[American Institute of Aeronautics and Astronautics 14th AIAA/CEAS Aeroacoustics Conference (29th AIAA Aeroacoustics Conference) - Vancouver, British Columbia, Canada ()] 14th AIAA/CEAS

American Institute of Aeronautics and Astronautics

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Enhanced Capabilities of the Aeroacoustic Wind Tunnel Braunschweig

Michael Pott-Pollenske1 and Jan Delfs.2 Deutsches Zentrum für Luft- und Raumfahrt, Braunschweig, 38108, Germany

The Aeroacoustic Wind Tunnel Braunschweig is DLR’s small, high-quality test facility for aero-acoustic noise measurements. After years of mainly pure acoustic measurements the actual and the future research work will focus on the combined acquisition of aerodynamic and acoustic data. In order to prepare the AWB for the next decades a modernization of this wind tunnel was initiated targeting the enhancement of the aerodynamic properties while the excellent acoustic properties should at least be preserved. Based on the assessment of AWB’s original acoustic and aerodynamic characteristics a new acoustic wall-treatment in combination with new sound absorbing turning vanes was installed in the flow circuit. In order to prevent the highly bended downwash flow caused by airfoils tested at high angles of attack from impinging on the test section floor a new collector was installed that can be moved upstream and vertically down. Finally the driven efforts resulted in an increase of the maximum flow velocity of about 8% while first the background noise levels for an empty test section were preserved and second the use of the adaptive collector lead to a significant background noise reduction for airfoil tests.

Nomenclature AD = nozzle exit cross-section b = slot width between silencer boxes B = nozzle width cm = average free stream velocity Dh = hydraulic diameter d = width of one silencer box f = free cross-section area H = nozzle height L = length

Dm& = mass flow Re = Reynolds Number V& = volume flow

λf = inflow friction coefficient pΔ = pressure loss

η = dynamic viscosity λ = friction coefficient

aλ = flow intake friction coefficient ν = kinematic viscosity ρ = density of air ς = drag coefficient Abbreviations: AWB = Aeroacoustic Wind Tunnel Braunschweig DLR = Deutsches Zentrum für Luft- und Raumfahrt

1 Research Scientist, Inst. of Aerodynamic and Flow Technology, [email protected], AIAA Member 2 Head of Technical Acoustics Dept., Inst. of Aerodynamic and Flow Technology, [email protected], AIAA Member

14th AIAA/CEAS Aeroacoustics Conference (29th AIAA Aeroacoustics Conference)5 - 7 May 2008, Vancouver, British Columbia Canada

AIAA 2008-2910

Copyright © 2008 by Deutsches Zentrum für Luft- und Raumfahrt e.V. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.


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I. Introduction HE Aeroacoustic Wind Tunnel Braunschweig (Figure 1) served since

the late 1970 years as small, high quality test facility for noise measurements on different subjects like propeller and rotor noise, vehicle noise and airframe noise. Especially the last 15 years were devoted to extensive airframe noise testing with a special focus on high lift devices’ noise. While in the first years of airframe noise testing mainly pure acoustic measurements on the identification and quantification of noise sources were conducted nowadays the combined acquisition of aerodynamic and acoustic data is necessary e.g. to perform validation experiments in order to support the development of numerical noise simulation tools.

After more than 3 decades of usage the program Update AWB was initiated in the year 2005 in order to enhance the capabilities of AWB with respect to future demands. The program targets two main objectives. First the excellent acoustic properties should be preserved or even improved. Besides the background noise levels for an empty measurement chamber special emphasis is on the reduction of noise originating from highly bended flow impinging on the test section’s floor while testing e.g. high lift systems at high angles of attack. Second the aerodynamic performance of the wind tunnel should be enhanced. The design effort therefore was focused on a new acoustic wall treatment in the entire flow circuit, new sets of turning vanes in order to replace both the existing hard walled turning vanes and all sets of silencers located in the flow circuit and finally an adaptive collector to optimize the flow intake especially for the highly bended downwash generated by high lift systems.

