9,366 research outputs found

    Challenges for Monocular 6D Object Pose Estimation in Robotics

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    Object pose estimation is a core perception task that enables, for example, object grasping and scene understanding. The widely available, inexpensive and high-resolution RGB sensors and CNNs that allow for fast inference based on this modality make monocular approaches especially well suited for robotics applications. We observe that previous surveys on object pose estimation establish the state of the art for varying modalities, single- and multi-view settings, and datasets and metrics that consider a multitude of applications. We argue, however, that those works' broad scope hinders the identification of open challenges that are specific to monocular approaches and the derivation of promising future challenges for their application in robotics. By providing a unified view on recent publications from both robotics and computer vision, we find that occlusion handling, novel pose representations, and formalizing and improving category-level pose estimation are still fundamental challenges that are highly relevant for robotics. Moreover, to further improve robotic performance, large object sets, novel objects, refractive materials, and uncertainty estimates are central, largely unsolved open challenges. In order to address them, ontological reasoning, deformability handling, scene-level reasoning, realistic datasets, and the ecological footprint of algorithms need to be improved.Comment: arXiv admin note: substantial text overlap with arXiv:2302.1182

    Μοντελοποίηση της διακινδύνευσης δασικών πυρκαγιών στον τομέα της γαλάζιας οικονομίας υπό συνθήκες κλιματικής αλλαγής: Εφαρμογή στον τουρισμό της νήσου Κρήτης

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    Οι συνέπειες της κλιματικής αλλαγής είναι αναμφίβολα πλέον περισσότερο εμφανείς από ποτέ άλλοτε στο παρελθόν, πλήττοντας ποικιλοτρόπως την οικολογική, κοινωνική και οικονομική βιωσιμότητα των περιοχών της Μεσογείου. Ειδικότερα, η διαρκώς αυξανόμενη συχνότητα εμφάνισης πυρκαγιάς συνιστά μια από τις σημαντικότερες απειλές των ηπειρωτικών και νησιωτικών οικοσυστημάτων, έχοντας παράλληλα άμεσα αρνητικό αντίκτυπο σε διάφορους τομείς της οικονομικής δραστηριότητας, συμπεριλαμβανομένου του τουρισμού. Υπό αυτό το πρίσμα, στην παρούσα διπλωματική εργασία επιχειρείται η εκτίμηση του βαθμού διακινδύνευσης, αναφορικά με την εκδήλωση συμβάντων δασικής πυρκαγιάς, που ενδέχεται να παρουσιάσει η νήσος Κρήτη στο εγγύς (2046 – 2065) και μακρινό μέλλον (2081 – 2100) σε αντιπαραβολή με τη χρονική περίοδο αναφοράς 1986 – 2005, δεδομένων των κλιματικών συνθηκών που αντανακλούν οι Αντιπροσωπευτικές Πορείες Συγκέντρωσης (Representative Concentration Pathways – RCPs) RCP2.6, RCP4.5 και RCP8.5. Η σχετική μέθοδος αναπτύσσεται σύμφωνα με το εννοιολογικό πλαίσιο της “αλυσίδας επιπτώσεων”. Συγκεκριμένα, αυτή βασίζεται στη συνδυαστική χρήση ποιοτικών και ποσοτικών μεταβλητών για την πλήρη και σαφή περιγραφή των κύριων συνιστωσών κινδύνου, έκθεσης και τρωτότητας, όπως αυτοί ορίζονται από τη Διακυβερνητική Επιτροπή για την Κλιματική Αλλαγή (Intergovernmental Panel on Climate Change – IPCC), με απώτερο στόχο τη διαμόρφωση του σύνθετου δείκτη διακινδύνευσης. Τα τελικά αποτελέσματα υποδηλώνουν τη συμβολή όλων των επιμέρους παραγόντων, ενώ παράλληλα επισημαίνονται οι κρισιμότερες περιβαλλοντικές και κοινωνικοοικονομικές παράμετροι που καθορίζουν τον βαθμό διακινδύνευσης. Η χωρική πολυκριτηριακή ανάλυση που υλοποιείται συμπληρωματικά, με την αξιοποίηση του λογισμικού ArcGIS, αναδεικνύει τις περιοχές που αναμένεται να πληγούν βαρύτατα στο μέλλον. Τέλος, υπογραμμίζεται εξίσου η χρησιμότητα της ιδίας μεθόδου στην ενίσχυση των αρμόδιων φορέων ως προς την εφαρμογή των απαραίτητων προληπτικών μέτρων ενάντια στις δυσμενείς επιπτώσεις της κλιματικής αλλαγής και τη λήψη των κατάλληλων μέτρων προσαρμογής αντίστοιχα.Nowadays, the consequences of climate change are undoubtedly more evident than ever, as they affect the ecological, social and economic sustainability of the Mediterranean regions in various ways. In particular, the ever-increasing frequency of fire incidents constitutes one of the most important threats to continental and island ecosystems, while having a direct negative impact on different sectors of the economy, including tourism. From this perspective, the present thesis attempts to assess forest fire risk that Crete island may present in the near (2046 – 2065) and distant future (2081 – 2100) in comparison with reference time period 1986 – 2005, given the climate conditions reflected by the Representative Concentration Pathways (RCPs) RCP2.6, RCP4.5 and RCP8.5. The relevant method is developed according to the conceptual framework of the "impact chain". Specifically, it is based on the combined use of qualitative and quantitative variables to fully describe the main factors of hazard, exposure and vulnerability, as defined by the Intergovernmental Panel on Climate Change (IPCC), aiming at the formulation of the composite risk index. The final results indicate the contribution of all individual factors while, at the same time, the most critical environmental and socio-economic parameters that determine the risk levels are highlighted. The multicriteria spatial analysis which is further implemented by using the ArcGIS software, highlights the areas that are expected to be heavily affected in the future. Finally, the usefulness of this method is also underlined, both in terms of implementing the necessary preventive measures against the adverse effects of climate change and in terms of strengthening the adaptation planning

