Investigation of Aeroacoustic Effects of Trailing-Edge Modifications to Blades of a Coaxial Counter-rotating Rotor
Type of DegreeMaster's Thesis
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Noise generated by the rotor blades of modern UAVs pose significant limitations to the widespread adoption of UAVs in the future. To mitigate noise production, rotor blades with a serrated trailing-edge have been used to reduce the overall sound pressure level (OASPL) generated by these systems. Past studies, primarily conducted for single rotor systems, have demonstrated moderate reductions in frequency specific sound pressure levels, and reductions as high as 3 dB in OASPL. However, the efficacy of such noise mitigation techniques for rotors operating in a coaxial configuration, a common design in modern UAVs, have yet to be studied. The primary objective of this study is to characterize the sound pressure level reduction capability of the addition of trailing-edge serrations on a coaxial counter-rotating rotor system. A counter-rotating coaxial rotor test stand was designed and fabricated to enable independent rotor thrust and torque measurements, while an array consisting of 8 microphones was used to map the acoustic field of the rotors in operation. The aerodynamic and aeroacoustic performance of the coaxial counter-rotating rotor system were evaluated with and without serrated trailing-edges. Results confirmed that serrations on the trailing-edge of the rotor blades of a coaxial rotor with a fixed pitch angle and rectangular planform area, attenuate OASPL by 0.9 dB collectively and the middle-to-high sound pressure level (MHSPL) by 1.8 dB. Twisted rotor blades with serration modifications displayed a benefit as high as 2.5 dB, with some combinations of rotational speed and azimuthal angles showing negative reductions. It is expected that increased turbulent mixing is responsible for the decrease in overall sound pressure level from the serrated blades. Particle image velocimetry was used to confirm the contribution of turbulence in the downwash. In summary, the modifications made to these coaxial rotors were found to be less beneficial to the overall noise produced by the system, when compared to a single rotor UAV.