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Comparison of microphone array measurements in the closed test section of LSWT and ETW

  • C. Spehr
  • T. AhlefeldtEmail author
Original Paper
  • 9 Downloads

Abstract

Aeroacoustic measurements in wind tunnels are a common tool in the determination of sound sources on scaled models. Measurements are performed worldwide in comparable wind tunnels. It is however not known to what degree a similar measurement in different wind tunnels will produce similar results. In this paper, therefore, a comparison of microphone array measurements using the very same aircraft half-model in the closed test section of two different wind tunnels with two different microphone arrays is made. For this, data from two measurement campaigns conducted in the LSWT in Bremen, Germany, and in the ETW in Cologne, Germany, were used. The comparisons are made by juxtaposing the two measurements in terms of source maps at individual frequencies and in terms of integrated spectra for different areas of the wing. Both comparison methods exhibit substantial agreement, but yet noticeable differences which are discussed afterward taking into account the model and model support, background noise, flow conditions, microphone array setup and post processing. Comparing and discussing the possible reasons yield insight about the repeatability of industrial microphone array measurements which will lead to a better interpretation of such experiments in the future.

Keywords

Airframe noise Microphone array Aeroacoustic testing Wind tunnel Closed test section High lift 

List of symbols

\(\alpha _{\mathrm{g}}\)

Geometric angle of attack

\(\delta\)

Mean aerodynamic chord length

\(\varphi\)

Radiation angle

\({\varvec{\xi }}\)

Focus point vector

\({\varvec{A}}\)

Source auto power estimate matrix

\({\varvec{C}}\)

Spectral cross-correlation matrix

\(C_{\mathrm{L}}\)

Lift coefficient of the model

\(C_{\mathrm{D}}\)

Drag coefficient of the model

\(C_{\mathrm{M}}\)

Pitching moment coefficient of the model

f

Frequency

\(f_{1/3\mathrm{Oct}}\)

3rd octave band center frequency

\({\varvec{g}}\)

Steering vector

\(L_\mathrm{p}\)

Sound pressure level

\(l_x,l_y\)

Array aperture

M

Mach number

N

Number of microphones

\({\varvec{p}}\)

Vector of Fourier-transformed pressure fluctuations

\(p_{\mathrm{s}}\)

Static pressure

r

Distance

\(r_0\)

Reference distance

\(Re_\delta\)

Reynolds number based on \(\delta\)

T

Temperature

\(u_\infty\)

Free stream velocity

V

Dihedral angle of the wing

\({\varvec{w}}\)

Weighted steering vector

xyz

Spatial coordinates

Abbreviations

\(\hbox {DR}\)

Diagonal removal

\(\hbox {ETW}\)

European transonic wind tunnel

\(\hbox {EWA}\)

European Windtunnel Association

\(\hbox {LSWT}\)

Low speed wind tunnel Bremen

\(\hbox {PSF}\)

Point spread function

\(\hbox {TBL}\)

Turbulent boundary layer

Notes

Acknowledgements

The authors would like to thank the following people and institutions for their assistance: The Federal Ministry of Economics and Technology (BMWi) for the financial support of ‘ALSA’ as part of the aerospace research program (LuFO IV). The European Windtunnel Association for their support of the ‘EWA Aero-Acoustic Benchmark’ test. The European Transonic Windtunnel GmbH for the provided infrastructure and help in performing the measurements. Special thanks go to Jürgen Quest, the ALSA project leader who from the beginning of the project showed great personal involvement in successfully carrying out this project. The Airbus company for the provision of the Airbus K3DY half-model and especially Iris Goldhahn for her helpful cooperation. Our colleague Florian Philipp for the development and the support of our beamforming software ‘SAGAS’. Our colleagues Stefan Haxter and Daniel Ernst for fruitful discussions and corrections.

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Copyright information

© Deutsches Zentrum für Luft- und Raumfahrt e.V. 2019

Authors and Affiliations

  1. 1.German Aerospace Center (DLR)GöttingenGermany

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