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See detailA simple error estimator for extended finite elements
Bordas, Stéphane UL; Duflot, Marc; Le, Phong

in Communications in Numerical Methods in Engineering (2008), 24(11), 961-971

This short communication presents the idea of an a posteriori error estimate for enriched (extended) finite elements (XFEM). The enhanced strain field against which the XFEM strains are compared, is ... [more ▼]

This short communication presents the idea of an a posteriori error estimate for enriched (extended) finite elements (XFEM). The enhanced strain field against which the XFEM strains are compared, is computed through extended moving least-squares smoothing constructed using the diffraction method to preserve the discontinuity. The error estimator is the L2 norm of the difference of the XFEM strain with the enhanced strain. We prove the concept of the proposed method on a 1D example with a singular solution and a 2D fracture mechanics example and conclude with some future work based on our paradigm. [less ▲]

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See detailA simple mass-action model for the eukaryotic heat shock response and its mathematical validation
Petre, Ion; Mizera, Andrzej UL; Hyder, Claire L. et al

in Natural Computing (2011), 10(1), 595-612

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See detailSimulating topological changes in real time for surgical assistance
Bordas, Stéphane UL; Kerfriden, Pierre; Courtecuisse, Hadrien et al

Speeches/Talks (2016)

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See detailSimulation of Shear Deformable Plates using Meshless Maximum Entropy Basis Functions
Hale, Jack UL; Baiz, P. M.

Scientific Conference (2011, June)

First-order Shear Deformable Plate Theory (FSDT) is widely used throughout engineering practice to simulate structures with planar dimensions much larger than their thickness. Meshless methods have seen ... [more ▼]

First-order Shear Deformable Plate Theory (FSDT) is widely used throughout engineering practice to simulate structures with planar dimensions much larger than their thickness. Meshless methods have seen use in the literature as a method for discretising the FSDT equations and hold numerous advantages over traditional mesh based techniques. A recent advance in the area of meshless methods are Maximum Entropy approximants (MaxEnt). MaxEnt combines many properties of various prior meshless approximants such as a weak Kronecker-delta property, seamless blending with Delaunay triangulations, high continuity, and convexity. In this work MaxEnt along with other meshless approximants have been implemented in a hybrid object-oriented Python/C++/Fortran computer simulation for the simulation of static deflection, free vibration and linear buckling of FSDT plates. The relative performance and ease of implementation of each of the methods will be discussed. The causes of shear locking along with the merits of various alleviation techniques will be covered, including matching fields method, mixed-variational formulations and construction of higher order polynomial basis via both intrinsic and extrinsic (partition of unity) methods. Convergence results show that MaxEnt provides in most cases similar and in some cases superior behaviour to MLS and RPIM approximants when used to discretise the FSDT equations. [less ▲]

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See detailA simulation-based design paradigm for complex cast components
Bordas, Stéphane UL; Conley, James. G.; Moran, Brian et al

in Engineering with Computers (2007), 23(1), 25-37

This paper describes and exercises a new design paradigm for cast components. The methodology integrates foundry process simulation, non-destructive evaluation (NDE), stress analysis and damage tolerance ... [more ▼]

This paper describes and exercises a new design paradigm for cast components. The methodology integrates foundry process simulation, non-destructive evaluation (NDE), stress analysis and damage tolerance simulations into the design process. Foundry process simulation is used to predict an array of porosity-related anomalies. The probability of detection of these anomalies is investigated with a radiographic inspection simulation tool (XRSIM). The likelihood that the predicted array of anomalies will lead to a failure is determined by a fatigue crack growth simulation based on the extended finite element method and therefore does not require meshing nor remeshing as the cracks grow. With this approach, the casting modeling provides initial anomaly information, the stress analysis provides a value for the critical size of an anomaly and the NDE assessment provides a detectability measure. The combination of these tools allows for accept/reject criteria to be determined at the early design stage and enables damage tolerant design philosophies. The methodology is applied to the design of a cast monolithic door used on the Boeing 757 aircraft. [less ▲]

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See detailSimultaneous Analysis of Strongly-Coupled Composite Energy Harvester-Circuit Systems Driven by Fluid-Structure Interaction
Zilian, Andreas UL; Ravi, Srivathsan UL

