Acoustic Modal Analysis Method for Helicopter Acoustic Scattering

Helicopter acoustic scattering can change the acoustic characteristics of the rotor noise and has an adverse influence on its prediction and control. Previous research has mainly focused on numerical simulations and principle experiments. In this study, an analytical method in the acoustic modal domain is proposed to predict the scattered sound field of the helicopter fuselage based on the T-matrix method. This method utilizes the advantages of modal analysis to investigate the global scattering process of the fuselage on steady and unsteady rotor noise from different sound sources. Specifically, the rotor noise is modeled as equivalent to noise from discrete rotating monopoles and dipoles. The acoustic modal components of the incident sound field are derived theoretically. Furthermore, the T-matrix method is used to establish the mapping relationship between the acoustic modal components of the incident and scattered sound fields. The acoustic modal components of the scattered sound field are predicted theoretically. In addition, the acoustic modal analysis method is verified by the simulation results calculated by the frequency-domain method. The T-matrix method is verified by the experimental and numerical results presented by DLR (German Aerospace Center). Finally, a mechanism analysis is performed in the acoustic modal domain to investigate the acoustic characteristics of the scattered sound field. Benefiting from high computational efficiency and low memory consumption of the proposed method, it has the potential for online use.