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See detailComparison of Several RANS Modelling for the Pavia TRIGA Mark II Research Reactor
Introini, Carolina; Cammi, Antonio; Lorenzi, Stefano et al

in Journal of Nuclear Engineering and Radiation Science (2018)

Aim of this work is the comparison of different turbulent models based on the Reynolds Averaged Navier-Stokes (RANS) equations in order to find out which model is the most suitable for the study of the ... [more ▼]

Aim of this work is the comparison of different turbulent models based on the Reynolds Averaged Navier-Stokes (RANS) equations in order to find out which model is the most suitable for the study of the channel thermal-hydraulics of the TRIGA Mark II reactor. Only the steady state behaviour (i.e. the full power stationary operational conditions) of the reactor has been considered. To this end, the RAS (Reynolds-Averaged Simulation) models available in the open source CFD software OpenFOAM have been applied to the most internal channel of the TRIGA and assessed against a Large Eddy Simulation (LES) model. The results of the latter approach, expressed in terms of axial velocity, turbulent viscosity, turbulent kinetic energy, and temperature have been compared with the results obtained by the RAS models available in OpenFOAM (k − ε, k − ω and Reynolds Stress Transport). Heat transfer is taken into account as well by means of the turbulent energy diffusivity parameter. The simulation results demonstrate how, amongst the RAS models, the k − ω SST is the one whose results are closer to the LES simulation. This model seems to be the best one for the treatment of turbulent flow within the TRIGA subchannel, offering a good compromise between accuracy and computational requirements. Since it is much less expensive than an LES model, it can be applied even to full core calculation, in order to obtain accurate results with less computational effort. [less ▲]

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See detailComparison of Several RANS Modelling for the Pavia TRIGA Mark II Research Reactor
Introini, Carolina; Baroli, Davide UL; Peters, Bernhard UL

Poster (2017)

In this study, a detailed analysis of the turbulent regime within the core of the Pavia TRIGA Mark II reactor is perfomed by means of an in-depth comparison of the RAS (Reynolds-Averaged Simulation ... [more ▼]

In this study, a detailed analysis of the turbulent regime within the core of the Pavia TRIGA Mark II reactor is perfomed by means of an in-depth comparison of the RAS (Reynolds-Averaged Simulation) turbulence models implemented in OpenFOAM. Aim of this analysis is to give some important information with respect to the flow regime within the core. The performance of the various models is tested against a LES (Large Eddy Simulation) of the innermost channel. [less ▲]

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See detailA comparison of two meso-models for dry-woven fabrics and their multiscale equivalents
Beex, Lars UL; Duflot, Marc; Adam, Laurent

Scientific Conference (2016, April)

In this presentation, an X-braced spring mesomodel will be compared to a mesomodel in which the diagonal springs are replaced by rotational springs. The results are signi cantly di fferent, but some ... [more ▼]

In this presentation, an X-braced spring mesomodel will be compared to a mesomodel in which the diagonal springs are replaced by rotational springs. The results are signi cantly di fferent, but some disadvantages of the use of rotational springs will also be mentioned. A substantial part of the presentation will furthermore be dedicated to the multiscale quasicontinuum method to upscale the mesomodels in order to achieve e fficient macroscale computations. macroscale computations [less ▲]

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See detailComparisons of design methodologies and process models across disciplines: A literature review
Gericke, Kilian UL; Blessing, Lucienne UL

in Proceedings of the 18th International Conference on Engineering Design (2011)

Challenges resulting from an environment characterised by complexity, competition on global markets, dynamics and ever-changing user wants and needs are increasing and affect all design processes ... [more ▼]

Challenges resulting from an environment characterised by complexity, competition on global markets, dynamics and ever-changing user wants and needs are increasing and affect all design processes, irrespective of whether the product or system designed is a machine, a building, software, service, etc.. These challenges require collaboration between disciplines, but current design processes of large systems, such as aircraft, essentially show separate development strands for each discipline. Research into methods is ongoing, but the original design methodologies have not been adapted accordingly. Based on a literature study this paper consolidates findings from different comparisons of design methodologies and process models across disciplines. A consensus can be found that at least on an abstract level design process models have a generic core of common stages. On a detailed level the picture is different. The extent to which design approaches appear similar depends on the perspective of the analyses. The literature study provides an overview of shortcomings of existing design methodologies which may serve as starting points for further research and development of design methodologies. [less ▲]