Scientific Conference (2016, July 27)

A specific class of energy harvester devices is investigated, that allow conversion of ambient fluid flow energy to electrical energy via flow-induced vibrations [1] of a piezo-ceramic composite structure ... [more ▼]

A specific class of energy harvester devices is investigated, that allow conversion of ambient fluid flow energy to electrical energy via flow-induced vibrations [1] of a piezo-ceramic composite structure positioned in the flow field. Potentially harmful flow fluctuations are harnessed to provide independent power supply to small electrical devices [2]. Such concept simultaneously involves the interaction of a composite structure and a surrounding fluid, the electric charge accumulated in the piezo-ceramic material and a controlling electrical circuit. In order to predict the efficiency and operational properties of these devices and to increase their robustness and performance, a predictive model of the complex physical system allows systematic computational investigation of the involved phenomena and coupling characteristics. A monolithic approach is proposed that provides simultaneous modelling and analysis of the harvester, which involves surface-coupled fluid-structure interaction, volume-coupled electro mechanics and a controlling energy harvesting circuit for applications in energy harvesting. A three dimensional space-time finite element approximation [3] is used for numerical solution of the weighted residual form of the governing equations of the flow-driven piezoelectric energy-harvesting device. This method enables time-domain investigation of different types of structures (plate, shells) subject to exterior/interior flow with varying cross sections, material compositions, and attached electrical circuits with respect to the electrical power output generated [4]. The space-time finite element model presented incorporates a novel method to enforce equipotentiality on the electrodes covering the piezoelectric patches, making the charge unknowns naturally appear in the formulation [5]. This enables to adapt any type of electrical circuit added to the electromechanical problem. [less ▲]

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See detailSimultaneous finite element analysis of circuit-integrated piezoelectric energy harvesting from fluid-structure interaction
Ravi, Srivathsan; Zilian, Andreas UL

in Mechanical Systems & Signal Processing (2019), 114

Flow-driven piezoelectric energy harvesting is a strongly coupled multiphysics phenomenon that involves complex three-way interaction between the fluid flow, the electromechanical effect of the ... [more ▼]

Flow-driven piezoelectric energy harvesting is a strongly coupled multiphysics phenomenon that involves complex three-way interaction between the fluid flow, the electromechanical effect of the piezoelectric material mounted on a deformable substrate structure and the controlling electrical circuit. High fidelity computational solution approaches are essential for the analysis of flow-driven energy harvesters in order to capture the main physical aspects of the coupled problem and to accurately predict the power output of a harvester. While there are some phenomenological and numerical models for flow-driven harvesters reported in the literature, a fully three-dimensional strongly coupled model has not yet been developed, especially in the context of flow-driven energy harvesting. The weighted residuals method is applied to establish a mixed integral equation describing the incompressible Newtonian flow, elastic substrate structure, piezoelectric patch, equipotential electrode and attached electric circuit that form the multiphysics fluid-structure interaction problem. A monolithic numerical solution method is derived that provides consistent and simultaneous solution to all physical fields as well as to fluid mesh deformation. The approximate solution is based on a mixed space-time finite element discretization with static condensation of the auxiliary fields. The discontinuous Galerkin method is utilized for integrating the monolithic model in time. The proposed solution scheme is illustrated in the example of a lid driven cavity with a flexible piezoelectric bottom wall, demonstrating quantification of the amount of electrical energy extractable from fluid flow by means of a piezoelectric harvester device. The results indicate that in order to make reliable predictions on the power output under varying operational states, the realization of strong multiphysics coupling is required for the mathematical model as well as the numerical solution scheme to capture the characteristics of flow-driven energy harvesters. [less ▲]

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See detailSimultaneous State and False-Data Injection Attacks Reconstruction for NonLinear Systems: an LPV Approach
Bezzaoucha, Souad UL; Voos, Holger UL

in Bezzaoucha, Souad (Ed.) International Conference on Automation, Control and Robots (2019, October)

The present contribution addresses simultaneous state and actuator/sensor false-data injection attacks reconstruction for nonlinear systems. The considered actuator/sensor attacks are modeled as time ... [more ▼]