    Intelligent computing : the latest advances, challenges and future

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    Computing is a critical driving force in the development of human civilization. In recent years, we have witnessed the emergence of intelligent computing, a new computing paradigm that is reshaping traditional computing and promoting digital revolution in the era of big data, artificial intelligence and internet-of-things with new computing theories, architectures, methods, systems, and applications. Intelligent computing has greatly broadened the scope of computing, extending it from traditional computing on data to increasingly diverse computing paradigms such as perceptual intelligence, cognitive intelligence, autonomous intelligence, and human computer fusion intelligence. Intelligence and computing have undergone paths of different evolution and development for a long time but have become increasingly intertwined in recent years: intelligent computing is not only intelligence-oriented but also intelligence-driven. Such cross-fertilization has prompted the emergence and rapid advancement of intelligent computing

    Efficient finite element methods for solving high-frequency time-harmonic acoustic wave problems in heterogeneous media

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    This thesis focuses on the efficient numerical solution of frequency-domain wave propagation problems using finite element methods. In the first part of the manuscript, the development of domain decomposition methods is addressed, with the aim of overcoming the limitations of state-of-the art direct and iterative solvers. To this end, a non-overlapping substructured domain decomposition method with high-order absorbing conditions used as transmission conditions (HABC DDM) is first extended to deal with cross-points, where more than two subdomains meet. The handling of cross-points is a well-known issue for non-overlapping HABC DDMs. Our methodology proposes an efficient solution for lattice-type domain partitions, where the domains meet at right angles. The method is based on the introduction of suitable relations and additional transmission variables at the cross-points, and its effectiveness is demonstrated on several test cases. A similar non-overlapping substructured DDM is then proposed with Perfectly Matched Layers instead of HABCs used as transmission conditions (PML DDM). The proposed approach naturally considers cross-points for two-dimensional checkerboard domain partitions through Lagrange multipliers used for the weak coupling between subproblems defined on rectangular subdomains and the surrounding PMLs. Two discretizations for the Lagrange multipliers and several stabilization strategies are proposed and compared. The performance of the HABC and PML DDM is then compared on test cases of increasing complexity, from two-dimensional wave scattering in homogeneous media to three-dimensional wave propagation in highly heterogeneous media. While the theoretical developments are carried out for the scalar Helmholtz equation for acoustic wave propagation, the extension to elastic wave problems is also considered, highlighting the potential for further generalizations to other physical contexts. The second part of the manuscript is devoted to the presentation of the computational tools developed during the thesis and which were used to produce all the numerical results: GmshFEM, a new C++ finite element library based on the application programming interface of the open-source finite element mesh generator Gmsh; and GmshDDM, a distributed domain decomposition library based on GmshFEM.Cette thèse porte sur la résolution numérique efficace de problèmes de propagation d'ondes dans le domaine fréquentiel avec la méthode des éléments finis. Dans la première partie du manuscrit, le développement de méthodes de décomposition de domaine est abordé, dans le but de surmonter les limitations des solveurs directs et itératifs de l'état de l'art. À cette fin, une méthode de décomposition de domaine sous-structurée sans recouvrement avec des conditions absorbante d'ordre élevé utilisées comme conditions de transmission (HABC DDM) est d'abord étendue pour traiter les points de jonction, où plus de deux sous-domaines se rencontrent. Le traitement des points de jonction est un problème bien connu pour les HABC DDM sans recouvrement. La méthodologie proposée mène à une solution efficace pour les partitions en damier, où les domaines se rencontrent à angle droit. La méthode est basée sur l'introduction de variables de transmission supplémentaires aux points de jonction, et son efficacité est démontrée sur plusieurs cas-tests. Une DDM sans recouvrement similaire est ensuite proposée avec des couches parfaitement adaptées au lieu des HABC (DDM PML). L'approche proposée prend naturellement en compte les points de jonction des partitions de domaine en damier par le biais de multiplicateurs de Lagrange couplant les sous-domaines et les couches PML adjacentes. Deux discrétisations pour les multiplicateurs de Lagrange et plusieurs stratégies de stabilisation sont proposées et comparées. Les performances des DDM HABC et PML sont ensuite comparées sur des cas-tests de complexité croissante, allant de la diffraction d'ondes dans des milieux homogènes bidimensionnelles à la propagation d'ondes tridimensionnelles dans des milieux hautement hétérogènes. Alors que les développements théoriques sont effectués pour l'équation scalaire de Helmholtz pour la simulation d'ondes acoustiques, l'extension aux problèmes d'ondes élastiques est également considérée, mettant en évidence le potentiel de généralisation des méthodes développées à d'autres contextes physiques. La deuxième partie du manuscrit est consacrée à la présentation des outils de calcul développés au cours de la thèse et qui ont été utilisés pour produire tous les résultats numériques : GmshFEM, une nouvelle bibliothèque d'éléments finis C++ basée sur le générateur de maillage open-source Gmsh ; et GmshDDM, une bibliothèque de décomposition de domaine distribuée basée sur GmshFEM

