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Sound in flow ducts with sharp edges
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.ORCID iD: 0000-0003-4103-0129
Växjö University, International Centre of Mathematical Modelling School of Mathematics and Systems Engineering, Sweden.
2007 (English)In: 13th AIAA/CEAS Aeroacoustics Conference (28th AIAA Aeroacoustics Conference), 2007Conference paper (Refereed)
Abstract [en]

Important transmission paths for the noise produced by fans, engines and other machinery are the connecting ducts used for transport of gases. Hence, reliable methods for calculating the acoustic attenuation in such systems are of great interest. In the presence of sharp edges strong interaction between sound and flow may occur even at low Mach numbers, which should be accounted for. The interaction has been successfully described using the vortex sheet model with an unexpanded and unstable jet. The current paper deals with the generalization to stable jets. By using the so-called Building Block Method, rather complex silencers can be modelled from the results of two canonical problems: the scattering at the trailing and leading bifurcations, respectively. The strong flowacoustic interaction occurs at the trailing edge only. Results are presented here for the bifurcation and the sudden area change at the trailing edge. The flow in the large part of the duct downstream and upstream of an area change is modelled in two regions where the acoustically thin shear layer is described by a newly proposed set of coupling conditions. We use a simple model with physically realistic stability properties for acoustically thin layers allowing for a hydrodynamically thick shear layer. In fact, the dynamic properties of the shear layer are changed continuously with the shear layer Strouhal number s from the unstable at vanishing s to a stable layer at high s. The transfer Strouhal number marks the border between the unstable and stable region. Like the vortex sheet model, two coupling conditions relate the fields on each side of the sheet, one of them being continuity of pressure. The second coupling condition means continuity of a variable ranging from displacement, similar to the vortex sheet model, at vanishing s via velocity to pressure gradient at infinite s. The used shear layer model is uniformly valid for all s and allows a straightforward generalization of a scattering theory for unstable shear layers, i.e. for small s. Analytic as well as numerical results for the acoustic scattering are presented.

Place, publisher, year, edition, pages
Keyword [en]
Bifurcation (mathematics), Ducts, Machinery, Scattering, Shear flow, Stability, Strouhal number, Vortex flow
National Category
Vehicle Engineering
URN: urn:nbn:se:kth:diva-154696ScopusID: 2-s2.0-84884778448ISBN: 978-162410003-1OAI: diva2:758572
13th AIAA/CEAS Aeroacoustics Conference (28th AIAA Aeroacoustics Conference), 21 May 2007 through 23 May 2007, Rome, Italy

QC 20141027

Available from: 2014-10-27 Created: 2014-10-27 Last updated: 2016-09-20Bibliographically approved

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Boij, Susann
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