The present contribution addresses simultaneous state and actuator/sensor false-data injection attacks reconstruction for nonlinear systems. The considered actuator/sensor attacks are modeled as time-varying parameters with a multiplicative effect on the actuator input signal and the sensor output signal, respectively. Based on the sector non-linearity approach and the convex polytopic transformation, the nonlinear model is written in a Linear Parameter-Varying (LPV) form, then an observer allowing both state and attack reconstruction is designed by solving an LMI optimization problem. [less ▲]

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See detailSingular spectrum analysis for modeling seasonal signals from GPS time series
Chen, Qiang; van Dam, Tonie UL; Sneeuw, Nico et al

in Journal of Geodynamics (2013), 72

Seasonal signals in GPS time series are of great importance for understanding the evolution of regional mass fluctuations, i.e., ice, hydrology, and ocean mass. Conventionally these signals quasi-annual ... [more ▼]

Seasonal signals in GPS time series are of great importance for understanding the evolution of regional mass fluctuations, i.e., ice, hydrology, and ocean mass. Conventionally these signals quasi-annual and semi-annual signals are modeled by least-squares fitting harmonic terms with a constant amplitude and phase. In reality, however, such seasonal signals are modulated, i.e., they will have a time-variable amplitude and phase. Recently, Davis et al.(2012) proposed a Kalman filter based approach to capture the stochastic seasonal behavior of geodetic time series. Singular Spectrum Analysis (SSA) is a non-parametric method, which uses time domain data to extract information from short and noisy time series without a priori knowledge of the dynamics affecting the time series. A prominent benefit is that trends obtained in this way are not necessarily linear. Further, true oscillations can be amplitude and phase modulated. In this work, we will assess the value of SSA for extracting time-variable seasonal signals from GPS time series. We compare our SSA-based results to those obtained using 1) least-squares analysis and 2) Kalman filtering. Our results demonstrate that SSA is a viable and complementary tool for extracting modulated oscillations from GPS time series. [less ▲]

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See detailSkew-symmetric Nitsche’s formulation in isogeometric analysis: Dirichlet and symmetry conditions, patch coupling and frictionless contact
Hu, Qingyuan; Chouly, Franz; Hu, Ping et al

in Computer Methods in Applied Mechanics and Engineering (2018), 341

A simple skew-symmetric Nitsche’s formulation is introduced into the framework of isogeometric analysis (IGA) to deal with various problems in small strain elasticity: essential boundary conditions ... [more ▼]

A simple skew-symmetric Nitsche’s formulation is introduced into the framework of isogeometric analysis (IGA) to deal with various problems in small strain elasticity: essential boundary conditions, symmetry conditions for Kirchhoff plates, patch coupling in statics and in modal analysis as well as Signorini contact conditions. For linear boundary or interface conditions, the skew-symmetric formulation is parameter-free. For contact conditions, it remains stable and accurate for a wide range of the stabilization parameter. Several numerical tests are performed to illustrate its accuracy, stability and convergence performance. We investigate particularly the effects introduced by Nitsche’s coupling, including the convergence performance and condition numbers in statics as well as the extra “outlier” frequencies and corresponding eigenmodes in structural dynamics. We present the Hertz test, the block test, and a 3D self-contact example showing that the skew-symmetric Nitsche’s formulation is a suitable approach to simulate contact problems in IGA. [less ▲]

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See detailSmart Cities, ‚big politics‘ und die Privatisierung der urbanen Governance
Carr, Constance UL; Hesse, Markus UL

Article for general public (2019)

Die Diskussion über Smart Cities hat in den letzten Jahren einen regelrechten Hype in Stadtpolitik, -forschung und -wirtschaft hervorgebracht. Die digitale Optimierung von Gebäuden, Quartieren oder ganzen ... [more ▼]

Die Diskussion über Smart Cities hat in den letzten Jahren einen regelrechten Hype in Stadtpolitik, -forschung und -wirtschaft hervorgebracht. Die digitale Optimierung von Gebäuden, Quartieren oder ganzen Stadträumen, so könnte man Smart Cities definieren, hat auch Luxemburg erfasst. Während das Wirtschaftsministerium die Vision einer intelligenten digitalen Spezialisierung verfolgt, präsentiert sich die Hauptstadt seit geraumer Zeit als Bühne der Smart City. Als Kontrast zu diesen Bildern analysieren wir ein prominentes Beispiel, das die Vision einer kontrollierten, technologisch und ökonomisch durchoptimierten Stadt vermittelt – Quayside, das Konversionsprojekt der Alphabet Inc. in Torontos Hafengebiet. [less ▲]

