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Aeroacoustics Studies of Duct Branches with Application to Silencers
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

New methodologies and concepts for developing compact and energy efficient automotive exhaust systems have been studied. This originates in the growing concern for global warming, to which road transportation is a major contributor. The focus has been on commercial vehicles—most often powered by diesel engines—for which the emission legislation has been dramatically increased over the last decade. The emissions of particulates and nitrogen oxides have been successfully reduced by the introduction of filters and catalytic converters, but the fuel consumption, which basically determines the emissions of carbon dioxides, has not been improved accordingly. The potential reduction of fuel consumption by optimising the exhaust after-treatment system (assuming fixed after-treatment components) of a typical heavy-duty commercial vehicle is ~4%, which would have a significant impact on both the environment and the overall economy of the vehicle.

First, methodologies to efficiently model complex flow duct networks such as exhaust systems are investigated. The well-established linear multiport approach is extended to include flow-acoustic interaction effects. This introduces an effective way of quantifying amplification and attenuation of incident sound, and, perhaps more importantly, the possibility of predicting nonlinear phenomena such as self-sustained oscillations—whistling—using linear models. The methodology is demonstrated on T-junctions, which is a configuration well known to be prone to self-sustained oscillations for grazing flow past the side branch orifice. It is shown, and validated experimentally, that the existence and frequency of self-sustained oscillations can be predicted using linear theory.

Further, the aeroacoustics of T-junctions are studied. A test rig for the full determination of the scattering matrix defining the linear three-port representing the T-junction is developed, allowing for any combination of grazing-bias flow. It is shown that the constructive flow-acoustic coupling not only varies with the flow configuration but also with the incidence of the acoustic disturbance. Configurations where flow from the side branch joins the grazing flow are still prone to whistling, while flow bleeding off from the main branch effectively cancels any constructive flow-acoustic coupling.

Two silencer concepts are evaluated: first the classic Herschel-Quincke tube and second a novel modified flow reversal silencer. The Herschel-Quincke tube is capable of providing effective attenuation with very low pressure loss penalty. The attenuation conditions are derived and their sensitivity to mean flow explained. Two implementations have been modelled using the multiport methodology and then validated experimentally. The first configuration, where the nodal points are composed of T-junctions, proves to be an example where internal reflections in the system can provide sufficient feedback for self-sustained oscillation. Again, this is predicted accurately by the linear theory. The second implementation, with nodal points made from Y-junctions, was designed to allow for equal flow distribution between the two parallel ducts, thus allowing for the demonstration of the passive properties of the system. Experimental results presented for these two configurations correlate well with the derived theory.

The second silencer concept studied consists of a flow reversal chamber that is converted to a resonator by acoustically short-circuiting the inlet and outlet ducts. The eigenfrequency of the resonator is easily shifted by varying the geometry of the short circuit, thus making the proposed concept ideal for implementation as a semi-active device. Again the concept is modelled using the multiport approach and validated experimentally. It is shown to provide significant attenuation over a wide frequency range with a very compact design, while adding little or no pressure loss to the system.

Place, publisher, year, edition, pages
Stockholm: KTH , 2010. , xi, 50 p.
Series
Trita-AVE, ISSN 1651-7660 ; 10:74
Keyword [en]
silencer, muffler, confined flows, flow duct, aeroacoustics, vortex sound, acoustic multiports, linear stability, self-sustained oscillations, whistling, Herschel-Quincke tube, acoustic resonator
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-29568ISBN: 978-91-7415-842-7 (print)OAI: oai:DiVA.org:kth-29568DiVA: diva2:395954
Public defence
2011-02-10, Sal F3, Lindstedtsvägen 26, KTH, Stockholm, 13:15 (English)
Opponent
Supervisors
Note
QC 20110208Available from: 2011-02-08 Created: 2011-02-08 Last updated: 2011-02-08Bibliographically approved
List of papers
1. Aeroacoustics of T-junctions: An experimental investigation
Open this publication in new window or tab >>Aeroacoustics of T-junctions: An experimental investigation
2010 (English)In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 329, no 10, 1793-1808 p.Article in journal (Refereed) Published
Abstract [en]

An experimental method for determining the aeroacoustic properties of side branch orifices allowing for any combination of grazing and bias flow is presented. The geometry studied, a T-junction, is treated as an active acoustic three-port. The passive properties, describing the reflection and transmission of an incident acoustic wave, are described by a system matrix while the active properties are described by a source vector. Expressions for the acoustic impedance under various mean flow and acoustic incidence configurations are developed. In addition, methods for identifying regions where the system can generate sound, by studying only the passive properties, are discussed. A self-sustained oscillation is triggered at one of the identified regions by coupling a resonant system to the three-port.

