Hot-Film Measurements on Rotor Tip Vortices in the High Pressure Wind Tunnel Göttingen (HDG)

Helicopter blades create helical vortices at the tip of the rotor, so the wake is characterized by a complex three-dimensional velocity field. These vortices affect both the aerodynamic and aeroacoustic performance of the rotor itself, and therefore the study of vortex core size, circulation and vortex pairing for a wide range of vortex Reynolds numbers is of great interest. Since the study of such vortices presents a challenge both with a numerical approach or with an experimental one on a full-scale helicopter, the High Pressure Wind Tunnel Göttingen (HDG) is employed. Such a facility makes it possible to work with a rotor scale model in a hyperbaric environment in order to obtain vortex Reynolds numbers characteristic of a full-scale helicopter rotor. As the interest of the present work concerns a rotor in a hovering condition and the analysis was carried out in a closed-loop wind tunnel, an optimally sized shield was placed downstream of the test chamber in order to control the inflow velocity of the rotor. The constant temperature hot-film anemometer technique was chosen for this study because its effectiveness is not affected by the increase in external pressure and provides a very high frequency response, necessary to capture the rapid velocity fluctuations in the small-scale structures that characterize rotor tip vortices.