Development of the Microphone-Array Measurement Technique for use in Cryogenic and Pressurized Wind Tunnels

In the development process of aircraft and rail vehicles, measurement techniques based on microphone-arrays are well-known and common practice on small-scaled models in wind tunnels with closed test section. Since conventional wind tunnels cannot generally achieve full-scale Reynolds numbers, measurements are often performed in cryogenic (down to 100 K) and/or pressurized (up to 450 kPa) wind tunnels which are capable of up to full scale Reynolds number flows. For the application in wind tunnels operating under these special environmental conditions microphone arrays were designed and constructed. Especially the selection of the sensor equipment, such as microphones, electronic devices, cables for signals and power supply is of major importance. For the microphones, the frequency response at different static pressures and different temperatures must be taken into account. During the development process, measurements have been conducted in the cryogenic wind tunnel (KKK) in Cologne and the European Transonic Windtunnel (ETW) using a single-rod configuration and airplane half models as aeroacoustic noise sources. In each case the results show significant Reynolds number dependencies of the measured sound pressure. To our knowledge, those developments leaded to the first time that airframe noise data of a small-scale model have been acquired at real-flight Reynolds numbers