| 1. |
- de Conchard, Antoine Vermeil, et al.
(författare)
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A perfectly matched layer formulation adapted for fast frequency sweeps of exterior acoustics finite element models
- 2019
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Ingår i: Journal of Computational Physics. - : ACADEMIC PRESS INC ELSEVIER SCIENCE. - 0021-9991 .- 1090-2716. ; 398
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Tidskriftsartikel (refereegranskat)abstract
- Effective treatment of unbounded domains using artificial truncating boundaries are essential in numerical simulation, e.g. using the Finite Element Method (FEM). Among these, Perfectly Matched Layers (PML) have proved to be particularly efficient and flexible. However, an efficient handling of frequency sweeps is not trivial with such absorbing layers since the formulation inherently contains coupled space-and frequency-dependent terms. Using the FEM, this may imply generating system matrices at each step of the frequency sweep. In this paper, an approximation is proposed in order to allow for efficient frequency sweeps. The performance and robustness of the proposed approximation is presented on 2D and 3D acoustic cases. A generic, robust way to truncate the acoustic domain efficiently is also proposed, tested on a range of test cases and for different frequency regions. All rights reserved.
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| 2. |
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| 3. |
- Gaborit, Mathieu, et al.
(författare)
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Controlled anisotropy materials and 3D printing: experimentations and analyses
- 2022
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Ingår i: Proceedings of ISMA 2022 - International Conference on Noise and Vibration Engineering and USD 2022 - International Conference on Uncertainty in Structural Dynamics. - : KU Leuven, Departement Werktuigkunde. ; , s. 477-481
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Konferensbidrag (refereegranskat)abstract
- Novel computational tools and optimisation strategies offer an unprecedented framework to explore large design spaces within a short time frame. In the scope of material design, these new possibilities have completely revolutionized the research horizon, leading amongst other things to controlled anisotropy media with a finer granularity than ever seen before. However, a question arises regarding the manufacturability of such media which most of the time relies on 3D printing and the agreement between modelled and printed geometry. In the recent years, the authors published several articles on the properties of Kelvin Cell packings and the possibility to control their anisotropy. In the last few months, an effort towards printing the designed media has been made in search for experimental validation of the numerical results. This contribution describes the printing process for kelvin cell packing samples with controlled anisotropy and analyses their agreement with the model both from a geometric and from a physical response standpoint. Depending on the advances of current research, information on the dynamic behaviour of such systems will be discussed.
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| 4. |
- Kleine-Wächter, Lukas, et al.
(författare)
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Numerical study of Kelvin cells for the design of periodic lattice metamaterials
- 2022
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Ingår i: Proceedings of ISMA 2022 - International Conference on Noise and Vibration Engineering and USD 2022 - International Conference on Uncertainty in Structural Dynamics. - : KU Leuven, Departement Werktuigkunde. ; , s. 2960-2974
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Konferensbidrag (refereegranskat)abstract
- Artificially-composed materials, often called metamaterials, are an increasingly considered measure for vibration control. By carefully arranging the material micro-structure, significant vibration attenuation is achievable in targeted frequency bands from resonant and wave scattering effects. An approach in designing materials for vibration control are micro-structures assembled from periodic cellular lattices. Such architectures result from the spatial repetition of cellular units that can be dynamically tuned by controlling the lattice characteristics. This contribution investigates the prospects of a three-dimensional lattice structure for application in vibration control. A unit cell design strategy is proposed based on the isometric Kelvin cell. By imposing twists on the faces of the Kelvin cell, a potential tuning mechanism for the cell's dispersive properties is introduced. Selected unit cell designs obtained from this approach are investigated in terms of the dispersion characteristics of 1D-infinite structures.
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| 5. |
- Li, Jiejie, et al.
(författare)
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Exact closed-form solution for vibration characteristics of multi-span beams on an elastic foundation subjected to axial force
- 2024
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Ingår i: Structures. - : Elsevier BV. - 2352-0124. ; 60
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Tidskriftsartikel (refereegranskat)abstract
- This paper investigates the vibration characteristics of multi-span beams resting on an elastic foundation and subjected to axial forces. A comprehensive analytical expression of the dynamic response of multi-span beams on an elastic foundation that is developed to address various boundary conditions. The vibration equation is derived by employing Newton's second law. By Laplace transformations and the Green's function method, the solution of this governing equation can be obtained. Subsequently, a unified description is implemented for distinct types of boundary conditions using matrix representations. The correctness is verified through reference results and finite element methods (FEM). The effects of different parameters such as support stiffness, foundation elastic and shear layer stiffness, and axial force on the vibration characteristics are analyzed. This study demonstrates two findings: First, there are two thresholds for support stiffness, and the stiffness value is divided into three intervals. In the same interval, multi-span beams show the same properties. Second, for a rigidly supported multi-span beam, the critical axial force with a natural frequency of zero is just the corresponding Euler's buckling load; for elastically supported multi-span beams, the critical axial force falls between the Euler's buckling load corresponding to single-span and multi-span beams.
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| 6. |
- Lundberg, Eva, 1964-, et al.
