ENCIA Advanced numerical techniques for the acoustic modelling of materials and noise control de

249 Pages·2016·4.26 MB·English

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UNIVERSITAT POLITECNICA DE VALENCIA
Department of Mechanical and Materials Engineering
PhD Thesis
Advanced numerical techniques for
the acoustic modelling of materials and
noise control devices in the exhaust system
of internal combustion engines
Presented by: MSc. Eva Mar´ıa S´anchez Orgaz
Supervised by: Dr. Francisco David Denia Guzma´n
Valencia, March, 2016

PhD Thesis
Advanced numerical techniques for
the acoustic modelling of materials and
noise control devices in the exhaust system
of internal combustion engines
for the degree of
Doctor in Engineering and Industrial Production
presented by
MSc. Eva Mar´ıa S´anchez Orgaz
at the
Department of Mechanical and Materials Engineering
Universitat Polit`ecnica de Val`encia
Supervised by
Dr. Francisco David Denia Guzm´an
Valencia, March, 2016

PhD Thesis
ADVANCED NUMERICAL TECHNIQUES FOR
THE ACOUSTIC MODELLING OF MATERIALS AND
NOISE CONTROL DEVICES IN THE EXHAUST SYSTEM
OF INTERNAL COMBUSTION ENGINES
Presented by: MSc. Eva Mar´ıa S´anchez Orgaz
Supervised by: Dr. Francisco David Denia Guzma´n
QUALIFYING COMMITTEE
President: Dr. Antonio J. Torregrosa Huguet
Vocal: Dr. Wenbo Duan
Secretary: Dr. Jordi Romeu Garb´ı
Valencia, March, 2016

ThisThesishasbeensupportedbytheMinisteriodeEconom´ıayCompetitividadand
the European Regional Development Fund (projects DPI2010-15412 and TRA2013-
45596-C2-1-R),aswellasbytheGeneralitatValenciana(projectPrometeo/2012/023).
E.M.S´anchez-Orgaz I

Abstract
ThisThesisisfocusedonthedevelopmentandimplementationofgeneralandefficient
numerical methods for the acoustic modelling and design of noise control devices in
the exhaust system of combustion engines. Special attention is paid to silencers, that
can be divided into reactive and dissipative configurations. For the latter, signifi-
cant differences are likely to appear in their acoustic behaviour, depending on the
temperature variations within the absorbent material. Also, material heterogeneities
associated with uneven filling processes and time degradation due to the exhaust
gases, can alter the silencer attenuation performance. Therefore, numerical tech-
niques considering all these features are required to guarantee an accurate prediction
of the acoustic behaviour.
First, a literature review is carried out. This is mainly related to one-dimensional
models and their associated matrix representation, as well as to acoustic models for
absorbentmaterialsandperforatedsurfaces. Itisworthnotingthatplanewavemodel
limitationsmakeindispensableusingalternativemultidimensionalmethods,sincethe
latter are valid for a wide frequency range and for silencers with complex geometries.
In addition to developing acoustic models to simulate exhaust silencers in general
terms, the possibility of using new acoustic elements is also explored in this Thesis.
These elements have as an objective being an interesting potential alternative to the
fibrous absorbent materials commonly used in practice, whose negative impact on
health is a current cause for concern. In this sense, and following earlier studies, the
Thesis considers the use of microperforated surfaces and goes into detail about the
study of surfaces manufactured by sintering. The latter present, in some cases, the
particularity of having a nearly constant acoustic impedance, whose value depends,
amongothers, onthethicknessandporosityoftheplates. Bothkindofsurfaceshave
been proved to be an interesting alternative to dissipative silencers; the microperfo-
ratedsurfacesinthelowfrequencyrangeandthesinteredscreensinthemidandhigh
frequency range.
General acoustic models, in which the wave equation permits to consider variable
properties and mean flow, are considered. Thus, a finite element approach is first
III
proposed for the acoustic analysis of perforated dissipative silencers including a per-
forated duct with uniform axial mean flow and an outer chamber with heterogeneous
absorbent material. The presence of material heterogeneities can be caused, for ex-
ample, by the manufacturing process as well as by the degradation due to the soot
particlescontainedintheexhaustgases. Inthisfirstapproach,avariablebulkdensity
is introduced in the model. On the other hand, property variations can be also pro-
duced by temperature gradients. In this second case, a hybrid finite element model
has been derived for the acoustic analysis of perforated dissipative silencers includ-
ing several effects simultaneously: (1) High temperature and thermal gradients in
the central duct and the outer absorbent material; (2) A perforated passage carrying
non-uniform axial mean flow. Independently of the reasons for the propagation me-
dia non-homogeneity, their properties vary with position. The material of the outer
chambercanbecharacterizedbymeansofitsequivalentacousticproperties(complex
density and speed of sound), considered coordinate-dependent in the context of the
current investigation. This can be achieved via the introduction of a heterogeneous
bulk density (and the corresponding material airflow resistivity variations) as well as
by computing the resistivity distribution if the thermal gradients are significant. A
finiteelementapproachhasbeenimplementedtosolvethepressure-basedwaveequa-
tion for a non-moving heterogeneous medium, associated with the problem of sound
propagation in the outer chamber. On the other hand, the governing equation in the
central duct has been written and solved in terms of an acoustic velocity potential to
allowthepresenceofanaxiallyinhomogeneousflowwhileovercomingsomenumerical
issuesofthepressureformulationfoundinearlierstudies. Thecouplingbetweenboth
regions and the corresponding acoustic fields has been carried out by means of a per-
forated duct and its acoustic impedance, adapted here to include absorbent material
heterogeneitiesandmeanfloweffectssimultaneously. Ithasbeenfoundthattempera-
turegradientsandbulkdensityheterogeneitiescanhaveasignificantinfluenceonthe
acoustic attenuation of an automotive silencer and so should be included in the theo-
retical models if accurate predictions are required. In some particular configurations,
itmayberelativelyprecisetoapproximatethetemperature/bulkdensitydistribution
by using a uniform profile with an average value, specially for low resistivity mate-
rials. It has been shown, however, that this is not always possible and attenuation
overestimation is likely to be predicted, mainly for high radial variations of tempera-
ture/bulkdensityandhighmaterialflowresistivities,ifthetemperature/bulkdensity
distribution is not taken into account.
Theuseofoptimizationtechniquesforindustrialdevicesisalsorelevant,sinceitleads
to the production of elements with better characteristics. From a practical point of
view, the material reduction usually means cost savings, which is very important, for
example, in the automotive sector, where large series are manufactured. In activities
relatedtotransportation ofpeopleandgoods, designcriteriatoachieve theoptimum
IV E.M.S´anchez-Orgaz