This paper investigates the aeroacoustic interactions of small hovering rotors, using both experiments and computations. The experiments were conducted in an anechoic chamber with arrays of microphones setup to evaluate the azimuthal and polar directivity. The computational methodology consists of high fidelity detached eddy simulations coupled to the Ffowcs-Williams and Hawkings equation, supplemented by a trailing edge broadband noise code. The aerodynamics and aeroacoustics of a single rotor are investigated first. The simulations capture a Reynolds number effect seen in the performance parameters that results in the coefficient of thrust changing with the RPM. The acoustic analysis enables the identification of self-induced noise sources. Next, dual side-by-side rotors are studied in both counter-rotating and co-rotating configurations to quantify the impact of their interactions. Higher harmonics appear due to the interactions and it is verified that the counter-rotating case leads to more noise and a less uniform azimuthal directivity. Difficulties that arise when trying to validate small rotor calculations against experiments are discussed. Comparisons of computational and experimental results yield further insight into the noise mechanisms that are captured by each methodology.

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Joby Aviation is developing a six propeller, all electric vertical takeoff and landing aircraft. The aircraft is designed to operate near high population areas such as residences and workplaces, so it is imperative that the acoustic emissions of the aircraft are minimized for community acceptance. The aircraft design process incorporated the usage of high-fidelity computational aeroacoustics methods. Acoustic flight tests of the Joby aircraft were conducted by the National Aeronautics and Space Administration as part of the Advanced Air Mobility National Campaign and confirmed the revolutionary low noise footprint. The measured noise levels were compared with high-fidelity computational aeroacoustics predictions for two steady, level flight conditions. The results provide insight into the noise sources of the aircraft in these flight conditions. Airframe noise was predicted to be dominant for both flight conditions in the A-weighted spectrum due to the low propeller loading. Modeling efforts were made to account for acoustic shielding and airframe broadband noise and resulted in an improvement in the predictions, although the true noise sources are to be confirmed with further investigation.

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