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Extracting the spatio-temporal variations in the gravity field recovered from GRACE spatial mission: methods and geophysical applications Prevost, Paoline Fleur Doctoral thesis (2019) Measurements of the spatio-temporal variations of Earth’s gravity field recovered from the Gravity Recovery and Climate Experiment (GRACE) mission have led to unprecedented insights into large spatial ... [more ▼] Measurements of the spatio-temporal variations of Earth’s gravity field recovered from the Gravity Recovery and Climate Experiment (GRACE) mission have led to unprecedented insights into large spatial mass redistribution at secular, seasonal, and sub-seasonal time scales. GRACE solutions from various processing centers, while adopting different processing strategies, result in rather coherent estimates. However, these solutions also exhibit random as well as systematic errors, with specific spatial and temporal patterns in the latter. In order to dampen the noise and enhance the geophysical signals in the GRACE data, several methods have been proposed. Among these, methods based on filtering techniques require a priori assumptions regarding the spatio-temporal structure of the errors. Despite the large effort to improve the quality of GRACE data for always finer geophysical applications, removing noise remains a problematic question as discussed in Chapter 1. In this thesis, we explore an alternative approach, using a spatio-temporal filter, namely the Multichannel Singular Spectrum Analysis (M-SSA) described in Chapter 2. M-SSA is a data-adaptive, multivariate, and non-parametric method that simultaneously exploits the spatial and temporal correlations of geophysical fields to extract common modes of variability. We perform an M-SSA simultaneously on 13 years of GRACE spherical harmonics solutions from five different processing centers. We show that the method allows for the extraction of common modes of variability between solutions, and removal of the solution-specific spatio-temporal errors arising from each processing strategies. In particular, the method filters out efficiently the spurious North-South stripes, most likely caused by aliasing of the imperfect geophysical correction models of known phenomena. In Chapter 3, we compare our GRACE solution to other spherical harmonics solutions and to mass concentration (mascon) solutions which use a priori information on the spatio-temporal pattern of geophysical signals. We also compare performance of our M-SSA GRACE solution with respect to others by predicting surface displacements induced by GRACE-derived mass loading and comparing results with independent displacement data from stations of the Global Navigation Satellite System (GNSS). Finally, in Chapter 4 we discuss the possible application of a refined GRACE solution to answer debated post-glacial rebound questions. More precisely, we focus on separating the post-glacial rebound signal related to past ice melting and the present ice melting in the region of South Georgia. [less ▲] Detailed reference viewed: 105 (8 UL)La Fabrication Additive Robotisée: Perspective et Recherche autour d'Impression 3D de Pièces Métalliques de Grandes Dimensions pour l'Industrie Aéronautique Mbodj, Natago Guilé ; Plapper, Peter Article for general public (2020) Detailed reference viewed: 134 (13 UL)Fast Stochastic Non-linear Model Predictive Control for Electric Vehicle Advanced Driver Assistance Systems Sajadi Alamdari, Seyed Amin ; Voos, Holger ; in 13th IEEE International Conference on Vehicular Electronics and Safety, Vienna, Austria 27-28 June 2017 (2017, June 27) Semi-autonomous driving assistance systems have a high potential to improve the safety and efficiency of the battery electric vehicles that are enduring limited cruising range. This paper presents an ... [more ▼] Semi-autonomous driving assistance systems have a high potential to improve the safety and efficiency of the battery electric vehicles that are enduring limited cruising range. This paper presents an ecologically advanced driver assistance system to extend the functionality of the adaptive cruise control system. A real-time stochastic non-linear model predictive controller with probabilistic constraints is presented to compute on-line the safe and energy-efficient cruising velocity profile. The individual chance-constraint is reformulated into a convex second-order cone constraint which is robust for a general class of probability distributions. Finally, the performance of proposed approach in terms of states regulation, constraints fulfilment, and energy efficiency is evaluated on a battery electric vehicle. [less ▲] Detailed reference viewed: 210 (9 UL)A fast, certified and "tuning free" two-field reduced basis method for the metamodelling of affinely-parametrised