True Farfield for Dual-Stream Jet Noise Measurements

摘要

I. Introduction THE minimum distance in the far field that corresponds to the true geometric and acoustic far field for jet noise measurements from dual-stream nozzles is established in this study. The practical significance of this minimum limit is the following: for measurements carried out beyond this distance, the assumption of a point source located at the nozzle exit plane to represent the distributed nature of the sources in ajet is valid. In most jet noise tests, the origin of the coordinate system is taken to be on the jet centerline and at the nozzle exit plane; the far-field microphones are set up at radiation angles measured from the origin of the coordinate system. The subsequent processing of the noise spectra over the entire frequency range becomes straightforward, if the assumption of point source is justified. Then, the distance to the source and the radiation angle are simply determined and appropriate corrections for the atmospheric attenuation, etc., may be applied easily. The minimum distance is determined through the examination of a large jet noise database; typical dual-stream exhaust geometries with extended primary nozzles of different diameters, together with several microphone arrays at different distances are used. Jets operating in a static environment as well as in the presence of a flight stream are considered.