II. Assessment of the original Aerodynamic and Acoustic Properties The closed circuit type wind tunnel provides an open jet

test section with a nozzle of 1.2 m height and 0.8 m width and a contraction ratio of 1:9. The maximum flow velocity with a free nozzle was 65 m/s while under standard test conditions with e.g. a high lift airfoil maximum flow speeds of up to 60 m/s could be operated. The suction side consisted of a collector with a diffuser channel followed by a set of 5 turning vanes, one silencer block and a second set of 8 turning vanes upstream of the fan intake. Both the collector and the first set of turning vanes were acoustically treated with porous foam. The flow downstream the fan was expanded by a full metal, hard walled diffuser. Two silencer blocks were installed up- and downstream of the third corner which was originally not equipped with turning vanes. In corner four a set of 14 turning vanes guided the flow into the flow straightener and the turbulence control screen at the nozzle inlet.

The first step towards a new design was the detailed analysis of the original acoustic properties and the key flow features inside the flow circuit as well as inside the measurement chamber between nozzle exit and collector intake.

T

Figure 1. Planform view of AWB in its original configuration.

1.5

m

1 m1 m0.5 m

M1 M2 M3

Microphone height: free stream centerline

1.5

m

1 m1 m0.5 m

M1 M2 M3

Microphone height: free stream centerline

Figure 2. Microphone positions for background noise measurements in the test section


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A. Acoustic Analysis1 The examination of the original acoustic properties was carried out in two steps. The background noise inside the

measurement chamber was determined at three locations and for different flow velocities in order to first characterize the background noise with respect to spectral shape and absolute sound power levels and second to identify potential noise sources like e.g. nozzle outflow noise. Afterwards the acoustic absorption of all components in the flow circuit was characterized by a measurement of the sound pressure level difference over each element which in addition leads to the total sound absorption installed by means of silencers and acoustic wall treatment in both the pressure and the suction side of the flow circuit.

Background noise data was acquired by means of

three ½” condenser microphones at three positions along the free stream as depicted in Figure 2 for free stream velocities of 20 m/s up to 60 m/s. It turned out that the spectral shape is very similar for all tested wind speeds. A sound pressure level maximum occurs at around 100 Hz followed by a rapid level decay of about 30 dB to frequencies around 2500 Hz. For frequencies above 5000 Hz a quasi linear level decay of 5 dB per 5000 Hz is observed (Figure 3). By comparing the individual microphone signals it turned out that except for very low frequencies the sound pressure levels acquired near the nozzle exit at position M1 are slightly higher than those for position M3 near the collector intake (Figure 4).

In order to identify the acoustic absorption of all components in the pressure and the suction side of the flow circuit a polyhedral loudspeaker was installed right in front of the fan intake and downstream of the diffuser channel. For each measurement one reference microphone was located nearby the loudspeaker while the second microphone was placed up- and downstream of the respective component in the flow circuit. By calculating the sound pressure level difference for the individual microphone positions with respect to the reference microphone the sound absorption of any individual component could be identified as well as the total sound absorption in the pressure and the suction side of the wind tunnel. As can be seen from Figure 5 the total sound absorption of up to 80 dB is mainly driven by the silencers while the effect of the full metal turning vanes in terms of sound absorption is of minor magnitude as could be expected.

1/3 Octave Band, Hz

1/3

Oct

ave

Ban

dLe

vel,

dB

102 103 104

20

30

40

50

60

70

80 U∞ = 20 m/sU∞ = 30 m/sU∞ = 40 m/sU∞ = 50 m/sU∞ = 60 m/s

Figure 3. Background noise level spectra acquired at position M1 for different free stream velocitiesranging from 20 m/s up to 60 m/s.

1/3 Octave Band, Hz

1/3

Oct

ave

Ban

dLe

vel,

dB

102 103 104

20

30

40

50

60

70

80 U∞ = 60 m/s - M1U∞ = 60 m/s - M3U∞ = 20 m/s - M1U∞ = 20 m/s - M3

Figure 4. Difference of background noise levels at positions M1 and M3 for U∞ = 20 m/s and U∞ = 60 m/s.


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Since the fan must be regarded as the most prominent noise source in a wind tunnel some effort was driven to evaluate the noise generation by the twelve-bladed fan in AWB. A microphone equipped with a nose cone was mounted in front of the fan spinner in a region of very low flow velocity in order to measure radiated fan noise. The sound pressure level spectra were again acquired for flow velocities of 20 m/s up to 60 m/s and afterwards compared to theoretical approximations for fan noise as documented e.g. in VDI 2081. As can be seen from Figure 6 the actual measured data show a reasonable agreement to the theoretical data which further on served as reference data for fan noise considerations.

