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See detailMeshless Methods for the Reissner-Mindlin Plate Problem based on Mixed Variational Forms
Hale, Jack UL

Presentation (2013, October 31)

Meshless numerical methods such as the element free Galerkin (EFG) method and $hp$-clouds method rely on a field of particles to construct a basis for the solution of partial differential equations (PDEs ... [more ▼]

Meshless numerical methods such as the element free Galerkin (EFG) method and $hp$-clouds method rely on a field of particles to construct a basis for the solution of partial differential equations (PDEs). This is in contrast with methods such as the finite element method (FEM) and finite difference method (FDM) which rely upon a mesh or grid. Because of this increased flexibility, meshfree methods have shown themselves to be effective tools for simulating difficult problems such as those with discontinuities, complex geometries and large deformations. The Reissner-Mindlin problem is widely used by engineers to describe the deformation of a plate including the effects of transverse shear. A well-known problem which must be overcome when designing an effective numerical method for the Reissner-Mindlin problem is shear-locking. Shear-locking is the inability of the constructed approximation space (meshless or otherwise) to richly represent the limiting Kirchhoff mode. This inability manifests itself as an entirely incorrect solution as the thickness of the plate approaches zero. We will demonstrate and explain the shear-locking problem and potential solutions to it using a simple one-dimensional example. The most effective, robust and general approaches to the shear-locking problem developed in the FEM literature are based on mixed variational forms, where a combination of displacements, stresses and strains are approximated directly. In our approach we start with a mixed variational form before eliminating the extra stress unknowns using the local patch projection technique of A Ortiz et. al. We will discuss the issues presented by the well-known LBB stability conditions and present a solution based upon the stabilising properties of both the augmented Lagrangian and additional `bubble' type functions. We will then show the good performance of the method and its shear-locking free properties. [less ▲]

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See detailMeshless methods: A review and computer implementation aspects
Nguyen, V. P.; Rabczuk, T.; Bordas, Stéphane UL et al

in Mathematics & Computers in Simulation (2008), 79(3), 763-813

The aim of this manuscript is to give a practical overview of meshless methods (for solid mechanics) based on global weak forms through a simple and well-structured MATLAB code, to illustrate our ... [more ▼]

The aim of this manuscript is to give a practical overview of meshless methods (for solid mechanics) based on global weak forms through a simple and well-structured MATLAB code, to illustrate our discourse. The source code is available for download on our website and should help students and researchers get started with some of the basic meshless methods; it includes intrinsic and extrinsic enrichment, point collocation methods, several boundary condition enforcement schemes and corresponding test cases. Several one and two-dimensional examples in elastostatics are given including weak and strong discontinuities and testing different ways of enforcing essential boundary conditions. © 2008 IMACS. [less ▲]

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See detailA Method for Designing Virtual Places
McCall, Roderick UL; O'Neill, Shaleph; Benyon, David et al

Scientific Conference (2004)

This paper discusses our experiences of using a range of methods and techniques to measure the sense of place in real and virtual environments. The paper presents a discussion on presence and how this is ... [more ▼]

This paper discusses our experiences of using a range of methods and techniques to measure the sense of place in real and virtual environments. The paper presents a discussion on presence and how this is linked directly with our sense of place. From here we discuss the development of ‘The Place Probe’ a bundle of measurement techniques that allows for the direct comparison of real and virtual scenes. We show how the data gathered from the probe can be used to inform the design of virtual places. The whole approach is predicated on the premise that by understanding place we can improve our sense of presence in virtual environments. [less ▲]

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See detailMethodenentwicklung zur numerischen Strömungsanalyse von Freispiegelströmungen bei Schaufelwasserrädern
Schippke, Henning; Seidel, Christian; Dinkler, Dieter et al

Scientific Conference (2014, March)

Der Ausbau der Wasserkraft gewinnt im Kontext der Energiewende weltweit an Bedeutung. Neben der Turbinentechnologie erweisen sich Wasserräder im Hinblick auf ihre hohe ökologische Verträglichkeit und ihr ... [more ▼]

