Numerical investigation of aeroacoustics for side-by-side rotor operating in proximity to the ground

The present study investigates the aerodynamic and aeroacoustic effects of the fountain switching flow phenomenon for small Unmanned Aerial System (sUAS) side-by-side rotor configurations in proximity to the ground. Scale-resolving computational fluid dynamics (CFD) simulations are conducted using the Lattice-Boltzmann/Very-Large-Eddy-Simulation (LBM/VLES) method to calculate the unsteady flow generated by the same configuration used in previous experimental studies. The simulations involve two APC 6×4 propellers positioned at one diameter height above the ground, with one diameter tip-to-tip clearance. The rotors operate at the same rotational speed in a counter-rotating manner. The simulated flow field reveals the deflection and radial expansion of the wake due to the presence of the ground, as well as the formation of an upward fountain-like flow between the rotors. Consistently with experimental observations, the fountain flow exhibits a bi-stable behavior characterized by a reciprocal movement between the rotors and subsequent re-ingestion by the rotor disks, occurring at a frequency that is two orders of magnitude lower than the rotor blade passing frequency (BPF). Furthermore, in order to highlight the aeroacoustic effects due to the rotor interaction, the noise spectra computed at several microphone locations are compared to those generated from an isolated rotor operating under identical conditions. The results indicate a significant increase in unsteady tonal noise and broadband noise components due to the flow re-ingestion effect.