Aerodynamic and acoustic investigations of rotor-rotor wake interaction

Abstract

The paper presents part of the experimental activities carried out in the GARTEUR Action Group RC/AG-26 to study the acoustic and aerodynamic characteristics of small rotor configurations, including the influence of the rotor-rotor interactions. Two rotors, equipped with two-bladed propellers of a diameter of D=330.2 mm, were operated at two rotating speeds of 8025 RPM and 10120 RPM and arranged in different configurations. Isolated rotor and two different coaxial configurations in hover conditions were assessed. The aerodynamic loads and the rotor slipstream characterisation, in terms of flow field velocity, tip vortex characterization as well as acoustics emissions, were performed using a six-component load cell, Particle Image Velocimetry and microphone measurements. The isolated pusher rotor presented better aerodynamic performance with respect to the puller one. The coaxial configurations result in a loss of thrust and an increase in torque compared to the isolated rotors. The upper rotor is barely affected by the presence of the lower one, while the lower rotor experiences a thrust loss ranging from 17 to 19%. The configuration with the larger rotor distance experiences greater performance loss. The mean velocity fields of the isolated rotors are measured and compared with the coaxial configurations for assessing the roto-rotor interaction, the results support the aerodynamic behaviour. The isolated rotor slipstream characterisation was carried out by phase-locked measurements covering the full rotor azimuth angle range from 0° to 350° with an increment of 30°. Tip vortex paths are followed up to a vortex age of 3.5 rotor revolutions. The 3D reconstruction of the tip vortices is conducted to gain insight into the slipstream behaviour and enhance the occurring vortex roll-up during the second revolution. The tip vortex jitter is investigated and the anisotropic distribution is confirmed along the slipstream. Furthermore, the noise emission intensifies in magnitude for the coaxial configuration with increasing relative gap between the rotors. The turbulent kinetic energy and the meandering of the blade tip vortices support the findings of the acoustics results.