An experimental investigation of the vortex wakes of hovering and descending rotors

Two sets of experiments are performed on a three-bladed rotor model, one in a towing tank to simulate descent, and the other in a stationary tank to simulate hover. The rotor's rotational speed, collective pitch angle, descent angle, and descent speed are all varied, simulating a wide range of rotorcraft operating states. For this work, circulation-based Reynolds numbers are of order 105 and chord-based Reynolds numbers are of order 104.; The focus of the first set of experiments is on descent speeds and angles where the rotor is operating in or near vortex ring state. Simultaneous flow visualization and thrust measurement allow the rotor's performance to be correlated to the development of the vortex wake. Periodic shedding of vorticity from the wake associated with vortex ring state is observed, resulting in peak-to-peak thrust fluctuations of up to 95% of the mean and occurring at regular intervals of 20--50 rotor rotations---with the oscillation period exhibiting a power law dependence on the advance speed and varying inversely with the collective angle.; The hover experiments use particle image velocimetry (PIV) and flow visualization to analyze the development of the wakes from either rectangular planform rotor blades or blades with triangular flaps attached near the blade tips. The dynamics of the two wakes differ substantially, with short-wave instabilities observed developing on the tip vortices of both wakes, while a long-wave instability develops only in the wake of the rectangular blades. The rectangular blade wake also decays somewhat more rapidly than the triangular-flap blade wake. While the wake can be altered significantly through modifications to the blade planform geometries, the trajectories of the tip vortices are not significantly affected by varying the rotor speed or collective angle. The effects of the experimental techniques employed here on the flow are also explored, with the injection of fluid from the blade tips in flow visualization experiments having a particularly strong effect on the strengths of the tip vortices.