    Understanding Trade-offs in Stellarator Design with Multi-objective Optimization

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    In designing stellarators, any design decision ultimately comes with a trade-off. Improvements in particle confinement, for instance, may increase the burden on engineers to build more complex coils, and the tightening of financial constraints may simplify the design and worsen some aspects of transport. Understanding trade-offs in stellarator designs is critical in designing high performance devices that satisfy the multitude of physical, engineering, and financial criteria. In this study we show how multi-objective optimization (MOO) can be used to investigate trade-offs and develop insight into the role of design parameters. We discuss the basics of MOO, as well as practical solution methods for solving MOO problems. We apply these methods to bring insight into the selection of two common design parameters: the aspect ratio of an ideal magnetohydrodynamic equilibrium, and the total length of the electromagnetic coils

    Reproducible and relocatable regional ocean modelling: Fundamentals and practices

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    In response to an increasing demand for bespoke or tailored regional ocean modelling configurations, we outline fundamental principles and practices that can expedite the process to generate new configurations. The paper develops the principle of Reproducibility and advocates adherence by presenting benefits to the community and user. The elements to this principle are reproducible workflows and standardised assessment, with additional effort over existing working practices being balanced against the added value generated. The paper then decomposes the complex build process, for a new regional ocean configuration, into stages and presents guidance, advice and insight on each component. This advice is compiled from across the user community, is presented in the context of NEMOv4, though aims to transcend NEMO version. Detail and region specific worked examples are linked in companion repositories and DOIs. The aim is to broaden the user community skill base, and to accelerate development of new configurations in order to increase available time exploiting the configurations

    Symmetry-based stability theory in fluid mechanics

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    The present work deals with the stability theory of fluid flows. The central subject is the question under which circumstances a flow becomes unstable. Instabilities are a frequent trigger of laminar-turbulent transitions. Stability theory helps to explain the emergence of structures, e.g. wave-like perturbation patterns. In this context, the use of Lie symmetries allows the classification of existing and the construction of new solutions within the framework of linear stability theory. In addition, a new nonlinear eigenvalue problem (NEVP) is presented, whose derivation is completely based on Lie symmetries. In classical linear stability theory, a normal ansatz is used for perturbations. Another ansatz that has been shown in early work is the Kelvin mode ansatz. In the work of Nold and Oberlack (2013) and Nold et al. (2015) it was shown that these ansätze can be traced back to the Lie symmetries of the linearized perturbation equations. Interestingly, knowledge of the symmetries also allows for the construction of new ansatz functions that go beyond the known ansätze. For a plane rotational shear flow, in addition to the normal mode ansatz, an algebraic mode ansatz with algebraic behavior in time t^s (eigenvalue s) can be constructed. The flow is stable according to Rayleigh's inflection point criterion, which is also confirmed by the algebraic mode ansatz. Furthermore, exact solutions of the eigenfunctions can be found and new stable modes can be determined by asymptotic methods. Thereby, spiral-like structures of the vorticity can be recognized, which propagate in the region with time. Another key result of this work is the formulation and solution of an NEVP based on the Lie symmetries of the Euler equation. It can is shown that an NEVP can be formulated for a class of flows with a constant velocity gradient. These include, for example, linear shear flows, strained flows, and rotating flows. The NEVP for linear shear flows shows a relation to experimental data from turbulent shear flows. It can be theoretically shown that the turbulent kinetic energy scales exponentially with the eigenvalue of the NEVP. The eigenvalue is determined numerically using a parallel spectral solver. Initially, nonlinear terms are neglected. The determined eigenvalues are in the range of known literature values for turbulent shear flows. Furthermore, the NEVPs for plane flows with pure rotation and pure strain are solved. It is shown that the flow is invariant to rotation, while oscillatory eigenfunctions are found in the case of strain. In addition, an algorithm to solve the NEVP including the nonlinear terms is presented. The results allow an exciting insight into a new stability theory and form the basis for further investigation and understanding of the full nonlinear dynamics of the fluid flows based on the NEVP