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See detailCompatibilités et incompatibilités liants cimentaires / superplastifiants
Nicolas, Edwige UL

Doctoral thesis (2010)

Currently required concrete strength imposes a lower water/cement ratio and use of superplasticizer to make easier the fresh material implementation. Polycarboxylate type products are very effective in ... [more ▼]

Currently required concrete strength imposes a lower water/cement ratio and use of superplasticizer to make easier the fresh material implementation. Polycarboxylate type products are very effective in the short term but can lead to a fast loss of workability in case of binder/superplasticizer incompatibility. The object of this work is the identification of cementitous phases involved in these rheological variations. Initially, flow of various superplasticized pastes is evaluated by a rheometer fitted with a ball measuring system. Compatible combinations flow like a Bingham’s fluid. An incompatibility results in an increase of yield stress and plastic viscosity or rheological properties evolution into Herschel-Bulkley’s fluid. Then, cement/superplasticizer combinations are characterized by rheological tests. Stiffening assessment of pastes and chemical characterisation of hydrated cements show that consistency is associated with amount and microstructure of formed ettringite. These parameters depend on initially present aluminate phase and calcium sulphates. Finally, study of synthetic cementitous systems states that calcium sulphates solubility, in presence of superplasticizer, is increased probably because Ca2+ ions complexation by polycarboxylates. As for the orthorhombic variety of aluminate phase, more reactive than cubic variety, it leads to a large amount of needle-shaped ettringite. [less ▲]

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See detailComputational heuristics for simplifying a biological model
Petre, Ion; Mizera, Andrzej UL; Back, Ralph-Johan

in Lecture Notes in Computer Science (2009), 5635

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See detailA computational library for multiscale modeling of material failure
Talebi, H.; Silani, M.; Bordas, Stéphane UL et al

in Computational Mechanics (2013)

We present an open-source software framework called PERMIX for multiscale modeling and simulation of fracture in solids. The framework is an object oriented open-source effort written primarily in Fortran ... [more ▼]

We present an open-source software framework called PERMIX for multiscale modeling and simulation of fracture in solids. The framework is an object oriented open-source effort written primarily in Fortran 2003 standard with Fortran/C++ interfaces to a number of other libraries such as LAMMPS, ABAQUS, LS-DYNA and GMSH. Fracture on the continuum level is modeled by the extended finite element method (XFEM). Using several novel or state of the art methods, the piece software handles semi-concurrent multiscale methods as well as concurrent multiscale methods for fracture, coupling two continuum domains or atomistic domains to continuum domains, respectively. The efficiency of our open-source software is shown through several simulations including a 3D crack modeling in clay nanocomposites, a semi-concurrent FE-FE coupling, a 3D Arlequin multiscale example and an MD-XFEM coupling for dynamic crack propagation. © 2013 Springer-Verlag Berlin Heidelberg. [less ▲]

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See detailComputational Mechanics Lab Report 2013-2014
Bordas, Stéphane UL

Report (2015)

This is the report of the Computational Mechanics Lab led by Prof. Stéphane Bordas

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See detailComputational mechanics of interfaces
Bordas, Stéphane UL

Presentation (2016, May 22)

The course will present an overview of recent developments, which will enable students to make informed choices in terms of discretization and model selection in solving numerical problems in mechanics ... [more ▼]

The course will present an overview of recent developments, which will enable students to make informed choices in terms of discretization and model selection in solving numerical problems in mechanics. We will cover discretization strategies ranging from the standard finite element method, the smoothed finite element method, the extended finite element method, polygonal and virtual element methods, meshfree methods. The applications range between fracture of heterogeneous materials and biomedical simulations. The intended learning outcomes of the course are such that the students will be: - able to critically assess discretization schemes in mechanics - able to implement simple error estimators for finite element methods - familiar with basic multi-scale methods for fracture and their limitations - able to follow basic literature in model error and model selection, in particular based on Bayesian inference Course participants will learn these topics through lectures and hands-on numerical experiments. [less ▲]

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See detailComputational methods for quantitative submodel comparison
Mizera, Andrzej UL; Czeizler, Elena; Petre, Ion

in Katz, Evgeny (Ed.) Biomolecular Information Processing. From Logic Systems to Smart Sensors and Actuators (2012)

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See detailComputational representation and hemodynamic characterization of in vivo acquired severe stenotic renal artery geometries using turbulence modeling
Kagadis, George; Skouras, Eugene; Bourantas, Georgios UL et al

in Medical Engineering & Physics (2008), 30(5), 647-660

The present study reports on computational fluid dynamics in the case of severe renal artery stenosis (RAS). An anatomically realistic model of a renal artery was reconstructed from CT scans, and used to ... [more ▼]

