THE STRUCTURE OF WOBBLING SOUND FIELDS

The acoustic field of rotating sources such as aeroplane propellers and helicopter rotors is known to develop a marked asymmetry when the incoming flow is not parallel to the axis of rotation. This effect has been modelled using a field composed of “wobbling modes”, azimuthal modes whose amplitude is a function of azimuth. A computationally efficient method is developed for the calculation of these wobbling modes and of the acoustic field at incidence. Results are presented for operating conditions representative of a range of aircraft from a high-speed propeller at low incidence to a helicopter rotor. Detailed contour plots of the acoustic pressure are presented and discussed in the context of the geometry of the acoustic field. The field structure is interpreted in terms of the tunnelling of acoustic radiation across a transition region around the sonic radius (where the source has a Mach number of unity) and the asymmetry of the field is shown to arise from variations in the thickness of this transition region as the sonic radius varies during a revolution of the rotor.