Der Ausbau der Wasserkraft gewinnt im Kontext der Energiewende weltweit an Bedeutung. Neben der Turbinentechnologie erweisen sich Wasserräder im Hinblick auf ihre hohe ökologische Verträglichkeit und ihr hohes ganzjähriges Arbeitsvermögen als besonders geeignet. Das sich drehende Wasserrad, umgeben von Wasser und Luft, stellt mechanisch ein gekoppeltes Drei- Feld-System bestehend aus einer Struktur und zwei Fluiden dar. Hinreichend genau ist es möglich, die Wasserradstruktur als Starrkörper zu beschreiben, während Luft und Wasser mit Hilfe der inkom- pressiblen Navier-Stokes Gleichungen gut modelliert werden können. Durch Anwendung der Raum-Zeit-Finite-Elemente-Methode zur Diskretisierung der inkompressiblen Navier-Stokes Gleichungen wird die Grenzfläche zwischen dem Starrkörper und den Fluiden explizit beschrieben und stets automatisch korrekt erfasst. Die Beschreibung der freien Wasseroberfläche als Grenzfläche zwischen den beiden Fluiden erfolgt implizit mit Hilfe der Level-Set Methode. Die Netzknoten der Fluid-Struktur-Grenzfläche verändern ihre Position infolge der Wasserraddrehung mit der Zeit, so dass ein Netzbewegungsalgorithmus notwendig ist. Die shear-slip mesh update method (SSMUM) ermöglicht als diskontinuierliches Netzbewegungsverfahren eine durchgehende Berechnung des Gesamtsystems, ohne dass das gesamte Gebiet neu vernetzt werden muss. Anhand verschiedener Testbeispiele wird die Güte der entwickelten numerischen Methode im Hinblick auf Ihre Erhaltungseigenschaften untersucht. Besonderes Augenmerk wird auf die Lösungsübertragung am shear-slip layer sowie auf die Abbildung der implizit beschriebenen freien Wasseroberfläche gelegt. [less ▲]

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See detailMethodenentwicklung zur numerischen Strömungsanalyse von Schaufelwasserrädern
Schippke, Henning; Zilian, Andreas UL; Seidel, Christian et al

in PAMM (2013)

Es wird ein Berechnungsmodell vorgestellt, mit dem eine numerischen Analyse der Strömungsvorgänge innerhalb von Schau- felwasserrädern möglich ist. Der Arbeitsschwerpunkt liegt dabei auf der ... [more ▼]

Es wird ein Berechnungsmodell vorgestellt, mit dem eine numerischen Analyse der Strömungsvorgänge innerhalb von Schau- felwasserrädern möglich ist. Der Arbeitsschwerpunkt liegt dabei auf der strömungsmechanischen Optimierung von Hochleis- tungswasserrädern der Weiterentwickelten Wasserradtechnologie. In dem Berechnungsmodell werden die inkompressiblen Navier-Stokes-Gleichungen in den unabhängigen Variablen Geschwindigkeiten und Druck mit Hilfe der zeit-diskontinuierlichen Raum-Zeit-Finite-Elemente-Methode diskretisiert und die auftretende Gebietsveränderung infolge der sich drehenden Struk- tur mit der Shear-Slip Mesh Update Methode als diskontinuierlichem Netzbewegungsverfahren erfasst. [less ▲]

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See detailA Methodological Framework to Design a Machine-Readable Privacy Icon Set
Palmirani, Monica; Rossi, Arianna UL; Martoni, Michele et al

in Schweighofer, Erich (Ed.) Data Protection / LegalTech Proceedings of the 21st International Legal Informatics Symposium IRIS 2018 (2018)

The GDPR suggests icons to convey data practices in a more straightforward way. Although vi- sualizations to represent legal terms have many benefits, there is fear that they could be misrep- resented by ... [more ▼]

The GDPR suggests icons to convey data practices in a more straightforward way. Although vi- sualizations to represent legal terms have many benefits, there is fear that they could be misrep- resented by designers and misinterpreted by individuals, thus hindering instead of facilitating the comprehension. In order to solve these issues, we present a methodology to generate legal visual representations that is based on an analysis of legal requirements, on an ontological representation of the legal knowledge, and on an iterative, multi-stakeholder design approach, followed by empirical evaluation. [less ▲]