Taking into account the measured sound absorption in the flow circuit and the respective fan noise generation it turned out that due to the high absorption level in both the pressure and the suction side the radiated fan noise will be cancelled out before entering the measurement chamber. Therefore the background noise levels measured inside the test section originate only from the noise generated by the nozzle outflow. This finding was proofed by a comparison of the measured noise levels at position M1 with calculated outflow noise data according to the method described by Stüber in Ref. 2. (Figure 7)

calculated fan noise

measured fan noise

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90

110

125 500 2000 8000 16000

1/3

Oct

ave

Ban

d Le

vel,

dB

1/3 Octave Band, Hz

U∞ = 60 m/s

calculated fan noise

measured fan noise

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90

110

125 500 2000 8000 16000

1/3

Oct

ave

Ban

d Le

vel,

dB

1/3 Octave Band, Hz

U∞ = 60 m/s

Figure 6. Comparison of measured fan noise soundpressure level spectra to fan noise spectrum derivedout of literature data.

U∞ = 60 m/s0125 500 2000 8000 16000

Frequency, Hz

20

40

60

80

100

Soun

d Po

wer

Lev

el, d

B

measured background noise @ M1calculated nozzle outflow noise

fan noise – absorption pressure sidefan noise – absorption suction side

U∞ = 60 m/s0125 500 2000 8000 16000

Frequency, Hz

20

40

60

80

100

Soun

d Po

wer

Lev

el, d

B



Figure 7. Measured noise levels in the test hall compared to calculated nozzle outflow noise and the respective fan noise absorption in the pressure and the suction side.

Suction side Pressure side

0

20

40

80

60

0

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40

80

60

Abs

orpt

ion,

dB

Abs

orpt

ion,

dB

32 63 125 250 500 1 k 2 k 4 k 8 kFrequency, Hz


Corner 2Corner 1

Silencer 1Collector

TotalX

TotalX Nozzle

Silencer 2Silencer 3

Corner 3Corner 4X


0

20

40

80

60

0

20

40

80

60

Abs

orpt

ion,

dB

Abs

orpt

ion,

dB



Corner 2Corner 1

Silencer 1Collector

TotalX

TotalX Nozzle

Silencer 2Silencer 3

Corner 3Corner 4X

Figure 5. Sound absorption of individual components and total sound absorption in the flow circuit for suction side (left hand graph) pressure side (right hand side).


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B. Aerodynamic Analysis3, 4 Due to the presence of turning vanes and silencers

the flow circuit in both the pressure and the suction side of the wind tunnel is influenced by several pressure gradients and corresponding velocity changes. By means of a detailed analytical and experimental study the pressure gradients and corresponding flow speeds for each component in each section of the wind tunnel were determined. The analytical study was based on the following reference data:

1) The free stream velocity in the nozzle exit cross-section smcm / 0.60= .

2) The respective volume flow skgVD / 60.57=& through the nozzle exit

cross-section of 0.96 m2 which corresponds to a mass flow of

skgmD / 12.69=& with ρ = 1.20 kg/m3. 3) The mass flow is considered to be

constant in the entire flow circuit. Therefore the respective flow velocity in each cross-section without any installed component is equal to smcm / 9.41 = .

The pressure loss in a flow is defined according to eq. (1) as:

22 mcp ⋅⋅=Δ ρς . (1)

The respective flow velocity e.g. in a silencer cross-section is derived by simply taking into account the narrowing of the free cross-section by the silencers as depicted in Figure 8. The flow velocity in each of the silencer slots can then be expressed according to eq. (2) as ratio of flow speed upstream the silencer and free cross-section between the silencer boxes.

bd

bfwithfCC SD

SD

mmSP +

== 11

11 . (2)

Applying e.g. the respective data of slot width b = 0.12 m and silencer width d = 0.2 m for the silencer set in the suction side the flow velocity is calculated to 13.1 m/s (eq. (3)):

smC

mmmf

mSP

SD

/ 1.13

375.0 120.0 20.0

120.0

1

1

=⇒

=+

=. (3)

The drag coefficient ς was derived out of literature databases as e.g. documented in Refs. 7, 8 and 9. Regarding once more the silencer set in the suction side the total drag coefficient for this silencer set was defined as the sum of the friction drag coefficient and the shape drag coefficient (eq.( 4)).

