Experiments on UAV rotor noise at low Reynolds and low Mach numbers

The present work outlines efforts at ISAE-SUPAERO to reduce the acoustic footprint of rotors operating at low Reynolds and low Mach numbers typical of small and medium scale drones. An experimental campaign that aims at characterizing rotor noise is presented. The latter serves as a basis for validation of numerical approaches and to investigate the noise mechanisms. Data are obtained in an anechoic room with dimensions (wedge tip to wedge tip) 5.02x5.24x5.34 m3 and cut-off frequency 80 Hz. Aerodynamic loads as well as acoustic fields are recorded for two off-the-shelf (APC 9x6 SF and 11x4.7 SF) and four canonical rotors with different numbers of blades. Both near and far acoustic fields are measured using one microphone located one diameter away from the rotor plane and 13 microphones mounted on a directivity antenna. A range of rotation speed from 1,000 to 10,000 RPM is addressed, corresponding to Mach and Reynolds numbers up to 0.23 and 130,000 respectively. Results presented in this article are related to two-bladed rotors and are validated using existing experimental data from the literature. They are then used to assess in-house numerical simulations (NL-VLM and CFD) and unravel the physics behind rotor noise at low Reynolds and low Mach numbers. This fundamental understanding will help draw general guidelines for the design of low noise rotors