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See detailMethods for Biochemical Model Decomposition and Quantitative Submodel Comparison
Mizera, Andrzej UL; Czeizler, Elena; Petre, Ion

in Israel Journal of Chemistry (2011), 51(1), 151164

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See detailMicro-structured materials: inhomogeneities and imperfect interfaces in plane micropolar elasticity, a boundary element approach
Atroshchenko, Elena; Hale, Jack UL; Videla, Javier A. et al

in Engineering Analysis with Boundary Elements (2017), 83

In this paper we tackle the simulation of microstructured materials modelled as heterogeneous Cosserat media with both perfect and imperfect interfaces. We formulate a boundary value problem for an ... [more ▼]

In this paper we tackle the simulation of microstructured materials modelled as heterogeneous Cosserat media with both perfect and imperfect interfaces. We formulate a boundary value problem for an inclusion of one plane strain micropolar phase into another micropolar phase and reduce the problem to a system of boundary integral equations, which is subsequently solved by the boundary element method. The inclusion interface condition is assumed to be imperfect, which permits jumps in both displacements/microrotations and tractions/couple tractions, as well as a linear dependence of jumps in displacements/microrotations on continuous across the interface tractions/couple traction (model known in elasticity as homogeneously imperfect interface). These features can be directly incorporated into the boundary element formulation. The BEM-results for a circular inclusion in an in finite plate are shown to be in excellent agreement with the analytical solutions. The BEM-results for inclusions in finite plates are compared with the FEM-results obtained with FEniCS. [less ▲]

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See detailMinimal dynamical structure realisations with application to network reconstruction from data
Yuan, Y.; Stan, G. B. V.; Warnick, S. et al

in The proceedings of the Joint 48th IEEE Conference on Decision and Control and 28th Chinese Control Conference (2009)

Network reconstruction, i.e., obtaining network structure from data, is a central theme in systems biology, economics, and engineering. Previous work introduced dynamical structure functions as a tool for ... [more ▼]

Network reconstruction, i.e., obtaining network structure from data, is a central theme in systems biology, economics, and engineering. Previous work introduced dynamical structure functions as a tool for posing and solving the problem of network reconstruction between measured states. While recovering the network structure between hidden states is not possible since they are not measured, in many situations it is important to estimate the number of hidden states in order to understand the complexity of the network under investigation and help identify potential targets for measurements. Estimating the number of hidden states is also crucial to obtain the simplest state-space model that captures the network structure and is coherent with the measured data. This paper characterises minimal order state-space realisations that are consistent with a given dynamical structure function by exploring properties of dynamical structure functions and developing algorithms to explicitly obtain a minimal reconstruction. [less ▲]

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See detailMinimal-time network reconstruction for DTLTI systems
Yuan, Y.; Goncalves, Jorge UL

in The proceedings of the 49th IEEE Conference on Decision and Control (CDC) (2010)

This paper considers the problem of obtaining in minimal time the “dynamical network structure” (DNS) from partial state observations of a discrete-time linear time-invariant system. From the DNS, we can ... [more ▼]

This paper considers the problem of obtaining in minimal time the “dynamical network structure” (DNS) from partial state observations of a discrete-time linear time-invariant system. From the DNS, we can not only obtain the network structure of the system at the measurement level, but also estimate the minimal number of hidden states which are not observed directly. First, we discuss when reconstruction of the DNS is and is not possible. Then, we give an algorithm to find the minimal number of successive outputs to find the DNS. Finally, we discuss extensions of the results to non-linear and noisy systems. These results can be directly applied to the decentralised network control problem of multi-agent systems to find network connections of the observed agents. [less ▲]

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See detailMinimal-time uncertain output final value of unknown DT-LTI systems with application to the decentralised network consensus problem
Yuan, Y.; Stan, G. B. V.; Shi, L. et al

Scientific Conference (2010)

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See detailMinimum energy multiple crack propagation Part I: Theory.
Sutula, Danas UL; Bordas, Stéphane UL

in Engineering Fracture Mechanics (n.d.)

