@article{ChauDinhZiLeeetal., author = {Chau-Dinh, T. and Zi, Goangseup and Lee, P.S. and Song, Jeong-Hoon and Rabczuk, Timon}, title = {Phantom-node method for shell models with arbitrary cracks}, series = {Computers \& Structures}, journal = {Computers \& Structures}, doi = {10.1016/j.compstruc.2011.10.021}, abstract = {A phantom-node method is developed for three-node shell elements to describe cracks. This method can treat arbitrary cracks independently of the mesh. The crack may cut elements completely or partially. Elements are overlapped on the position of the crack, and they are partially integrated to implement the discontinuous displacement across the crack. To consider the element containing a crack tip, a new kinematical relation between the overlapped elements is developed. There is no enrichment function for the discontinuous displacement field. Several numerical examples are presented to illustrate the proposed method.}, subject = {Angewandte Mathematik}, language = {en} } @article{TalebiSamaniegoSamaniegoetal., author = {Talebi, Hossein and Samaniego, C. and Samaniego, Esteban and Rabczuk, Timon}, title = {On the numerical stability and mass-lumping schemes for explicit enriched meshfree methods}, series = {International Journal for Numerical Methods in Engineering}, journal = {International Journal for Numerical Methods in Engineering}, doi = {10.1002/nme.3275}, pages = {1009 -- 1027}, abstract = {Meshfree methods (MMs) such as the element free Galerkin (EFG)method have gained popularity because of some advantages over other numerical methods such as the finite element method (FEM). A group of problems that have attracted a great deal of attention from the EFG method community includes the treatment of large deformations and dealing with strong discontinuities such as cracks. One efficient solution to model cracks is adding special enrichment functions to the standard shape functions such as extended FEM, within the FEM context, and the cracking particles method, based on EFG method. It is well known that explicit time integration in dynamic applications is conditionally stable. Furthermore, in enriched methods, the critical time step may tend to very small values leading to computationally expensive simulations. In this work, we study the stability of enriched MMs and propose two mass-lumping strategies. Then we show that the critical time step for enriched MMs based on lumped mass matrices is of the same order as the critical time step of MMs without enrichment. Moreover, we show that, in contrast to extended FEM, even with a consistent mass matrix, the critical time step does not vanish even when the crack directly crosses a node.}, subject = {Angewandte Mathematik}, language = {en} } @inproceedings{Schmeikal, author = {Schmeikal, Bernd Anton}, title = {BAUHAUS ISOMETRY AND FIELDS}, series = {Digital Proceedings, International Conference on the Applications of Computer Science and Mathematics in Architecture and Civil Engineering : July 04 - 06 2012, Bauhaus-University Weimar}, booktitle = {Digital Proceedings, International Conference on the Applications of Computer Science and Mathematics in Architecture and Civil Engineering : July 04 - 06 2012, Bauhaus-University Weimar}, editor = {G{\"u}rlebeck, Klaus and Lahmer, Tom and Werner, Frank}, organization = {Bauhaus-Universit{\"a}t Weimar}, issn = {1611-4086}, doi = {10.25643/bauhaus-universitaet.2785}, url = {http://nbn-resolving.de/urn:nbn:de:gbv:wim2-20170314-27859}, pages = {9}, abstract = {While integration increases by networking, segregation strides ahead too. Most of us fixate our mind on special topics. Yet we are relying on our intuition too. We are sometimes waiting for the inflow of new ideas or valuable information that we hold in high esteem, although we are not entirely conscious of its origin. We may even say the most precious intuitions are rooting in deep subconscious, collective layers of the mind. Take as a simple example the emergence of orientation in paleolithic events and its relation to the dihedral symmetry of the compass. Consider also the extension of this algebraic matter into the operational structures of the mind on the one hand and into the algebra of geometry, Clifford algebra as we use to call it today, on the other. Culture and mind, and even the individual act of creation may be connected with transient events that are subconscious and inaccessible to cognition in principle. Other events causative for our work may be merely invisible too us, though in principle they should turn out attainable. In this case we are just ignorant of the whole creative process. Sometimes we begin to use unusual tools or turn into handicraft enthusiasts. Then our small institutes turn into workshops and factories. All this is indeed joining with the Bauhaus and its spirit. We shall go together into this, and we shall present a record of this session.