Coaxial Rotor Wake Measurements in Hover Using Phase-Resolved and Time-Resolved PIV

Abstract

Flow field measurements were performed on a 2-m-diameter coaxial counter-rotating rotor at a blade loading coefficient of 0.085. Two sets of experiments were performed: phase-resolved measurements on the coaxial rotor system with single blades on each rotor, and time-resolved measurements on the coaxial rotor system with two blades on each rotor. The goal of the measurements was to investigate the interactional aerodynamics of the coaxial rotors as well as the variation of rotor inflow over rotor radius and azimuth. The phase-resolved measurements were done at index angles of 0°, 10°, 45°, 90°, 180°, and 270° with 500 instantaneous flow realizations per azimuth captured over 500 rotor revolutions. Time-resolved measurements were done at 64/rev over 520 rotor revolutions. Tip vortex characteristics in terms of vortex trajectories and axial velocity profiles through the vortices were investigated. Aerodynamic interactions between the upper and lower rotor vortex wakes were apparent, mutually inducing velocities on each other. The tip vortices from the upper rotor were found to convect radially inward and axially downstream significantly faster than the tip vortices from the lower rotor, and their vortex core radius grew more quickly. Mean axial velocity profiles and their variation in the rotor spanwise and axial directions were measured above, below, and in between the rotor planes. The inflow velocities for each of the rotors as a function of rotor azimuth were approximated by interpolating the axial velocity at a single radial location immediately above and below the coaxial rotor planes. The measured data clearly showed the effects of the blade passage and the aerodynamic interactions between the upper and lower rotors on the inflow.