The three-part paper deals with energy-minimal multiple crack propagation in a linear elastic solid under quasi-static conditions. The principle of minimum total energy, i.e. the sum of the potential and ... [more ▼]

The three-part paper deals with energy-minimal multiple crack propagation in a linear elastic solid under quasi-static conditions. The principle of minimum total energy, i.e. the sum of the potential and fracture energies, which stems directly from the Griffith's theory of cracks, is applied to the problem of arbitrary crack growth in 2D. The proposed formulation enables minimisation of the total energy of the mechanical system with respect to the crack extension directions and crack extension lengths to solve for the evolution of the mechanical system over time. The three parts focus, in turn, on (I) the theory of multiple crack growth including competing cracks, (II) the discrete solution by the extended finite element method using the minimum-energy formulation, and (III) the aspects of computer implementation within the Matlab programming language. The key contributions of Part-I of this three-part paper are: (1) formulation of the total energy functional governing multiple crack behaviour, (2) three solution methods to the problem of competing crack growth for different fracture front stabilities (e.g. stable, unstable, or a partially stable configuration of crack tips), and (3) the minimum energy criterion for a set of crack tip extensions is posed as the criterion of vanishing rotational dissipation rates with respect to the rotations of the crack extensions. The formulation lends itself to a straightforward application within a discrete framework for determining the crack extension directions of multiple finite-length crack tip increments, which is tackled in Part-II, using the extended finite element method. In Part-III, we discuss various applications and benchmark problems. The open-source Matlab code, documentation, benchmark/example cases are included as supplementary material. [less ▲]

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See detailMinimum energy multiple crack propagation. Part II: Discrete Solution with XFEM.
Sutula, Danas UL; Bordas, Stéphane UL

in Engineering Fracture Mechanics (n.d.)

The three-part paper deals with energy-minimal multiple crack propagation in a linear elastic solid under quasi-static conditions. The principle of minimum total energy, i.e. the sum of the potential and ... [more ▼]

The three-part paper deals with energy-minimal multiple crack propagation in a linear elastic solid under quasi-static conditions. The principle of minimum total energy, i.e. the sum of the potential and fracture energies, which stems directly from the Griffith's theory of cracks, is applied to the problem of arbitrary crack growth in 2D. The proposed formulation enables minimisation of the total energy of the mechanical system with respect to the crack extension directions and crack extension lengths to solve for the evolution of the mechanical system over time. The three parts focus, in turn, on (I) the theory of multiple crack growth including competing cracks, (II) the discrete solution by the extended finite element method using the minimum-energy formulation, and (III) the aspects of computer implementation within the Matlab programming language. This Part-II of our three-part paper examines three discrete solution methods for solving fracture mechanics problems based on the principle of minimum total energy. The discrete solution approach is chosen based on the stability property of the fracture configuration at hand. The first method is based on external load-control. It is suitable for stable crack growth and stable fracture configurations. The second method is based on fractured area-control. This method is applicable to stable or unstable fracture growth but it is required that the fracture front be stable. The third solution method is based on a gradient-descent approach. This approach can be applied to arbitrary crack growth problems; however, the gradient-descent formulation cannot be guaranteed to yield the optimal solution in the case of competing crack growth and an unstable fracture front configuration. The main focus is on the gradient-descent solution approach within the framework of the extended finite element discretisation. Although a viable solution method is finally proposed for resolving competing crack growth in the case of an unstable fracture front configuration, the method is not implemented within the present XFEM code but rather exists as a separate proof-of-concept algorithm that is tested against several fabricated benchmark problems. The open-source Matlab code, documentation and example cases are included as supplementary material. [less ▲]

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See detailMinimum energy multiple crack propagation. Part III: XFEM computer implementation and applications.
Sutula, Danas UL; Bordas, Stéphane UL

in Engineering Fracture Mechanics (n.d.)

The three-part paper deals with energy-minimal multiple crack propagation in a linear elastic solid under quasi-static conditions. The principle of minimum total energy, i.e. the sum of the potential and ... [more ▼]

