Prediction of Noise from Low Reynolds Number Rotors with Different Number of Blades using a Non-Linear Vortex Lattice Method

The demand of micro air vehicles (MAV) with multiple rotors is increasing in both military and civil applications because of their versatility on various missions. However, the use of MAVs for some missions still has limited success because of their noise pollution. One of the main noise sources is aeroacoustic sound produced by the low Reynolds number flows around the rotors. There have been many previous studies about small-scale rotor systems of MAVs during the past decades, but they mainly focused on investigations of the aerodynamics rather than the acoustics. Several studies considering the acoustics have started recently. However, only steady loading forces computed by using Blade Element Momentum Theory (BEMT) were considered in the previous studies, and the noise from unsteady flow phenomena were not taken into account. The main objective of the current study is to investigate the noise mechanisms and further to find ways to reduce the noise levels in unsteady low Reynolds number flows. A Non-linear Vortex Lattice Method (NVLM) is used to simulate the unsteady low Reynolds number flow and model the corresponding noise sources. The tonal components of far-field noise is predicted by using an acoustic analogy based on Ffowcs Williams-Hawkings (FW-H) equations. These numerical methods are applied to low Reynolds number propeller and rotor cases, and validated upon experimental data. Then, they are used to investigate the influence of the number of blades on both aerodynamic and aeroacoustic performance and provide further insight into prominent sources of noise

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