}, subject = {Angewandte Informatik}, language = {en} } @inproceedings{Wudtke, author = {Wudtke, Idna}, title = {CONSTITUTIVE MODELING OF CRYSTALLINE MATERIALS WITH TEXTURE CHARACTERISTICS}, series = {Digital Proceedings, International Conference on the Applications of Computer Science and Mathematics in Architecture and Civil Engineering : July 04 - 06 2012, Bauhaus-University Weimar}, booktitle = {Digital Proceedings, International Conference on the Applications of Computer Science and Mathematics in Architecture and Civil Engineering : July 04 - 06 2012, Bauhaus-University Weimar}, editor = {G{\"u}rlebeck, Klaus and Lahmer, Tom and Werner, Frank}, organization = {Bauhaus-Universit{\"a}t Weimar}, issn = {1611-4086}, doi = {10.25643/bauhaus-universitaet.2791}, url = {http://nbn-resolving.de/urn:nbn:de:gbv:wim2-20170314-27910}, pages = {9}, abstract = {The analysis of the response of complex structural systems requires the description of the material constitutive relations by means of an appropriate material model. The level of abstraction of such model may strongly affect the quality of the prognosis of the whole structure. In context to this fact, it is necessary to describe the material in a convenient sense as exact but as simple as possible. All material phenomena of crystalline materials e.g. steel, affecting the behavior of the structure, rely on physical effects which are interacting over spatial scales from subatomic to macroscopic range. Nevertheless, if the material is microscopically heterogenic, it might be appropriate to use phenomenological models for the purpose of civil engineering. Although constantly applied, these models are insufficient for steel materials with microscopic characteristics such as texture, typically occurring in hot rolled steel members or heat affected zones of welded joints. Hence, texture is manifested in crystalline materials as a regular crystallographic structure and crystallite orientation, influencing macroscopic material properties. The analysis of structural response of material with texture (e.g. rolled steel or heat affected zone of a welded joint) obliges the extension of the phenomenological material description of macroscopic scale by means of microscopic information. This paper introduces an enrichment approach for material models based on a hierarchical multiscale methodology. This has been done by describing the grain texture on a mesoscopic scale and coupling it with macroscopic constitutive relations by means of homogenization. Due to a variety of available homogenization methods, the question of an assessment of coupling quality arises. The applicability of the method and the effect of the coupling method on the reliability of the response are presented on an example.}, subject = {Angewandte Informatik}, language = {en} } @phdthesis{Ifesanya, author = {Ifesanya, Kunle}, title = {The Role of Government Agencies in Urban Housing Delivery in Lagos}, doi = {10.25643/bauhaus-universitaet.1761}, url = {http://nbn-resolving.de/urn:nbn:de:gbv:wim2-20121115-17619}, school = {Bauhaus-Universit{\"a}t Weimar}, pages = {386}, abstract = {There is a continuous exacerbation of environmental problems in big cities of today's world, thereby, diminishing the quality of life in them. Of particular concern is the fact that today's megacities are evolving in the developing world without corresponding growth in the economy, infrastructure and other human development indices. As urban population continues to grow in these cities of the Global South, governing institutions are usually unable to keep pace with their social responsibilities, thus, making the issue of urban governance very critical. This is because effective and efficient urban governance is highly essential for the creation, strengthening and sustenance of governing institutions. Lagos, a mega-city of over 15.45 million people and the most populous metropolitan area on the African continent epitomizes the fundamental grave characteristics of the emerging megacities of the Global South, thereby, constituting an apt choice in understanding the emerging megacities of the next generation. Two out of every three Lagos residents live in slums and de-humanizing physical and social conditions. Many of them sleep, work, eat and cook under highway bridges, at the mercy of weather elements. This research, therefore, evaluated urban governance through housing administration in Africa's largest megacity. It examines the extent of housing problems in the city, the causal factors and the culpability of government agencies statutorily responsible for the provision, control and management of housing development in Lagos - the tenth largest city in the world. A representative geographic part of the city which manifests classic characteristics of slum life, listed by Mike Davis as the largest slum in Africa and the 6th largest in the world - Ajegunle - was adopted for case study. The research design combined rigorous literature search (desk research) with quantitative and, especially, qualitative approaches to data collection. The qualitative approach was more intensely adopted because government officials often respond to enquiries with 'official answers and data' which may not be reliable and the study had to rely on keen observation of physical traces, social interaction and personal investigation. The cross-sectional research method was adopted. Information was solicited from house-owners, building industry professionals, sociologists and officials of relevant government agencies, through research tools like questionnaires, interviews, focused group discussions and personal observations. The analysis and discussion of these field data, in conjunction with the information from the desk