Numerical Investigation of Counter-Rotating Open Rotor Noise Emission in Different Flight Conditions

Abstract

Noise emission of a 9×7 and 8×8 open rotor configuration in cruise and in take-off conditions is examined by 3D unsteady numerical simulations utilising the chimera method to represent rotor movement. The acoustic analysis has been carried out with a Ffowcs Williams-Hawkings code over one rotor revolution with a resolution of 360 time steps. To ensure covering of all sources while keeping numerical losses low different hull surfaces have been examined. The comparison of two configurations at different flight conditions shows two main noise generating effects: the single rotor emission and emission caused by interaction of the rotors. The single rotor emission can mostly be seen in the rotor plane whereas the interaction can be examined at an angle of 20 to 45 and 135 to 155 degrees to the rotating axis with approximately the same share of total noise in take-off conditions. In cruise conditions the single rotor emission prevails over the interaction. This can be explained by the transonic blade tip speeds during cruise. Due to the reduced tip speeds in take-off interaction noise contributes to the total noise with a higher share than in cruise conditions. The 8×8-configuration shows higher noise emissions by interaction since the rotor-rotor interactions occur simultaneously.