1,691 research outputs found
Analysis of Boundary Element Methods
No full textInternational audienceThe workshop \emph{Analysis of Boundary Element Methods}, organized by Martin Costabel (Rennes) and Ernst P. Stephan (Hannover). This meeting brought together 46 experts in numerical analysis. Boundary element methods (BEM) are well established numerical methods with a wide range of applications. There are still many challenging problems, the aim of this workshop was to coordinate the efforts to tackle these problems. A central theme of many talks was the error analysis. In the following we focus on some central aspects
Higher-order time domain boundary elements for elastodynamics: gradedmeshes and hp versions
No full textEfficient coupling of finite elements and boundary elements---adaptive procedures and preconditioners
No full textThe article is split into four parts. First, we present the symmetric finite element/boundary element-coupling method. Second, we address the choices of appropriate preconditioners for the resulting discrete system when h- and p-versions are performed. Third, we discuss contact problems which are reduced to variational inequalities. Finally, we show the practical applicability of the finite element/boundary element-coupling method by applying it to a metal turning process. Here the viscoplastic work piece is modelled with finite elements and the linear elastic work tool (milling cutter) is modelled with boundary elements. This leads to an efficient and fast numerical method to simulate the metal turning process and to predict failure of the thermal shrink fit which holds the milling cutter.
References A. Chernov, M. Maischak and E. P. Stephan. A priori error estimates for hp penalty bem for contact problems in elasticity. Comput. Methods Appl. Mech. Engrg. 196 (2007), no. 37-40, 3871--3880. doi:10.1016/j.cma.2006.10.044 A. Chernov, M. Maischak and E. P. Stephan. hp-mortar boundary element methodfor two-body contact problems with friction. Math. Methods Appl. Sci., published online, doi:10.1002/mma.1005 A. Chernov and E. P. Stephan. Adaptive bem for contact problems with friction. IUTAM Symposium on Computational Methods in Contact Mechanics, 113--122, IUTAM Bookser., 3, Springer, Dordrecht, 2007. M. Costabel and E. P. Stephan. Coupling of finite and boundary element methods for an elastoplastic interface problem. SIAM J. Numer. Anal. 27 (1990), no. 5, 1212--1226. B. Denkena, E. P. Stephan, M. Maischak, D. Heinisch, M. Andres. Numerical computation methods for process-oriented structures in metal chipping. Proceedings of 1st International Conference on Process Machine Interaction (PMI). (2008), 247--258. B. Guo and E. P. Stephan. The hp-version of the coupling of finite element and boundary element methods for transmission problems in polyhedral domains. Numer. Math. 80 (1998), no. 1, 87--107. E. W. Hart. Constitutive relations for the nonelastic deformation of metals, Journal of Enginering Materials and Technology. 98 (1976), 193--202. N. Heuer, F. Leydecker and E. P. Stephan. An iterative substructuring method for the hp-version of the bem on quasi-uniform triangular meshes, Num.Meth.PDEs. 23 (2007), 879--903. doi:10.1002/num.20259 N. Heuer, M. Maischak and E. P. Stephan. Preconditioned minimum residual iteration for the hp-version of the coupled fem/bem with quasi-uniform meshes. Numer. Linear Algebra Appl. 6 (1999), no. 6, 435--456. N. Heuer and E. P. Stephan. An additive Schwarz method for the h-p version of the boundary element method for hypersingular integral equations in , IMA J. Numer. Analysis. 21 (2001), 265--283. doi:10.1093/imanum/21.1.265 I. Babuska, A. Craig, J. Mandel and J. Pitkaeranta. Efficient preconditioning for the p-version finite element method in two dimensions. SIAM J. Numer. Anal. 28 (1991), 624--661. N. Heuer, E. P. Stephan and T. Tran. Multilevel additive Schwarz method for the hp-version of the Galerkin boundary element method. Math.Comp.. 67 (1998), no. 222, 501--518. M. Maischak and E. P. Stephan. The hp-version of the boundary element method in . The basic approximation results. Math. Meth. Appl. Sci. 20 (1997), 461--476. M. Maischak and E. P. Stephan. Adaptive hp-versions of boundary element methods for elastic contact problems, Comp.Mech. 39 (2007), 597--607. doi:10.1007/s00466-006-0109-y S. Mukherjee and A. Chandra. Boundary element formulations for large strain-large deformation problems of plasticity and viscoplasticity, Developments in Boundary Element Methods, 1984, editors P. K. Banerjee and S. Mukherjee, 27--58. P. Mund and E. P. Stephan. Adaptive coupling and fast solution of fem-bem equations for parabolic-elliptic interface problems. Math. Methods Appl. Sci. 20 (1997), no. 5, 403--423. P. Mund and E. P. Stephan. An adaptive two-level method for the coupling of nonlinear fem-bem equations. SIAM J. Numer. Anal. 36 (1999), no. 4, 1001--1021. P. Seshaiyer and M. Suri. Uniform hp convergence results for the mortar finite element method. Math. Comp. 69 (2000), no. 230, 521--546. E. P. Stephan. Coupling of finite elements and boundary elements for some nonlinear interface problems, Comput. Methods Appl. Mech. Engrg. 