Aeroacoustic investigations of a rotor-beam configuration in small-size drones.

Various aeroacoustic mechanisms involved in a rotor-beam configuration typically encountered in small-size drones in hover conditions are investigated both numerically and analytically, complemented with experimental data. High-fidelity lattice-Boltzmann method (LBM) simulations are performed on the complete experimental setup, capturing both the aerodynamic and the acoustic features of the configuration. The far-field noise is obtained by applying the Ffowcs Williams and Hawkings (FW-H) acoustic analogy. The rotor noise is also modeled as the sum of thickness noise, steady and unsteady loading noise corresponding to potential interactions between the blades and the beam. The analytical model of rotor noise relies on a strip theory, combining input velocity profiles from LBM and Sears's blade response function for each strip, and the FW-H analogy formulated in the frequency domain. The beam noise is modeled using a similar strip theory and a response model to the circulation of passing blades, based on the incompressible potential flow theory around a circular cylinder. Aerodynamic and acoustic results from the simulation and the models are in good agreement with measurements. Unsteady loading noise is found dominant for all tones for the present rotor-beam configuration corresponding to a small chord-to-beam diameter ratio. The three-dimensional directivities of some sound harmonics also have a unique wavy pattern in the rotor plane.