27 november 2013 - Seminar Porous materials

Beatrixgebouw Jaarbeurs Utrecht
Dagvoorzitter: Prof.dr.ir. N.B. (Bert) Roozen

Aanmelding

Leden van de NAG kunnen zich aanmelden door middel van het aanmeldingsformulier dat zij hebben ontvangen. Niet-leden wordt verzocht contact op te nemen met het NAG. De kosten voor de lunch bedragen € 10,00 per persoon, te voldoen ter plaatse. U wordt verzocht alleen dan voor de lunch in te schrijven als u daaraan ook met zekerheid zult deelnemen, dit in verband met de aan de organisatie verbonden financiële verplichtingen. Niet-leden zijn eveneens welkom op deze lezingendag. De kosten voor deelname en lunch voor niet-leden bedragen € 65,00, te voldoen ter plaatse. Bij betaling van dit bedrag hebben deelnemers recht op vrijstelling van het 1e jaar contributie van het NAG bij aanmelding als lid. Studerende niet-leden kunnen gratis deelnemen aan de lezingendag en betalen uitsluitend, voor zover van toepassing, € 10,00 voor de lunch, te voldoen ter plaatse.

PROGRAMMA

09:00 – 9:30 Ontvangst en koffie/thee

09:30 - 09:40 Welcom and introduction

09:40 – 10:30 Using sound absorbing material as a mitigation measure for shooting noise
Frits van der Eerden, TNO

In order to reduce noise around military training areas, the Ministry of Defense (MoD-NL) applies mitigation measures. The mitigation of sound by shielding is possible because the firing of weapons or demolition charges can take place at or around fixed locations. In a research program the mitigation of shock waves by means of a sound absorbing barrier close to the source has been investigated and tested. The sound propagation and the interaction with a barrier close to the source is non-linear. In this region, a dedicated model is used to describe the non-linear phenomena. It is demonstrated and validated that a better shielding of strong shock waves can be obtained by adding absorbing material to the barrier. In a second research program, the sound propagation from a semi-open shooting range has been investigated. When a weapon is fired, the muzzle blast reflects against the side-walls and safety screens and creates a complex sound field, which depends on the weapon/ammunition used and the firing position in the range. As a result the sound propagation from the semi-open shooting range strongly depends on the angle with respect to the line of fire. In order to predict the sound emission and to determine the effect of measures with sound absorption, a numerical model is being developed. Results are shown and compared to measurements.

10:30 – 11:20 The in-situ measurement of sound absorption coefficients for arbitrary sound sources.
Y.H. Wijnant and E.R. Kuipers, Structural Dynamics and Acoustics, Faculty of Engineering Technology, University of Twente

The L(S)PW-method are in-situ, power-based methods, to determine the sound absorption coefficient of a surface for an a priori unknown sound source. In this paper, we present the methods and discuss the pro- and cons. In particular we focus on cases, which are generally difficult to measure; non-plane waves incident upon poorly absorbing planar surfaces. It is shown that the active acoustic power determined at the measurement surface, just in front of the actual absorbing surface, may deviate from the absorbed acoustic power. It is shown that the accuracy can be improved by also measuring the active acoustic power flowing parallel to the surface, even for well-absorbing surfaces. Experiments carried out at two different samples confirm the improved accuracy. These experiments furthermore indicate that an accurate determination of the active acoustic power parallel to the surface is difficult at high frequencies. 11:20 – 12:20 Porous material for acoustical, thermal or mechanical purposes: from theory to practice in an industrial context Francois-Xavier Bécot, Matelys, Vaulx-en-Velin, France This talk will discuss on the importance of understanding the basic functioning of porous material in an industrial context. After some basic theoretical presentation, case studies which have been carried out in an industrial context will be reviewed. Applications will cover acoustical, thermal and mechanical applications, together with the problematic of flow in porous media.