101 (1992), no. 1--3, 61--72. E. P. Stephan. The hp-boundary element method for solving - and 3-dimensional problems. Comput. Methods Appl. Mech. Engrg. 133 (1996), no. 3--4, 183--208. E. P. Stephan. Coupling of Boundary Element Methods and Finite Element Methods, Encyclopedia of Computational Mechanics. Vol. 1, Chapter 13. 2004, John Wiley and Sons. ISBN: 0-470-84699-2. I. Babuska, B. Q. Guo and E. P. Stephan. The hp-version of the boundary element method with geometric mesh on polygonal domains. Computer Methods in Applied Mechanics and Engineering 80 (1990), 319--325. E. P. Stephan, S. Geyn, M. Maischak and M. Andres. A boundary element / finite element procedure for metal chipping. CMM-2007, June 19--22, 2007, \unhbox \voidb@x \setbox \z@ \hbox {L}\hbox to\wd \z@ {\hss \@xxxii L}odz--Spala, Poland. T. Tran and E. P. Stephan. Additive Schwarz methods for the h-version boundary element method. Applicable Analysis. 60 (1996), no. 1--2, 63--84. T. Tran and E. P. Stephan. Additive {Schwarz} algorithms for the p-version of the Galerkin boundary element method. Numer. Math.. 85 (2000), no. 3, 433--468. T. Tran and E. P. Stephan. An overlapping additive Schwarz preconditioner for boundary element approximations to the Laplace screen and Lame crack problems, J. Numer. Math. 12 (2004), 311--330. doi:10.1515/1569395042571265 P. Wriggers. Computational Contact Mechanics, 2002, John Wiley and Sons. E. Zeidler. Nonlinear Functional Analysis and its Applications. IV. Springer, New York, 1988. C. Carstensen and E. P. Stephan. Adaptive coupling of boundary elements and finite elements. RAIRO Modl. Math. Anal. Numer. 29 (1995), no. 7, 779--817. A. Chernov. Nonconforming boundary elements and finite elements for interface and contact problems with friction; hp-version for mortar, penalty and Nitsche's methods, PhD Thesis, Universitaet Hannover, 2007
De Caussis Quibusdam Praematurae Senectutis Praecipuis
No full textHalle, Univ., Med. Diss., 1765Enth. 4 Gedichte der BeiträgerAutopsie nach Ex. der ULB Sachsen-AnhaltVorlageform des Erscheinungsvermerks: Halae Ad Salam Aere Stephaniano
Spectral field study of the Makgadikgadi Salt Pans in Botswana as a planetary analog for ancient fluvio-lacustrine environments on Mars
No full textThe Makgadikgadi Salt Pans in northern Botswana offer a unique opportunity to study the mineralogy of evaporates and clays derived from fluvio-lacustrine sediments in their geological context. During the rainy season the pans are usually filled with water, but the level never exceeds ~50 cm in height. During the dry season, from June to October, an expanse of salt deposits dries in the sun above a layer of clay and sand. The pans are completely flat. However, locally, some exposed rocks such as granite, dolerite, exist. A field campaign taking place in August 2022, funded by Europlanet 2024 RI (grant agreement No 871149) were performed in order to 1) obtain a horizontal profile of the mineralogical diversity throughout the Pans from the topographic center to the rim/shoreline; and (2) investigate variations in the mineralogical composition of the evaporates and clays due to the influence of neighboring and/or underlying (bedrock) units. Spectral measurements were performed directly in the field with a portable visible/near-infrared spectrometer that samples the surface in the visible and near-infrared (VIS-NIR) wavelength range between 0.35 and 2.5μm. This range is known to be best suited for mineralogic research and most commonly used on planetary spacecrafts.
The acquired spectra reveal that salts dominate a more or less fresh, white to light brown, several mm-thick uppermost crust throughout the pans. They are particularly prominent where the salts themselves or at least the clays underneath this layer are still wet from the rainy season. The special shape of the water-related feature at 2μm implies that sodium hydrogen carbonates such as trona dominate the salt layer. Although halite should be also present, its spectral signature might be masked by the signature of trona. In the wettest location, a thin greenish layer of organic material has been found, which causes a characteristic feature near 0.7 μm. In regions that have been dry for a prolonged period, clays such as montmorillonite dominate over salts. Bed rocks that are in direct contact with the pan deposits often show a distinct greenish color. Spectra of these rocks are dominated by glauconite, sometimes in combination with illite, which possibly develop as a consequence of alteration of sedimentary deposits associated with low-oxygen conditions. The collected spectra in combination with the knowledge of their geologic context will be extremely useful for identifying and mapping similar environments on Mars with potential for future landing sites by spectrometers working in the visible-near infrared (VNIR) wavelength range such as Mars Express OMEGA and MRO CRISM.
