Abstract: The boundary element method is applied to analyse three-dimensional acoustic characteristics of ducts and expansion chambers with mean flow, The boundary integral equation and fundamental solution of wave propagation in the presence of mean flow have been obtaind. The nine nodes quadratic isoparametric elements areused to discretize boundary surface. The polar coordinate transformation method and indirect method are jointly used to treat singular integration problem, the different partical vibrating velocities are supposed at points of corner, the four-pole parameters of a stright duct and transmission loss of an expansion chamber with and without mean flow have been calculated and compared with results obtained by means of one-dimentional theory and other methods.

Key words: boundary element method, expansion chamber, acoustic characteristics, mean flow, three-dimensional effect