局所運動量理論を用いた回転翼騒音の計算法

抄録

A method to estimate the dipole source noise of a rotor was presented. The method utilizes the Local Momentum Theory for the prediction of the angle of attack distribution on the disk. The influence of the angle of attack distribution on the rotor disk upon the acoustic waveform was discussed. In the present method, a new parameter which specifies the non-linear acoustic directivity caused by large blade pitch angle, twist, camber, or thickness ratio was introduced. This parameter has shown to give the noise level more than 20% larger than the previous result for the rotor having zero-twist angle, 4° pitch angle, and NACA 0010 section shaped blade. The noise level and the acoustic waveform computed from the present theory were compared with those computed from the previous theory for the same rotor thrust. The most distinctive feature of the present method comparing with the previous one is to use the variable angle of attack distribution on the disk instead of the uniform angle of attack distribution. The difference in noise level was not so big but the waveform computed from the present theory has shown many peaks caused by blade-vortex interactions whereas the previous one was smooth and simple. Strong correlation between the sudden increase in the angle of attack and the negative peak in the observed waveform was shown for the rotor in the upwash field where the interaction effect was intense especially when the direction of the tip vortex shedded from the former blade coinside with the span direction of the following blade. Then it was concluded that the non-uniform distribution of the angle of attack on the rotor disk should be adopted for the estimation of the rotor noise of helicopter in forward flight or in descending flight to account intense blade-vortex interactions.