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See detailh- and p-adaptivity driven by recovery and residual-based error estimators for PHT-splines applied to time-harmonic acoustics
Videla, Javier; Anitescu, Cosmin; Khajah, Tahsin et al

in Computers and Mathematics with Applications (2018), 77(9), 2369-2395

In this work, we demonstrate the application of PHT-splines for time-harmonic acoustic problems, modeled by the Helmholtz equation. Solutions of the Helmholtz equation have two features: global ... [more ▼]

In this work, we demonstrate the application of PHT-splines for time-harmonic acoustic problems, modeled by the Helmholtz equation. Solutions of the Helmholtz equation have two features: global oscillations associated with the wave number and local gradients caused by geometrical irregularities. We show that after a sufficient number of degrees of freedom is used to approximate global oscillations, adaptive refinement can capture local features of the solution. We compare residual-based and recovery-based error estimators and investigate the performance of -refinement. The simulations are done in the context of recently introduced Geometry Independent Field approximaTion (GIFT), where PHT-splines are only used to approximate the solution, while the computational domain is parameterized with NURBS. This approach builds on the natural adaptation ability of PHT-splines and avoids the re-parameterization of the NURBS geometry during the solution refinement process. [less ▲]

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See detailH2 Norm Based Network Volatility Measures
Huang, Qingqing; Yuan, Ye; Goncalves, Jorge UL et al

in The proceedings of the American Control Conference (2014)

Motivated by applications in biology and economics, we propose new volatility measures based on the H2 system norm for linear networks stimulated by independent or correlated noise. We identify critical ... [more ▼]

Motivated by applications in biology and economics, we propose new volatility measures based on the H2 system norm for linear networks stimulated by independent or correlated noise. We identify critical links in a network, where relatively small improvements can lead to large reductions in network volatility measures. We also examine volatility measures of individual nodes and their dependence on the topological position in the network. Finally, we investigate the dependence of the volatility on different network interconnections, weights of the edges and other network properties. Hence in an intuitive and efficient way, we can identify critical links, nodes and interconnections in network which can shed light in the network design to make it more robust. [less ▲]

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See detailHeterogeneous agent models in economics: a study of heterogeneous productivity of sectors
Tonita, R. J.; Goncalves, Jorge UL; Vinnicombe, G.

in Proceedings of the 2008 American Control Conference (2008)

Macroeconomic modeling is undergoing a change from the ground up. Previously models based on fully rational representative agents were constructed to give macroeconomics soli microeconomic foundations ... [more ▼]

Macroeconomic modeling is undergoing a change from the ground up. Previously models based on fully rational representative agents were constructed to give macroeconomics soli microeconomic foundations. However the representative agent models have been shown to be inconsistent with empirical evidence and a new method of approach has emerged, one based on heterogeneity of agents. Recently heterogenous models have been used to simulate expected outcomes but due to their complexity little analytic work has been done. In this paper a basic model of the macro economy, with heterogeneous sectors differentiated by productivity, and driven by a jump Markov process, is investigated and steady state solutions for a sector’s output variance are discovered. We adjust the model to include a gain term, to represent a sector’s reaction to its error signal, excess demand, and then linearize the transition rates and apply the fluctuation dissipation theorem to solve the model. [less ▲]

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See detailHeuristic methods for minimal controller location set problem in transportation networks
Mazur, Xavier UL; Rinaldi, Marco UL; Viti, Francesco UL

in Transportation Research Procedia (2020), 52

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See detailHierarchical a posteriori error estimation of Bank-Weiser type in the FEniCS Project
Bulle, Raphaël UL; Hale, Jack UL; Lozinski, Alexei et al

E-print/Working paper (2021)

In the seminal paper of Bank and Weiser [Math. Comp., 44 (1985), pp.283-301] a new a posteriori estimator was introduced. This estimator requires the solution of a local Neumann problem on every cell of ... [more ▼]

