Papers by Andreas Wiegmann
Modeling Curved Fibers by Fitting R-vine Copulas to their Frenet Representations
Microscopy and Microanalysis
In the present paper, we propose a novel single-fiber model which exploits a description of fiber... more In the present paper, we propose a novel single-fiber model which exploits a description of fibers as a sequence of bond and torsion angles. Using the Frenet–Serret formulas, this representation can be translated into three-dimensional (3D) space and vice-versa. While the precise locations of points along a fiber do not directly convey information about the inner material properties of the fiber, the distribution of bond, and torsion angles may be related to various material characteristics and, thus, our model may form a direct link between inner material properties and emerging microstructure properties. More precisely, we model curved fibers in the 3D Euclidean space R3 as polygonal tracks that we represent by their local curvature and torsion at each sampling point. The 2D sequences of curvatures and torsions obtained in this way are then considered as realizations of a Markov chain with finite memory which takes its values in R2. The transition kernel of this Markov chain is gi...
Journal of the European Ceramic Society, 2018
Numerical calculations of relative Young's modulus and thermal conductivity have been performed o... more Numerical calculations of relative Young's modulus and thermal conductivity have been performed on computer-generated microstructures of wall-based (closed-cell) and strut-based (open-cell) cellular materials (foams) and inverse foams. The results are compared to rigorous upper bounds (Wiener-Paul, Hashin-Shtrikman), modelbased predictions (power-law, exponential) and cross-property predictions (CPRs). It is shown that closed-cell foams exhibit higher property values than open-cell and inverse foams, Kelvin foams higher than random foams, and the difference between closed-cell and open-cell foams is larger than that between Kelvin and random foams. While the properties of closed-cell foams are higher than the power-law prediction, those of inverse and opencell random foams are between the exponential and power-law predictions, and open-cell Kelvin foams follow the Gibson-Ashby power-law prediction for open-cell foams. The Pabst-Gregorová CPR is shown to predict Young's modulus with accuracy better than ±0.02 relative property units (better than any model-based relation and any other CPR).
Advances in Water Resources, 2016
Pore-scale images obtained from a synchrotron-based X-ray computed micro-tomography (CT) imbibit... more Pore-scale images obtained from a synchrotron-based X-ray computed micro-tomography (CT) imbibition experiment in sandstone rock were used to conduct Navier-Stokes flow simulations on the connected pathways of water and oil phases. The resulting relative permeability was compared with steady-state Darcy-scale imbibition experiments on 5 cm large twin samples from the same outcrop sandstone material. While the relative permeability curves display a large degree of similarity, the endpoint saturations for the CT data are 10% in saturation units higher than the experimental data. However, the two datasets match well when normalizing to the mobile saturation range. The agreement is particularly good at low water saturations, where the oil is predominantly connected. Apart from different saturation endpoints, in this particular experiment where connected pathway flow dominates, the discrepancies between pore-scale connected pathway flow simulations and Darcy-scale steady-state data are minor overall and have very little impact on fractional flow. The results also indicate that if the pore-scale fluid distributions are available and the amount of disconnected non-wetting phase is low, quasi-static flow simulations may be sufficient to compute relative permeability. When pore-scale fluid distributions are not available, fluid distributions can be obtained from a morphological approach, which approximates capillary-dominated displacement. The relative permeability obtained from the morphological approach compare well to drainage steady state whereas major discrepancies to the imbibition steady-state experimental data are observed. The morphological approach does not represent the imbibition process very well and experimental data for the spatial arrangement of the phases are required. Presumably for modelling imbibition relative permeability an approach is needed that captures moving liquid-liquid interfaces, which requires viscous and capillary forces simultaneously.
