Microphone-Array Measurements on a Scaled Model at Real-Flight Reynolds Numbers

Abstract

Aerodynamic measurements are often performed in cryogenic and/or pressurized wind tunnels which are capable of increased (up to real-flight) Reynolds number flows - since conventional wind tunnels cannot generally achieve real-flight Reynolds numbers. Based on previous research, at the DLR Institute of Aerodynamics and Flow Technology the microphone array measurement technique has now been further developed to acquire aeroacoustic data in an environment which is both pressurized and fully cryogenic and thus at real-flight Reynolds numbers. The measurements were conducted at the facility of the European Transonic Windtunnel GmbH which can provide real-flight Reynolds numbers by virtue of both decreased temperature and increased pressure. Measurements were carried out for a wide range of operational parameters and different Reynolds numbers up to real-flight Reynolds numbers using a scaled Airbus K3DY half-model. The results showed a significant Reynolds number dependency for various sources: sources on the slat with a strong tonal character, which disappear with a rise of the Reynolds number and various peaks with combined Strouhal- and Reynolds number dependencies. Of particular note are various dominant sources appearing on the flap at real-flight Reynolds numbers, giving a major contribution to the overall level. To our knowledge, this is the first time that airframe noise data of a small-scale model have 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.