In the seminal paper of Bank and Weiser [Math. Comp., 44 (1985), pp.283-301] a new a posteriori estimator was introduced. This estimator requires the solution of a local Neumann problem on every cell of the finite element mesh. Despite the promise of Bank-Weiser type estimators, namely locality, computational efficiency, and asymptotic sharpness, they have seen little use in practical computational problems. The focus of this contribution is to describe a novel implementation of hierarchical estimators of the Bank-Weiser type in a modern high-level finite element software with automatic code generation capabilities. We show how to use the estimator to drive (goal-oriented) adaptive mesh refinement and to mixed approximations of the nearly-incompressible elasticity problems. We provide comparisons with various other used estimators. An open-source implementation based on the FEniCS Project finite element software is provided as supplementary material. [less ▲]

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See detailHigh Performance Parallel Coupling of OpenFOAM+XDEM
Besseron, Xavier UL; Pozzetti, Gabriele; Rousset, Alban UL et al

Presentation (2019, June 21)

Detailed reference viewed: 227 (25 UL)
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See detailHigher-order quasicontinuum methods for elastic and dissipative lattice models: uniaxial deformation and pure bending
Beex, Lars UL; Rokos, Ondrej; Zeman, Jan et al

in GAMM Mitteilungen (2015), 38(2), 344-368

The quasicontinuum (QC) method is a numerical strategy to reduce the computational cost of direct lattice computations - in this study we achieve a speed up of a factor of 40. It has successfully been ... [more ▼]

The quasicontinuum (QC) method is a numerical strategy to reduce the computational cost of direct lattice computations - in this study we achieve a speed up of a factor of 40. It has successfully been applied to (conservative) atomistic lattices in the past, but using a virtual-power-statement it was recently shown that QC approaches can also be used for spring and beam lattice models that include dissipation. Recent results have shown that QC approaches for planar beam lattices experiencing in-plane and out-of-plane deformation require higher-order interpolation. Higher-order QC frameworks are scarce nevertheless. In this contribution, the possibilities of a second-order and third-order QC framework are investigated for an elastoplastic spring lattice. The higher-order QC frameworks are compared to the results of the direct lattice computations and to those of a linear QC scheme. Examples are chosen so that both a macroscale and a microscale quantity influences the results. The two multiscale examples focused on are (i) macroscopically prescribed uniaxial deformation and (ii) macroscopically prescribed pure bending. Furthermore, the examples include an individual inclusion in a large lattice and hence, are concurrent in nature. [less ▲]

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See detailA holding control strategy for diverging bus lines
Laskaris, Georgios UL; Cats, Oded; Jenelius, Erik et al

Scientific Conference (2018, July 24)

We introduce a holding criterion for network configurations with lines that operate jointly along a common corridor and then individually diverge. The proposed holding decision rule accounts for all ... [more ▼]

We introduce a holding criterion for network configurations with lines that operate jointly along a common corridor and then individually diverge. The proposed holding decision rule accounts for all different passengers groups in the overlapping segment and takes care of the transition to individual line operation. The holding rule is evaluated using simulation for different demand levels and segmentations and compared with other control schemes for a real-world network. Results show that gains in overall network performance as well as for specific passenger groups can be achieved under specific demand distributions. [less ▲]

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See detailHolo Study – Mixed Reality Framework for Industrial Engineering Education and Training
Minoufekr, Meysam UL; Plapper, Peter UL

in Trends in Technical & Scientific Research (2020), 4(5), 0131-0135

Nowadays, students struggle with understanding complex production processes and kinematics of machine tools and robots. While professors mostly have been dealing with these subjects for many years ... [more ▼]

Nowadays, students struggle with understanding complex production processes and kinematics of machine tools and robots. While professors mostly have been dealing with these subjects for many years, students have only short time to be introduced to the complex nature of these topics. Mixed Reality allows students to engage hands-on in their subjects to reach new dimensions of understanding. Making challenging topics for students accessible by creating a more visual and tactile experience. The applications for entertainment and gaming are off the charts, but the implications for augmented reality in education and science are also undoubtedly massive. The project focusses on making education as interactive as possible, which helps both, teachers and students receive a unique tool for communicating phenomena and processes that are difficult to describe verbally. As a result, a complex knowledge becomes simple, an involvement of students grows, the quality of education increases [less ▲]

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See detailHow companies learn from design flaws: results from an empirical study of the german manufacturing industry
Gries, Bruno; Gericke, Kilian UL; Blessing, Lucienne UL

in Proceedings of the 15th International Conference on Engineering Design (2005)

Design flaws often become apparent at a time when the product is already in use and its development process, which in many cases includes extensive testing of parts, components and prototypes, is ... [more ▼]

