Rotor-on-Rotor Aeroacoustic Interactions of Multirotor in Hover

Abstract

Multirotor configurations introduce complicated aerodynamic and aeroacoustic interactions that must be considered during aircraft design. In this paper we explore two numerical methods to model the acoustic noise caused by aerodynamic rotor-on-rotor interactions of rotors in hover. The first method uses a conventional mesh-based unsteady Reynolds-average Navier-Stokes (URANS) solver, while the second consists of a meshless Lagrangian solver based on the viscous vortex particle method (VPM). Both methods are coupled with an aeroacoustics solver for tonal and broadband noise predictions. Noise predictions are validated for single and multi-rotor configurations, obtaining with the VPMasimilar accuracy than URANS while being two orders of magnitude faster. We characterize the interactions of two side-by-side rotors in hover as the tip-to-tip distance and downstream spacing are varied. At an observer located six diameters away, multirotor noise is the strongest above and below the rotors, increasing by about 10 dBA directly underneath as the rotors are brought closer together. The interactions show no sensitivity to blade loading distribution, indicating that multirotor interactions are not alleviated with a lighter tip loading. We found that noise can be mitigated by spacing the rotors in the downstream direction - with the optimal spacing being about half a diameter - achieving a noise decrease of about 4 dBA without any aerodynamic penalties.

Aerodynamic and Acoustic Interaction Effects of Adjacent Propellers in Forward Flight

“…Studies in the literature involving multiple propellers have mainly focused on the hover condition. It is shown that proximity between propellers causes, depending on configuration, a detrimental effect on thrust generation, corresponding to a loss of up to 8% [9,10]. The higher percentages appear especially for hovering conditions.…”