This paper presents a detailed investigation of tonal noise produced by an unmanned aerial vehicle propeller operating with a circular strut mounted just downstream. Experimental measurements of the acoustic pressure in near- and far-fields are presented, and it is observed that the measured pressure signals contain a strong impulse caused by the propeller–strut interaction. The magnitude and shape of this impulse vary significantly with an observer location. It was also observed that the magnitude of these impulses was reduced by increasing the distance between the propeller and the strut. In order to investigate the physics of the noise generation mechanism, a number of computational fluid dynamics (CFD) simulations were performed, and a numerical method was developed to predict the radiated acoustic tones using the CFD data. This analysis showed that the unsteady loading sources on the strut and the propeller both made significant contributions to the total impulse, and the directivity of the sound radiated from the strut was different from that radiated from the propeller. Therefore, the addition of the signals from the propeller and the strut produced a pressure impulse, which had a complex directivity. Analytical models were also developed to estimate the unsteady loading on the propeller and strut from which estimates of the radiated noise field could be made. Predictions made using these models are compared with CFD predictions and experimental measurements of the unsteady loading on the strut and also the total radiated noise field and show reasonable agreement.
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