    Verification of the generalized reduced-order particle-in-cell scheme in a radial-azimuthal E×B plasma configuration

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    In this article, we present an in-depth verification of the generalized electrostatic reduced-order particle-in-cell (PIC) scheme in a cross electric and magnetic field configuration representative of a radial-azimuthal section of a Hall thruster. The setup of the simulations follows a well-established benchmark case. The main purpose of this effort is to demonstrate that our novel PIC scheme can reliably resolve the complex two-dimensional dynamics and interactions of the plasma instabilities in the radial-azimuthal coordinates of a Hall thruster at a fraction of the computational cost compared to full-2D PIC codes. To this end, we first present the benchmarking of our newly developed full-2D PIC code. Next, we provide an overview of the reduced-order PIC scheme and the resulting “quasi-2D” code, specifying that the degree of order reduction in the quasi-2D PIC is defined in terms of the number of “regions” along the simulation’s directions used to divide the computational domain. We compare the predictions of the quasi-2D simulation in various approximation degrees of the 2D problem against our full-2D simulation results. We show that, by increasing the number of regions in the Q2D simulations, the quasi-2D results converge to the 2D ones. Nonetheless, we also highlight that a quasi-2D simulation that provides a factor of 5 reduction in the computational cost resolves the underlying physical processes in an almost indistinguishable manner with respect to the full-2D simulation and incurs a L2-norm error of only about 2 % in the ion number density and below 1 % in the electron temperature

    Towards fully covariant machine learning

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    Any representation of data involves arbitrary investigator choices. Because those choices are external to the data-generating process, each choice leads to an exact symmetry, corresponding to the group of transformations that takes one possible representation to another. These are the passive symmetries; they include coordinate freedom, gauge symmetry, and units covariance, all of which have led to important results in physics. In machine learning, the most visible passive symmetry is the relabeling or permutation symmetry of graphs. Our goal is to understand the implications for machine learning of the many passive symmetries in play. We discuss dos and don'ts for machine learning practice if passive symmetries are to be respected. We discuss links to causal modeling, and argue that the implementation of passive symmetries is particularly valuable when the goal of the learning problem is to generalize out of sample. This paper is conceptual: It translates among the languages of physics, mathematics, and machine-learning. We believe that consideration and implementation of passive symmetries might help machine learning in the same ways that it transformed physics in the twentieth century.Comment: substantial revision from v1; submitted to TML

    Siena Galaxy Atlas 2020

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    We present the 2020 version of the Siena Galaxy Atlas (SGA-2020), a multi-wavelength optical and infrared imaging atlas of 383,620 nearby galaxies. The SGA-2020 uses optical grzgrz imaging over 20,000\approx20,000 deg2^{2} from the DESI Legacy Imaging Surveys Data Release 9 and infrared imaging in four bands (spanning 3.4-22 μ\mum) from the six-year unWISE coadds; it is more than 95% complete for galaxies larger than R(26)25R(26)\approx25 arcsec and r<18r<18 measured at the 26 mag arcsec2^{-2} isophote in the rr-band. The atlas delivers precise coordinates, multi-wavelength mosaics, azimuthally averaged optical surface brightness profiles, model images and photometry, and additional ancillary metadata for the full sample. Coupled with existing and forthcoming optical spectroscopy from the Dark Energy Spectroscopic Instrument (DESI), the SGA-2020 will facilitate new detailed studies of the star formation and mass assembly histories of nearby galaxies; enable precise measurements of the local velocity field via the Tully-Fisher and Fundamental Plane relations; serve as a reference sample of lasting legacy value for time-domain and multi-messenger astronomical events; and more.Comment: 47 pages, 13 figures, 5 tables; submitted to ApJS. All catalogs and data are publicly available through the SGA web-portal at https://sga.legacysurvey.or
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