The present study reports on computational fluid dynamics in the case of severe renal artery stenosis (RAS). An anatomically realistic model of a renal artery was reconstructed from CT scans, and used to conduct CFD simulations of blood flow across RAS. The recently developed Shear Stress Transport turbulence model was pivotally applied in the simulation of blood flow in the region of interest. Blood flow was studied in vivo under the presence of RAS and subsequently in simulated cases before the development of RAS, and after endovascular stent implantation. The pressure gradients in the RAS case were many orders of magnitude larger than in the healthy case. The presence of RAS increased flow resistance, which led to considerably lower blood flow rates. A simulated stent in place of the RAS decreased the flow resistance at levels proportional to, and even lower than, the simulated healthy case without the RAS. The wall shear stresses, differential pressure profiles, and net forces exerted on the surface of the atherosclerotic plaque at peak pulse were shown to be of relevant high distinctiveness, so as to be considered potential indicators of hemodynamically significant RAS. [less ▲]

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See detailComputational Sciences Year 2016 Activity Report
Bordas, Stéphane UL

Report (2016)

Born from a bottom-up initiative of Mathematics, Computer Science, Physics and Computational Engineering, Computational Sciences (CoSc) have contributed to create at UL a positive and symbiotic research ... [more ▼]

Born from a bottom-up initiative of Mathematics, Computer Science, Physics and Computational Engineering, Computational Sciences (CoSc) have contributed to create at UL a positive and symbiotic research environment relying on a strong fundamental scientific research core. CoSc will continue to rationalize research efforts across a range of strategic innovation domains by centralizing research and development tools and building upon the existing strengths of the Luxembourgish research and socio-economic landscape. [less ▲]

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See detailCondition assessment of concrete structures and bridges using vibration monitoring in comparison to changes in their static properties
Bungard, Volker UL

Doctoral thesis (2010)

The thesis deals with the non-destructive condition assessment of civil engineering structures made of reinforced and prestressed concrete as well as bridges and concentrates on a comparison of static and ... [more ▼]

The thesis deals with the non-destructive condition assessment of civil engineering structures made of reinforced and prestressed concrete as well as bridges and concentrates on a comparison of static and dynamic investigation methods. The main questions discuss the sensitivity of changes in static as well as in dynamic properties during the mechanical lifetime of those types of structures under laboratory conditions, the sensitivity of these changes when testing in-situ and the dimension of undesired environmental influences on the dynamic properties. Therefore, a gradually loaded laboratory reinforced concrete beam (cp. chapter 7) and a reinforced as well as a prestressed concrete slab (cp. chapter 8) are investigated for changes in their static and dynamic parameters until reaching their ultimate loads. Furthermore, a successively damaged and stepwise loaded laboratory reinforced concrete slab and a prestressed concrete slab (cp. also chapter 8) are under investigation and also compared in changes in their static as well as in dynamic properties during their lifetimes. By means of static and dynamic tests on a successively damaged and stepwise loaded prestressed concrete bridge (cp. chapter 9) it was possible to investigate the amount of changes in static and dynamic properties for a real in-situ object. A long-term measurement of a composite bridge (cp. chapter 10) demonstrates the environmental influences on the static and dynamic properties of a system which have to be considered when conducting comparative non-destructive condition assessment techniques on real objects. All tests are calculated respectively simulated by the method of the integration of stress (cp. chapter 6) or the finite element method by using the softwares ANSYS and SOFISTIK. Furthermore, measured eigenfrequencies and modeshapes are used to adapt finite element models with the aim to improve the model or to locate and quantify damaged areas. The work is a contribution to the application of non-destructive static and dynamic condition assessment methods on civil engineering structures made of concrete in the sector of civil engineering. [less ▲]

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See detailConfined in a Fiber: Realizing Flexible Gas Sensors by Electrospinning Liquid Crystals
Reyes, Catherine UL

Doctoral thesis (2019)

Liquid crystalline phases (LCs) readily exhibit optical responsivity to small fluctuations in their immediate environment. By encapsulating LC phase forming compounds within polymer fibers through the ... [more ▼]

