Sound from rotors in non-uniform flow

An analysis is presented for the evaluation of the acoustic field of a rotating source in a non-uniform potential flow. Other than the restriction to low flow Mach numbers, the method is exact and general. The variation in radiation properties with source position is handled by representation as a Fourier series in source angle, giving rise to an asymmetrically varying acoustic field evaluated by summation of the series. The method is used to develop an exact solution for the model problem of a rotor operating near a cylinder in cross-flow and sample calculations demonstrate the accuracy of the technique when compared to full numerical evaluation. The calculations show changes of greater than one decibel in the acoustic field due to flow non-uniformity at a flow Mach number of 0.15, a typical speed for propeller aircraft at take-off.