1,720,994 research outputs found
determinazione dei parametri non acustici di materiali porosi sulla base di un ottimizzazione parametrica
No full textTransport parameters for sound propagation in air saturated motionless porous materials: A review
No full textTransport parameters play a key role in characterizing the thermo-viscous behaviour of the microgeometry. Semi-phenomenological models provide valuable tools to establish a connection between the dynamic behaviour of porous materials and these transport parameters. However, each model has its limitations in terms of the frequency range and material types it can accurately represent. One of the most used semi-phenomenological acoustic models in the literature is the Johnson-Champoux-Allard-Lafarge (JCAL) model [J. Fluid. Mech. 176 (1987) 379–402, J. App. Phy. 70 (1998) 1975, J. Ac. Soc. Am. 102 (1998) 1995]. This model requires the knowledge of six transport parameters, known as the porosity φ, airflow resistivity σ, thermal characteristic length Λ′, viscous characteristic length Λ, high-frequency limit of tortuosity α∞, and static thermal permeability k0′, which establish a connection between the micro-geometrical features of the porous material and its macroscopic behaviour when subjected to sound waves. The JCAL model is applicable to all types of porous materials, and the required transport parameters can be measured using suitable devices. With recent advancements in additive manufacturing, it is now possible to create porous materials with precise and controlled geometries. Therefore, understanding the relationships between microgeometry and transport parameters is crucial for designing porous materials with specific acoustic properties. This study provides a comprehensive overview of all the transport parameters involved in characterizing the JCAL model. It synthesizes various direct, indirect, and inverse measurement techniques used to assess these parameters. Additionally, computational approaches for evaluating the transport parameters from representative elementary volumes (REV) of materials are presented. Finally, the study compiles the existing correlations between transport parameters and the microgeometry of the unit cell from the available literature
Insertion loss of a heap of gravel outdoors
No full textThe work reported in this paper deals with the measurement of the insertion loss IL of a wedge-shaped heap of
limestone chips on the ground outdoors. Measurements were carried out to investigate about the potential use of
these stocked granular materials as noise barriers. Since the measurements revealed it is worth considering this
opportunity, a semi-empirical model was implemented for the calculation of the IL for the observed situation. This
model stems from a theoretical model for the diffraction by the straight edge of an hard-wedge of infinite extent.
Modifications were applied to the original model in order to take into account the finite acoustic impedance of both
the surface of the wedge and the ground. The predicted and the measured values of the insertion loss appear to be in
an acceptable agreement
Experimental characterization of thermal and viscous powers in porous media under oscillating flow
No full textPorous materials are integrated components across various industries, offering unique properties such as high surface area, low density, and good permeability. They have a wide range of applications including energy conversion, with relevance in sound absorption and thermoacoustic phenomena. Understanding the intricate energy conversion mechanisms within the microstructure of porous materials under oscillating flows, such as sound waves, is paramount for optimizing their performance in these applications. The techniques currently used for testing porous materials enable the characterization of the behaviour of the porous matrix when subjected to an acoustic wave, without consideration to energetic quantities. Here, this paper presents two novel measurement techniques allowing for the experimental quantification of the power dissipated within the porous material, by making an explicit distinction between thermal relaxation and viscous dissipation effects. The study involves a model to quantify the viscous and thermal energetic behaviours from which analytical expressions guiding the elaboration of the proposed experimental techniques are derived, and finally validated through experimental data. Experimental tests have been carried out on three different samples (polyester fibers, wire mesh and triangular pores sample) largely used both in acoustic and thermoacoustic fields. The experimental data compared with the theoretical prediction for each sample allow to validate the measurement methodologies. By bridging theoretical modelling with experimental validation, this work contributes to the broader understanding and utilization of porous materials in energy conversion applications
A study on the energy and the reflection angle of the sound reflected by a porous material
No full textMany noise control applications are based on the use of porous materials; therefore, it is important to have a tool to simulate acoustic behaviours. Plane-wave acoustic properties such as the sound absorption or reflection coefficient at normal incidence can be quickly obtained using standing wave tube or theoretical models and can be used to select the type of porous material and its dimensions. However, in real situations, no plane wave exists, and the sound field is more complex. Therefore, it is interesting to understand the real behaviour of porous materials. In this paper, a simple case of a point source above a uniform infinite layer of porous material has been analysed. Conversely, to the case of a plane wave excitation, on the porous material surface different incidence angles can be observed and the phase relation between the pressure and the particles velocity should be considered. The aim of this work is to define the far and the near field above a porous material where the results obtained considering a plane excitation may or may not be respectively extended. For this purpose, two parameters have been introduced: the energetic deviations index and the true reflection angle
A study on the sound transmission loss of a new lightweight hemp/bio-epoxy sandwich structure
No full textDue to the increasing awareness around the environmental crisis and the depletion of petroleum resources, together with the inherent issues connected with the use of composite systems such as their disposal and recycling and the health problems associated with the use of some raw materials, new legislative rules on global scale have been promulgated to fight environmental issues and will be hoping adopted by several countries. Based on these considerations, the new generation of composite systems will have to minimize the environmental impact with a more efficient use of energy resources and materials and where possible, replace synthetic or petroleum materials with more sustainable components, like natural ones. Therefore, the aim of this paper is to investigate the sound transmission loss of a new sandwich structure in bio-composite material based on hemp fibers and bio-epoxy resin in order to further extend the application fields of natural fibers composite (NFC) materials. Measurements in the impedance tube were first used to assess the sound transmission loss at normal incidence. By using measured mechanical properties, a Finite Element Method was used to predict sound transmission loss in the same boundary condition. Finally, the so determined mechanical model has been used to predict the sound transmission loss in a diffuse field
Porous cores in small thermoacoustic devices for building applications
No full textThe thermoacoustic behavior of different typologies of porous cores is studied in this paper with the goal of finding the most suitable solution for small thermoacoustic devices, including solar driven air coolers and generators, which can be used in future buildings. Cores provided with circular pores, with rectangular slits and with arrays of parallel cylindrical pins are investigated. For the type of applications in focus, the main design constraints are represented by the reduced amount of the input heat power and the size limitations of the device. In this paper, a numerical procedure has been implemented to assess the behavior of the different core typologies. For a fixed input heat power, the maximum acoustic power delivered by each core is computed and the corresponding engine configuration (length of the resonator and position of the core) is provided. It has been found that cores with parallel pins provide the largest amount of acoustic power with the smallest resonator length. This conclusion has been confirmed by experiments where additive manufactured cores have been tested in a small, light-driven, thermoacoustic prime mover
Acoustic and thermoacoustic properties of an additive manufactured lattice structure
No full textWith the advent of additive manufacturing, lattice structures can be printed with precisely controlled geometries. In this way, it is possible to realize porous samples with specific acoustic and thermoacoustic characteristics. However, to this aim and prior to the manufacturing process, it is fundamental to have a design tool that can predict the behaviour of the lattices. In the literature, Luu, Perrot, and Panneton [Acta Acust. United Ac. 103, 1050 (2017)] provide a model to characterize transport parameters of fibrous material with a certain fiber orientation with respect to the direction of wave propagation. In this work, finite element numerical simulations are used to improve their model in order to compute the thermoviscous functions of lattice structures composed of cylindrical struts arranged in Tetragonal Body Centred cells. New correlations for transport parameters are suggested, which are finally coupled with the semi-phenomenological model of Johnson-Champoux-Allard-Lafarge to obtain the complex density and bulk modulus of the equivalent fluid. These results are compared with the measurements carried out on two 3-dimensional-printed samples with hybrid impedance tube techniques
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