research gave a better understanding of the status-quo, which informed the recommendations proposed in the dissertation for mitigating the problems. The research discovered that many of the statutory housing agencies have the capacity to effectively discharge their responsibilities. However, it was also shown that corruption and abdication of responsibilities by the staff of these agencies constitute primary causes of the chasm between the anticipated lofty outcome from the laudable building regulations/bye-laws and the appalling reality. It also discovered that lack of political will and apathy on the part of successive Governments of Lagos State to the improvement of housing conditions of the poor masses are major causes of the housing debacle in Lagos. Several germane and realistic recommendations for redressing the situation were subsequently proffered. These include amongst others, the conduction of an accurate census for Lagos, in conjunction with credible international agencies, as a requisite basis for effective planning of any sort. The process of obtaining legal titles for land should also be made less cumbersome, while the housing administration process should be computerized; in order to reduce inter-personal contacts between applicants and government officials to the barest minimum, as a means of curbing the wide spread corruption in the system.}, subject = {Housing}, language = {en} } @phdthesis{UrbinaCazenave, author = {Urbina Cazenave, Mario Humberto}, title = {Gaze Controlled Applications and Optical-See-Through Displays - General Conditions for Gaze Driven Companion Technologies}, doi = {10.25643/bauhaus-universitaet.1749}, url = {http://nbn-resolving.de/urn:nbn:de:gbv:wim2-20121107-17492}, school = {Bauhaus-Universit{\"a}t Weimar}, pages = {106}, abstract = {Gaze based human-computer-interaction has been a research topic for over a quarter century. Since then, the main scenario for gaze interaction has been helping handicapped people to communicate an interact with their environment. With the rapid development of mobile and wearable display technologies, a new application field for gaze interaction has appeared, opening new research questions. This thesis investigates the feasibility of mobile gaze based interaction, studying deeply the use of pie menus as a generic and robust widget for gaze interaction as well as visual and perceptual issues on head mounted (wearable) optical see-through displays. It reviews conventional gaze-based selection methods and investigates in detail the use of pie menus for gaze control. It studies and discusses layout issues, selection methods and applications. Results show that pie menus can allocate up to six items in width and multiple depth layers, allowing a fast and accurate navigation through hierarchical levels by using or combining multiple selection methods. Based on these results, several text entry methods based on pie menus are proposed. Character-by-character text entry, text entry with bigrams and with text entry with bigrams derived by word prediction, as well as possible selection methods, are examined in a longitudinal study. Data showed large advantages of the bigram entry methods over single character text entry in speed and accuracy. Participants preferred the novel selection method based on saccades (selecting by borders) over the conventional and well established dwell time method. On the one hand, pie menus showed to be a feasible and robust widget, which may enable the efficient use of mobile eye tracking systems that may not be accurate enough for controlling elements on conventional interface. On the other hand, visual perception on mobile displays technologies need to be examined in order to deduce if the mentioned results can be transported to mobile devices. Optical see-through devices enable observers to see additional information embedded in real environments. There is already some evidence of increasing visual load on the respective systems. We investigated visual performance on participants with a visual search tasks and dual tasks presenting visual stimuli on the optical see-through device, only on a computer screen, and simultaneously on both devices. Results showed that switching between the presentation devices (i.e. perceiving information simultaneously from both devices) produced costs in visual performance. The implications of these costs and of further perceptual and technical factors for mobile gaze-based interaction are discussed and solutions are proposed.}, subject = {Eye tracking movement}, language = {en} } @inproceedings{Krausshar, author = {Kraußhar, Rolf S{\"o}ren}, title = {SOME HARMONIC ANALYSIS ON M{\"O}BIUS STRIP DOMAINS AND THE KLEIN BOTTLE IN Rn}, series = {Digital Proceedings, International Conference on the Applications of Computer Science and Mathematics in Architecture and Civil Engineering : July 04 - 06 2012, Bauhaus-University Weimar}, booktitle = {Digital Proceedings, International Conference on the Applications of Computer Science and Mathematics in Architecture and Civil Engineering : July 04 - 06 2012, Bauhaus-University Weimar}, editor = {G{\"u}rlebeck, Klaus and Lahmer, Tom and Werner, Frank}, organization = {Bauhaus-Universit{\"a}t Weimar}, issn = {1611-4086}, doi = {10.25643/bauhaus-universitaet.2769}, url = {http://nbn-resolving.de/urn:nbn:de:gbv:wim2-20170314-27692}, pages = {10}, abstract = {The aim of this paper we discuss explicit series constructions for the fundamental solution of the Helmholtz operator on some important examples non-orientable conformally at manifolds. In the context of this paper we focus on higher dimensional generalizations of the Klein bottle which in turn generalize higher dimensional M{\"o}bius strips that we discussed in preceding works. We discuss some basic properties of pinor valued solutions to the Helmholtz equation on these manifolds.