Stochastic Approach for Estimating the Sound Power of Tonal Mode Components in Flow Ducts based on Azimuthal Mode Analysis

Abstract

The accurate experimental determination of the sound power in the ducts of aircraft engines, engine demonstrators and turbomachinery test rigs is challenging due to limited installation space for sensor arrays. Particularly in the inlet, a high number of propagating modes requires sensor arrays with multiple axial positions in order to decompose the measured sound field into its radial mode constituents. As an alternative, a sensor ring array requires a minimum of axial installation space and provides insight into the mode distribution using an azimuthal mode analysis (AMA). In this case, it is necessary to use radial mode distribution models, such as "equal mode energy" in order to estimate the sound power. However, the difficulty with tonal components, e.g. those generated by rotor-stator interaction, is that the radial modes are mutually coherent. This leads to a complex relation between the azimuthal mode amplitudes and the underlying mode distribution, which varies with the individual case, e.g. operating parameters of the rotor-stator configuration. To account for this variability, a stochastic approach is proposed in which, in particular, the phase relationships of the modes are varied and a distribution of sound power is determined. Its validity is evaluated by comparison with the results of a radial mode analysis obtained at a fan test rig.