A comparison between mode-matching and finite-element for sound propagation with mean swirl

Sound transmission in a finite-length lined annular duct with mean swirling flow is investigated using both Mode-Matching Method (MMM) and Finite-Element Method (FEM). The main application of this work is the acoustics of aeroengine duct systems, especially the prediction of sound transmission behind a fan/rotor stage where the swirl velocity could be comparable to the axial component. First, a spectral collocation technique is used to determine the eigenmodes of three-dimensional Linearized Euler Equations (LES), then MMM and FEM are utilized for calculating the sound transmission in ducts. The two methods are compared with each other for different mean flows, frequencies and circumferential mode orders. A good agreement is obtained. Meanwhile, the effect of mean swirl, frequency and circumferential mode order on the acoustic field is investigated briefly.