elasticity problems ; ; Bordas, Stéphane in Computer Methods in Applied Mechanics and Engineering (2016), 298 This paper proposes a new reduced basis algorithm for the metamodelling of parametrised elliptic problems. The developments rely on the Constitutive Relation Error (CRE), and the construction of separate ... [more ▼] This paper proposes a new reduced basis algorithm for the metamodelling of parametrised elliptic problems. The developments rely on the Constitutive Relation Error (CRE), and the construction of separate reduced order models for the primal variable (displacement) and flux (stress) fields. A two field greedy sampling strategy is proposed to construct these two fields simultaneously and in an efficient manner: at each iteration, one of the two fields is enriched by increasing the dimension of its reduced space in such a way that the CRE is minimised. This sampling strategy is then used as a basis to construct goal-oriented reduced order modelling. The resulting algorithm is certified and “tuning free”: the only requirement from the engineer is the level of accuracy that is desired for each of the outputs of the surrogate. It is also shown to be significantly more efficient in terms of computational expense than competing methodologies. [less ▲] Detailed reference viewed: 177 (18 UL)A fast, certified and "tuning-free" two-field reduced basis method for the metamodelling of parametrised elasticity problems ; ; Bordas, Stéphane in Computer Methods in Applied Mechanics and Engineering (2015) This paper proposes a new reduced basis algorithm for the metamodelling of parametrised elliptic problems. The developments rely on the Constitutive Relation Error (CRE), and the construction of separate ... [more ▼] This paper proposes a new reduced basis algorithm for the metamodelling of parametrised elliptic problems. The developments rely on the Constitutive Relation Error (CRE), and the construction of separate reduced order models for the primal variable (displacement) and flux (stress) fields. A two-field Greedy sampling strategy is proposed to construct these two fields simultaneously and efficient manner: at each iteration, one of the two fields is enriched by increasing the dimension of its reduced space in such a way that the CRE is minimised. This sampling strategy is then used as a basis to construct goal-oriented reduced order modelling. The resulting algorithm is certified and "tuning-free": the only requirement from the engineer is the level of accuracy that is desired for each of the outputs of the surrogate. It is also one order of magnitude more efficient in terms of computational expenses than competing methodologies. [less ▲] Detailed reference viewed: 416 (12 UL)FastMotif: Spectral Sequence Motif Discovery Colombo, Nicolo ; Vlassis, Nikos in Bioinformatics (2015) Motivation: Sequence discovery tools play a central role in several fields of computational biology. In the framework of Transcription Factor binding studies, most of the existing motif finding algorithms ... [more ▼] Motivation: Sequence discovery tools play a central role in several fields of computational biology. In the framework of Transcription Factor binding studies, most of the existing motif finding algorithms are computationally demanding, and they may not be able to support the increasingly large datasets produced by modern high-throughput sequencing technologies. Results: We present FastMotif, a new motif discovery algorithm that is built on a recent machine learning technique referred to as Method of Moments. Based on spectral decompositions, our method is robust to model misspecifications and is not prone to locally optimal solutions. We obtain an algorithm that is extremely fast and designed for the analysis of big sequencing data. On HT-Selex data, FastMotif extracts motif profiles that match those computed by various state-of- the-art algorithms, but one order of magnitude faster. We provide a theoretical and numerical analysis of the algorithm’s robustness and discuss its sensitivity with respect to the free parameters. [less ▲] Detailed reference viewed: 153 (9 UL)Federal Blockchain Infrastructure Asylum (FLORA) - Piloting and evaluation of the FLORA support system in the context of the AnkER facility Dresden ; ; et al Report (2022) Detailed reference viewed: 41 (0 UL)Federated Learning as a Solution for Problems Related to Intergovernmental Data Sharing ; Delgado Fernandez, Joaquin ; et al in Proceedings of the 56th Hawaii International Conference on System Sciences (2023, January 03) To address global problems, intergovernmental collaboration is needed. Modern solutions to these problems often include data-driven methods like artificial intelligence (AI), which require large amounts ... [more ▼] To address global problems, intergovernmental collaboration is