Modal analysis of the influence of freestream on global rotor noise

Abstract

The exact frequency domain solutions of the noise for rotating point sources are extended to acoustic modal domain. Moreover, the modal analysis method from the aerodynamic loading in the source modal domain to the aeroacoustic noise in the acoustic modal domain is established. A large amount of work has been done to directly predict the rotor noise at the observation points. However, there is very little work to consider the acoustic characteristics of the rotor contained in a large amount of data, such as the evolution of global noise in the space. In this study, the modal analysis method is used to quantify the acoustic characteristics of the rotor under different flight conditions. It is concluded that the acoustic modal components of the noise from the in-plane sources (mass source, radial force and circumferential force) are similar, but they are cross distributed with that from the out-of-plane source (axial force). The out-of-plane source becomes the main contribution source with the increase of forward speed, owning to the high radiation capacity of high-order harmonic components.