Acoustic Measurements at Real-Flight Reynolds Numbers

Kurzfassung

The use of microphone phased arrays to acquire acoustic data of scaled models in standard wind tunnels has become a standard measurement technique. As shown by several authors, results obtained in a standard wind tunnel show differences when compared with results obtained from real flight tests: the differences can be attributed to lack of model-fidelity, installation effects, a discrepancy in Reynolds number, and the assumptions made in phased array processing. This work will address the effect of the Reynolds number in such measurements. Aerodynamic measurements are often performed in cryogenic and/or pressurized wind tunnels which are capable of higher (viz. real-flight) Reynolds number flows - conventional wind tunnels cannot generally achieve real-flight Reynolds numbers. Aeroacoustic measurements, however, have so far not been performed at real-flight Reynolds numbers on a small-scale aircraft model (i.e. under combined cryogenic and pressurized conditions). This study consists of data acquired with a microphone array in the European Transonic Windtunnel (ETW). The facility of the European Transonic Windtunnel GmbH (ETW) can provide real-flight Reynolds numbers by virtue of both decreased temperature and increased pressure. At the DLR, the microphone-array measurement technique has been further developed to perform measurements under combined cryogenic and pressurized conditions. For this purpose, a microphone-array consisting of 96 microphones was designed and constructed. Acoustic array measurements performed at the ETW wind tunnel are shown for various Reynolds numbers in the range 1 million to 22 million (based on the aerodynamic chord length) using a 13.6% Airbus K3DY half model in high-lift configuration. The results showed a significant Reynolds number dependency with various sources. Of particular note are various dominant sources appearing on the flap at real-flight Reynolds numbers. To our knowledge, this is the first time that airframe noise data of a small scaled aircraft model has been acquired at real-flight Reynolds numbers. The ability of measuring airframe noise at real-flight Reynolds numbers gives now the possibility of separating the effect of the Reynolds number from the effects of model-fidelity and Mach number on aeroacoustic behavior. In order to demonstrate the suitability of aeroacoustic small-scale model measurements, it is necessary to compare the results from these tests with wind tunnel testing on a full scale model, and also, ultimately, with real flyover tests. This encompasses several issues: for example, the attainment of comparable conditions and the related determination of the most important factors (e.g. pressure distribution, flyover/wind tunnel corrections, model-fidelity, and deformation).