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See detailThe Arabidopsis circadian clock incorporates a cADPR-based feedback loop
Dodd, A. N.; Gardner, M. J.; Hotta, C. T. et al

in SCIENCE (2007), 318(5857), 1789-1792

Transcriptional feedback loops are a feature of circadian clocks in both animals and plants. We show that the plant circadian clock also incorporates the cytosolic signaling molecule cyclic adenosine ... [more ▼]

Transcriptional feedback loops are a feature of circadian clocks in both animals and plants. We show that the plant circadian clock also incorporates the cytosolic signaling molecule cyclic adenosine diphosphate ribose (cADPR). cADPR modulates the circadian oscillator’s transcriptional feedback loops and drives circadian oscillations of Ca2+ release. The effects of antagonists of cADPR signaling, manipulation of cADPR synthesis, and mathematical simulation of the interaction of cADPR with the circadian clock indicate that cADPR forms a feedback loop within the plant circadian clock. [less ▲]

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See detailKE Tableaux for Public Announcement Logic
de Boer, Mathijs UL

Scientific Conference (2007, September 06)

Public announcement logic (PAL) is a simple dynamic epistemic logic extending reasoning about knowledge of agents with a modal operator for simultaneous and transparent knowledge updates. This logic is no ... [more ▼]

Public announcement logic (PAL) is a simple dynamic epistemic logic extending reasoning about knowledge of agents with a modal operator for simultaneous and transparent knowledge updates. This logic is no more expressive than epistemic logic (EL) without updates, but exhibits compact representation of a number of complex epistemic situations. A labeled tableau proof system to reason with these updates directly is presented here. This system can analyse and present well-known epistemic puzzles like `muddy children' and `three wise men'. Using the KE tableau system as a basis, the modal and propositional characteristics of epistemic updates can be separated. [less ▲]

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See detailMathematical Modelling of Flux Decline due to Concentration Polarisation and Cake Layer Formation in Crossflow Filtration Systems
Hale, Jack UL; Li, Qilin; Harris, Alison et al

Scientific Conference (2007, May)

Crossflow membrane filtration is an effective way of removing both colloidal and dissolved organic matter from contaminated water supplies. Two phenomena domimate solute flux in crossflow systems ... [more ▼]

Crossflow membrane filtration is an effective way of removing both colloidal and dissolved organic matter from contaminated water supplies. Two phenomena domimate solute flux in crossflow systems; concentration polarization and cake layer formation. Many innovative mathematical models for predicting both flux decline and quasi-steady state flux have been produced in the literature. However, limited regime applicability and conflicting physical predictions have made choosing optimal performance parameters in design a challenging process. An overview of the current field of models was undertaken, including an assessment of mathematical assumptions, numerical computation workload and number and complexity of system constants. A new model incorporating viscosity dependence on concentration is developed for the concentration polarization regime. Preliminary results will also be presented from a Monte Carlo based molecular dynamics simulation of volume packing fraction in the cake layer regime. The novel aspects of these models will be compared with existing models and the experimental results of our collaborators. Laboratory ultrafiltration tests were undertaken using varying concentrations of Dextran. An outline of future model directions and refinements will be presented. [less ▲]

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See detailThree-dimensional non-linear fracture mechanics by enriched meshfree methods without asymptotic enrichment
Bordas, Stéphane UL; Zi, G.; Rabczuk, T.

in Proceedings of the IUTAM Symposium on Discretization Methods for Evolving Discontinuities (2007)

This paper presents a three-dimensional, extrinsically enriched meshfree method for initiation, growth and coalescence of an arbitrary number of cracks in non-linear solids including large deformations ... [more ▼]

This paper presents a three-dimensional, extrinsically enriched meshfree method for initiation, growth and coalescence of an arbitrary number of cracks in non-linear solids including large deformations, for statics and dynamics. The novelty of the methodology fashioned in this work is that only an extrinsic discontinuous enrichment and no near-tip/asymptotic enrichment is required. Instead, a Lagrange multiplier field is added along the crack front to close the crack along the front. This decreases the computational cost and removes difficulties involved with a branch enrichment. Numerical examples treated include the pull-out of a reinforcement bar from a concrete block, and a Taylor bar impact with very large deformation and fragmentation. The results are compared to experimental results, and other simulations from the literature, which shows the robustness and accuracy of the method. © 2007 Springer. [less ▲]

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See detailDynamical structure functions for the reverse engineering of LTI networks
Goncalves, Jorge UL; Howes, R.; Warnick, S.

in Proceedings of the 46th IEEE Conference on Decision and Control (2007)

This research explores the role and representation of network structure for LTI systems with partial state observations. We demonstrate that input-output representations, i.e. transfer functions, contain ... [more ▼]