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See detailSmart Cities: Selbstzweck oder zum Wohl der Städte?
Carr, Constance UL

Article for general public (2019)

Die Idee der intelligenten Stadt – „Smart City“ – hat in den letzten Jahren unsere Fantasie beflügelt. Viele glauben, dass hier die Zukunft der Stadt liege. Die digitale Optimierung von Gebäuden ... [more ▼]

Die Idee der intelligenten Stadt – „Smart City“ – hat in den letzten Jahren unsere Fantasie beflügelt. Viele glauben, dass hier die Zukunft der Stadt liege. Die digitale Optimierung von Gebäuden, Kommunikations- und Verkehrsinfrastruktur begeistert Politiker, Wirtschaftsführer und die breite Öffentlichkeit. Alle wollen smart sein – ohne dass man jeweils wüsste, was genau eine Smart City ist. Von Constance Carr* Dies hat einen gewissen Wettlauf der Innovationen ausgelöst, wobei die Entwicklungsgeschwindigkeit ein solches Tempo erreicht hat, dass der Appetit der Technologieindustrie und der Städte, hier mitzuspielen, unersättlich erscheint. Die Digitalisierung ist insofern ein wichtiger Aspekt der Stadtentwicklungspolitik weltweit. Luxemburg bildet hier keine Ausnahme und strebt eine führende Position in der digitalen Entwicklung an. [less ▲]

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See detailSmartphone-based Adaptive Driving Maneuver Detection: A large-scale Evaluation Study
Castignani, German UL; Derrmann, Thierry UL; Frank, Raphaël UL et al

in IEEE Transactions on Intelligent Transportation Systems (2017)

The proliferation of connected mobile devices together with advances in their sensing capacity has enabled a new distributed telematics platform. In particular, smartphones can be used as driving sensors ... [more ▼]

The proliferation of connected mobile devices together with advances in their sensing capacity has enabled a new distributed telematics platform. In particular, smartphones can be used as driving sensors to identify individual driver behavior and risky maneuvers. However, in order to estimate driver behavior with smartphones, the system must deal with different vehicle characteristics. This is the main limitation of existing sensing platforms, which are principally based on fixed thresholds for different sensing parameters. In this paper, we propose an adaptive driving maneuver detection mechanism that iteratively builds a statistical model of the driver, vehicle, and smartphone combination using a multivariate normal model. By means of experimentation over a test track and public roads, we first explore the capacity of different sensor input combinations to detect risky driving maneuvers, and we propose a training mechanism that adapts the profiling model to the vehicle, driver, and road topology. A large-scale evaluation study is conducted, showing that the model for maneuver detection and scoring is able to adapt to different drivers, vehicles, and road conditions. [less ▲]

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See detailThe smoothed extended finite element method for strong discontinuities
Natarajan, S.; Bordas, Stéphane UL; Rabczuk, Timon

Scientific Conference (2009, June)

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See detailThe smoothed extended finite element method
Natarajan, S.; Bordas, Stéphane UL; Minh, Q. D. et al

in Proceedings of the 6th International Conference on Engineering Computational Technology (2008)

This paper shows how the strain smoothing technique recently proposed by G.R.Liu [1] coined as smoothed finite element method (SFEM) can be coupled to partition of unity methods, namely extended finite ... [more ▼]

This paper shows how the strain smoothing technique recently proposed by G.R.Liu [1] coined as smoothed finite element method (SFEM) can be coupled to partition of unity methods, namely extended finite element method (XFEM) [2] to give birth to the smoothed extended finite element method (SmXFEM), which shares properties both with the SFEM and the XFEM. The proposed method suppresses the need to compute and integrate the derivatives of shape functions (which are singular at the tip in linear elastic fracture mechanics). Additionally, integration is performed along the boundary of the finite elements or smoothing cells and no isoparametric mapping is required, which allows elements of arbitrary shape. We present numerical results for cracks in linear elastic fracture mechanics problems. The method is verified on several examples and comparisons are made to the conventional XFEM. © 2008 Civil-Comp Press. [less ▲]

