Acoustic mode detection of high-frequency broadband noise in the aero-engine interstage

What is it about?

This paper presents a theoretical and numerical investigation into the development of in-situ interstage measurement techniques for separating the rotor and the stator broadband noise sources on a modal basis. The paper investigates a fundamental challenge to broadband acoustic mode detection brought about by the presence of a mean swirling flow in the engine interstage, which renders the existing phased array methods unreliable for use in the engine interstage. Using the idealized rigid body swirl model, the distinctive facets of a multi-modal sound field previously observed in the swirling flows have been explained with the resulting inferences validated using numerical computations. Based on the ensuing inferences, a new phased array technique has been developed, which consists of two close-by wall microphone rings to measure the broadband duct modes in the swirling flows. The new technique has been shown to sufficiently overcome the fundamental challenge to mode detection imposed by the swirling flow, and thereby deliver a cogent and reliable prediction in complex swirling flow conditions, rendering it suitable for use in the engine interstage.

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Photo by Felix Berger on Unsplash

Photo by Felix Berger on Unsplash

Why is it important?

Investigating the fan broadband (BB) noise requires the estimation of the relative contributions of its rotor (fan) and stator (Outlet Guide Vane (OGV)) components. However, despite several years of research, the balance between these major noise sources has still not been identified. Existing approaches to measuring fan BB noise have focussed on measurements in the engine intake and bypass ducts, which do not allow the balance between the fan and OGV to be determined. Therefore, estimating the relative levels of the rotor and the stator noise sources on a first approximation makes the engine interstage more suitable for an in-duct BB modal analysis, where the mean axial flow is accompanied by a comparable swirl component. Hence, developing suitable phased array methods for use in the swirling flows will be extremely valuable for making reliable modal measurements of the fan broadband noise in the engine interstage, across a wide range of fan speeds, to estimate the relative balance between the fan and the OGV noise sources. The present paper is a work in that direction. Also, a knowledge of the relative contribution of the rotor and the OGV sources will be invaluable towards guiding design decisions for noise abatement solutions as well as in the development of prediction models for aeroacoustic analysis.

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