needed. Modern solutions to these problems often include data-driven methods like artificial intelligence (AI), which require large amounts of data to perform well. However, data sharing between governments is limited. A possible solution is federated learning (FL), a decentralised AI method created to utilise personal information on edge devices. Instead of sharing data, governments can build their own models and just share the model parameters with a centralised server aggregating all parameters, resulting in a superior overall model. By conducting a structured literature review, we show how major intergovernmental data sharing challenges like disincentives, legal and ethical issues as well as technical constraints can be solved through FL. Enhanced AI while maintaining privacy through FL thus allows governments to collaboratively address global problems, which will positively impact governments and citizens. [less ▲] Detailed reference viewed: 43 (1 UL)Federated Learning for Credit Risk Assessment Lee, Chul Min ; Delgado Fernandez, Joaquin ; Potenciano Menci, Sergio et al in Proceedings of the 56th Hawaii International Conference on System Sciences (2023, January 03) Credit risk assessment is a standard procedure for financial institutions (FIs) when estimating their credit risk exposure. It involves the gathering and processing quantitative and qualitative datasets ... [more ▼] Credit risk assessment is a standard procedure for financial institutions (FIs) when estimating their credit risk exposure. It involves the gathering and processing quantitative and qualitative datasets to estimate whether an individual or entity will be able to make future required payments. To ensure effective processing of this data, FIs increasingly use machine learning methods. Large FIs often have more powerful models as they can access larger datasets. In this paper, we present a Federated Learning prototype that allows smaller FIs to compete by training in a cooperative fashion a machine learning model which combines key data derived from several smaller datasets. We test our prototype on an historical mortgage dataset and empirically demonstrate the benefits of Federated Learning for smaller FIs. We conclude that smaller FIs can expect a significant performance increase in their credit risk assessment models by using collaborative machine learning. [less ▲] Detailed reference viewed: 69 (17 UL)Final report for the project "European Data Science Conference" Krolak-Schwerdt, Sabine ; Böhmer, Matthias Report (2017) Detailed reference viewed: 116 (1 UL)Finding surface Lyapunov functions through sum-of-squares Programming ; Goncalves, Jorge in Proceedings of the American Control Conference 2004 (2004) This work presents a sum-of-squares method to construct polynomial surface Lyapunov functions (SuLF) of arbitrary order for the impact maps of limit cycles in piecewise linear systems (PLS). This work ... [more ▼] This work presents a sum-of-squares method to construct polynomial surface Lyapunov functions (SuLF) of arbitrary order for the impact maps of limit cycles in piecewise linear systems (PLS). This work extends previous results on stability analysis of such limit cycles, which utilized quadratic SuLFs. This paper also discusses an initial study of hierarchical jump linear systems where the switching is driven by feedback of low-level dynamical system states and a Markovian process. [less ▲] Detailed reference viewed: 103 (1 UL)Finite deformations govern the anisotropic shear-induced area reduction of soft elastic contacts Lengiewicz, Jakub ; ; et al in Journal of the Mechanics and Physics of Solids (2020), 143 Solid contacts involving soft materials are important in mechanical engineering or biomechanics. Experimentally, such contacts have been shown to shrink significantly under shear, an effect which is ... [more ▼] Solid contacts involving soft materials are important in mechanical engineering or biomechanics. Experimentally, such contacts have been shown to shrink significantly under shear, an effect which is usually explained using adhesion models. Here we show that quantitative agreement with recent high-load experiments can be obtained, with no adjustable parameter, using a non-adhesive model, provided that finite deformations are taken into account. Analysis of the model uncovers the basic mechanisms underlying anisotropic shear-induced area reduction, local contact lifting being the dominant one. We confirm experimentally the relevance of all those mechanisms, by tracking the shear-induced evolution of tracers inserted close to the surface of a smooth elastomer sphere in contact with a smooth glass plate. Our results suggest that finite deformations are an alternative to adhesion, when interpreting a variety of sheared contact experiments involving soft materials. [less ▲] Detailed reference viewed: 126 (8 UL)Finite element analysis on implicitly defined domains: An accurate representation based on arbitrary parametric surfaces ; ; et al in Computer Methods in Applied Mechanics and Engineering (2011), 200(5-8), 774-796 In this paper, we present some novel results and ideas for robust and accurate implicit representation of geometric surfaces in finite element analysis. The novel contributions of this paper are threefold ... [more ▼] In this paper, we present some novel results and ideas for robust and accurate implicit representation of geometric surfaces in finite element analysis. The novel contributions of this paper are threefold: (1) describe and validate a method to represent arbitrary parametric surfaces implicitly; (2) represent arbitrary solids implicitly, including sharp features using level sets and boolean operations; (3) impose arbitrary Dirichlet and Neumann boundary conditions on the resulting implicitly defined boundaries. The methods proposed do not require local refinement of the finite element mesh in regions of high curvature, ensure the independence of the domain's volume on the mesh, do not rely on boundary regularization, and are well suited to methods based on fixed grids such as the extended finite element method (XFEM). Numerical examples are presented to demonstrate the robustness and effectiveness of the proposed approach and show that it is possible to achieve optimal convergence rates using a fully implicit representation of object boundaries. This approach is one step in the desired direction of tying numerical simulations to computer aided design (CAD), similarly to the isogeometric analysis paradigm. © 2010 Elsevier B.V. [less ▲] Detailed reference viewed: 504 (9 UL)Finite strain poro-hyperelasticity: an asymptotic multi-scale ALE-FSI approach supported by ANNs Dehghani, Hamidreza ; Zilian, Andreas in Computational Mechanics (2023) This contribution introduces and discusses a formulation of poro-hyperelasticity at finite strains. The prediction of the time-dependent response of such media requires consideration of their ... [more ▼] This contribution introduces and discusses a formulation of poro-hyperelasticity at finite strains. The prediction of the time-dependent response of such media requires consideration of their characteristic multi-scale and multi-physics parameters. In the present work this is achieved by formulating a non-dimensionalised fluid–solid interaction problem (FSI) at the pore level using an arbitrary Lagrange–Euler description (ALE). The resulting coupled systems of PDEs on the reference configuration are expanded and analysed using the asymptotic homogenisation technique. This approach yields three partially novel systems of PDEs: the macroscopic/effective problem and two supplementary microscale problems (fluid and solid). The latter two provide the microscopic response fields whose average value is required in real-time/online form to determine the macroscale response (a concurrent multi-scale approach). In order to overcome the computational challenges related to the above multi-scale closure, this work introduces a surrogate approach for replacing the direct numerical simulation with an artificial neural network. This methodology allows for solving finite strain (multi-scale) porohyperelastic problems accurately using direct automated differentiation through the strain energy. Optimal and reliable training data sets are produced from direct numerical simulations of the fully-resolved problem by including a simple real-time output density check for adaptive sampling step refinement. The data-driven approach is complemented by a sensitivity analysis of the RVE response. The significance of the presented approach for finite strain poro-elasticity/poro-hyperelasticity is shown in the numerical benchmark of a multi-scale confined consolidation problem. Finally, to show the robustness of the method, the system response is dimensionalised using characteristic values of soil and brain mechanics scenarios. [less ▲] Detailed reference viewed: 28 (3 UL)Finite-time road grade computation for a vehicle platoon ; ; et al in IEEE (2014) Given a platoon of vehicles traveling uphill, this paper considers the finite-time road grade computation problem. We propose a decentralized algorithm for an arbitrarily chosen vehicle to compute the ... [more ▼] Given a platoon of vehicles traveling uphill, this paper considers the finite-time road grade computation problem. We propose a decentralized algorithm for an arbitrarily chosen vehicle to compute the road grade in a finite number of time-steps by using only its own successive velocity measurements. Simulations then illustrate the theoretical results. These new results can be applied to real-world vehicle platooning problems to reduce fuel consumption