Keyword
GRAZING MEAN FLOW, ACOUSTIC-IMPEDANCE, PERFORATED PLATES, CIRCULAR ORIFICE, BOUNDARY-LAYER, FLUID MACHINES, PIPE, SYSTEMS, DUCT, POTENTIALITY
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-28712 (URN)10.1016/j.jsv.2009.11.024 (DOI)000274926800012 ()2-s2.0-77954372075 (Scopus ID)
Note
QC 20110121Available from: 2011-01-21 Created: 2011-01-19 Last updated: 2017-12-11Bibliographically approved
2. Quasisteady model of the acoustic scattering of a T- Junction
Open this publication in new window or tab >>Quasisteady model of the acoustic scattering of a T- Junction
2011 (English)In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 330, no 21, 5131-5137 p.Article in journal (Refereed) Published
Abstract [en]

T-junctions are common elements in flow duct networks. It is shown that for low Strouhal numbers, based on the duct diameter, their acoustic scattering properties can be described using an incompressible quasi-steady model. Models are derived for systems subjected to combinations of grazing and bias mean flow, and are shown to work in both the limiting cases that is, grazing or bias flow alone as well as for the joining or dividing flow configurations. The upper Strouhal number for which the quasi-steady models are applicable is limited by flow-acoustic interaction effects, which differ significantly between the different flow configurations. Generally the models are applicable up to Strouhal numbers of approximately 0.1-0.2, which is comparable with previous published models for other configurations such as bends and orifices. This range is sufficient to make the model useful in many important engineering applications.

Keyword
aeroacoustic response, duct
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-29567 (URN)10.1016/j.jsv.2011.05.012 (DOI)000293725500012 ()2-s2.0-79960555852 (Scopus ID)
Funder
TrenOp, Transport Research Environment with Novel Perspectives
Note
QC 20110208. Updated from submitted to published 20120327Available from: 2011-02-08 Created: 2011-02-08 Last updated: 2017-12-11Bibliographically approved
3. On the Use of Linear Acoustic Multiports to Predict Whistling in Confined Flows
Open this publication in new window or tab >>On the Use of Linear Acoustic Multiports to Predict Whistling in Confined Flows
2011 (English)In: Acta Acoustica united with Acustica, ISSN 1610-1928, E-ISSN 1861-9959, Vol. 97, no 1, 24-33 p.Article in journal (Refereed) Published
Abstract [en]

The use of linear acoustic multiport models to analyse the existence of flow driven instabilities, that is - self-sustained oscillators or whistles - is addressed. By combining the scattering and reflection matrices for a system and searching for eigenfrequencies (zeros) in the critical half-plane, the existence of exponentially growing instabilities can be determined. In practice, the available frequency domain data are only known along the real axis; then the search for zeros may be done via the so-called Nyquist stability criterion generalized to an N-degree of freedom system. The method has been validated by two test cases, a T-junction and a Herschel-Quincke tube. In both cases, the occurrence and the frequencies of the self-sustained oscillations were correctly predicted.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-29564 (URN)10.3813/AAA.918383 (DOI)000286113700004 ()2-s2.0-78651483482 (Scopus ID)
Funder
TrenOp, Transport Research Environment with Novel Perspectives
Note
QC 20110208Available from: 2011-02-08 Created: 2011-02-08 Last updated: 2017-12-11Bibliographically approved
4. The Herschel-Quincke tube: The attenuation conditions and their sensitivity to mean flow
Open this publication in new window or tab >>The Herschel-Quincke tube: The attenuation conditions and their sensitivity to mean flow
2008 (English)In: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 124, no 2, 723-732 p.Article in journal (Refereed) Published
Abstract [en]

The classic Herschel-Quincke tube is a parallel connection of two ducts yielding multiple noise attenuation maxima via destructive interference. This problem has been discussed to different degrees by a number of authors over the years. This study returns to the basics of the system for the purpose of furthering the understanding of the conditions necessary for noise attenuation and especially their sensitivity to mean flow. First, the transmission loss for an N-duct system with mean flow and arbitrary conditions of state in the different ducts is derived. Next, the two types of conditions yielding the attenuation maxima are studied. In addition to a discussion of the underlying physics, generic expressions for frequencies at which maximum attenuation occur are presented. Experiments without mean flow generally show good agreement with theory based on straight duct elements. However, more detailed models may be required for accurate simulations in the presence of mean flow. A simple model compensating for the losses associated with bends is shown to improve the results significantly for the geometry studied.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2008
Keyword
ACOUSTIC PERFORMANCE, PIPE, TRANSMISSION
National Category
Fluid Mechanics and Acoustics Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-29565 (URN)10.1121/1.2940580 (DOI)000258230500014 ()2-s2.0-49249123013 (Scopus ID)
Note

QC 20110208. QC 20160129

Available from: 2011-02-08 Created: 2011-02-08 Last updated: 2017-12-11Bibliographically approved
5. The flow reversal resonator: basic concept and influence of mean flow
Open this publication in new window or tab >>The flow reversal resonator: basic concept and influence of mean flow
2007 (English)In: SAE technical paper series, ISSN 0148-7191Article in journal (Refereed) Published
Abstract [en]

The flow reversal chamber is a commonly used element in practical silencer design. To lower its fundamental eigenfrequency, it is suggested to acoustically short circuit the inlet and outlet duct. In the low frequency limit such a configuration will correspond to a Helmholtz resonator, but with a choked flow through the short circuit, the main flow will be forced through the expansion volume. For the proposed concept, the flow reversal resonator, a theoretical model is derived and presented together with transfer matrix simulations. The possible extension to a semi active device as well as the influence of mean flow on the system is investigated experimentally. Finally the concept is implemented on a truck silencer. The results indicate that the flow reversal resonator would prove an interesting complement to traditional side branch resonators. The attenuation bandwidth is broader and it can be packaged very efficiently. Mean flow effects are still an issue and should be studied further.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-29566 (URN)10.4271/2007-01-2203 (DOI)2-s2.0-84877440385 (Scopus ID)
Note

QC 20110208

Available from: 2011-02-08 Created: 2011-02-08 Last updated: 2017-12-11Bibliographically approved

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