}, subject = {Angewandte Informatik}, language = {en} } @phdthesis{Potthast, author = {Potthast, Martin}, title = {Technologies for Reusing Text from the Web}, doi = {10.25643/bauhaus-universitaet.1566}, url = {http://nbn-resolving.de/urn:nbn:de:gbv:wim2-20120217-15663}, school = {Bauhaus-Universit{\"a}t Weimar}, pages = {237}, abstract = {Texts from the web can be reused individually or in large quantities. The former is called text reuse and the latter language reuse. We first present a comprehensive overview of the different ways in which text and language is reused today, and how exactly information retrieval technologies can be applied in this respect. The remainder of the thesis then deals with specific retrieval tasks. In general, our contributions consist of models and algorithms, their evaluation, and for that purpose, large-scale corpus construction. The thesis divides into two parts. The first part introduces technologies for text reuse detection, and our contributions are as follows: (1) A unified view of projecting-based and embedding-based fingerprinting for near-duplicate detection and the first time evaluation of fingerprint algorithms on Wikipedia revision histories as a new, large-scale corpus of near-duplicates. (2) A new retrieval model for the quantification of cross-language text similarity, which gets by without parallel corpora. We have evaluated the model in comparison to other models on many different pairs of languages. (3) An evaluation framework for text reuse and particularly plagiarism detectors, which consists of tailored detection performance measures and a large-scale corpus of automatically generated and manually written plagiarism cases. The latter have been obtained via crowdsourcing. This framework has been successfully applied to evaluate many different state-of-the-art plagiarism detection approaches within three international evaluation competitions. The second part introduces technologies that solve three retrieval tasks based on language reuse, and our contributions are as follows: (4) A new model for the comparison of textual and non-textual web items across media, which exploits web comments as a source of information about the topic of an item. In this connection, we identify web comments as a largely neglected information source and introduce the rationale of comment retrieval. (5) Two new algorithms for query segmentation, which exploit web n-grams and Wikipedia as a means of discerning the user intent of a keyword query. Moreover, we crowdsource a new corpus for the evaluation of query segmentation which surpasses existing corpora by two orders of magnitude. (6) A new writing assistance tool called Netspeak, which is a search engine for commonly used language. Netspeak indexes the web in the form of web n-grams as a source of writing examples and implements a wildcard query processor on top of it.}, subject = {Information Retrieval}, language = {en} } @inproceedings{LahmerGhorashi, author = {Lahmer, Tom and Ghorashi, Seyed Shahram}, title = {XFEM-BASED CRACK IDENTIFICATION APPLYING REGULARIZING METHODS IN A MULTILEVEL APPROACH}, series = {Digital Proceedings, International Conference on the Applications of Computer Science and Mathematics in Architecture and Civil Engineering : July 04 - 06 2012, Bauhaus-University Weimar}, booktitle = {Digital Proceedings, International Conference on the Applications of Computer Science and Mathematics in Architecture and Civil Engineering : July 04 - 06 2012, Bauhaus-University Weimar}, editor = {G{\"u}rlebeck, Klaus and Lahmer, Tom and Werner, Frank}, organization = {Bauhaus-Universit{\"a}t Weimar}, issn = {1611-4086}, doi = {10.25643/bauhaus-universitaet.2771}, url = {http://nbn-resolving.de/urn:nbn:de:gbv:wim2-20170314-27717}, pages = {9}, abstract = {Many structures in different engineering applications suffer from cracking. In order to make reliable prognosis about the serviceability of those structures it is of utmost importance to identify cracks as precisely as possible by non-destructive testing. A novel approach (XIGA), which combines the Isogeometric Analysis (IGA) and the Extended Finite Element Method (XFEM) is used for the forward problem, namely the analysis of a cracked material, see [1]. Applying the NURBS (Non-Uniform Rational B-Spline) based approach from IGA together with the XFEM allows to describe effectively arbitrarily shaped cracks and avoids the necessity of remeshing during the crack identification problem. We want to exploit these advantages for the inverse problem of detecting existing cracks by non-destructive testing, see e.g. [2]. The quality of the reconstructed cracks however depends on two major issues, namely the quality of the measured data (measurement error) and the discretization of the crack model. The first one will be taken into account by applying regularizing methods with a posteriori stopping criteria. The second one is critical in the sense that too few degrees of freedom, i.e. the number of control points of the NURBS, do not allow for a precise description of the crack. An increased number of control points, however, increases the number of unknowns in the inverse analysis and intensifies the ill-posedness. The trade-off between accuracy and stability is aimed to be found by applying an inverse multilevel algorithm [3, 4] where the identification is started with short knot vectors which successively will be enlarged during the identification process.