Design flaws often become apparent at a time when the product is already in use and its development process, which in many cases includes extensive testing of parts, components and prototypes, is considered complete. Such flaws may reach from poor ergonomics to the total failure of the product. Often, especially when user safety is at risk, design flaws are so severe that companies are forced to announce a product callback. Petroski suggests that many (if not most) products, which we are familiar with today, have a long history of previously flawed designs [3]. This implies that designers did indeed learn from design flaws in both senses of the word “learn”: discovering the flaw and utilizing the knowledge gained about it to find a solution. As far as discovering a design flaw is concerned, it can be assumed that the feedback from those who interact with the physical products in practice – the individuals who maintain, repair, recycle but essentially use the products – plays an important role. In their previous work, the authors pointed out hat this feedback information could not only be vital for identifying potential product hazards but helps designers to review the effects of their design measures and therefore to improve their products from generation to generation [4]. In order to obtain a better understanding of how designers learn from design flaws, a mail survey was conducted that aimed at investigating company-, process- and product-related factors of this phenomenon and to answer (among others) the following research questions: • To what extent are design flaws of a company’s (or a competitor’s) product a driving force in the development of new products? • How do the designers of a company become aware of design flaws of their products? • How successful are companies in correcting design flaws? • How do successful and unsuccessful companies differ in terms of size, activity profile of their designers and characteristics of their products? • What are possible factors that influence the success in correcting a design fault? [less ▲]

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See detailHPC Multi-physics Biomass Furnace simulations as a Service
Besseron, Xavier UL; Rusche, Henrik; Peters, Bernhard UL et al

Scientific Conference (2020, November)

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See detailA hybrid MGA-MSGD ANN training approach for approximate solution of linear elliptic PDEs
Dehghani, Hamidreza UL; Zilian, Andreas UL

E-print/Working paper (2020)

We introduce a hybrid "Modified Genetic Algorithm-Multilevel Stochastic Gradient Descent" (MGA-MSGD) training algorithm that considerably improves accuracy and efficiency of solving 3D mechanical problems ... [more ▼]

We introduce a hybrid "Modified Genetic Algorithm-Multilevel Stochastic Gradient Descent" (MGA-MSGD) training algorithm that considerably improves accuracy and efficiency of solving 3D mechanical problems described, in strong-form, by PDEs via ANNs (Artificial Neural Networks). This presented approach allows the selection of a number of locations of interest at which the state variables are expected to fulfil the governing equations associated with a physical problem. Unlike classical PDE approximation methods such as finite differences or the finite element method, there is no need to establish and reconstruct the physical field quantity throughout the computational domain in order to predict the mechanical response at specific locations of interest. The basic idea of MGA-MSGD is the manipulation of the learnable parameters’ components responsible for the error explosion so that we can train the network with relatively larger learning rates which avoids trapping in local minima. The proposed training approach is less sensitive to the learning rate value, training points density and distribution, and the random initial parameters. The distance function to minimise is where we introduce the PDEs including any physical laws and conditions (so-called, Physics Informed ANN). The Genetic algorithm is modified to be suitable for this type of ANN in which a Coarse-level Stochastic Gradient Descent (CSGD) is exploited to make the decision of the offspring qualification. Employing the presented approach, a considerable improvement in both accuracy and efficiency, compared with standard training algorithms such classical SGD and Adam optimiser, is observed. The local displacement accuracy is studied and ensured by introducing the results of Finite Element Method (FEM) at sufficiently fine mesh as the reference displacements. A slightly more complex problem is solved ensuring the feasibility of the methodology [less ▲]

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See detailA hybrid T-Trefftz polygonal finite element for linear elasticity
Bhattacharjee, Kalyan; Natarajan, Sundararajan; Bordas, Stéphane UL

E-print/Working paper (2014)

In this paper, we construct hybrid T-Trefftz polygonal finite elements. The displacement field within the polygon is repre- sented by the homogeneous solution to the governing differential equation, also ... [more ▼]

