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Optimal damping and slow sound in ducts
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
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: urn:nbn:se:kth:diva-251443ISBN: 978-91-7873-176-3 (print)OAI: oai:DiVA.org:kth-251443DiVA, id: diva2:1315635
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
List of papers
1. Revisiting the cremer impedance
Open this publication in new window or tab >>Revisiting the cremer impedance
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
Keywords
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:nbn:se:kth:diva-236779 (URN)10.1121/2.0000619 (DOI)2-s2.0-85049487496 (Scopus ID)
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
2. Experimental Analysis on the 'Exact' Cremer Impedance in Rectangular Ducts
Open this publication in new window or tab >>Experimental Analysis on the 'Exact' Cremer Impedance in Rectangular Ducts
2018 (English)In: SAE technical paper series, ISSN 0148-7191, Vol. 2018-June, no JuneArticle in journal (Refereed) Published
Abstract [en]

Cremer impedance, first proposed by Cremer (Acustica 3, 1953) and then improved by Tester (JSV 28, 1973), refers to the locally reacting boundary condition that can maximize the attenuation of a certain acoustic mode in a uniform waveguide. One limitation in Tester's work is that it simplified the analysis on the effect of flow by only considering high frequencies or the 'well cut-on' modes. This approximation is reasonable for large duct applications, e.g., aero-engines, but not for many other cases of interest, with the vehicle intake and exhaust system included. A recent modification done by Kabral et al. (Acta Acustica united with Acustica 102, 2016) has removed this limitation and investigated the 'exact' solution of Cremer impedance for circular waveguides, which reveals an appreciable difference between the exact and classic solution in the low frequency range. Consequently, the exact solution can lead to a much higher low-frequency attenuation level. In addition, the exact solution is found to exhibit some special properties at very low frequencies, e.g., a negative resistance. In this paper, liners designed on the basis of the exact solution are tested and the difference between the exact and classic solution in the low frequency range (not low enough to go into the negative resistance region) is experimentally investigated.

Place, publisher, year, edition, pages
SAE International, 2018
National Category
Other Engineering and Technologies
Identifiers
urn:nbn:se:kth:diva-238194 (URN)10.4271/2018-01-1523 (DOI)2-s2.0-85050564046 (Scopus ID)
Conference
SAE 10th International Styrian Noise, Vibration and Harshness Congress: The European Automotive Noise Conference, SNVH 2018, Congress GrazSparkassenplatz 1Graz, Austria, 20 June 2018 through 22 June 2018
Note

QC 20181120

Available from: 2018-11-20 Created: 2018-11-20 Last updated: 2019-05-14Bibliographically approved
3. The Cremer Impedance: An Investigation of the Low Frequency Behavior
Open this publication in new window or tab >>The Cremer Impedance: An Investigation of the Low Frequency Behavior
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The Cremer impedance, first proposed by Cremer (1953) and then extended by Tester (1973), is supposed to give the maximum propagation damping in an infinitely long waveguide. Previous works including a uniform grazing flow have shown negative resistance in the low frequency range for both circular and 2-D rectangular waveguides, i.e., implying an active boundary. In order to further analyze the low frequency behaviour of the Cremer impedance, especially the negative resistance, two investigations are conducted in the current work. First, the previously used Ingard-Myers boundary condition is replaced by the Brambley boundary condition with the introduction of a thin inviscid boundary layer, and results obtained with the two boundary conditions are compared to see the effect of a sheared flow. The frequency range where the two boundary conditions can be applied is also analyzed. Second, discussions regarding the validity of the low frequency result in both the up- and downstream directions from the perspective of mode-merging are presented. This analysis is further extended from the fundamental mode to higher order modes in the frequency range where they are ‘just cut-on’.

Keywords
the Cremer impedance; low frequency range; negative resistance; boundary condition; mode-merging
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-251437 (URN)
Note

QC 20190520

Available from: 2019-05-14 Created: 2019-05-14 Last updated: 2019-05-20Bibliographically approved
4. ‘Double-’ and ‘Triple-root’ Cremer Impedancefor a Rectangular Duct with Opposite Lined Walls
Open this publication in new window or tab >>‘Double-’ and ‘Triple-root’ Cremer Impedancefor a Rectangular Duct with Opposite Lined Walls
(English)Manuscript (preprint) (Other academic)
Abstract [en]

