Numerical studies of modal scattering within turbofan bypass ducts

The accurate prediction of the effects of geometry and acoustic treatment on noise propagation and attenuation within bypass ducts is critical for the acoustic design of modern turbofan aeroengines. A large number of propagating modes are generally present in such ducts, and complex acoustic interactions occur between the duct geometry and the liner placement. In this paper, a numerical study of these effects is presented. Finite element models are used to predict modal scattering for multimode sources and complex geometries. A shell program is used for efficient implementation. The mean flow field within the duct is calculated by an embedded compressible Euler flow solver prior to the acoustic analysis. A modal representation is used at each end of the computational domain to generate a modal power transmission matrix. A parametric study is performed in which the duct configuration is systematically varied to illustrate the effect of duct geometry and liner specification.