Acoustic Shock Waves Generated by a Transonic Helicopter Blade

Recent experimental data show the importance of nonlinear effects in the propagation of sound by high-speed main rotor blades. Progressive distortion of a wave and the appearance of a shock in the far field wave form are effects that cannot be accounted for by linear acoustic theory. Calculations presented here are based on the usual linear monopole term that is modified by a variable local speed of sound in the rotor far field. The method, called nonlinearization, is similar to the technique used in sonic boom theory. Numerical results show that the rotor acoustic wave form becomes progressively more distorted with distance from the blade tip, and that the wave will steepen so that an acoustic shock forms. Calculations are given for a hovering rotor and compared with model data taken in an anechoic facility.