111 fSPRSPSP ςςς += (4)

200

mm

120

mm

2030 mm34

40 m

m

200

mm

120

mm

2030 mm34

40 m

m

Figure 8. Dimensions of the silencer in suction side of the original AWB flow circuit


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The selection of an applicable coefficient from databases is based on flow characteristics and the dimension of the component in question. The flow through the silencer slot is therefore characterized by its Reynolds number given in eq. (5).

526-

11 100.2

/1015.1 120.02/ 1.132Re ⋅=

⋅⋅⋅

=⋅⋅

=sm

msmbCmSPSP ν

. (5)

Furthermore the friction drag coefficient is defined as product of friction number and wetted area as denoted in eq. (6)

bDfwithDL

SP

SP

haSPh

SDSPR ⋅=⋅=⋅= 2 and 1

11 λλλλς . (6)

The friction coefficient 1SPλ was calculated by literature data for the friction number in an flow intake 023.0=aλ and the friction coefficient for intake flows 26.1=λf to a value of 0289.01 =SPλ .

With the hydraulic diameter mDSPh 24.0

1= and the wetted length of one silencer box being mLSD 93.11 = the

friction drag coefficient is calculated to

231.01 =RSPς (7)

Taking the shape drag coefficient from literature as

48.01 =FSPς (8)

the total drag coefficient is derived to

711.01 =SPς . (9)

Finally the respective pressure loss for the silencer set in the suction side was calculated to be

PasmmkgpSD 73)/ 1.13(/ 20.15.0711.0 231 =⋅⋅⋅=Δ (10)

By application of the described method on the remaining silencer and equivalent methods on all other components in the flow circuit the pressure loss for each component of the wind tunnel was figured out so that consequently the pressure distribution along the entire flow circuit including the test section was known.

The detailed analysis of the pressure distribution along the flow circuit revealed that the pressure loss in the flow circuit is mainly caused by the silencers and was calculated to about 150 Pa. In addition it turned out that the pressure loss in corner 3 is much higher as for e.g. in corner 2 or corner 4 which clearly indicates the need for an additional set of turning vanes at this position.

In parallel to the analytical study pressure measurements were conducted in AWB in order to broaden the database on which on the one hand the flow conditions of the AWB in its original layout are judged and on the other hand predictions for a new layout of the flow circuit are based on.


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III. Aerodynamic and Acoustic Specifications of new AWB Flow Circuit Layout1,2,3 Based on the findings of the aerodynamic analysis

the flow circuit was first equipped with a sufficient number of turning vanes in every corner in order to guide the flow with a minimum pressure loss. In a second step the acoustic wall treatment and the porous foam cover on the turning vanes was dimensioned. All turning vanes are shaped according to the so called Kröber profile depicted in Figure 9. The sheet metal core is covered with porous foam on both the suction and the pressure side. A rounded leading edge allows for safety against flow separation and a well defined stagnation point while the long chord trailing edge with a small taper angle provides a minimum pressure jump which consequently reduces the risk of flow separation at the trailing edge. The foam layer on the suction side is of constant thickness. The total sound absorption in terms of frequency range and level that is achievable by means of such turning vanes is a function a the distance between each profile and the thickness of the foam layer on the pressure side. Due to the fact that AWB is predominantly used for model scale experiments the actual turning vane configurations for each corner in AWB are optimized in order to provide a maximum sound absorption for frequencies above 600 Hz to 800 Hz.

Finally the new flow circuit layout was defined as depicted in Figure 10 incorporating the following key features:

1) Corner 1: 11 turning vanes, 1350 mm chord length, 100 mm foam layer on pressure side, gap width 108 mm to 127 mm, acts as diffuser.

2) Corner 2: 5 turning vanes, 1350 mm chord length, 160 mm foam layer on pressure side, constant gap width of 422 mm for minimum pressure loss.

3) Corner 3: 7 turning vanes, 1800 mm chord length, 190 mm foam layer on pressure side, constant gap width of 340 mm for minimum pressure loss.

