Abstract: Numerical simulation of propagation of rotor / stator interaction noise in turbofan exhaust duct which is assumed to be made up of annular ducts with axially segmented treatments is developed by the mode analysis method. By quantitatively predicting the change of the mode coefficient distribution of forward and backward propagating acoustic energy at the segment interfaces, the attenuation of interaction noise is also investigated. The results manifest that when the forward travelling sound waves pass through the admittance discontinuity planes, the modal reflection will increase and the forward modes of waves will decrease. So the modal reflection at the interfaces can be utilized to suppress propagation of the in-duct noise. In addition, the effects of the change of resistance and reactance of liners on the suppression are theoretically calculated. It can be concluded that in the presence of high frequency source and high Mach number flow, whether for the change of resistance or for the change of reactance, there exist local peak values and fluctuation of the in-duct suppression, and at this moment, the monotonicity of suppression with the change of resistance and of reactance will no longer come into existence. Therefore, it is necessary for the engine noise reduction to choose adequate impedance of liners.

Key words: aeroacoustics annular exhaust duct, sound propagation, mode analysis, uniform flow