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See detailA smoothed finite element method for plate analysis
Nguyen-Xuan, H.; Rabczuk, T.; Bordas, Stéphane UL et al

in Computer Methods in Applied Mechanics & Engineering (2008), 197(13-16), 1184-1203

A quadrilateral element with smoothed curvatures for Mindlin-Reissner plates is proposed. The curvature at each point is obtained by a non-local approximation via a smoothing function. The bending ... [more ▼]

A quadrilateral element with smoothed curvatures for Mindlin-Reissner plates is proposed. The curvature at each point is obtained by a non-local approximation via a smoothing function. The bending stiffness matrix is calculated by a boundary integral along the boundaries of the smoothing elements (smoothing cells). Numerical results show that the proposed element is robust, computational inexpensive and simultaneously very accurate and free of locking, even for very thin plates. The most promising feature of our elements is their insensitivity to mesh distortion. [less ▲]

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See detailA smoothed finite element method for shell analysis
Nguyen-Thanh, N.; Rabczuk, T.; Nguyen-Xuan, H. et al

in Computer Methods in Applied Mechanics & Engineering (2008), 198(2), 165-177

A four-node quadrilateral shell element with smoothed membrane-bending based on Mindlin-Reissner theory is proposed. The element is a combination of a plate bending and membrane element. It is based on ... [more ▼]

A four-node quadrilateral shell element with smoothed membrane-bending based on Mindlin-Reissner theory is proposed. The element is a combination of a plate bending and membrane element. It is based on mixed interpolation where the bending and membrane stiffness matrices are calculated on the boundaries of the smoothing cells while the shear terms are approximated by independent interpolation functions in natural coordinates. The proposed element is robust, computationally inexpensive and free of locking. Since the integration is done on the element boundaries for the bending and membrane terms, the element is more accurate than the MITC4 element for distorted meshes. This will be demonstrated for several numerical examples. [less ▲]

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See detailSocially assistive robots for teaching emotional abilities to children with autism spectrum disorder
Pinto Costa, Andreia UL; Steffgen, Georges UL; Rodriguez Lera, Francisco Javier UL et al

Scientific Conference (2017, March)

Socially assistive robots, when used in a way that takes into consideration children’s needs and developmental characteristics, can be useful tools to enable children’s development. More specifically, due ... [more ▼]

Socially assistive robots, when used in a way that takes into consideration children’s needs and developmental characteristics, can be useful tools to enable children’s development. More specifically, due to their characteristics (predictability, simplicity, and repetition) robots can be especially helpful to teach emotional abilities to children with autism spectrum disorder (ASD). Previous research has provided preliminary evidence that robots can help children improve in some domains such as joint attention and imitation. However, no studies have examined how robots can be integrated in intervention protocols targeting the acquisition of emotional abilities in children with ASD. This paper presents a work in progress on an emotional ability training developed for children with ASD using the QT socially assistive robot. This training aims to test whether children with ASD can benefit from a robot-mediated training to improve emotional ability. [less ▲]

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See detailA solution for Multi-Alignment by Transformation Synchronisation
Bernard, Florian UL; Thunberg, Johan UL; Gemmar, Peter et al

in The proceedings of the IEEE Conference on Computer Vision and Pattern Recognition (CVPR) (2015)

The alignment of a set of objects by means of transformations plays an important role in computer vision. Whilst the case for only two objects can be solved globally, when multiple objects are considered ... [more ▼]

The alignment of a set of objects by means of transformations plays an important role in computer vision. Whilst the case for only two objects can be solved globally, when multiple objects are considered usually iterative methods are used. In practice the iterative methods perform well if the relative transformations between any pair of objects are free of noise. However, if only noisy relative transformations are available (e.g. due to missing data or wrong correspondences) the iterative methods may fail. Based on the observation that the underlying noise-free transformations lie in the null space of a matrix that can directly be obtained from pairwise alignments, this paper presents a novel method for the synchronisation of pairwise transformations such that they are globally consistent. Simulations demonstrate that for a high amount of noise, a large proportion of missing data and even for wrong correspondence assignments the method delivers encouraging results. [less ▲]

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