4) Corner 4: 9 turning vanes, 950 mm chord length, 130 mm, foam layer on pressure side, constant gap width of 130 mm for minimum pressure loss.

5) Acoustic wall treatment of 100 mm thickness on vertical walls and ceiling. 6) A diffuser channel with a 6°opening angle downstream of corner 1 in order to allow a controlled flow

expansion towards corner 2.

Flow

Flow

Flow

Flow

Figure 9. Principle design of an acoustically treated Kröber profile as suggested for installation in AWB.Flow direction from bottom to left hand side.

Figure 10. Planform view of the new AWB flow circuit layout .


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According of the defined layout of the AWB flow circuit the background noise level in the test section was predicted. Therefore the potential sound absorption which actually only depends on the turning vanes and the absorptive wall treatment inside the flow circuit was determined. As can be seen from Figure 11 the predicted total sound absorption in both the pressure and the suction side of the wind tunnel compares very well to the data acquired for the original AWB. As pointed out almost all measurements in AWB are conducted in small model scale thus it is reasonable to accept a reduced absorption for low frequencies below 800 Hz.

Based on the predicted total sound absorption in the

pressure and the suction side of the wind tunnel a prediction of the background noise levels in the test section was conducted. As presented in Figure 12 the predicted background noise levels show up equivalent levels compared to those determined for the original AWB. Therefore the proposed installation of turning vanes and absorptive wall treatment was considered to be a suitable means to realize the desired improvements of the wind tunnel from the acoustics point of view.

The design of the new collector was mainly based

on the good experience with the existing one. Therefore the intake geometry including the large 45° inclined brim was taken over for the new collector. The entire collector is completely covered with a 50 mm thick porous foam layer. In order to move the collector about 600 mm vertically down it is mounted on an electro-hydraulic powered lift. By means of insert cassettes the length of the diffuser channel can be increased in steps of 200 mm in order to move the intake section horizontally up to 1000 m upstream towards the nozzle.

U∞ = 60 m/s0

20

40

60

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100

Soun

d Po

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B



125 500 2000 8000 16000Frequency, Hz

U∞ = 60 m/s0

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100

Soun

d Po

wer

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el, d

B



125 500 2000 8000 16000Frequency, Hz

Figure 12. Predicted background noise level in the measurement chamber based on final layout proposal.


0

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60

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dB


measured absorption in origial AWB layout Prediceted absortion for new AWB layout

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dB


measured absorption in origial AWB layout Prediceted absortion for new AWB layout

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orpt

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dB


Figure 11. Predicted sound absorption for proposed AWB layout for suction side (left hand side)and pressure side (right hand side).


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The aerodynamic effectiveness of the proposed turning vanes was worked out thoroughly by means of analytical methods as described in section II. Additionally a numerical flow simulation of the corner flow was conducted by means of the DLR flow solver TAU. Assuming that the flow conditions in the turning vane cross-section are almost constant along the profiles spanwise direction it seems reasonable to perform a 2D flow simulation in the horizontal plane on the wind tunnel center axis. The inflow velocity was defined according to the calculated average flow velocity cm1 = 4.9 m/s. The total pressure distribution computed for corner 4 upstream of the flow straightener is given in Figure 13. As can be seen the total pressure loss is calculated to about 3 Pa. which in fact is an excellent agreement to the pressure loss of 4 Pa worked out by means of analytical methods.

From the aerodynamics point of view the major benefit of the new design is a significant reduction of the overall

pressure loss of about 150 Pa compared to original AWB design. Based on these findings the total pressure increase that has to be provided by the fan is reduced from about 2490 Pa to about 2340 Pa thus the operating point of the fan is shifted to lower rotational speed for the reference volume flow of skgVD / 60.57=& at smcm / 0.60= . This fact yields an increased volume flow for the maximum rotational speed of the fan finally providing higher flow velocities which in fact is one of the results that should be achieved by all measures taken during the program Update AWB.

IV. Acoustic and Aerodynamic Characteristics of the modernized AWB After the modernization work in the wind tunnel was finished extensive test runs were conducted. The test

objectives were 1) to identify the background noise levels in the test section for different flow speeds with the collector in

its original position, 2) to determine the pressure distribution along the entire flow circuit in order to validate to proper function

of all turning vanes and 3) to demonstrate the benefit of adjusted collector positions on both the background noise level and the

flow velocity.

