Inversion Technique for Determining the Strength of Rotating Broadband Sources in Ducts

Abstract

Aeroengine broadband fan noise is a major contributor to the community noise exposure from aircraft. Currently there are no measurement techniques that allow the localisation and quantification of rotor-based broadband noise sources. This paper presents an inversion technique for estimating the broadband acoustic source strength distribution over a ducted rotor using pressure measurements made at the duct wall. It is shown that the rotation of acoustic sources in a duct prevents the use of standard acoustic inversion techniques. The technique presented here makes use of a new Green function that takes into account the effect of source rotation. The new Green function is used together with a modal decomposition technique to remove the effect of source rotation, thereby allowing an estimation of the rotor-based source strengths in the rotating reference frame. It is shown that the pressure measured at the sensors after application of this technique is identical to that measured by sensors rotating at the same speed as the rotor. Results from numerical simulations are presented to investigate the resolution limits of the inversion technique. The azimuthal resolution limit, namely the ability of the measurement technique to discriminate between sources on adjacent blades, is shown to improve as the speed of rotation increases. To improve the robustness of the inversion technique a simplifying assumption is made whereby the sources on different blades are assumed to be identical. It is also shown that the accuracy and robustness of the inversion procedure improve as the axial separation between the rotor and sensors decreases. Simulations demonstrate that for a 26-bladed fan, rotating at Mt = 0.5, the aerodynamic source strengths can be estimated with acceptable robustness and approximately 1dB accuracy, when measurements are made 0.1 acoustic wavelengths from the rotor.