Vortex ring state of a shrouded rotor: an experimental survey

The vortex ring state (VRS) is a flow condition typical of rotors operating in axial descent flight which may lead to large unpredictable thrust oscillations and possibly the loss of control of the rotorcraft. Despite the dangers associated with this flight condition, there is a distinct lack of detailed experimental data related to shrouded rotors operating in axial descent, which is of relevance owing to the number of novel unmanned aerial vehicles that incorporate this technology. This manuscript presents an experimental investigation designed to assess if and how the presence of the shroud affects the development of the vortex ring state. To this end, laser Doppler anemometry and particle image velocimetry were used to investigate the flow for a range of descent velocities and results were compared with those obtained without the shroud. Time-averaged data were used to assess the general structures of the flow fields, whilst statistical analysis of the velocity fluctuations and modal analysis of the velocity field using proper orthogonal decomposition highlighted the unsteady features of the flow. The investigation showed that shrouded rotors enter the VRS similarly to their isolated counterparts, and the presence of the shroud may be responsible for a slight delay of its onset.