Investigation of Vortex Ring State Onset in eVTOL Aircraft using Viscous Vortex Particle Method

This paper investigates the feasibility of using the Viscous Vortex Particle Method (VVPM) to simulate Vortex Ring State (VRS) and analyzes the detailed flow physics. Evaluating various rotor configurations, including isolated rotor, dual side-by-side rotors, coaxial rotors, and multiple rotors of a full eVTOL air vehicle configuration, the study found that VRS onset is accurately captured using VVPM. Statistical analysis and flowfield investigation help identify VRS onset, showing good agreement with literature for isolated rotor configurations. Roll rate effects on dual side-by-side rotors and mutual wake interactions in a coaxial rotor configuration were examined, and revealed valuable insights into the system behavior upon VRS onset. This study also investigated multi-rotor wake interaction in a full flight lift and cruise configuration, which exhibited larger thrust fluctuations in installed rotors compared to an isolated rotor in the same flow condition. More specifically, a detailed analysis of the full aircraft flowfield indicated a sideward trajectory of rotor wake vortices on the installed rotors, which interacted with vortices on the opposite end of the rotor disk, resulting in VRS onset. Additionally, the frequency content of the rotor thrust was evaluated, and the unsteady nature of the rotor wake induced various peaks at multiples of the blade passage frequency just prior to, and within the VRS regime for all configurations.