This research explores the role and representation of network structure for LTI systems with partial state observations. We demonstrate that input-output representations, i.e. transfer functions, contain no internal structural information of the system. We further show that neither the additional knowledge of system order nor minimality of the true realization is generally sufficient to characterize network structure. We then introduce dynamical structure functions as an alternative, graphical-model based representation of LTI systems that contain both dynamical and structural information of the system. The main result uses dynamical structure to precisely characterize the additional information required to obtain network structure from the transfer function of the system. [less ▲]

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See detailMathematical models of circadian Ca2+ oscillations
Neil, D.; Goncalves, Jorge UL; Webb, A.R.

in Proceedings of the eighth International Conference on Systems Biology (2007)

In the model plant Arabidopsis thaliana, the concentration of cytosolic-free Ca2+ ([Ca2+]cyt) oscillates with a circadian rhythm. We are investigating the regulation and role of these oscillations both ... [more ▼]

In the model plant Arabidopsis thaliana, the concentration of cytosolic-free Ca2+ ([Ca2+]cyt) oscillates with a circadian rhythm. We are investigating the regulation and role of these oscillations both experimentally and mathematically. Through systems identification, we have developed simple mathematical models from a single experiment measuring [Ca2+]cyt and the promoter activity of CIRCADIAN CLOCK ASSOCIATED 1 (CCA1). Through validation with 4 contrasting datasets, including a clock-arrhythmic transgenic line (CCA1 overexpressor), we demonstrate the necessity for a light input pathway to regulate basal [Ca2+]cyt levels. [less ▲]

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See detailDerivative recovery and a posteriori error estimate for extended finite elements
Bordas, Stéphane UL; Duflot, M.

in Computer Methods in Applied Mechanics & Engineering (2007), 196(35-36), 3381-3399

This paper is the first attempt at error estimation for extended finite elements. The goal of this work is to devise a simple and effective local a posteriori error estimate for partition of unity ... [more ▼]

This paper is the first attempt at error estimation for extended finite elements. The goal of this work is to devise a simple and effective local a posteriori error estimate for partition of unity enriched finite element methods such as the extended finite element method (XFEM). In each element, the local estimator is the L2 norm of the difference between the raw XFEM strain field and an enhanced strain field computed by extended moving least squares (XMLS) derivative recovery obtained from the raw nodal XFEM displacements. The XMLS construction is tailored to the nature of the solution. The technique is applied to linear elastic fracture mechanics, in which near-tip asymptotic functions are added to the MLS basis. The XMLS shape functions are constructed from weight functions following the diffraction criterion to represent the discontinuity. The result is a very smooth enhanced strain solution including the singularity at the crack tip. Results are shown for two- and three-dimensional linear elastic fracture mechanics problems in mode I and mixed mode. The effectivity index of the estimator is close to 1 and improves upon mesh refinement for the studied near-tip problem. It is also shown that for the linear elastic fracture mechanics problems treated, the proposed estimator outperforms one of the superconvergent patch recovery technique of Zienkiewicz and Zhu, which is only C0. Parametric studies of the general performance of the estimator are also carried out. © 2007 Elsevier B.V. All rights reserved. [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 detailA three-dimensional meshfree method for continuous multiple-crack initiation, propagation and junction in statics and dynamics
Rabczuk, Timon; Bordas, Stéphane UL; Zi, Goangseup

in Computational Mechanics (2007), 40(3), 473-495

This paper proposes a three-dimensional meshfree method for arbitrary crack initiation and propagation that ensures crack path continuity for non-linear material models and cohesive laws. The method is ... [more ▼]

This paper proposes a three-dimensional meshfree method for arbitrary crack initiation and propagation that ensures crack path continuity for non-linear material models and cohesive laws. The method is based on a local partition of unity. An extrinsic enrichment of the meshfree shape functions is used with discontinuous and near-front branch functions to close the crack front and improve accuracy. The crack is hereby modeled as a jump in the displacement field. The initiation and propagation of a crack is determined by the loss of hyperbolicity or the loss of material stability criterion. The method is applied to several static, quasi-static and dynamic crack problems. The numerical results very precisely replicate available experimental and analytical results. [less ▲]

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See detailDealing with requirements: Influences on Idea Generation in the Early Stages of Product Development
Gericke, Kilian UL; Schmidt-Kretschmer, Michael; Blessing, Lucienne UL

in Proceedings of DTRS7: Design Meeting Protocols (2007)

This paper aims at identifying factors which influence the number of ideas generated during a brainstorming meeting as part of an industrial mechanical engineering design project. A framework for ... [more ▼]