In addition, spectra acquired in the field provide the spectral endmembers, which are now used to classify the currently available data of the pans provided by the Environmental Mapping and Analysis Program (EnMAP) of the German hyperspectral satellite mission. EnMAP data cover the same wavelength range in the VIS-NIR such as the field instrument and cover the major portions of the pans at the same seasonal period of the year. We present the distribution of the minerals detected in the field across the observed areas.
Furthermore, samples collected in the field are now analyzed by additional types of spectroscopy such as Laser-induced breakdown spectroscopy (LIBS) and Raman, a combination, which have proven to significantly enhance the scientific potential of studying the mineralogy of planetary analog materials (Stephan et al., 2022). Results of this multi-instrument approach will be presented.
References:
Stephan, K., Gwinner, K., Schröder, S., Rammelkamp, K., Baque, M., Grindrod, P. (2022): Spectral investigation of volcanic alteration deposits on Vulcano island /Italy as planetary analog for acid alteration conditions on Mars, EPSC, DOI: 10.5194/epsc2022-974
Changing incentives to publish
No full textMany national governments have implemented policies providing incentives for researchers to publish, especially in highly ranked international journals. Although still the top publishing nation, the United States has seen its share of publications decline from 34.2% in 1995 to 27.6% in 2007 as the number of articles published by U.S. scientists and engineers has plateaued and that of other countries has grown (1, 2). Hicks (3) argues that the two events are not unrelated: The decline in the relative performance of the United States relates to increased international competition engendered by newly adopted incentives that have crowded out some work by U.S. author
Multilevel methods for the h-, p-, and hp-versions of the boundary element method
No full textAbstractIn this paper we give an overview on the definition of finite element spaces for the h-, p-, and hp-version of the BEM along with preconditioners of additive Schwarz type. We consider screen problems (with a hypersingular or a weakly singular integral equation of first kind on an open surface Γ) as model problems. For the hypersingular integral equation and the h-version with piecewise bilinear functions on a coarse and a fine grid we analyze a preconditioner by iterative substructuring based on a non-overlapping decomposition of Γ. We prove that the condition number of the preconditioned linear system behaves polylogarithmically in H/h. Here H is the size of the subdomains and h is the size of the elements. For the hp-version and the hypersingular integral equation we comment in detail on an additive Schwarz preconditioner which uses piecewise polynomials of high degree on the fine grid and yields also a polylogarithmically growing condition number. For the weakly singular integral equation, where no continuity of test and trial functions across the element boundaries has to been enforced, the method works for nonuniform degree distributions as well. Numerical results supporting our theory are reported
On domain derivatives of the solution to the electromagnetic transmission problem
No full textReport No. 19/2008 Analysis of Boundary Element Methods Organised by Martin Costabel, Rennes; Ernst P. Stephan, Hannover April 13th - April 19th, 2008International audienc
Telecommunication-Telemedia-Assessment/360_testcontent: v1.0.0
No full text<p>This repository contains some publicly available 360° videos.</p>
<p>If you use any or parts of the material included in this dataset, please cite the following paper:</p>
<p>For material in <code>/EI2019</code>:</p>
<pre><code>@article{hofmeyer2019impacts,
title={Impacts of internal HMD playback processing on subjective quality perception},
author={Hofmeyer, Frank and Fremerey, Stephan and Cohrs, Thaden and Raake, Alexander},
journal={Electronic Imaging},
volume={31},
pages={1--7},
year={2019},
publisher={Society for Imaging Science and Technology}
}
</code></pre>
High-Order Integral Equation Methods for Diffraction Problems Involving Screens and Apertures
No full textThis thesis presents a novel approach for the numerical solution of problems of diffraction by infinitely thin screens and apertures. The new methodology relies on combination of weighted versions of the classical operators associated with the Dirichlet and Neumann open-surface problems. In the two-dimensional case, a rigorous proof is presented, establishing that the new weighted formulations give rise to second-kind Fredholm integral equations, thus providing a generalization to open surfaces of the classical closed-surface Calderon formulae. High-order quadrature rules are introduced for the new weighted operators, both in the two-dimensional case as well as the scalar three-dimensional case. Used in conjunction with Krylov subspace iterative methods, these rules give rise to efficient and accurate numerical solvers which produce highly accurate solutions in small numbers of iterations, and whose performance is comparable to that arising from efficient high-order integral solvers recently introduced for closed-surface problems. Numerical results are presented for a wide range of frequencies and a variety of geometries in two- and three-dimensional space, including complex resonating structures as well as, for the first time, accurate numerical solutions of classical diffraction problems considered by the 19th-century pioneers: diffraction of high-frequency waves by the infinitely thin disc, the circular aperture, and the two-hole geometry inherent in Young's experiment
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