Non-Axisymmetric Acoustic Propagation in and Radiation from Lined Ducts in a Subsonic Uniform Mean Flow: An Axisymmetric Boundary Element Formulation

A direct collocation boundary element formulation for the calculation of acoustic propagation in a subsonic uniform mean flow is presented. The model is directed towards calculation of the propagation inside and radiation from axisymmetric lined ducts. The model takes its starting point in an axisymmetric boundary element formulation, but is extended to non-axisymmetric boundary conditions so non-axisymmetric excitation is feasible (spinning modes). The singularities in the Green's function and its normal derivative are integrated analytically over the angle of revolution and expressed as sums of elliptic integrals in order to minimize the computational work. An iterative scheme allows the modelling of infinite and semiinfinite circular ducts.