}, subject = {Angewandte Informatik}, language = {en} } @phdthesis{Schrader, author = {Schrader, Kai}, title = {Hybrid 3D simulation methods for the damage analysis of multiphase composites}, doi = {10.25643/bauhaus-universitaet.2059}, url = {http://nbn-resolving.de/urn:nbn:de:gbv:wim2-20131021-20595}, school = {Bauhaus-Universit{\"a}t Weimar}, pages = {174}, abstract = {Modern digital material approaches for the visualization and simulation of heterogeneous materials allow to investigate the behavior of complex multiphase materials with their physical nonlinear material response at various scales. However, these computational techniques require extensive hardware resources with respect to computing power and main memory to solve numerically large-scale discretized models in 3D. Due to a very high number of degrees of freedom, which may rapidly be increased to the two-digit million range, the limited hardware ressources are to be utilized in a most efficient way to enable an execution of the numerical algorithms in minimal computation time. Hence, in the field of computational mechanics, various methods and algorithms can lead to an optimized runtime behavior of nonlinear simulation models, where several approaches are proposed and investigated in this thesis. Today, the numerical simulation of damage effects in heterogeneous materials is performed by the adaption of multiscale methods. A consistent modeling in the three-dimensional space with an appropriate discretization resolution on each scale (based on a hierarchical or concurrent multiscale model), however, still contains computational challenges in respect to the convergence behavior, the scale transition or the solver performance of the weak coupled problems. The computational efficiency and the distribution among available hardware resources (often based on a parallel hardware architecture) can significantly be improved. In the past years, high-performance computing (HPC) and graphics processing unit (GPU) based computation techniques were established for the investigationof scientific objectives. Their application results in the modification of existing and the development of new computational methods for the numerical implementation, which enables to take advantage of massively clustered computer hardware resources. In the field of numerical simulation in material science, e.g. within the investigation of damage effects in multiphase composites, the suitability of such models is often restricted by the number of degrees of freedom (d.o.f.s) in the three-dimensional spatial discretization. This proves to be difficult for the type of implementation method used for the nonlinear simulation procedure and, simultaneously has a great influence on memory demand and computational time. In this thesis, a hybrid discretization technique has been developed for the three-dimensional discretization of a three-phase material, which is respecting the numerical efficiency of nonlinear (damage) simulations of these materials. The increase of the computational efficiency is enabled by the improved scalability of the numerical algorithms. Consequently, substructuring methods for partitioning the hybrid mesh were implemented, tested and adapted to the HPC computing framework using several hundred CPU (central processing units) nodes for building the finite element assembly. A memory-efficient iterative and parallelized equation solver combined with a special preconditioning technique for solving the underlying equation system was modified and adapted to enable combined CPU and GPU based computations. Hence, it is recommended by the author to apply the substructuring method for hybrid meshes, which respects different material phases and their mechanical behavior and which enables to split the structure in elastic and inelastic parts. However, the consideration of the nonlinear material behavior, specified for the corresponding phase, is limited to the inelastic domains only, and by that causes a decreased computing time for the nonlinear procedure. Due to the high numerical effort for such simulations, an alternative approach for the nonlinear finite element analysis, based on the sequential linear analysis, was implemented in respect to scalable HPC. The incremental-iterative procedure in finite element analysis (FEA) during the nonlinear step was then replaced by a sequence of linear FE analysis when damage in critical regions occured, known in literature as saw-tooth approach. As a result, qualitative (smeared) crack initiation in 3D multiphase specimens has efficiently been simulated.}, subject = {high-performance computing}, language = {en} }