In this paper, we construct hybrid T-Trefftz polygonal finite elements. The displacement field within the polygon is repre- sented by the homogeneous solution to the governing differential equation, also called as the T-complete set. On the boundary of the polygon, a conforming displacement field is independently defined to enforce continuity of the displacements across the element boundary. An optimal number of T-complete functions are chosen based on the number of nodes of the polygon and degrees of freedom per node. The stiffness matrix is computed by the hybrid formulation with auxiliary displacement frame. Results from the numerical studies presented for a few benchmark problems in the context of linear elasticity shows that the proposed method yield highly accurate results. [less ▲]

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See detailA hyper-reduction method using adaptivity to cut the assembly costs of reduced order models
Hale, Jack UL; Schenone, Elisa; Baroli, Davide UL et al

in Computer Methods in Applied Mechanics and Engineering (in press)

At every iteration or timestep of the online phase of some reduced-order modelling schemes, large linear systems must be assembled and then projected onto a reduced order basis of small dimension. The ... [more ▼]

At every iteration or timestep of the online phase of some reduced-order modelling schemes, large linear systems must be assembled and then projected onto a reduced order basis of small dimension. The projected small linear systems are cheap to solve, but assembly and projection are now the dominant computational cost. In this paper we introduce a new hyper-reduction strategy called reduced assembly (RA) that drastically cuts these costs. RA consists of a triangulation adaptation algorithm that uses a local error indicator to con- struct a reduced assembly triangulation specially suited to the reduced order basis. Crucially, this reduced assembly triangulation has fewer cells than the original one, resulting in lower assembly and projection costs. We demonstrate the efficacy of RA on a Galerkin-POD type reduced order model (RAPOD). We show performance increases of up to five times over the baseline Galerkin-POD method on a non-linear reaction-diffusion problem solved with a semi-implicit time-stepping scheme and up to seven times for a 3D hyperelasticity problem solved with a continuation Newton-Raphson algorithm. The examples are implemented in the DOLFIN finite element solver using PETSc and SLEPc for linear algebra. Full code and data files to produce the results in this paper are provided as supplementary material. [less ▲]

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See detail"I Personally Relate It to the Traffic Light": A User Study on Security & Privacy Indicators in a Secure Email System Committed to Privacy by Default
Stojkovski, Borce UL; Lenzini, Gabriele UL; Koenig, Vincent UL

in Proceedings of the 36th Annual ACM Symposium on Applied Computing (2021)

Improving the usability and adoption of secure (i.e. end-to-end encrypted) email systems has been a notorious challenge for over two decades. One of the open questions concerns the amount and format of ... [more ▼]

Improving the usability and adoption of secure (i.e. end-to-end encrypted) email systems has been a notorious challenge for over two decades. One of the open questions concerns the amount and format of information that should be communicated to users to inform them of the security and privacy properties with respect to different messages or correspondents. Contributing to the ongoing discussion on the usability and effectiveness of security and privacy indicators, particularly in the context of systems targeting non-expert users, this paper sheds light on users' evaluation of traffic light-inspired indicators, as a metaphor to represent different privacy states and guarantees, provided by a new system for email end-to-end encryption called p≡p. Using a mixed-methods approach, based on input gathered from 150 participants in three online studies, we highlight the pros and cons of the traffic light semantic in p≡p's context and beyond, and discuss the potential implications on the perceived security and use of such systems. [less ▲]

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See detailIdentifying elastoplastic parameters with Bayes' theorem considering double error sources and model uncertainty
Rappel, Hussein UL; Beex, Lars UL; Noels, Ludovic et al

in Probabilistic Engineering Mechanics (2019), 55

We discuss Bayesian inference for the identi cation of elastoplastic material parameters. In addition to errors in the stress measurements, which are commonly considered, we furthermore consider errors in ... [more ▼]

We discuss Bayesian inference for the identi cation of elastoplastic material parameters. In addition to errors in the stress measurements, which are commonly considered, we furthermore consider errors in the strain measurements. Since a difference between the model and the experimental data may still be present if the data is not contaminated by noise, we also incorporate the possible error of the model itself. The three formulations to describe model uncertainty in this contribution are: (1) a random variable which is taken from a normal distribution with constant parameters, (2) a random variable which is taken from a normal distribution with an input-dependent mean, and (3) a Gaussian random process with a stationary covariance function. Our results show that incorporating model uncertainty often, but not always, improves the results. If the error in the strain is considered as well, the results improve even more. [less ▲]

Detailed reference viewed: 349 (67 UL)