The three-part paper deals with energy-minimal multiple crack propagation in a linear elastic solid under quasi-static conditions. The principle of minimum total energy, i.e. the sum of the potential and fracture energies, which stems directly from the Griffith's theory of cracks, is applied to the problem of arbitrary crack growth in 2D. The proposed formulation enables minimisation of the total energy of the mechanical system with respect to the crack extension directions and crack extension lengths to solve for the evolution of the mechanical system over time. The three parts focus, in turn, on (I) the theory of multiple crack growth including competing cracks, (II) the discrete solution by the extended finite element method using the minimum-energy formulation, and (III) the aspects of computer implementation within the Matlab programming language. The key contributions of Part-III of the three-part paper are as follows: (1) implementation of XFEM in Matlab with emphasis on the design of the code to enable fast and efficient computational times of fracture problems involving multiple cracks and arbitrary crack intersections, (2) verification of the minimum energy criterion and comparison with the maximum tension criterion via multiple benchmark studies, and (3) we propose a numerical improvement to the crack growth direction criterion that gives significant improvements in accuracy and convergence rates of the fracture paths, especially on coarse meshes. The comparisons of the fracture paths obtained by the maximum tension (or maximum hoop-stress) criterion and the energy minimisation approach via a multitude of numerical case studies show that both criteria converge to virtually the same fracture solutions albeit from opposite directions. In other words, it is found that the converged fracture path lies in between those obtained by each criterion on coarser meshes. Thus, a modified crack growth direction criterion is proposed that assumes the average direction of the directions obtained by the maximum tension and the minimum energy criteria. The numerical results show significant improvements in accuracy (especially on coarse discretisations) and convergence rates of the fracture paths. Finally, the open-source Matlab code, documentation, benchmarks and example cases are included as supplementary material. [less ▲]

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See detailMitarbeiterführung und Social-Media-Nutzung im Führungsalltag von Generation-Y-Führungskräften - Eine explorative Analyse mittels Mixed-Methods-Ansatz
Feltes, Florian UL

Doctoral thesis (2016)

The topic of this thesis is the qualitative and quantitative evaluation of leadership behaviour and therefore the leadership style of Generation Y (GenY) considering the use of social media in day-to-day ... [more ▼]

The topic of this thesis is the qualitative and quantitative evaluation of leadership behaviour and therefore the leadership style of Generation Y (GenY) considering the use of social media in day-to-day management. It examines the question of how GenY leaders lead and how they use social media in this context. It explores the topic based on a sequential mixed methods approach of qualitative interviews and a quantitative online questionnaire. Using the qualitative content analysis, it examines 25 qualitative interviews concerning the following aspects: leadership behaviour of generation Y, generation-based differences in the leadership and different strength of leadership styles, influence of contextual factors like hierarchies, sector and company size on the leadership style and use of social media, use of social media on day-to-day management, and, finally, connections between applied leadership styles and social media usage of GenY leaders. The findings and tendencies were then verified in an online questionnaire. The results of the online questionnaire [self-evaluation of leaders (N=406), bottom-up evaluation by employees (N=622)] show a significant discrepancy between the leaders’ statements and those of the employees. However, there are clear results and tendencies that confirm the findings of the qualitative study. It was established that GenY leaders show characteristics of task-oriented, person-oriented, transactional and transformational leadership. GenY leadership is characterised by clear outcome orientation, flat hierarchies and feedback. The use of social media varies considerably, depending for example on the context in which the leader works, e. g. sector and level of management. In summary, it can be stated that there is a connection between the strength of the leadership style and the usage of social media in day-to-day management. [less ▲]

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See detailMixed hybrid and electric bus dynamic fleet management in urban networks: a model predictive control approach
Rinaldi, Marco UL; Picarelli, Erika UL; D'Ariano, Andrea et al

Scientific Conference (2019, June)

Abstract—Reducing pollutant emissions and promoting sustainable mobility solutions, including Public Transport, are increasingly becoming key objectives for policymakers worldwide. In order to jointly ... [more ▼]

Abstract—Reducing pollutant emissions and promoting sustainable mobility solutions, including Public Transport, are increasingly becoming key objectives for policymakers worldwide. In order to jointly achieve these goals, careful consideration should be put on the operational cost and management of PT services, in order to promote the adoption of green mobility solutions and advanced management techniques by operators. In this work we develop a dynamic fleet management approach for next generation Public Transportation systems, considering the instance of mixed electric / hybrid fleet. Our objective is that of investigating to what extent electrification, coupled with optimal fleet management, can yield operational cost savings for PT operators, explicitly considering real-time disturbances, including delays, service disruptions etc. We propose a Mixed Integer Linear Program to address the problem of optimal scheduling of a mixed fleet of electric and hybrid / non-electric buses, and employ it as predictor in a Model Predictive Control approach. Test results based upon a real-life scenario showcase how the proposed approach is indeed capable of yielding a sizable reduction in operational costs, even when considerable disturbances arise from the underlying system. [less ▲]

