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An experimental campaign was undertaken to identify the effects of flow recirculation on an isolated rotor's acoustic emissions in a closed anechoic chamber. It is shown that flow recirculation results in a significant increase in higher harmonic noise, with an increase of more than 15 dB in some harmonics. This increase in noise is due to the nature of testing in a closed facility and does not represent the acoustic emissions of a similar rotor in hovering flight outdoors. Future measurements of rotors and full unmanned aerial vehicles in closed anechoic chambers must acknowledge recirculation effects and attempt to mitigate the effect on reported measurements.
Stephenson et al. (Fri,) studied this question.