Suppression of instability waves in numerical simulations of sound propagation from aft ducts

The scattering effects from mean flow shear layer should be accounted for in the numerical simulation of sound propagation from aft ducts. However, the time domain simulation of the linearized Euler equation has to face the difficulties of the generation and amplification of Kelvin-Helmholtz instability waves in non-uniform shear flows. The available methods for the suppression of the instability waves are difficult to give satisfactory results. The main objective of this paper is to check the feasibility of a hybrid method. Firstly, a broadband sound source wave packet with limited time span is introduced for the separation of acoustic waves from instability waves. Then, a source filtering technique is adopted to suppress the instability waves. Half infinitely length annular ducts are selected for numerical validations. The 2.5D linearized Euler equations are solved in the time domain with a computational aeroacoustics approach. Numerical results agree fairly well with analytical solutions for no mean flow cases which show the accuracy and reliability. Furthermore, it is demonstrated that the wave packet method with the help of source filtering technique can remarkably suppress the instability waves without the influence to the sound field for non-uniform mean flow cases.