(författare)
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Analytical method for predicting micro-geometry based flow resistivity in anisotropic foams to improve sound absorption of vehicle panels
- 2021
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Ingår i: Proceedings of the Resource Efficient Vehicles Conference - 2021 (rev2021). - Stockholm, Sweden.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Vehicle structures such as train floors or car roofs are usually built as multi-layer panels, where a foam is placed between a load-carrying structure and an interior panel. The foam adds acoustical and thermal performance, but very little weight. In most contributions introducing foams for acoustic treatment, these have been considered isotropic, with acoustic losses mainly dependingon properties in the thickness direction. Another mechanism investigated here is the possibilityfor the acoustic flow in the foam to change from acting only in the thickness direction but rather to be re-directed to also travel in-plane, where dimensions are substantially larger than in the thickness direction, permitting more losses as the wave travels through the material. That kind of effect would result in higher acoustic losses without increasing the thickness of the vehicle panel and better use of the allowable space to achieve acoustic and functional requirements, i.e. a better functional density. A first step is to investigate how the absorption properties of an anisotropic foam differs from an isotropic foam. The chosen approach is to use an analytical micro-modelto calculate the dynamic drag impedance (flow resistivity on micro-scale) for an anisotropic opencell foam material. Based on a simple micro-scale geometry of Kelvin cells, it has been shown that simple cell alterations to the micro-geometry, such as stretching, twisting and tilting results in an anisotropic foam structure. The anisotropic flow resistivity tensor is not diagonal and uniform, but different directions can have different magnitudes and it can display off-diagonal coupling terms. The influence of such micro-scale distortions on the flow resistivity, and on the resulting sound absorption is investigated with the purpose of improving the acoustic performance without adding volume. Future steps include to modify the functional density and tailor the sound transmission loss to a specific application.
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| 7. |
- Lundberg, Eva, 1964-, et al.
(författare)
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Tunable absorption of micro-structure based anisotropic opencell materials
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- A simplified analytical model based on the Kelvin cell micro geometry has been developed for estimating the dynamic drag impedance of a periodic open cell material based on a Kelvincell based micro structure. The calculated dynamic drag impedance estimates similar properties as the static flow resistivity, but is based on dynamic micro scale estimates of the viscous losses neglecting interactions between struts. The Kelvin cell model can have a controlled degree ofanisotropy. Implementing the micro model in a state space transfer matrix method allows for absorption calculations of anisotropic micro material including stiffness to be calculated with limited input information. In addition to the solid constituent material parameters only cell height,strut thickness and twist angle, determining the degree of anisotropy, are required. The modelling allows for fast computations making optimization feasible, which is demonstrated by optimizing a two-layer porous material with the degree of anistropy as a design variable. An optimized design with enhanced absorption at lower frequencies, where the two layers have different anisotropic cell geoemetry, is discussed.
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| 8. |
- Lundberg, Eva, 1964-, et al.
(författare)
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Tuning sound transmission loss for multi-layer panels with anisotropic foams
- 2022
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Multilayer panels consisting of a load carrying structure, a porous material for thermal and acoustic insulation and an interior trim panel is a very common type of design for vehicles. Weight as well as total build height are usually limiting constraints on the design. The idea of using an anisotropic porous material instead of an isotropic one to improve the sound transmission loss without adding a lot of weight or thickness is explored in the paper. By using a state space formulation of the transfer matrix method transmission loss it is possible to include anisotropic material properties in the calculation. The anisotropic material is modelled by a combination of a simplified analytical model for the acoustic losses and inverse estimation of the 21 independent elastic constants of the Hooke’s tensor. The porous material, which has typical dimensions possible to 3D print, is based on a Kelvin cell micro model that has a controlled degree of anisotropy.
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| 9. |
- Mallol, Pau, et al.
(författare)
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Experiments and simulations of the deployment of a bistable composite boom
- 2018
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Ingår i: Journal of Spacecraft and Rockets. - : American Institute of Aeronautics and Astronautics Inc.. - 0022-4650 .- 1533-6794. ; 55:2, s. 292-302
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Tidskriftsartikel (refereegranskat)abstract
- The rapidly growing use of small satellites for space missions requires deployable systems to be highly storable yet large and with adequate mechanical properties when deployed. This paper focuses on the modeling and simulation of a meter-class passively deployable boom, based on the self-contained linear meter-class deployable boom, exploiting the bistable nature of composite shells. Experimental tests were performed on a boom prototype suspended in a gravity offloading system. The strain energy level, deployment time, and spacecraft displacements calculated from the finite element method agree well with analytical analyses, confirming the theoretical accuracy of the finite element method. Because friction and strain energy relaxation were not included in the model, the finite element simulations predicted deployment times up to five times shorter than those of the gravity offloaded boom experiments. The quick deployment and violent end-of-deployment shock created boom deployment dynamics that were not seen in the experiments. The observed differences between the finite element model and the tests were mainly due to inaccurate material and friction models.
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| 10. |
- Mallol, Pau, et al.
(författare)
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Experiments and Simulations of the Deploymentof a Bi-stable Composite Boom
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Annan publikation (populärvet., debatt m.m.)abstract
- The rapidly growing use of nano- and pico-satellites for space missions requires de-ployable systems to be highly storable yet large and with adequate mechanical properties when deployed. This paper focuses on the modeling and simulation of a meter-class passively deployable boom, based on the self-contained linear meter-class deployable(SIMPLE) boom by Thomas W. Murphey, exploiting the bi-stable nature of compositeshells. Experimental tests were carried on a boom prototype suspended in a gravityo-offloading system. The strain energy level, deployment time and spacecraft displacements calculated from the finite element method agree well with analytical analyses, confirming the theoretical accuracy of the finite element method. Since friction and strain energy relaxation were not accurately included in the model, the finite element simulations predict deployment times up to five times shorter than those of the gravity off-loaded boom experiments. The quick deployment and violent end-of-deployment shock create boom deployment dynamics which are not seen in the experiments.
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