Noise generation by transonic open rotors

The problem of predicting the noise signature of open transonic rotors is considered. Certain experimental features which cannot be explained by linear aero-acoustic theory are highlighted. It is argued that the acoustic analogy cannot always be used effectively on this problem because of difficulties concerning the specification and computation of the quadrupole field. Instead, an alternative approach is described, based on the non-linear equation of transonic rotor blade flow. Although this equation is non-linear, it is shown to possess properties which can account for these unexplained experimental features. A method for calculating the acoustic field is described which avoids the pitfalls of the acoustic analogy. Examples are presented which suggest that good practical predictions are possible in this speed range.