Rotor Wake Investigation Under Autorotation Condition in Water Tunnel

Abstract

This paper focuses on the wake features of autorotation status mainly including the induced velocity and tip vortex trajectory in order to preliminary explanation the principle of rotation. In this work, the flow features of autorotating rotor were investigated in water tunnel via particle image velocimetry (PIV) technique. The experiments were conducted with a two-blade single rotor which can freely rotate. The stable rotational speed with various pitch and shaft backward angle was recorded and analyzed. The results show that there is a threshold value of stable rotational speed at shaft backward angle and an optimal region at pitch angle. The PIV results showed that the velocity of the internal wake was much lower than the external, indicating a consuming of energy from the flow, which was in accord with momentum theory. The vortices have three prominent parts: tip vortices, wake sheets and root vortices. There is no apparent interference between wake sheets and tip vortices. The vorticity of the tip was observed to increase in near wake due to lesser interference with the sheets. The Beddoes's model was employed to calculate the tip vortex location with respect to the rotor. The experimental results are in good agreement with the calculated results. It is meaningful for the study to enrich the autorotation theory of rotor and develop underwater rotorcraft.