A classification of duct modes based on surface waves

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For the relatively high frequencies relevant in a turbofan engine duct the modes of a lined section may be classified in two categories: genuine acoustic 3D duct modes resulting from the finiteness of the duct geometry, and 2D surface waves that exist only near the wall surface in a way essentially independent of the rest of the duct. Per frequency and circumferential order there are at most 4 surface waves. They occur in two kinds: 2 acoustic surface waves that exist with and without mean flow, and 2 hydrodynamic surface waves that exist only with mean flow. The number and location of the surface waves depends on the wall impedance Z and mean flow Mach number. When Z is varied, an acoustic mode may change via small transition zones into a surface waves and vice versa. Compared to the acoustic modes, the surface waves behave-for example as a function of the wall impedance-rather differently as they have their own dynamics. They are therefore more difficult to find. A method is described to trace all modes by continuation in Z from the hard-wall values, by starting in an area of the complex Z plane without surface waves. Keywords: Acoustic surface waves; Duct geometries; Duct modes; High frequency HF; Mean flow; Transition zones; Wall impedance; Wall surfaces
Original languageEnglish
Title of host publication7th AIAA/CEAS Aeroacoustics Conference (Maastricht, The Netherlands, May 28-30, 2001)
Number of pages11
Publication statusPublished - 2001
Event7th AIAA/CEAS Aeroacoustics Conference - Maastricht, Netherlands
Duration: 28 May 200130 May 2001


Conference7th AIAA/CEAS Aeroacoustics Conference


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