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

Jet installation noise is a relevant acoustic noise source in aviation. It is further emerging compared to other engine noise sources, due to the increasing size of modern turbo-fan engines which on the one side brings in a reduction on the nozzle-exit velocity and on the other side makes a closer coupling with the air frame necessary. Wind-tunnel experiments are a key component in the evaluation of relevant source mechanisms in jet-installation noise and their relevance in the definition of the environmental noise generated by aircraft. The effect of the forward flight of the aircraft is simulated by embedding the sound-emitting aeroacoustic sources in the wind-tunnel stream. The acoustic measurements in wind tunnels are often made in the geometrical near field of the aeroacoustic source. With this we mean that a microphone is seen at appreciably different polar coordinates from different positions within the source region. The evaluation of environmental noise associated with a given test configuration is needed for positions that are far away from the source region. Such observation positions are seen from within the source region at nearly constant spherical polar coordinates. This implies that while environmental noise estimates require a balanced superposition of the aeroacoustic-source effects, wind tunnel measurements only offer, for a given microphone, a biased superposition where one region of the source is added to a different region with different weighting factors. 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 defined by using the DLR method SODIX and a free space Green's function. Resulting source maps are corrected by using a plug-flow model for the wind-tunnel stream. We present the result of this a-posteriori correction on an example source map and on different experimental configurations, with and without forward-flight effect. The SODIX source distribution is also used to effectively 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.