and carbon dioxide emissions. [less ▲] Detailed reference viewed: 446 (6 UL)Fleet readiness: Stocking spare parts and high tech assets ; Arts, Joachim in IISE Transactions (2017), 49(4), 429-441 Detailed reference viewed: 260 (6 UL)FLEXCAP – Cost competitive flexible process for highly aesthetic closures manufacturing Kakogiannos, Ioannis ; Hichri, Bassem ; Plapper, Peter in Kakogiannos, Ioannis; Hichri, Bassem; Plapper, Peter (Eds.) Robotix-Academy Conference for Industrial Robotics (RACIR) 2018 (2018, November) Detailed reference viewed: 193 (4 UL)Flexible Resource Optimization for GEO Multibeam Satellite Communication System Abdu, Tedros Salih ; Kisseleff, Steven ; Lagunas, Eva et al in IEEE Transactions on Wireless Communications (2021) Conventional GEO satellite communication systems rely on a multibeam foot-print with a uniform resource allocation to provide connectivity to users. However, applying uniform resource allocation is ... [more ▼] Conventional GEO satellite communication systems rely on a multibeam foot-print with a uniform resource allocation to provide connectivity to users. However, applying uniform resource allocation is inefficient in presence of non-uniform demand distribution. To overcome this limitation, the next generation of broadband GEO satellite systems will enable flexibility in terms of power and bandwidth assignment, enabling on-demand resource allocation. In this paper, we propose a novel satellite resource assignment design whose goal is to satisfy the beam traffic demand by making use of the minimum transmit power and utilized bandwidth. The motivation behind the proposed design is to maximize the satellite spectrum utilization by pushing the spectrum reuse to affordable limits in terms of tolerable interference. The proposed problem formulation results in a non-convex optimization structure, for which we propose an efficient tractable solution. We validate the proposed method with extensive numerical results, which demonstrate the efficiency of the proposed approach with respect to benchmark schemes. [less ▲] Detailed reference viewed: 371 (102 UL)Flood Detection On Low Cost Orbital Hardware ; ; et al Scientific Conference (2019, October) Satellite imaging is a critical technology for monitoring and responding to natural disasters such as flooding. Despite the capabilities of modern satellites, there is still much to be desired from the ... [more ▼] Satellite imaging is a critical technology for monitoring and responding to natural disasters such as flooding. Despite the capabilities of modern satellites, there is still much to be desired from the perspective of first response organisations like UNICEF. Two main challenges are rapid access to data, and the ability to automatically identify flooded regions in images. We describe a prototypical flood segmentation system, identifying cloud, water and land, that could be deployed on a constellation of small satellites, performing processing on board to reduce downlink bandwidth by 2 orders of magnitude. We target PhiSat-1, part of the FSSCAT mission, which is planned to be launched by the European Space Agency (ESA) near the start of 2020 as a proof of concept for this new technology. [less ▲] Detailed reference viewed: 49 (10 UL)Fluctuations and Limitations of a Multi-Sector Economic Model with Delays ; ; Goncalves, Jorge et al in The proceedings of the 2010 American Control Conference (ACC) (2010) A general multi-sector model of the economy is investigated. A sector's input to production, labor, evolves according to a jump Markov process. Labor jumps between sectors to balance supply and demand ... [more ▼] A general multi-sector model of the economy is investigated. A sector's input to production, labor, evolves according to a jump Markov process. Labor jumps between sectors to balance supply and demand, where each sector differs by its productivity. The jump model captures the intrinsic noise of the micro agents on the macro level, which is represented by the random timing of labor jumps. Quantifying this noise is a central theme of this paper. An operator theoretic approach is utilized to capture the fluctuations of a linearized jump system exactly. As an illustrative example two sector and three sector economies are studied. In each case the optimal aggressiveness, gain, of a sector is determined for minimal variance. Delays are then introduced into the model. It is shown that the presence of a delay creates a limit on the minimum variance achievable and that high gain is destabilizing. For both the two and three sector models the nonlinear jump systems are simulated. It is shown that the operator theoretic approach is an appropriate method for quantifying the second moments. [less ▲] Detailed reference viewed: 97 (0 UL) |
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