This paper aims at identifying factors which influence the number of ideas generated during a brainstorming meeting as part of an industrial mechanical engineering design project. A framework for describing groups of influencing factors and their relationships is used. As a result of an explorative, comparative protocol analysis of two design meetings the influence of some factors is described e.g. the formulation of the design task description and the sequence of the process steps. [less ▲]

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See detailArchitecture tradeoffs of integrating a mesh generator to partition of unity enriched object-oriented finite element software
Dunant, C.; Nguyen, V. P.; Belgasmia, M. et al

in European Journal of Computational Mechanics (2007), 16(2), 237-258

We explore the tradeoffs of using an internal mesher in a XFEM code. We show that it allows an efficient enrichement detection scheme, while retaining the ability to have welladapted meshes. We provide ... [more ▼]

We explore the tradeoffs of using an internal mesher in a XFEM code. We show that it allows an efficient enrichement detection scheme, while retaining the ability to have welladapted meshes. We provide benchmarks highlighting the considerable gains which can be expected from a well designed architecture. The efficiency of the proposed algorithm is shown by solving fracture mechanics problems of densely micro-cracked bodies including adaptive mesh refinement. [less ▲]

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See detailMöglichkeiten und Grenzen generischer Ansätze als Beitrag zur Steigerung der Robustheit von Produktentwicklungsprojekten
Gericke, Kilian UL; Schmidt-Kretschmer, Michael; Blessing, Lucienne UL

in Proceedings of 18th Symposium Design for X (2007)

Zur Darstellung des theoretischen Rahmens dieser Arbeit werden ein Modell zur Visualisierung der Vernetzung von Einflussfaktoren auf den Projekterfolg aus den Domänen Projekt-Management, Produkt und ... [more ▼]

Zur Darstellung des theoretischen Rahmens dieser Arbeit werden ein Modell zur Visualisierung der Vernetzung von Einflussfaktoren auf den Projekterfolg aus den Domänen Projekt-Management, Produkt und Stakeholder und ein Verbesserungsprozess zur Steigerung der Robustheit von Produktentwicklungsprojekten vorgestellt. Vor diesem Hintergrund werden die Grenzen aber auch die Möglichkeiten generischer Lösungsansätze diskutiert. Anhand einer Fallstudie wird untersucht welche Barrieren die Durchführung des Verbesserungsprozesses und somit auch die Anwendung der Lösungsansätze in der Praxis erschweren. Die Analysen zeigen, dass die Kenntnis der Ursachen von Problemen, die zu Planungsabweichungen in Produktenwicklungsprojekten führen, und eine entsprechende Anpassung der Handlungsempfehlungen Voraussetzungen zur Implementierung daraus abgeleiteter Maßnahmen sind. Die domänenübergreifende Betrachtung möglicher Einflussfaktoren gemäß dem Konzept der Robustheit führt während der Ursachenanalyse zu einem fundierten Verständnis der Problemlage und ermöglicht eine bessere Anpassung der Lösungsansätze. Aus den theoretischen Vorüberlegungen und den Ergebnissen der Fallstudie werden Anforderungen an die Weiterentwicklung des Ansatzes zur Steigerung der Robustheit von Produktentwicklungsprojekten abgeleitet, der die Anwender unterstützen soll, den Transfer empirischer Erkenntnisse und Handlungsempfehlungen auf individuelle Problemstellungen zu vollziehen. [less ▲]

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See detailMechanical failure in microstructural heterogeneous materials
Bordas, Stéphane UL; Hoppe, R. H. W.; Petrova, S. I.

in Lecture Notes in Computer Science (2007), 4310 LNCS

Various heterogeneous materials with multiple scales and multiple phases in the microstructure have been produced in the recent years. We consider a mechanical failure due to the initiation and ... [more ▼]

Various heterogeneous materials with multiple scales and multiple phases in the microstructure have been produced in the recent years. We consider a mechanical failure due to the initiation and propagation of cracks in places of high pore density in the microstructures. A multi-scale method based on the asymptotic homogenization theory together with the mesh superposition method (s-version of FEM) is presented for modeling of cracks. The homogenization approach is used on the global domain excluding the vicinity of the crack where the periodicity of the microstructures is lost and this approach fails. The multiple scale method relies on efficient combination of both macroscopic and microscopic models. The mesh superposition method uses two independent (global and local) finite element meshes and the concept of superposing the local mesh onto the global continuous mesh in such a way that both meshes not necessarily coincide. The homogenized material model is considered on the global mesh while the crack is analyzed in the local domain (patch) which allows to have an arbitrary geometry with respect to the underlying global finite elements. Numerical experiments for biomorphic cellular ceramics with porous microstructures produced from natural wood are presented. [less ▲]

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