Feasibility of Duct Source Identification in the Rotating Frame From External Measurement

Current methods of inner duct source identification work by measurements from the duct wall, causing damage to the duct wall and limited with immutable microphone positions. To overcome these defects, a methodology has been proposed by the authors recently to identify static sound sources inside a cylindrical duct with the microphone array placed and measured not on the duct wall but on the external of the duct. This approach has now been extended to include rotating sound sources. The forward model considers a rotating monopole source within a semi-infinite cylindrical duct and a static observer in outer space. The expressions for near-field and far-field radiation from the duct termination are derived through duct mode decomposition and the Rayleigh integral. Near-field and far-field sound pressure stimulated by the rotating monopole are compared to in-duct measurement cases, indicating the same periodicity in the azimuthal direction with in-duct cases and the advantage for rotating sound source identification in the external with fewer microphones in the circumference. The Doppler effect at out-duct observations is eliminated by the mode decomposition method for broadband rotating sources, followed by source identification using beamforming and the equivalent source method. Experiments are conducted to validate the possibility of rotating duct source identification by out-duct measurements, whose results indicate that rotating duct sources are well identified with much fewer azimuthal microphones compared to in-duct measurements. Additionally, the propagation model is extended to dipolar force sources, and the method is applied to identify the loading noise source of a ducted axial flow fan. The proposed method successfully distinguishes broadband noise sources on the fan blades in the source maps.