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ALSA

Experimental setup of an acoustic application study in the ETW (courtesy of ETW GmbH)

ALSA ("Akustische Lokalisation von Strömungsablösung im ETW") is a project which is funded by the German Ministry of Economy and Technology (BMWi) as part of the LUFO4 framework program under the topic “Development of technological elements for the climate-friendly air transport system of the future”. Project partners are the European Transonic Windtunnel GmbH (ETW) and Airbus Germany.

The project has two main objectives: first, the development of a non-intrusive measurement technique for the localization of laminar separation, and secondly, the development of an acoustic measurement technique for use in cryogenic and pressurized environments.

Most measurement techniques available today for the detection of laminar flow separation are not effectually applicable to the ETW. Here, in contrast to conventional wind tunnels, the boundary layers are very thin (1 to 2 mm) and the accessibility to the model is limited due to the extremely low temperatures (down to 115 K) and the pressurized (up to 4.5 bar) environment. Thus, the Reynolds number for the small model can be increased through the cryogenic and/or pressurized fluid.

In this project the relationship between the near-field (pressure fluctuations on the airfoil) and the far-field (acoustic pressure fluctuations on wall of the test section) will be investigated. It is anticipated that the measurement of the far-field data to-gether with knowledge of the transmission mechanisms will be a helpful tool to identify and detect separation regions on an airfoil.

For measurements of pressure fluctuations in the far-field, the microphone-array measurement technique is to be used. The use of microphone phased arrays to acquire acoustic data on scaled models in conventional wind tunnels is a standard tool in industrial wind tunnels. Here, the special challenge is the use of this technique under the extreme ETW temperature and pressure conditions. This development has also a second outcome: the ability for the first time worldwide to be able to measure airframe noise on small scaled models in wind tunnels at actual flight Reynolds numbers.

Acoustic source maps of an aeroplane half-model at different Reynolds numbers Left: T = 272 K, p = 2 bar, right: T = 115 K, p = 4 bar

Literature

1. Ahlefeldt, T., Koop, L.: Microphone Array Measurements in a Cryogenic Wind Tunnel, AIAA Journal Vol. 48 (2010), Juli, Nr. 7, S. 1470–1479
2. Ahlefeldt, T.: Aeroacoustic Measurements of a Scaled Half-Model at High Reynolds Numbers, AIAA Journal (in press, accepted for publication 6 May 2013)
3. Ahlefeldt, T., Quest, J.: High-Reynolds Number Aeroacoustic Testing Under Pressurised Cryogenic Conditions in PETW, 50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2012
4. Ahlefeldt, T., Sopranzetti F,, Spehr C., Henning A.: Investigation of laminar detachment by means of simultaneous microphone and surface hot wire measurements, 16th AIAA/CEAS Aeroacoustics Conference, AIAA-2013-2110