Noise source separation in low-Reynolds multi-propeller systems: experimental techniques and post-processing approaches

Small propellers and rotors operating at low Reynolds numbers exhibit predominantly broadband noise, so that the tonal noise at the blade passing frequency and its higher harmonics (more significant in large-scale propellers and rotors) is masked by broadband noise in acoustic spectra obtained from reduced-scale wind tunnel tests. Thus, to evaluate the decay of blade passing frequency harmonics in rotating noise sources obtained from reduced-scale tests, it is necessary to apply techniques capable of synchronously measuring the angular position of each blade along with the acoustic pressure. In this context, techniques such as ensemble averaging can be employed to enable the measurement of noise associated with the blade passing frequency and its higher harmonics even when broadband noise predominates. This work describes the experimental setup and post-processing techniques used at the Experimental Aerodynamics Laboratory of the São Carlos School of Engineering, University of São Paulo, to perform noise source separation in small, low-Reynolds-number propellers and rotors in multi-propeller and multi-rotor systems.