Using Phased Array Beamforming to Identify Broadband Noise Sources in a Turbofan Engine

Abstract

Fan broadband noise is a major component of the total noise emitted by turbofan engines, especially at lower shaft speeds. It is generated in the rotor/stator region, but the exact origin is not always known. This article discusses the application of phased array beamforming techniques for a better understanding of the source mechanisms of fan broadband noise. The Conventional Beamforming technique was applied, as well as the deconvolution technique CLEAN-SC and the beamforming technique ROSI for rotating sources. Beamforming was applied to acoustic data measured by two circular microphone arrays that were mounted in the intake and in the bypass of a Rolls-Royce fan rig. These arrays are normally used for the detection of azimuthal modes. The merits of beamforming are discussed by considering a number of typical low shaft speed cases. Using the intake array, in one of the cases forward radiating broadband noise sources were found that were coherent over a large area. These could have been due to a rotor instability. In an other case, the forward radiating broadband noise seemed to have its origin at the stator vanes. This could be made plausible with the help of the intake mode detection results. Using the bypass array, stator bound noise sources were found that seemed to be distributed along the span of the vanes. In other words, tip sources seemed to be of minor importance for aft radiating fan broadband noise. The bypass mode detection results seemed to indicate that the spanwise distributed noise sources were located at the trailing edges of the stator vanes.