Random process-based stochastic analysis of multirotor hovering noise under rotational speed fluctuations

Abstract

A stochastic numerical analysis of a multirotor was performed considering the rotational speed fluctuation to investigate the acoustic characteristics. To validate the analysis, the noise was measured in an anechoic chamber at different azimuth angles (from 0° to 45°) and polar angles (from 0° to 67.5°) in revolutions per minute (RPM) assuming a multirotor hovering maneuver. Frequency and amplitude modulation characteristics due to RPM fluctuations were observed despite the considered hovering condition. Moreover, an azimuthal noise directivity pattern in a circular shape was observed, which corresponds to the collapse of the phase effect due to the RPM fluctuation of each rotor. In the existing numerical studies, the RPM fluctuation could not be considered due to the high computational cost. In this study, a random process was applied to reflect the RPM fluctuation effects through a validated multirotor noise assessment framework. To perform the stochastic analysis, ensemble averaging, a concept of random process, was applied to analyze the acoustic effects of the multirotor considering generalized RPM fluctuations. A quantitative analysis was conducted considering the spectrum, azimuthal directivity, polar directivity, and noise signal similarity. The results indicated that the proposed stochastic analysis could effectively predict the multirotor noise by taking into account the RPM fluctuation effect.