12:20 – 13:20 Lunch

13:20 – 14:10 Polyrethanes Foams as Acoustics materials: Modelling, Characterisation and Application
Jan Vandenbroeck and Mark Brennan, Huntsman Polyurethanes, Everberg, Belgium

The flexibility of polyurethane chemistry enables a wide variety of foam cell microstructures and polymer viscoelastic properties in polyurethane foam materials. These structures and properties can be tuned to aid energy dissipation of sound waves propagating through polyurethane foams and so enable these materials to be used in different sound absorption and insulation applications. This talk will present a numerical approach based on micro- CFD and which links the microstructure of the open cell foams to acoustic performance. The results of this approach are compared with and used to interpret impedance tube measurements. It is shown that this approach opens the route to novel material development and to new characterisation methods and material models. The work is supported by characterisations methods such as micro-CT to enable 3D reconstruction of polyurethane foams.

14:10 – 15:00 The application of efficient axisymmetric wave based models for the evaluation of the reflection coefficient of poro-elastic materials
Elke Deckers, Dirk Vandepitte, Wim Desmet, Department of Mechanical Engineering, K.U.Leuven, Belgium

Performance metrics of poroelastic materials such as sound absorption and the reflection coefficient are often obtained from impedance tube measurements. By measuring the acoustic pressure in two microphone locations, these frequency-dependent values can be obtained using simple expressions. However, these expressions are based on the assumption that the dynamic fields behave unidimensionally, which only holds in the case the sample is loosely fitting in the measurement tube and no air gaps are being present. As has been shown in various studies, the mounting conditions of the sample in the tube may have an important impact. Recently, a Wave Based Method has been developed to predict the dynamic behaviour of poroelastic materials in a computationally efficient manner as compared to standard element based prediction techniques. This method can be straightforwardly applied to axisymmetric problems and allows evaluating the effect of mounting conditions. The potential of the proposed method is illustrated using analytical expressions, finite element calculations and measurements.

15:00 – 15:25 Coffee/tea pause

15:25 – 16:15 Robust estimation of the mechanical properties of foams, based on dispersion measurements
N.B. Roozen, B. Verstraeten and C. Glorieux, Katholieke Universiteit Leuven, Belgium

In most porous materials the frame elasticity and its coupling with the fluid (e.g. air) is important for a correct description of the sound absorption and transmission of those materials. In this contribution the physical properties of the elastic guided waves in the porous material are determined. To this end the propagation velocities of the longitudinal and shear wave velocities of the porous material are estimated, fitting a numerical model on to experimentally obtained dispersion curves. Multiple elastic modes, as measured over a wide frequency range are employed for this purpose, yielding a robust estimate of the structural wave velocities for longitudinal and shear motions. Details about the experiment and the fit procedure will be given.

16:10 – 17:00 Microstructure-based numerical modeling of the solid-fluid coupling interaction in acoustic foam
K. Gao¹, J.A.W. van Dommelen¹, P. Göransson², M.G.D. Geers¹

¹ Eindhoven University of Technology, Mechanical Engineering, Materials Technology, Eindhoven, The Netherlands
² MWL, Department of Aeronautical and Vehicle Engineering, KTH Royal Institute of Technology, Stockholm, Sweden

High-tech systems operating in a noisy environment must be protected from acoustic excitations. Passive absorbing materials, such as acoustic foams, can be used in acoustic shielding covers to improve their performance, which depends on the interaction between the acoustic wave and the microstructure of the foam. Biot’s poroelastic equations, which describe the macroscopic sound propagation in a porous medium, can be used to design the cover. In this paper, a numerical approach is proposed to obtain Biot’s parameters based on the structure of an isotropic porous material. It is assumed that a macroscopic point can be presented by a microscopic representative volume element (RVE) consisting of the solid skeleton and the air. The macroscopic properties are controlled by Biot’s equations and the microscopic RVE is governed by balance equations for momentum and linear constitutive laws. With suitable boundary conditions, the micro-macro relation is built up on consistency of energy. Then, Biot’s parameters are calculated through the response of the microscopic RVE. An example of a 3D cubic RVE is given and it is shown that the parameters obtained from this new approach behave qualitatively the same as associated phenomenological models. Furthermore, simulations of sound propagation in a foam are conducted by using Biot’s equations with Biot’s parameters and they are also compared with the direct numerical simulation (numerical experiment).

17:00 – 18.00 Drink

Bron: NAG