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Revisiting the cremer impedance
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.ORCID iD: 0000-0001-7898-8643
2017 (English)In: Proceedings of Meetings on Acoustics, Acoustical Society of America , 2017, no 1Conference paper, Published paper (Refereed)
Abstract [en]

In a classical paper (Acustica 3, 1953) Cremer demonstrated that in a rectangular duct, with locally reacting walls, there exits an impedance (”the Cremer impedance”) that maximizes the propagational damping for the lowest mode. Later (JSV 28, 1973) Tester extended the analysis to include a plug flow and ducts of both circular and rectangular cross-section. One limitation in the work of Tester is that it simplified the analysis of the effect of flow only considering high frequencies or well cut-on modes. This approximation is reasonable for large duct applications, e.g., aero-engines, but not for many other cases of interest. Kabral et al. (Acta Acustica united with Acustica 102, 2016) removed this limitation and investigated the’exact’ Cremer impedance for circular ducts including flow effects. As demonstrated in that paper the exact solution exhibits some special properties at low frequencies, e.g., a negative real part of the wall impedance. In this paper the exact Cremer impedance is further analyzed and discussed. Also, the exact solution for rectangular ducts is presented. 

Place, publisher, year, edition, pages
Acoustical Society of America , 2017. no 1
Keywords [en]
Acoustic impedance, Acoustics, Aircraft engines, Circular and rectangular cross-section, Circular ducts, Exact solution, Flow effects, High frequency HF, Rectangular ducts, Special properties, Wall impedance, Ducts
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-236779DOI: 10.1121/2.0000619Scopus ID: 2-s2.0-85049487496OAI: oai:DiVA.org:kth-236779DiVA, id: diva2:1277150
Conference
173rd Meeting of Acoustical Society of America, Acoustics 2017 and 8th Forum Acusticum, 25 June 2017 through 29 June 2017
Note

QC 20190109

Available from: 2019-01-09 Created: 2019-01-09 Last updated: 2019-05-14Bibliographically approved
In thesis
1. Optimal damping and slow sound in ducts
Open this publication in new window or tab >>Optimal damping and slow sound in ducts
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The thesis is dedicated to expanding knowledge on two duct acoustic issues including: 1) the optimal damping of low frequency sound and 2) the development and application of ‘slow sound’.

To address the first issue, the ‘Cremer impedance’ proposed more than half a century ago has been revisited and further developed. The original motivation is to extend the concept from large duct applications, such as aero-engines, to low frequency applications including vehicle intake and exhaust or cooling and ventilation systems. This leads to the derivation of the ‘exact’ solution of the Cremer impedance for single-lined rectangular ducts valid in the low frequency range in the presence of a ‘plug’ flow. A substantial improvement in the low frequency damping is achieved with the exact solution and a measurement campaign is carried out to validate this.

However, for both circular and rectangular ducts (including single-lined and double-lined types) the exact solution of the Cremer impedance has a negative real part in the low frequency range. This indicates that an active boundary is required to provide the optimal damping. Two investigations on the negative resistance are conducted. First, the ‘plug’ flow is replaced by a sheared flow by changing the boundary condition in the optimization model. With this modification, the Cremer impedance is recalculated and the negative resistance is still found in most cases, demonstrating that the negative resistance is not necessarily an artefact of the boundary condition. Second, since the Cremer impedance is based on mode-merging, a mode-merging analysis is carried out. The merging result shows that the downstream results are always valid, but some of the upstream results in the low frequency range are invalid in the sense that unexpected mode pairs merge, and the corresponding damping is smaller than expected. This finding is true for both the fundamental mode and higher order modes.

Regarding the second issue, ‘slow sound’ or sound with a much reduced ‘phase velocity’ is investigated using a resonant periodic system in the low frequency range. This can be seen as an acoustic metamaterial where sound propagates at a much smaller-than-normal speed around its resonance frequency. Following a hydrodynamic particle agglomeration model, the slow sound is applied to manipulate the distribution of small particles in the vehicle exhaust system. Although in principle this acoustic agglomeration method can work, it will only be efficient if the wave damping in the metamaterial is kept small. 

Abstract [sv]

Denna avhandling har till syfte att öka kunskapen om akustiska problem i kanaler, inklusive: 1) optimal dämpning av lågfrekvent ljud och; 2) utveckling och tillämpning av ‘slow sound’.När det gäller det första problemet, har ‘Cremer-impedansen’, som föreslogs för mer än ett halvt sekel sedan studerats och vidareutvecklats. Detta möjliggör nya tillämpningar svarande mot lågfrekvent ljud som insugnings- och avgassystem för fordon samt ventilationssystem. En ‘exakt’ lösning av Cremer-impedansen för rektangulära kanaler med en ljuddämpande vägg giltig i lågfrekvensområdet har härletts. En väsentlig förbättring av lågfrekvent dämpning har uppnåtts med denna lösning vilket även validerats med mätningar.Emellertid har den exakta lösningen av Cremer-impedansen en negativ realdel (‘resistans’) i lågfrekvensområdet, vilket betyder att en aktiv väggbeklädnad är nödvändig för att åstadkomma optimal dämpning. Två undersökningar av den negativa resistansen har genomförts för att studera om dessa lösningar är realiserbara. I den första ändrades randvillkoret för att inkludera gränsskikt i strömningen. Resultatet visade att negativ resistans erhålls i de flesta fall även med det modifierade randvillkoret. I den andra studerades lösningens giltighet i det komplexa vågtalsplanet. Resultatet visade att lösningen nedströms alltid är giltig medan vissa lösningar uppströms i lågfrekvensområdet är ogiltiga. Detta resultat gäller i princip för alla vågor eller moder i en kanal.När det gäller det andra problemet undersöktes möjligheterna att skapa ‘slow sound’, dvs ljud som utbreder sig mycket långsammare än normalt, genom att utnyttja ett resonant periodiskt system i en kanal i lågfrekvensområdet. Detta kan ses som ett s.k. akustiskt metamaterial och kan nyttjas för att med hjälp av starka ljudvågor påverka små partiklar som tvingas att kollidera och bli större. En studie om denna metod för sammanslagning av partiklar (‘particle agglomeration’) kan nyttjas för avgasrening har genomförts. Metoden är teoretiskt möjlig men begränsad av att tekniken för att skapa ‘slow sound’ inte bara saktar ned ljudvågen utan även dämpar dess amplitud och därigenom minska ljudvågens påverkan på partiklar.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019. p. 75
Series
TRITA-SCI-FOU ; 2019:22
National Category
Fluid Mechanics and Acoustics
Research subject
Vehicle and Maritime Engineering
Identifiers
urn:nbn:se:kth:diva-251443 (URN)978-91-7873-176-3 (ISBN)
Public defence
2019-06-04, F3, Lindstedtsvägen 26, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC20190514

Available from: 2019-05-14 Created: 2019-05-14 Last updated: 2019-05-14Bibliographically approved

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