Comparison of Computational Aeroacoustic Prediction Methods for Transonic Rotor Noise

This paper compares two methods for predicting transonic rotor noise for helicopters in hover and forward e ight. Both methods rely on a computational e uid dynamics (CFD) solution as input to predict the acoustic near and far e elds. For this work, the same full-potential rotor code has been used to compute the CFD solution for both acoustic methods. The e rst method employs the acoustic analogy as embodied in the Ffowcs Williams› Hawkings equation, including the quadrupole term. The second method uses a rotating Kirchhoff formulation. Computed results from both methods are compared with one another and with experimental data for both hover and advancing rotor cases. The results are quite good for all cases tested. The Kirchhoff method was somewhat sensitive to the location of Kirchhoff surface, if the surface was positioned too close to the rotor blade. The acoustic analogy method was not as sensitive to the extent of volume included in the quadrupole calculation. The computational requirements of both methods are comparable; in both cases these requirements are much less than the requirements for the CFD solution.