Effect of rotor–fuselage spacing on the development of the vortex ring state

A wind tunnel investigation to assess the impact of rotor–fuselage spacing on the development of the vortex ring state (VRS) and flow topology is presented. Particle Image Velocimetry was utilized to investigate flow mechanisms across a range of rotor–fuselage spacings (xs/R=0.34−1.56) and descent ratios (α=0.0−2.0), which were compared to the isolated rotor configuration. Mean velocity flow field data were utilized to identify coherent flow structures and features within the rotor wake, while flow unsteadiness was quantified through statistical analysis of the velocity fluctuations. For cases of VRS onset, the presence of the fuselage delays the development of the VRS for all rotor–fuselage separation distances tested. Furthermore, at an intermediate rotor–fuselage separation distance, a strong recirculation region between the rotor and fuselage is identified, resulting in a change in the VRS onset boundary. Moreover, at large descent ratios a decoupling between the development of the VRS and rotor–fuselage separation distance is observed.