Acoustic imaging of the broadband noise arising in contra-rotating co-axial propellers

The interaction between two contra-rotating co-axial propellers leads to variations in noise strength during the rotation and presents challenges for traditional acoustic measurement methods. In this work, we propose using a wavelet-based beamforming method to study the transient noise generation characteristics of the contra-rotating co-axial propellers. Compared to the conventional beamforming method, the wavelet-based beamforming method can address unsteady and high-speed moving noise sources in the time-frequency domain. A representative set-up is considered herein. The acoustic imaging results show that the transient noise sources increase after the overlap of the propellers and are dominant on the aft propeller, particularly at high frequencies and low rotating speeds. At relatively high rotating speeds, the results show that the contra-rotating propellers noise can be generated at the trailing edge near the hub of the propellers. Overall, the wavelet-based beamforming method deepens the physical understanding of transient aerodynamic and aeroacoustic processes, and could find applications in the design of low-noise contra-rotating propellers.