Liquid crystalline phases (LCs) readily exhibit optical responsivity to small fluctuations in their immediate environment. By encapsulating LC phase forming compounds within polymer fibers through the electrospinning process (a fiber spinning method known for being a fast way of forming chemically diverse non-woven mats), it is possible to create functionalized LC-polymer fiber mats that are responsive as well. As these fiber mats can be handled macroscopically, a usercan observe the responses of the mats macroscopically without the need for bulky electronics. This thesis presents several non-woven fiber mats that were coaxially electrospun to contain LC within their individual polymer fibers cores for use as novel volatile organic compound (VOC) sensors. The mats are flexible, lightweight, and shown to both macroscopically and microscopically respond to toluene gas. Such gas responsive mats may be incorporated into garments for visually alerting the wearer when they are exposed to harmful levels of VOCs for example. Additionally, the interaction and re-prioritization of several electrospinning variables (from the chemistry based to the processing based) for forming the LC-mats are also discussed. The balance of these variables determines whether a wide range of phenomena occur during fiber formation. For instance, unexpected phase separation between the polymer sheath solution and the LC core can mean the difference between forming fully dried fibrous mats and wet/meshed films. A chapter is devoted to discussing the impact that solvent miscibility with an LC can have on fiber production, including also the effect that water can have when condensed into the electrospinning coaxial jet. The fiber shapes that the polymer fiber sheaths adopt (beaded versus non-beaded), as well as the continuity of the LC core, will influence the visual app earance of the mats. These optical properties, in turn, influence the mats’ responsivity to gases and whether the responses can be macroscopically observed with or without additional polarizers. During two types of gas sensing experiments --mats exposed to gas when contained in a cell, and mats exposed to gas diffused in ambient air without containment, we see that not all fibers within a mat respond at the same time. Moreover, different segments of the fibers within the same non-woven mat also show slightly different rates of response due to variations in fiber thickness, LC content, and whether the fiber cores had variations in LC filling (i.e. LC director twists, and gaps). [less ▲]

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See detailConsensus and Pursuit-Evasion in Nonlinear Multi-Agent Systems
Thunberg, Johan UL

Doctoral thesis (2014)

Within the field of multi-agent systems theory, we study the problems of consensus and pursuit-evasion. In our study of the consensus problem, we first provide some theoretical results and then consider ... [more ▼]

Within the field of multi-agent systems theory, we study the problems of consensus and pursuit-evasion. In our study of the consensus problem, we first provide some theoretical results and then consider the problem of consensus on SO(3) or attitude synchronization. In Chapter 2, for agents with states in Rm, we present two theorems along the lines of Lyapunov’s second method that, under different conditions, guarantee asymptotic state consensus in multi-agent systems where the interconnection topologies are switching. The first theorem is formulated by using the states of the agents in the multiagent system, whereas the second theorem is formulated by using the pairwise states for pairs of agents in the multi-agent system. In Chapter 3, the problem of consensus on SO(3) for a multi-agent system with directed and switching interconnection topologies is addressed. We provide two different types of kinematic control laws for a broad class of local representations of SO(3). The first control law consists of a weighted sum of pairwise differences between positions of neighboring agents, expressed as coordinates in a local representation. The structure of the control law is well known in the consensus community for being used in systems of agents in the Euclidean space, and here we show that the same type of control law can be used in the context of consensus on SO(3). In a later part of this chapter, based on the kinematic control laws, we introduce torque control laws for a system of rigid bodies in space and show that the system reaches consensus when these control laws are used. Chapter 4 addresses the problem of consensus on SO(3) for networks of uncalibrated cameras. Under the assumption that each agent uses a camera in order to measure its rotation, we prove convergence to the consensus set for two types of kinematic control laws, where only conjugate rotation matrices are available for the agents. In these conjugate rotations, the rotation matrix can be seen as distorted by the (unknown) intrinsic parameters of the camera. For the conjugate rotations we introduce distorted versions of well known local parameterizations of SO(3) and show consensus by using control laws that are similar to the ones in Chapter 3, with the difference that the distorted local representations are used instead. In Chapter 5, we study the output consensus problem for homogeneous systems of agents with linear continuous time-invariant dynamics. We derive control laws that solve the problem, while minimizing a cost functional of the control signal. Instead of considering a fixed communication topology for the system, we derive the optimal control law without any restrictions on the topology. We show that for all linear output controllable homogeneous systems, the optimal control law uses only relative information but requires the connectivity graph to be complete and in general requires measurements of the state errors. We identify cases where the optimal control law is only based on output errors. In Chapter 6, we address the multi-pursuer version of the visibility pursuit-evasion problem in polygonal environments. By discretizing the problem and applying a Mixed Integer Linear Programming (MILP) framework, we are able to address problems requiring so-called recontamination and also impose additional constraints, such as connectivity between the pursuers. The proposed MILP formulation is less conservative than solutions based on graph discretizations of the environment, but still somewhat more conservative than the original underlying problem. It is well known that MILPs, as well as multi-pursuer pursuit-evasion problems, are NP-hard. Therefore we apply an iterative Receding Horizon Control (RHC) scheme, where a number of smaller MILPs are solved over shorter planning horizons. The proposed approach is illustrated by a number of solved examples. [less ▲]

