Searching for sound-source mechanisms in the flow equations
Peter Jordan
Université de Poitiers, France
Abstract:
The local physical mechanisms by which compressible fluid in turbulent motion drives an 'internally-generated' sound-field are currently very poorly understood; this is largely due to the doubtful theoretical foundations on which aeroacoustics is currently built. This fact, however, is not always clearly admitted in the aeroacoustic literature, and this has led to a rather unquestioning acceptance of the physical validity of acoustic analogies. When used to understand local mechanisms, most of these analogies can only lead to internal contradictions, or entirely unphysical interpretations.
The seminar will outline the essential ideas behind the acoustic-analogy, with a particular focus on the problem of local source-mechanism identification – to what extent can these theoretical tools be useful in this regard ? The question of the separation of a compressible turbulent field into acoustic and non-acoustic types of motion will be discussed, as this appears to be the central problem; and it will be demonstrated how, even in the simplest, irrotational, linear acoustic systems, this question is perhaps not as clear as one might think. The seminar will include analytical, numerical and experimental examples of the problem of source-mechanism identification.