Experimental Assessment of the Flow Recirculation Effect on the Noise Measurement of a Free-Flying Multi-rotor UAS in a Closed Anechoic Chamber

The flow recirculation effect on the noise measurement of a multi-rotor unmanned aircraft system (UAS) hovering in a closed anechoic chamber is experimentally characterized in this work. The measured acoustic spectrogram reveals that the recirculation forms around 30 s after the UAS’s take-off, manifested as prominent fluctuations in blade passage frequency and its harmonics. However, the instantaneous overall sound pressure level shows no obvious increase with the development of the recirculation. The result indicates that the recirculation effect does not significantly change the total acoustic energy but increases the uncertainties in the spectral distribution, which can be quantified by spectral entropy. The quantitative analysis of different noise components shows that the recirculation has a minimal effect on the tonal noise levels but slightly increases the broadband noise level out of the rotors’ plane. The results from parametric tests suggest that this broadband noise increment has a positive correlation with the UAS’s hover height but a negative correlation with the UAS’s gross mass. The comparison with existing studies highlights the difference in the recirculation effect on the noise of isolated rotor(s) and free-flying multi-rotor UAS in confined spaces.