The vortex ring state of a shrouded rotor

Abstract

An experimental investigation into the flow field produced by a shrouded rotor, operating in axial descent is presented. Laser Doppler Anemometry (LDA) was used to determine the induced velocity of the rotors, whilst Particle Image Velocimetry (PIV) was applied to investigate the dynamics of the unsteady flow field produced. Tests were repeated using the same rotor, without the shroud, to assess its influence on the observed phenomena. At low descent velocity ratios, the mean flow field resembled that produced by a shrouded rotor operating in hover. At slightly higher descent velocity ratios, the wake from the shrouded rotor broke down, leading to the formation of a large region of recirculation outboard of the shroud which aperiodically sheds into the free stream. Further increases in descent velocity lead the centre of recirculation to form around the external surface of the shroud. The flow topology appears to be generally similar to the one arising from an isolated rotor operating in descent flight, however the results suggest that the presence of the shroud might slightly delay the onset of the Vortex Ring State (VRS).