Wake characteristics of a twin rotor in axial descent

In the present study, we investigate how the wake characteristics of a twin rotor change as the descending speed and the spacing between the rotors change. For this purpose, the velocity fields measurements are performed with varying the normalized rotor separation distance (0.25 ≤ s/R ≤ 2.5, where s is the distance between the adjacent rotor tips and R is the radius of the rotor) and the descent rate (0.87 ≤ V_D/ V_h ≤ 1.52, where V_D is the descent speed and V_h is the induced velocity at the rotor disk in hover). For the isolated rotor, the wake generates extensive recirculation zones near each rotor tip at a high descent rate of more than 1.08, entering the vortex ring state (VRS). For the twin rotor, the wake has different flow characteristics depending on the descent speed and the rotor separation distance. In the region between rotors (inner region), the rotor-rotor interaction collapses the vortex ring structure at the critical s/R, which has been lowered as the descent rate increases. On the other hand, the flow state in the outer region does not change regardless of the rotor separation distance. These results reveal that the strong rotor-rotor interaction causes the flow states in the inner and outer regions to differ from each other, leading to an asymmetrical wake structure about the rotational axis. Some more details will be discussed in the presentation.