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Micro-Structure Modelling of Acoustics of Open Porous Material
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. (MWL)ORCID iD: 0000-0001-9948-249X
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Transportation is a large and growing part of the world’s energy consumption. This drives a need for reduced weight of rail vehicles, just as it does for road vehicles. In spite of weight reductions, the vehicle still has to provide the same level of acoustic comfort for the passengers. Porous materials, with more than 90% air, are often included in multi-layer vehicle panels, contributing to acoustic performance without adding much weight. Here the acoustic performance of open cell porous materials, with focus on flow resistivity, is evaluated based on simplified micro-structure models to investigate the effect of anisotropy on the performance In order to evaluate how the redistribution of material affects the flow resistivity, the porosity of the material is kept constant. Two micro-geometries are analysed and compared: the hexahedral model and the tetrakaidecahedron (Kelvin cell). For flow resistivity calculations the solid frame is assumed to be rigid. The models are elongated in one direction to study the influence of micro-structural anisotropy on the macro level flow resistivity. To keep porosity constant, two different approaches are investigated. The first approach is to let strut thickness be uniform and adjust the volume of the cell to a constant ratio compared to the isotropic case. The second approach is to let the strut volume, and cell volume, be constant. For an anisotropic hexahedral cell with uniform strut thickness, the flow resistivity increases substantially with increasing height to width ratio for the hexahedral model, while the flow resistivity for the tetrakaidecahedron model with uniform strut thickness decreases with increasing height to width ratio. For both geometries and constant strut volume, the average flow resistivity is close to the same constant value. For uniform strut thickness the relative volume of anisotropic to isotropic volume is very important.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , 46 p.
Series
TRITA-AVE, ISSN 1651-7660 ; 2016:24
Keyword [en]
acoustics, porous material, flow resistivity, micro-structure, anisotropic, foam, strut
National Category
Fluid Mechanics and Acoustics
Research subject
Vehicle and Maritime Engineering
Identifiers
URN: urn:nbn:se:kth:diva-187322ISBN: 978-91-7729-040-7 (print)OAI: oai:DiVA.org:kth-187322DiVA: diva2:929844
Presentation
2016-06-15, Munin, KTH Royal Institute of Technology, Teknikringen 8, Stockholm, 09:00 (English)
Opponent
Supervisors
Note

The work has been carried out within the Centre for ECO2 Vehicle Design.

QC 20160523

Available from: 2016-05-23 Created: 2016-05-20 Last updated: 2016-05-24Bibliographically approved
List of papers
1. Sensitivity of macroscopic properties of a multi-layer panel including porous material on the micro-level parameters of an open cell porous material
Open this publication in new window or tab >>Sensitivity of macroscopic properties of a multi-layer panel including porous material on the micro-level parameters of an open cell porous material
2014 (English)In: PROCEEDINGS OF INTERNATIONAL CONFERENCE ON NOISE AND VIBRATION ENGINEERING (ISMA2014) AND INTERNATIONAL CONFERENCE ON UNCERTAINTY IN STRUCTURAL DYNAMICS (USD2014), 2014, 2135-2149 p.Conference paper, Published paper (Refereed)
Abstract [en]

The micro-structure of an open cell porous material is modelled as an idealized, periodic structure, allowing anisotropic properties to form on the micro-scale. Using simple analytic descriptions of acoustic and elastic properties calculated from micro-structure geometry, the microstructural properties can be linked to averaged macroscopic elasto-acoustic properties, which can be measured and observed. These macro level properties may be deduced from measurements on a sample of a porous material which is at least a few centimetres across. The most common of the acoustic properties are the flow resistivity or the (dynamic) permeability, the porosity and the viscous and thermal characteristic lengths, together with the tortuosity. For the elasticity, the moduli of the Hooke's law are the most important. The underlying motivation for the current work is that the quantities are interdependent since they all depend on the micro geometry. Thus, to design the macro level acoustic properties for a required performance, physically relevant models linking these to the micro structure would be necessary. In this paper, a set of such models are proposed and utilising these, the influence of the anisotropy on acoustic properties of the foam as well as the transmission loss of a multi-layer plate is investigated. Typical macro level acoustic properties of open cell poro-elastic materials calculated from micro structure parameters are found to give results of the same order of magnitude as measured data found in the literature. Further, the model is applied to evaluate transmission loss of roof constructions with isotropic and anisotropic foam. It is shown that the sound reduction can be improved without changing the overall surface weight of the structure by making use of the anisotropy of the foam. The results Underline the importance of anisotropy and in addition provide guidance for further research.

National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-164938 (URN)000352201002045 ()2-s2.0-84913591667 (Scopus ID)978-907380291-9 (ISBN)
Conference
International Conference on Noise and Vibration Engineering (ISMA), Leuven, BELGIUM, SEP 15-17, 2014
Note

QC 20150512

Available from: 2015-04-20 Created: 2015-04-20 Last updated: 2016-12-05Bibliographically approved
2. Predicted and measured anisotropic acoustic and elastic properties for open cell porous material and their influence on typical train applications
Open this publication in new window or tab >>Predicted and measured anisotropic acoustic and elastic properties for open cell porous material and their influence on typical train applications
2015 (English)Conference paper, Published paper (Refereed)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-165039 (URN)
Conference
EURONOISE 2015
Note

NQC 2015

Available from: 2015-04-21 Created: 2015-04-21 Last updated: 2016-05-23Bibliographically approved
3. Micro model for calculating flow resistivity for anisotropic open cell materials with high aspect ratio of struts
Open this publication in new window or tab >>Micro model for calculating flow resistivity for anisotropic open cell materials with high aspect ratio of struts
2016 (English)Report (Other academic)
Abstract [en]

Micro-structure models for calculating flow resistivity for open cell materials with high aspect ratio of struts are presented.  Two micro-geometries are analyzed and compared: the hexahedral model, with the solid frame in shape of a cube for the isotropic case, and the tetrakaidecahedron (Kelvin cell). For flow resistivity calculations the solid frame is assumed to be rigid. The models are elongated in one direction to study the influence of micro-structural anisotropy on the macro level flow resistivity. In order to evaluate how the redistribution of material affects the flow resistivity, the porosity of the material is kept constant. Since the porosity is defined as unity minus the ratio of strut volume of the cell to the total volume of the cell, there are different ways of linking the micro level properties to the macro level. The first approach presented here is to let strut thickness be uniform and adjust the volume of the cell to a constant ratio compared to the isotropic case. The second approach is to let the strut volume be constant, i.e. if a strut is elongated, it is also thinner. In this case the cell volume must equal the isotropic volume. For uniform strut thickness the flow resistivity increases substantially with increasing height to width ratio for the hexahedral model, and decreases for the Kelvin cell. For constant strut volume the flow resistivity perpendicular to the flow direction increases, and the other direction (where the strut thickness increases) will be reduced. The average flow resistivity will in this case be almost constant. The method of scaling the micro-cell properties has a high influence on the resulting average flow resistivity, as does the cell volume in relation to the isotropic volume. For open porous materials which are approximately isotropic, the choice of micro geometry is not so critical and can be determined by other considerations, such as elasticity.

Publisher
19 p.
Keyword
acoustics, foam, micro-structure, flow resistivity, strut, anisotropy
National Category
Fluid Mechanics and Acoustics
Research subject
Vehicle and Maritime Engineering
Identifiers
urn:nbn:se:kth:diva-187321 (URN)
Note

QC 20160524

Available from: 2016-05-20 Created: 2016-05-20 Last updated: 2016-11-11Bibliographically approved

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