WITHDRAWN: Effect of Diameter Ratio on Aerodynamic and Aeroacoustic Performance of Contra-Rotating Rotors

This research experimentally investigates the effect of diameter ratio on the aerodynamic and aeroacoustic performance of coaxial contra-rotating (CCR) rotors in hover. The diameter ratio $DR$ of the upstream and downstream rotors ranges from 1.0 to 2.0 with a fixed upstream rotor diameter. The experiments were conducted separately under the fixed conditions of rotational speed and combined thrust, as well as varying the rotor axial spacing. The results show that increasing the diameter ratio significantly reduces noise emission at the fixed rotational speed, yet with a loss in thrust. At the fixed combined thrust, the CCR rotors exhibit the lowest noise at $DR=1.5$ with a total noise reduction of up to 2.8 dB, as a result of attenuated interaction noise and downstream rotor self noise, yet with a loss of aerodynamic efficiency. It was also found that broadband noise dominates the total noise and hence the noise reduction. Notably, the optimal diameter ratio achieves a noise reduction of about 1 dB, compared to the isolated single rotor with the same thrust. The optimal rotor spacing with minimum noise was found to be $0.25 D_1$ for most configurations, where $D_1$ is the upstream rotor diameter, though the noise level becomes nearly insensitive to spacing at very high diameter ratios. These findings provide new physical insights into the noise-thrust trade-off mechanisms and offer practical guidance on designing low-noise CCR systems in urban air mobility applications.