To achieve high low frequency damping in an infinitely long double-lined rectangular duct with zero mean flow, the optimization of axial sound attenuation in the sense defined by Cremer (i.e., using the ‘Cremer impedance/solution’ concept) is examined for the fundamental mode. Two double-root Cremer impedance solutions that merge a mode pair into a single mode are presented. Different mode-merging patterns due to symmetry are found for these two solutions. As an extension, two triple-root Cremer impedance solutions (including one with a negative resistance) that merge three modes are also provided by extending the optimum condition proposed by Cremer. The two triple root solutions are theoretically advantageous in damping compared with the double root solutions, and the advantage is explained from the perspective of mode-merging.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-251440 (URN)
Note

QC 20190520

Available from: 2019-05-14 Created: 2019-05-14 Last updated: 2019-05-20Bibliographically approved
5. Particle Number Reduction in Automotive Exhausts Using Acoustic Metamaterials
Open this publication in new window or tab >>Particle Number Reduction in Automotive Exhausts Using Acoustic Metamaterials
Show others...
2017 (English)In: SAE International Journal of Engines, ISSN 1946-3936, E-ISSN 1946-3944, Vol. 10, no 4, p. 1566-1572Article in journal (Refereed) Published
Abstract [en]

Air pollution caused by exhaust particulate matter (PM) from vehicular traffic is a major health issue. Increasingly strict regulations of vehicle emission have been introduced and efforts have been put on both the suppression of particulate formation inside the engine cylinders and the development of after-treatment technologies such as filters. With modern direct injected engines that produce a large number of really small sub-micron particles, the focus has increased even further and now also includes a number count.The problem of calculating particle trajectories in flow ducts like vehicle exhaust systems is challenging but important to further improve the technology. The interaction between particles and oscillating flows may lead to the formation of particle groups (regions where the particle concentration is increased), yielding a possibility of realizing particle agglomeration. The oscillating flow may simply be hydrodynamic or as assumed here: the flow oscillations are created by sound propagation rather than hydrodynamic approaches. An analysis is presented which gives the relationship between the speed of sound, the mean flow velocity and the amplitude of the acoustic particle velocity for particle agglomeration to be feasible. It is shown that it can be achieved if the convective speed of sound is reduced to the same order as the mean flow velocity. It is therefore suggested to use the so-called acoustic metamaterials, which can help control, direct and manipulate sound waves. At this stage a phenomenological 1D model is used for the analysis, which allows the authors to build an understanding of the effect of the sound waves and flow oscillations on particle motion and paves the way for further analysis on particle agglomeration.

Place, publisher, year, edition, pages
SAE International, 2017
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-216542 (URN)10.4271/2017-01-0909 (DOI)000416807900016 ()2-s2.0-85018319516 (Scopus ID)
Note

QC 20171124

Available from: 2017-11-24 Created: 2017-11-24 Last updated: 2019-05-14Bibliographically approved
6. Experimental analysis of whistle noise in a particle agglomeration pipe
Open this publication in new window or tab >>Experimental analysis of whistle noise in a particle agglomeration pipe
2018 (English)In: INTER-NOISE 2018 - 47th International Congress and Exposition on Noise Control Engineering: Impact of Noise Control Engineering, Institute of Noise Control Engineering , 2018Conference paper, Published paper (Refereed)
Abstract [en]

A self-sustained sound, more usually known as a whistle, refers to a distinct tonal noise created due to the interaction between the sound and flow field. When a positive feedback loop is formed between the two fields, the energy in the mean flow will be transferred into the sound wave, thus giving rise to a whistle. In engineering practice, whistles are destructive as they can produce high sound and vibration levels and may result in risk for mechanical failures. In this work, a flow-related high level tonal noise was found during a measurement on a particle agglomeration pipe, which is a quasi-periodic corrugated structure designed for the exhaust system of heavy-duty trucks. The purpose of the pipe is to enhance particle agglomeration to increase the size of exhaust gas particles. To investigate the origin of the detected tonal noise additional measurements were carried out. Based on the measurement result, the aero-acoustic coupling in the agglomeration pipe was analyzed, revealing that the pipe has a large potentiality to amplify the incident sound power in the presence of a mean flow. Furthermore, the Nyquist stability criterion was applied to confirm the existence of exponentially growing modes in the system at certain conditions.

Place, publisher, year, edition, pages
Institute of Noise Control Engineering, 2018
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-241859 (URN)000456356800019 ()2-s2.0-85059372519 (Scopus ID)
Conference
47th International Congress and Exposition on Noise Control Engineering: Impact of Noise Control Engineering, INTER-NOISE 2018, Marriott Magnificent Mile DowntownChicago, United States, 26 August 2018 through 29 August 2018
Note

QC 20190122

Available from: 2019-01-25 Created: 2019-01-25 Last updated: 2019-05-14Bibliographically approved

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Citation style
  • apa
  • ieee
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