High-fidelity Aerodynamic Investigation of Rotors within the Vortex Ring State

The Vortex Ring State (VRS) is an intriguing phenomenon where rotary wings are trapped in their own wake. It is inherently difficult to model with the classic momentum theory due to the breakdown of slipstream assumptions. In practice, it is still a critical safety concern for helicopters and emerging multi-rotor platforms. Despite extensive wind tunnel tests, flight tests, and modelling over the past decades, our quantitative understanding of the underpinning flow details is still limited, because of limitations in measurements and modelling resolution. First-principles-based, high-resolution simulations could uncover the flow details, but the modelling is still rare and challenging due to complexities in the flow and flight physics, and particularly the associated high computational costs. Nevertheless, in this work, a series of high-resolution simulations of the VRS phenomenon are presented. Fully blade-resolved and unsteady simulations of an isolated helicopter rotor within the VRS were carried out for over 20 revolutions. The simulations were performed using the Helicopter Multi-Block 3 (HMB3) CFD framework developed at the University of Glasgow, solving the fully compressible and unsteady Reynolds-averaged Navier-Stokes (RANS) equations with a Scale-Adaptive Simulation (SAS) closure. The modelling revealed the formation of the large vortex ring stemming from the discrete tip vortices, and its evolution over the 20 revolutions. The blade loading distribution and evolution were analysed and compared with test data. Moreover, we extracted the inflow features from the CFD results via direct extraction and inverse Blade Element Theory (BET). The results highlighted the strong induction of the vortex ring, and the influence of secondary flow features besides the inflow. It was also noted, with the correct inflow information, the BET was able to reconstruct the VRS loading with reasonable accuracy. These high-fidelity results provide unique insight into the flow and flight physics underpinning the VRS, and contribute to the ongoing GARTEUR AG28 collaboration on multi-rotor VRS investigation.