOA and
Flow
50 m/s, and this value was reasonable comparing with VT-AWT's results
Fig. 14: Deflection image around
which was about 5cm. This agreement of the deflection was interesting,
the OTOMO model
because size and tension of the Kevlar walls between two wind tunnels
were different and it was affected by tension of the Kevlar wall which has
some uncertainty due to the difficulty to keep in accurate value on all area of the Kevlar sheet.
Fig. 15: Kevlar deflection for the OTOMO model measured using IR sensors in VT-AWT at 16deg AoA
and 50m/s.
Cp
Kevlar Wall
D. Correction of wall interference using anechoic
chamber pressure
Test Section
Considering Kevlar wall deflection, conventional hardwall interference correction is not suitable to correct the
Positive static
Negative static
aerodynamic data in Kevlar wall test section because of the
pressure
pressure
flow deflected by the flow induced by the model. Here, we
propose new correction method that is suitable for our
anechoic closed test section. The deflection of the flow and
Main Anechoic
Sub Anechoic
Kevlar wall has to be caused by the aerodynamic effect
Flow
Chamber
Chamber
between the model and Kevlar wall. It is important that the
-0.25
Kevlar wall deflection has strong relationship to the
-0.20
pressure in the anechoic chamber behind the Kevlar wall.
We measured the static pressure at several locations in the
-0.15
Sub chamber
both anechoic chambers. Now, the aerodynamic correction
-0.10
of wall interference is shown using these pressures in the
-0.05
anechoic chambers.
0.00
Main chamber
The static pressure distribution is changing according to
0.05
changing the angle of attack. Main anechoic chamber,
which was pressure side of the model, showed positive
0.10
0
5
10
15
20
25
pressure, and sub anechoic chamber showed negative
AoA[deg]
pressure as shown in Fig. 16. It means that higher pressure
of the pressure side, which is lower surface of the model, Fig. 16: Chamber pressure change according to AOA.
9
American Institute of Aeronautics and Astronautics
CL
increased lift of the model by pushing up the
3.5
model. If a control volume surface along the
Kevlar wall is assumed, it finds that the
3.0
pressure on the Kevlar wall surface can be
added to the control volume. Note that the
2.5
correction of drag coefficient was not needed in
the Kevlar-wall test section, as shown above.
2.0
Therefore, the correction can be considered
Hard wall
about only lift direction and the correction was
Hard wall Corrected
made very simple. Here, this correction for the
1.5
'Kevlar wall
CD is considered to be negligible because the
'Kevlar wall + dCp
drag component by the pressure to Kevlar wall
1.0
is almost canceled by considering the
-5
0
5
10
15
20
25
30
deformation of Kevlar. So, CD was mainly
AoA[deg]
affected by Kevlar tension and its deformation,
Fig. 17: Aerodynamics of OTOMO model. Here, CL was
and the Kevlar wall worked to correct the wall
corrected by pressure difference dP between two chambers.
interference considering these experimental
dF = dP * S = q * S * dCL, CL_corrected = CL-dCL
results.
The corrected results are shown in Fig, 17.
The correction method for CL was very simple as shown in Fig.17. The CL was corrected by pressure difference dP
between two chambers. Using dF = dP * S = q * S * dCL, and CL_corrected = CL - dCL, we can obtain new corrected
CL. The results showed completely same as the corrected CL of hard wall. This value considered as free stream CL,
and this correction for the Kevlar wall interference was reasonable. Now we were able to find one of the
aerodynamic correction method for the Kevlar-wall test section for CL. On the other hand, Cm correction seems to
have a little difficulty. The moment correction by chamber pressure effect had to be caused by moment due to
pressure which acted on pressure center. The estimation of Xcp for correction might include some error for this kind
of high-lift device model which consists of three parts of wings. Although the correction for Cm is not able to show
good agreement quantitatively here, the Kevlar wall test section has interesting aerodynamic characteristics as
considering CL correction with chamber pressure. Definitely, this test section showed very good performance for
aeroacoustic measurement for high-lift device model. We can conclude the anechoic closed test section is very
useful and promising facility for the aerodynamic and aeroacoustic measurements.
IV. Concluding remarks
A new anechoic closed test section at JAXA has been completed for the 2 m x 2 m Low-speed wind tunnel. This
test section has stretched Kevlar cloth as the side walls of the closed test section. Anechoic chambers are installed
behind the Kevlar walls. Aerodynamic noise sources from test section can be measured in the anechoic chamber
through the Kevlar wall. By using a high-lift device model, the JAXA's new test section was evaluated and validated
acoustically and aerodynamically by comparing to VT’s anechoic test section and JAXA’s closed test section results.
Specifically, aerodynamic characteristics by applying new wall interference correction procedure using chamber
pressure in the anechoic closed test section showed remarkable agreement with corrected closed test section results
which should be the true value. This anechoic test section is useful and a promising tool not only for an aeroacoustic
but also for an aerodynamic testing ability in Low-speed wind tunnel.
Acknowledgments
Authors greatly thank for Dr. Taro IMAMURA and Mr. Hiroshi UCHIDA in Commercial Transport Team in
JAXA, and Mr. Hugo Camargo and Mr. Marcel Remillieux, Graduate student at Virginia Tech, Mr. Zackary Boor,
undergraduate student at Virginia Tech, who supported the wind tunnel tests and data analyses, and Prof. William J,
Devenport, who greatly helped us in the wind tunnel test at VT.
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11
American Institute of Aeronautics and Astronautics