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See detailMixed hybrid and electric bus dynamic fleet management in urban networks: a model predictive control approach
Rinaldi, Marco UL; Picarelli, Erika; Laskaris, Georgios UL et al

Scientific Conference (2019, January)

Reducing pollutant emissions and promoting sustainable mobility solutions, including Public Transport, are increasingly becoming key objectives for policymakers worldwide. In order to jointly achieve ... [more ▼]

Reducing pollutant emissions and promoting sustainable mobility solutions, including Public Transport, are increasingly becoming key objectives for policymakers worldwide. In order to jointly achieve these goals, careful consideration should be put on the operational cost and management of PT services, in order to promote the adoption of green mobility solutions and advanced management techniques by operators. In this work we develop a dynamic fleet management approach for next generation Public Transportation systems, considering the instance of mixed electric / hybrid fleet. Our objective is that of investigating to what extent electrification, coupled with optimal fleet management, can yield operational cost savings for PT operators, explicitly considering real-time disturbances, including delays, service disruptions etc. We propose a Mixed Integer Linear Program to address the problem of optimal scheduling of a mixed fleet of electric and hybrid / non-electric buses, and employ it as predictor in a Model Predictive Control approach. Test results based upon a real-life scenario showcase how the proposed approach is indeed capable of yielding a sizable reduction in operational costs, even when considerable disturbances arise from the underlying system. [less ▲]

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See detailA mixed integer linear programming approach to pursuit evasion problems with optional connectivity constraints
Thunberg, Johan UL; Ögren, P.

in Autonomous Robots (2011), 31(4), 333-343

In this paper, we address the multi pursuer version of the pursuit evasion problem in polygonal environments. By discretizing the problem, and applying a Mixed Integer Linear Programming (MILP) framework ... [more ▼]

In this paper, we address the multi pursuer version of the 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 implemented in Matlab/Cplex and illustrated by a number of solved examples. [less ▲]

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See detailA Mixed Similarity Measure for Data with Numeric, Symbolic and Ordinal Attributes
Nguyen, Ngoc Binh; Than, Van Cuong; Nguyen, Thanh Phuong UL et al

in The proceeding of Japan-Vietnam Joint Workshop on Active Mining (Artificial Intelligence I) (2004)

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See detailMixed-fleet single-terminal bus scheduling problem: Modelling, solution scheme and potential applications
Rinaldi, Marco UL; Picarelli, Erika UL; D'Ariano, Andrea et al

in Omega: the International Journal of Management Science (in press)

Reducing pollutant emissions and promoting sustainable mobility solutions, including Public Transport (PT), are increasingly becoming key objectives for policymakers worldwide. In this work we develop an ... [more ▼]

Reducing pollutant emissions and promoting sustainable mobility solutions, including Public Transport (PT), are increasingly becoming key objectives for policymakers worldwide. In this work we develop an optimal vehicle scheduling approach for next generation PT systems, considering the instance of mixed electric / hybrid fleet. Our objective is that of investigating to what extent electrification, coupled with optimal fleet management, can yield operational cost savings for PT operators. We propose a Mixed In- teger Linear Program (MILP) to address the problem of optimal scheduling of a mixed fleet of electric and hybrid / non-electric buses, coupled with an ad-hoc decomposition scheme aimed at enhancing the scalability of the proposed MILP. Two case studies arising from the PT network of the city of Luxem- bourg are employed in order to validate the model; sensitivity analysis to fleet design parameters is performed, specifically in terms of fleet size and fleet composition. Conclusions point to the fact that careful modelling and handling of mixed-fleet conditions are necessary to achieve operational savings, and that marginal savings gradually reduce as more conventional buses are replaced by their electric counterparts. We believe the methodology proposed may be a key part of advanced decision support systems for policymakers and operators that are dealing with the on-going transition from conventional bus fleets towards greener transport solutions. [less ▲]

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