Sound transmission in a finite-length lined annular duct with mean swirling flow is investigated using mode-matching methods. 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 velocity. First, a spectral collocation technique is used to determine the eigenmodes of three-dimensional linearized Euler equations, then two mode-matching schemes are utilized for calculating the sound transmission in ducts. The two schemes are compared with each other and also with the finite element method. The modified matching scheme is believed to deal with the impedance discontinuity more accurately at the interface. A sound power definition for high frequencies in the presence of mean swirling flow is used for the analysis of sound transmission characteristics. The modified matching scheme is then used to analyze the effect of mean swirling flow on the sound power transmission loss and conduct liner optimization in the impedance plane, compared with the uniform axial mean flow case. Finally, sound attenuation due to inner wall lining, outer wall lining, and combined inner and outer wall lining, respectively, is investigated.

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