A. Validation of Predicted Acoustic and Aerodynamic Properties Since AWB is an aeroacoustic test facility the noise level inside the measurement chamber is of vital importance

for the quality of this wind tunnel. The background noise level measurements were conducted at the positions described in section II, Figure 2 in order to determine the original AWB background noise levels. As stated above the background noise levels inside the measurement chamber should at least remain constant after the modernization. A comparison of the original and the newly acquired background noise data is presented in Figure 14 for microphone position M1 near the nozzle exit and microphone position M3 in the vicinity of the collector. At position M1 the original values are kept up to frequencies of about 8 kHz. Going to higher frequencies a significant noise level reduction is observed. The data acquired at microphone position M3 reveals a broadband noise reduction ranging from up to 1 dB at frequencies between 2 kHz and 5 kHz to about up to 10 dB for high frequencies. Since AWB due to its

1/3 Octave Band, kHz

1/3

Oct

ave

Ban

dLe

vel,

dB

5 10 15 20 2540

50

60

70

80

original AWBactual AWB

U∞ = 60 m/s

microphone near collector

microphone near nozzlewith offest of + 5 dB

Figure 14. Comparison of background noise levelsmeasured in the original and the modernized AWB.

Figure 13. Total pressure distribution at corner 4 derived by means of a numerical flow simulation with DLR TAU code


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small size is mainly used for model scale investigations with model scales ranging from 1:6 up to 1:10 especially the background noise reduction for the high frequency domain is of mayor importance and must be considered as successful improvement of AWB.

By means of dedicated pressure measurements the analytically determined pressure distribution along the flow circuit was validated. Regarding e.g. the total pressure loss due to the turning vanes in corner 3 and corner 4 a value of about 4 to 5 Pa was acquired which nicely fits to the predicted data. For the AWB in its original configuration the total pressure increase produced by the fan in order to provide a free stream velocity of 60 m/s was determined to 2300 Pa. The actual measurements revealed that after the modernization a total pressure increase of 2230 Pa is sufficient to provide a flow speed of 60 m/s. This finding again shows good agreement with the analytical data on which the modernization was based and proofed further more the success of the driven efforts to enhance the capabilities of AWB.

B. Effect of the Enhanced Capabilities of AWB on Airframe Noise Investigations For any kind of aeroacoustic measurement it is eligible to conduct all tests for the identical flow velocity which

was up to now restricted to speeds of 60 m/s or less. In case of single airfoils or high lift systems the free jet is deflected downwards due to aerodynamic and geometric reasons. By means of a NACA0012 airfoil of 400 mm chord length installed between vertical endplates downstream of the nozzle in a horizontal position. The airfoil was tested at 65 m/s free stream velocity for different angles of attack. According to the deflection of the free stream caused by the single NACA0012 airfoil the collector position was vertically adapted in order to allow for an optimized air intake into the collector with respect to the actual angle of attack. The pressure distribution depicted in Figure 15 belongs to an 11° geometric angle of attack at 65 m/s free stream velocity with the collector moved about 300 mm vertically down and indicates the potential to perform noise measurements on constantly high free stream velocities above 60 m/s.

The effect of adjusted collector positions is not only important for the aerodynamic test conditions but also e.g. in terms of background noise reduction for acoustic measurements. In case a high lift system, installed in the same manner as the NACA0012 airfoil, deflects the free stream in way that it impinges on the floor upstream of the collector in its original position and due to this fact additional noise is generated that of course will be recorded by free field microphones. This circumstance was demonstrated by means of a high lift system with slat and flap fully deployed tested at different free stream velocities between 30 m/s and 60 m/s at constant geometric angle of attack of 10 degrees. With a microphone located at position M2 (see Figure 2) the noise level at a sideline location beneath the free stream was recorded. Afterwards the collector position was adjusted in order to allow for a complete flow intake into the collector which was a position 600 mm horizontally upstream towards the nozzle and about 400 mm vertically down. The significant noise reduction received due to this measure is depicted in Figure 16 and can be regarded as success of the installation of an adaptive collector.

