Microphone-Array Measurements of a Model Scale Contra-Rotating Open Rotor in a Reverberant Open Wind-Tunnel

Abstract

The DLR department of engine acoustics performed acoustic measurements of a contra-rotating open-rotor (CROR) model with a linear microphone array in the T-104 open wind-tunnel at TsAGI. The microphone data is corrected for shear-layer effects using the model of an infinitely thin shear layer by Amiet. Investigations of the room-acoustics of the test hall show strong reflections from the ceiling and two side walls, which cause strong interference patterns at the microphone positions. The acoustic change between different rotor confgurations, like different rotor spacings and an upstream mounted mock pylon, is shown with spatially averaged spectra. The evaluation of the source directivities is strongly influenced by the interference patterns of the reflections in the test hall. The rotor tones can be localized at the correct position with classical beamforming. The inverse method SODIX is used to model the directivities of the broadband sound-sources of the CROR. Green's functions that take the shear layer and the first-order reflections into account were implemented. The accuracy of these correction methods could be investigated with the directive SODIX results. The shear-layer model gives accurate results for emission angles < 100° while all modeled sources move downstream for larger emission angles. Corrections for the first-order reflections of the test hall show an increased spatial resolution of the SODIX results and a suppression of artificial sources.