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See detailConsistent Estimation of Gibbs Energy Using Component Contributions
Noor, Elad; Haraldsdottir, Hulda UL; Milo, Ron et al

in PLoS Computational Biology (2013), 9(7), 1003098

Standard Gibbs energies of reactions are increasingly being used in metabolic modeling for applying thermodynamic constraints on reaction rates, metabolite concentrations and kinetic parameters. The ... [more ▼]

Standard Gibbs energies of reactions are increasingly being used in metabolic modeling for applying thermodynamic constraints on reaction rates, metabolite concentrations and kinetic parameters. The increasing scope and diversity of metabolic models has led scientists to look for genome-scale solutions that can estimate the standard Gibbs energy of all the reactions in metabolism. Group contribution methods greatly increase coverage, albeit at the price of decreased precision. We present here a way to combine the estimations of group contribution with the more accurate reactant contributions by decomposing each reaction into two parts and applying one of the methods on each of them. This method gives priority to the reactant contributions over group contributions while guaranteeing that all estimations will be consistent, i.e. will not violate the first law of thermodynamics. We show that there is a significant increase in the accuracy of our estimations compared to standard group contribution. Specifically, our cross-validation results show an 80% reduction in the median absolute residual for reactions that can be derived by reactant contributions only. We provide the full framework and source code for deriving estimates of standard reaction Gibbs energy, as well as confidence intervals, and believe this will facilitate the wide use of thermodynamic data for a better understanding of metabolism. [less ▲]

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See detailConstructing IGA-suitable planar parameterization from complex CAD boundary by domain partition and global/local optimization
Xu, Gang; Li, Ming; Mourrain, Bernard et al

in Computer Methods in Applied Mechanics & Engineering (2018), 328

In this paper, we propose a general framework for constructing IGA-suitable planar B-spline parameterizations from given complex CAD boundaries. Instead of the computational domain bounded by four B ... [more ▼]

In this paper, we propose a general framework for constructing IGA-suitable planar B-spline parameterizations from given complex CAD boundaries. Instead of the computational domain bounded by four B-spline curves, planar domains with high genus and more complex boundary curves are considered. Firstly, some pre-processing operations including B´ezier extraction and subdivision are performed on each boundary curve in order to generate a high-quality planar parameterization; then a robust planar domain partition framework is proposed to construct high-quality patch-meshing results with few singularities from the discrete boundary formed by connecting the end points of the resulting boundary segments. After the topology information generation of quadrilateral decomposition, the optimal placement of interior B´ezier curves corresponding to the interior edges of the quadrangulation is constructed by a global optimization method to achieve a patch-partition with high quality. Finally, after the imposition of C1/G1-continuity constraints on the interface of neighboring Bezier patches with respect to each quad in the quadrangulation, the high-quality Bezier patch parameterization is obtained by a local optimization method to achieve uniform and orthogonal iso-parametric structures while keeping the continuity conditions between patches. The efficiency and robustness of the proposed method are demonstrated by several examples which are compared to results obtained by the skeleton-based parameterization approach. [less ▲]

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See detailA constructive conditional logic for access control: a preliminary report
Genovese, Valerio UL; Giordano, Laura; Gliozzi, Valentina et al

in ECAI 2010 (2010)

We define an Intuitionistic Conditional Logic for Access Control called CICL. The logic CICL is based on a conditional language allowing principals to be defined as arbitrary formulas and it includes few ... [more ▼]

We define an Intuitionistic Conditional Logic for Access Control called CICL. The logic CICL is based on a conditional language allowing principals to be defined as arbitrary formulas and it includes few uncontroversial axioms of access control logics. We provide an axiomatization and a Kripke model semantics for the logic CICL, and we prove that the axiomatization is sound and complete with respect to the semantics. [less ▲]

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