On Source Localization and Far Field Extrapolation for Jet Aeroacoustics Measurements in Wind Tunnels

Kurzfassung

Jet installation noise is a relevant acoustic noise source in aviation. The increased size of modern turbo-fan engines reduces the nozzle-exit velocity but also brings the jet closer to the air frame. Wind-tunnel experiments are performed to investigate the source mechanisms in jet-installation noise. The effect of forward flight is simulated by the wind-tunnel stream. Acoustic measurements in wind tunnels are often made in the geometrical near field of the aeroacoustic sources, but the contributions of the sources to the overall radiated noise are needed for positions that are far away from the source region. We use microphone array measurements acquired in the Aeroacoustic Wind Tunnel Braunschweig (AWB) as part of the project Decrease Jet Installation Noise (DJINN). The array measurements are used to define a distribution of equivalent directive point sources on the axis of the source region. The source distribution is calculated using the DLR method SODIX and a free space Green’s function. Resulting source maps are corrected for shear layer refreaction using a plug-flow model for the wind-tunnel stream. We present the result of this a-posteriori correction for different experimental configurations, with and without simulated forward-flight. The SODIX source distribution is also used to reconstruct sound pressure levels for far-away positions, beyond the dimension of the AWB anechoic chamber. A measurement for an isolated jet in static conditions is compared to reference measurements, taken at a distance about ten times greater than the AWB nozzle to microphone distance.