7.1 Introduction

The term aero-acoustics has come to mean the part of acoustics which deals with problems encountered in aerodynamics, for example, sound generation by flow and by devices such as fans and compressors. Sometimes the term aero-thermo-acoustics is used to indicate that also acoustical problems related to heat release and other thermal effects are included.

The second part in the title of this chapter, instabilities, indicates that we are going to treat problems which result from the intrinsic instability of fluid flow. It means that oscillations (and sound) can be produced without any external oscillatory driving force, but merely as a result of the flow breaking into spontaneous oscillations. The sound or vibrations produced as a result can feed back on the flow and promote or stimulate the instability so that the amplitude will grow exponentially with time until some damping mechanism eventually limits the amplitude. Such vibrations often cause mechanical (acoustic fatigue) failure.

In regard to sound generation, in general, we have considered so far mainly vibrating boundaries or piston sources producing density fluctuations and corresponding pressure fluctuations. In the case of a pulsating sphere, for example, the acoustic flow source strength, the rate of mass transfer to the surrounding fluid, was denoted q _f and the corresponding acoustic source by q = _f (or q( ?) = ? i ?q _f( ?) for harmonic time dependence). The oscillating acoustically compact sphere was found to be equivalent to a point...