Measurement techniques for mode detection in aeroengine inter-stage sections

The sound field within an aeroengine duct can be expressed as a superimposition of acoustic modes. Knowledge of the modal pressure amplitude is useful for providing insight into the noise generation mechanism, assist in the design of sound absorbing liners, and is invaluable for determining the sound power. In-duct modal analysis allows the amplitude of each modal component to be determined from the sound pressure measured at the duct wall. Previous research has demonstrated the feasibility of using axial microphone arrays to detect the modes in the tonal and broadband sound fields. Broadband noise generally comprises all propagating modes, which would require at least as many microphones to deduce the amplitude for each mode index pair (m,n). Most existing studies have focussed on the modal analysis of broadband noise at the inlet or bypass sections of aeroengines. Meanwhile, all existing modal analysis techniques assume that the modes in the sound field are mutually uncorrelated. It is commonly believed that the broadband noise is mainly generated from the interaction of wake turbulence from the rotor with the leading edge of the Outlet Guide Vane (OGV), and the interaction of the boundary turbulence with the trailing edge of rotor blade. However, no work has been undertaken into modal analysis based on measurements in the engine inter-stage section aimed at understanding this interaction noise mechanism. The space restriction of the inter-stage section constrains the number of microphones that can be used. Therefore, innovative measurement techniques must be developed which can detect the modes in the limited spacing between the rotor and the OGV. This paper investigates two different measurement techniques suitable for this purpose. The first is based on measurements of the coherence function of the acoustic pressure between two measurement positions at the duct wall. The second uses a beamformer formed from an axial array of microphones at the duct wall. In this paper we present a simple acoustic model for the sound field in the engine inter-stage due to a rotating fan and an OGV. The model has a number of simplifying assumptions but includes realistic spanwise correlation characteristics through the use of simple semi-empirical turbulence models, which is necessary for predicting the correct modal correlation behaviour. The swirl is treated simply as a rigid body rotation. Based on the simulated acoustic pressure at the duct wall, the actual mode amplitude distribution and the estimated mode amplitude distribution from two techniques are compared. The modal information is obtained in the form of the mode amplitude versus modal cuton ratio for both rotor and OGV. Thus, the methods are effective as a means of determining the dominant noise source in the engine. The relationship between the two methods is explored.