Hot-Film Measurements and Data Evaluation of Rotor Tip Vortices in a High Pressure Wind Tunnel

Abstract

Helicopter rotors generate helical vortex systems that emerge at the blade tips, impacting both the aerodynamic and aeroacoustic performance of the rotor. Vortex properties are of significant interest, and previous research indicates they are influenced by the vortex Reynolds number. Studies of blade tip vortices at high Reynolds numbers are challenging for both numerical approaches and full-scale experiments; therefore, the Hoch Druck Windkanal Göttingen (High Pressure Wind Tunnel Göttingen) (HDG) is employed. The HDG was used to conduct measurements on a rotor scale model in a hyperbaric environment in order to obtain vortex Reynolds numbers similar to a full-scale helicopter rotor.

As the present work focuses on a rotor in hovering condition and the experiments were carried out in a closed-loop wind tunnel, an optimally sized shield was placed downstream of the test section in order to control the inflow velocity of the rotor. The costant temperature hot-film anemometery technique was chosen for this study due to its applicability to hyperbaric conditions and high frequency response, necessary to capture the rapid velocity fluctutations in the small-scale structures that characterize rotor tip vortices.

The results are compared to analytical vortex models to determine vortex characteristics such as position and core size when varying the Reynolds number. The findings demonstrate that the trajectory of the rotor tip vortices changes with the Reynolds number. Forced blade boundary layer transition showed an overall greater effect on the vortex trajectories than changing the Reynolds number in the range studied.