The helicopter blade flow for majority of helicopter flight operation conditions is unsteady. This is due to periodically changing, blade angle of attack and background flow velocity variations. Two cases of unsteady background flow existing in the rotor blade aerodynamics are investigated experimentally and numericaly in the present paper: (i) harmonic oscillations and (ii) a vortex convected in a uniform flow field (BVI problem). The oscillating background flow is characterized by periodically appearing, accelerations and decelerations. It was observed in the present study that the acceleration of the background flow causes the initially separated flow to be attached to the airfoil surface along its entire length. This effect occurs also for relatively high airfoil angle of attack. In contrary to this the deceleration induces separation which for some extremal cases reaches the leading edge of the airfoil. In this case the weak shock waves, which are shed periodically, move upstream like a transonic sound waves. The sound waves produced due to the BVI strongly depends on the vortex trajectory. They are much stronger when the vortex approaching the airfoil accelerates the flow at the pressure surface of the airfoil than in the case when the vortex decelerates the flow at the suction surface. The former corresponds to the clockwise rotating vortex passing under the blade.
Acoustical aspect of helicopter rotor blade flow instabilities
International Congress on Sound and Vibration, 7 ; 1363-1370
2000
8 Seiten, 6 Bilder, 11 Quellen
Conference paper
English