Microstructure reconstruction and transport simulation in polymer electrolyte membrane fuel cells
Polymer Electrolyte Membrane and Direct Methanol Fuel Cell Technology, 2012
Abstract: The polymer electrolyte fuel cell (PEFC) consists of disparate porous media microstruct... more Abstract: The polymer electrolyte fuel cell (PEFC) consists of disparate porous media microstructures, e.g. catalyst layer, microporous layer, gas diffusion layer, as the key components for achieving the desired performance attributes. The microstructure-transport interactions are of paramount importance to the performance and durability of the PEFC. In this chapter, a systematic description of the stochastic microstructure reconstruction techniques along with the numerical methods to estimate effective transport properties and to study the influence of the porous structures on the underlying transport behavior is presented.
Nordic Pulp & Paper Research Journal, May 1, 2012
When liquid packaging board is made aseptic in the filling machine the unsealed edges of the boar... more When liquid packaging board is made aseptic in the filling machine the unsealed edges of the board are exposed to hydrogen peroxide. A high level of liquid penetration may lead to aesthetic as well as functional defects. To be able to make a priori predictions of the edge wicking properties of a certain paperboard material is therefore of great interest to paper industry as well as to packaging manufacturers. The aim of this paper is to present a new analytical theory for prediction of the edge wicking properties of paperboard. The theory is based on Darcy's law and the ideal gas law to describe the physical behavior of water flow in paperboard. The theory is compared to a recently published multi-scale framework and with pressurized edge wick experiments. The agreement is very good for paperboard samples of different sizes. The conclusion from the work is that both analytical theory and detailed simulations are useful to predict edge wicking properties of paperboard material.
Multifunctional optimization of random hollow sphere stackings
Scripta Materialia, 2013
ABSTRACT Random hollow sphere stackings present good performance for the seemingly contradictory ... more ABSTRACT Random hollow sphere stackings present good performance for the seemingly contradictory requirements of a light structural material and good acoustic damping properties. A way to optimize the architectured materials parameters is described. The structural properties of random hollow sphere stacking have been measured using X-ray tomography and compared to an Artz compaction model. The mechanical properties are optimized according to the architectural parameters by way of coupled finite elements and discrete elements simulations, and acoustical properties are discussed based on tube absorption models.
International Journal of Hydrogen Energy, May 1, 2011
Water management in polymer electrolyte membrane (PEM) fuel cells is of importance due to its imp... more Water management in polymer electrolyte membrane (PEM) fuel cells is of importance due to its impact on the performance, durability and ultimately the cost of the cell. In the gas diffusion layer (GDL), liquid water has a direct effect on species and heat transport. The amount of liquid water in the GDL affects the relative permeability and capillary pressure, which govern the convective and diffusive transport of liquid water. Liquid water acts as a barrier to the diffusion of gases through the void region and facilitates in heat transfer. In this study, the full morphology model was used in order to investigate the effects of liquid water presence on the transport properties of the carbon paper GDL and examine the applicability of using various laws to estimate the transport properties in the presence of liquid water. The numerical results were compared against published experimental data. Further, the method of standard porosimetry was used to experimentally measure the effect of Teflon treatment on the capillary pressure of carbon paper. It was found that the addition of PTFE to the GDL results in the increase of capillary pressure; however, further increases to the PTFE loading did not result in additional changes to the capillary pressure.
SIAM Journal on Numerical Analysis, 2000
Many boundary value problems (BVPs) or initial BVPs have non-smooth solutions, with jumps along l... more Many boundary value problems (BVPs) or initial BVPs have non-smooth solutions, with jumps along lower-dimensional interfaces. The Explicit-Jump Immersed Interface Method (EJIIM) was developed following Li's Fast Iterative IIM (FIIIM), recognizing that the foundation for the efficient solution of many such problems is a good solver for elliptic BVPs. EJIIM generalizes the class of problems for which FIIIM is applicable. It handles interfaces between constant and variable coefficients and extends the IIM to BVPs on irregular domains with Neumann and Dirichlet boundary conditions. Proofs of second order convergence for a 1D problem with piecewise constant coefficients and for 2D problems with singular sources are given. Other problems are reduced to the singular sources case, with additional equations determining the source strengths. The advantages of EJIIM are high quality of solutions even on coarse grids and easy adaptation to many problems with complicated geometries, while still maintaining the efficiency of the FIIIM.
