Dept. of Mech. Eng., Univ. of Houston, Houston, TX 77204-4792
After a brief historical account introducing some lasting concepts, the origins and main features of aerodynamically generated sound are described in terms of the melding of some pertinent elements of fluid dynamics and of acoustics. These are, for example, flow separation, flow instability, and vortices on the one hand, coupled with the hydrodynamic flows of acoustic monopoles, dipoles, and quadrupoles on the other. With emphasis on the vortex theory of aerodynamic sound, various theoretical approaches are generally described in physical terms and are illustrated by a variety of sound-generating flows. These may be classified as free flows with no solid surfaces present (spinning vortices, turbulent jet noise, supersonic jet screech), flows over rigid surfaces (boundary layer noise, whistling telephone wires, edge tones, pipe tones, vocal fricatives, and whistling, organ pipes), interaction with steadily moving surfaces (helicopter blade slap, fan blade interaction, but not including the rotational noise of fans and propellers), flow with excited surfaces (interior aircraft boundary layer noise, aeolian tones, reed and lip-driven musical instruments, vocal chord vibration, snoring and raspberries). Some of these involved no resonance at all, while others have flow resonance, acoustic resonance or mechanical vibration or resonance.