Normalised Chord Length x/c

Pre

ssur

e,P

a

0 0.2 0.4 0.6 0.8 1

-8000

-6000

-4000

-2000

0

2000

4000

6000

8000

NACA0012 - U∞ = 65 m/s - angle of attack = 11 deg

Figure 15. Pressure distribution for NACA0012 airfoil tested at 65 m/s and 11° geometric angle of attack. Collector position vertically adjusted by about 300 mm

1/3 Octave Band, kHz

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ave

Ban

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B

5 10 15 2040

50

60

70

80

adapted collector positionoriginal collector position

U∞ = 30 m/s

U∞ = 60 m/s

U∞ = 40 m/s

U∞ = 50 m/s

Figure 16. Comparison of background noise levels atmicrophone position M2 for the collector in its originaland in an optimized position.


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V. Conclusion The program Update AWB was initiated in order to enhance the capabilities of the Aeroacoustic Wind Tunnel

Braunschweig and to prepare the wind tunnel for future demands especially with respect to airframe noise research. By means of dedicated investigations the aerodynamic and acoustic properties of the AWB in its original configuration were worked out. Based on a high level of expertise and experience a new layout for the flow circuit and an adaptive collector was designed and installed in AWB. It could be demonstrated that due to the applied changes reduction of background noise levels in the measurement chamber could be achieved and in addition due to reduced pressure losses in the flow circuit and by means of the adaptive collector the wind tunnel can be operated at higher free stream velocities.

Acknowledgments Thanks to the combined aerodynamic and acoustic design effort the project Update AWB was successfully

finished. The success of this program was mainly based on the expertise, experience and excellent cooperation of P. Brandstätt from Fraunhofer Institute für Bauphysik responsible for the acoustic design, D. Pecornic and H. Zimmermann from Mannheim University responsible for the aerodynamic assessment and P. Schneider, A. Lecheler and especially A. Rampp from FAIST Anlagenbau as project leader and manufacturer.

References 1Brandstätt, P., Babuke, G., Schneider, W. “Akustische Messungen und Auslegungen für die Modernisierung des aero-

akustischen Windkanals Braunschweig AWB des DLR“, Fraunhofer Institut für Bauphysik, Stuttgart, 2007. 2Heckl, M., Müller, H.A., “Taschenbuch der Technischen Akustik”, Springer Verlag Berlin, 1995, Chapter 7, pp 206, 207 3Pecornik, D., Zimmermann, H., Michel, H. „Bericht über Strömungstechnische Optimierung des bestehenden

Aeroakustischen Windkanals Braunschweig AWB“, Institut für Angewandte Thermo- und Fluiddynamik der Hochschule Mannheim, Mannheim, 2007

4Pecornik, D., Zimmermann, H., „Anhang 2 Konventionelle Druckverlustberechnung für den Istzustand und für den Umbau des bestehenden Aeroakustischen Windkanals Braunschweig AWB“, Institut für Angewandte Thermo- und Fluiddynamik der Hochschule Mannheim, Mannheim, 2007

5Michel, H. „Berechnung der Strömungsverhältnisse im Plenum des aeroakustischen Windkanals Braunschweig mit dem CFD-Programm FLUENT“, Ingenieurberatung Prof. Dr. Hartmut Michel, Strömungssimulation und Wärmetechnik, Mannheim, 2007

6Michel, H. „Berechnung der Strömungsverhältnisse im aeroakustischen Windkanal Braunschweig im Istzustand und nach den (geplanten) Änderungen mit dem CFD-Programm FLUENT“, Ingenieurberatung Prof. Dr. Hartmut Michel, Strömungssimulation und Wärmetechnik, Mannheim, 2007

7Idelchik, I.E., „Handbook of Hydraulic Resistance (Second Edition)“, Springer Verlag, 1986 8Kerber, W., Hoyningen-Huene, D.v., Pecornik, D., Zimmermann, H.,“Handbuch für Widerstandsbeiwerte von Bauteilen für

Leitungssystem“, Forschungsgemeinschaft für Luft- und Trocknungstechnik Frankfurt e.V., 1992 9Kerber, W., Pecornik, D., Zimmermann, H., “Druckverlustmessungen an Schalldämmkulissen“, Forschungsbericht, 1983

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[American Institute of Aeronautics and Astronautics 14th AIAA/CEAS Aeroacoustics Conference (29th AIAA Aeroacoustics Conference) - Vancouver, British Columbia, Canada ()] 14th AIAA/CEAS