Normal mode analysis for the sound propagation in a lined annular duct with mean swirling flow

Sound propagation in a lined annular duct with mean swirling flow was investigated via the normal mode analysis of three-dimensional (3-D) Linearized Euler equations, and was calculated as a generalized eigenvalue problem by a spectral collocation technique. Axial eigenmodes spectrum which are mainly composed of nearly/purely acoustic modes and nearly/purely convected modes with mean swirl and relative unsteady disturbances can then be determined. Mean swirl may even change the number of the propagating acoustic modes, and consequently affect the cut-on/cut-off characteristics of sound propagation. After validation of the approach, the effects of flow parameters and liner on the acoustic modes and convected modes are discussed.