Characterization and source separation of turbofan broadband noise using intra-stage measurements

Abstract

The broadband (BB) noise generated in the fan stage is a significant contributor to the total engine noise with the dominant noise generation mechanisms attributed to the self-noise at the trailing edges (TE) of the fan blades and the rotor-stator interaction noise at the leading edges (LE) of the outlet guide vanes (OGV). Despite more than 50 years of research, the fan BB noise is not properly understood, and its rotor (fan) and stator (OGV) components haven’t been properly quantified. The noise measurements in the engine intra-stage between the fan TE and OGV LE permit a direct comparison between the BB noise characteristics and the relative levels generated by the fan and OGV, unlike the engine intake and bypass sections, wherein the noise from the fan and OGV arrive simultaneously in one direction and suffer some degree of acoustic blockage
due to one another. In this thesis, we exclusively focus on the noise measurements made in the intra-stage region to identify and reveal the fundamental characteristics of the fan BB noise and develop suitable phased array techniques to overcome the measurement challenges and accomplish the desired source separation.

In our present work, we propose a theoretical point source model for the engine intra-stage with empirically determined source parameters, which effectively reproduces the measured relative BB modal pressure distribution characteristics due to the fan and OGV. The consistency of these source parameters is demonstrated across different turbofan rigs and operating conditions to bring out their universality. The problem of acoustic mode detection and wave splitting in the engine intra-stage are systematically investigated with suitable phased array methods developed and implemented, which reveal the OGV to dominate the fan noise spectrum by more than 6 dB at both approach and cutback. The concept of modal axial group velocity is investigated for ducted swirling flows, based on which modal sound power distributions have been estimated for measured intra-stage noise data at approach and cutback, which show conformity as well as deviation from the ideal equal-power-per-mode concept.

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