Journal of Computational Physics, Sep 1, 2000
We develop and test an algorithmic approach to the boundary design of elastic structures. The goa... more We develop and test an algorithmic approach to the boundary design of elastic structures. The goal of our approach is two-fold: first, to develop a method which allows one to rapidly solve the two-dimensional Lamé equations in arbitrary domains and compute, for example, the stresses, and second, to develop a systematic way of modifying the design to optimize chosen properties. At the core, our approach relies on two distinct steps. Given a design, we first apply an explicit jump immersed interface method to compute the stresses for a given design shape. We then use a narrow band level set method to perturb this shape and progress towards an improved design. The equations of 2D linear elastostatics in the displacement formulation on arbitrary domains are solved quickly by domain embedding and the use of fast elastostatic solvers. This effectively reduces the dimensionality of the problem by one. Once the stresses are found, the level set method, which represents the design structure through an embedded implicit function, is used in the second step to alter the shape, with velocities depending on the stresses in the current design. Criteria are provided for advancing the shape in an appropriate direction and for correcting the evolving shape when given constraints are violated.
The well known discretization of the Dirichlet boundary condition for the Laplace equation on a r... more The well known discretization of the Dirichlet boundary condition for the Laplace equation on a rectangle is shown to be a special case of the Explicit Jump Immersed Interface Method. For one-dimensional boundary value problems, Schur-complements for finite difference discretizations are pointwise discretizations of integral formulas. The analogy requires a discretization of the delta and dipole, which may occur even on the domain boundary.
OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information), 2009
Los Alamos National Laboratory. an affirmative action/equal opporiunity employer, is operated by ... more Los Alamos National Laboratory. an affirmative action/equal opporiunity employer, is operated by the Los Alamos Nc~lional Security, LLC for the National Nuclear Security Administration of the US. Depariment of Energy under contract DE•AC52•06NA25396 By acceptance of this ariicle, the publisher recognizes that the U.S. Government retains a nonexclusive, royalty•free license to publish or reproduce the published form of this contribution, or to allow others to do so, for U.S. Government purposes. Los Alamos National Laboratory requests that the publisher identity this ariicle as work performed under the auspices of the U.S. Depariment of Energy. Los Alamos National Laboratory strongly supports aCcJemic freedom and a researcher's right to publish; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness. Form 836 (7/06) Two-Pha.~ Behavior Dod Compression tfr~cl in the PEFC Ca, l>irru,iu D \lediun• 1
Sintered hollow spheres random stackings : mechanical properties and acoustic damping optimization
HAL (Le Centre pour la Communication Scientifique Directe), Nov 7, 2012
International audienc
Struktur und Druckverlust realer und virtueller Drahtgewebe
Four aspects are important in the design of hydraulic filters. We distinguish between two cost fa... more Four aspects are important in the design of hydraulic filters. We distinguish between two cost factors and two performance factors. Regarding performance, filter efficiency and filter capacity are of interest. Regarding cost, there are production considerations such as spatial restrictions, material cost and the cost of manufacturing the filter. The second type of cost is the operation cost, namely the pressure drop. Albeit simulations should and will ultimately deal with all 4 aspects, for the moment our work is focused on cost. The PleatGeo Module generates three-dimensional computer models of a single pleat of a hydraulic filter interactively. PleatDict computes the pressure drop that will result for the particular design by direct numerical simulation. The evaluation of a new pleat design takes only a few hours on a standard PC compared to days or weeks used for manufacturing and testing a new prototype of a hydraulic filter. The design parameters are the shape of the pleat, the permeabilities of one or several layers of filter media and the geometry of a supporting netting structure that is used to keep the outflow area open. Besides the underlying structure generation and CFD technology, we present some trends regarding the dependence of pressure drop on design parameters that can serve as guide lines for the design of hydraulic filters. Compared to earlier two-dimensional models, the three-dimensional models can include a support structure.
An efficient mathematical model to virtually generate woven metal wire meshes is presented. The a... more An efficient mathematical model to virtually generate woven metal wire meshes is presented. The accuracy of this model is verified by the comparison of virtual structures with threedimensional images of real meshes, which are produced via computer tomography. Virtual structures are generated for three types of metal wire meshes using only easy to measure parameters. For these geometries the velocity-dependent pressure drop is simulated and compared with measurements performed by the GKD-Gebr. Kufferath AG. The simulation results lie within the tolerances of the measurements. The generation of the structures and the numerical simulations were done at GKD using the Fraunhofer GeoDict software.
Journal of The Electrochemical Society, 2007
A full morphology ͑FM͒ model has been developed for studying the two-phase characteristics of the... more A full morphology ͑FM͒ model has been developed for studying the two-phase characteristics of the gas diffusion medium in a polymer electrolyte fuel cell ͑PEFC͒. The three-dimensional ͑3D͒ fibrous microstructure for the nonwoven gas diffusion layer ͑GDL͒ microstructure has been reconstructed using a stochastic technique for Toray090 and SGL10BA carbon papers. The FM model directly solves for the capillary pressure-saturation relations on the detailed morphology of the reconstructed GDL from drainage simulations. The estimated capillary pressure-saturation curves can be used as valuable inputs to macroscopic two-phase models. Additionally, 3D visualization of the water distribution in the gas diffusion medium suggests that only a small number of pores are occupied by liquid water at breakthrough. Based on a reduced compression model, the two-phase behavior of the GDL under mechanical load is also investigated and the capillary pressure-saturation relations are evaluated for different compression levels.
e-Journal of Nondestructive Testing, Mar 1, 2019
The properties of granular or sintered manufactured materials or natural materials depend on thei... more The properties of granular or sintered manufactured materials or natural materials depend on their microstructure. Obtaining meaningful insights on the microstructure of these materials often relies on the correct identification of individual grains and pores. Their direct identification on 3D digital images is challenging but opens the possibility to detailed simulations and accurate predition of material behavior, such as mechanical simulations that depend on the grain orientation. Such simulations give vital information to figure out the most promising designs of manufactured materials such as electrodes or to reach decisions based on reservoir rock properties and determined by Digital Rock Physics. With this motivation, several new algorithms have been integrated into the GeoDict ® [1] software. GeoDict's capabilities to segment 3D scans, e.g. CT or FIB-SEM scans have been expanded to identify also individual grains and pores through the new modules GrainFind and PoreFind that are presented here, together with the option to use the results of GrainFind for the creation of digital twins of granular materials.
Comparison of methods for measuring and investigating water permeability of woven filter media
Chemical engineering research & design, Feb 1, 2023
FFT-based fast Poisson and fast Helmholtz solvers on rectangular parallelepipeds for periodic bou... more FFT-based fast Poisson and fast Helmholtz solvers on rectangular parallelepipeds for periodic boundary conditions in one-, two and three space dimensions can also be used to solve Dirichlet and Neumann boundary value problems. For non-zero boundary conditions, this is the special, grid-aligned case of jump corrections used in the Explicit Jump Immersed Interface method. Fast elastostatic solvers for periodic boundary conditions in two and three dimensions can also be based on the FFT. From the periodic solvers we derive fast solvers for the new "normal" boundary conditions and essential boundary conditions on rectangular parallelepipeds. The periodic case allows a simple proof of existence and uniqueness of the solutions to the discretization of normal boundary conditions. Numerical examples demonstrate the efficiency of the fast elastostatic solvers for non-periodic boundary conditions. More importantly, the fast solvers on rectangular parallelepipeds can be used together with the Immersed Interface Method to solve problems on non-rectangular domains with general boundary conditions. Details of this are reported in the preprint The Explicit Jump Immersed Interface Method for 2D Linear Elastostatics by the author.
Effective Properties of Nonwoven Textiles from Microstructure Simulations
Springer eBooks, 2008
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Papers by Andreas Wiegmann