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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
81

Interfaces between Competing Patterns in Reaction-diffusion Systems with Nonlocal Coupling

Nicola, Ernesto Miguel 27 February 2002 (has links)
In this thesis we investigate the formation of patterns in a simple activator-inhibitor model supplemented with an inhibitory nonlocal coupling term. This model exhibits a wave instability for slow inhibitor diffusion, while, for fast inhibitor diffusion, a Turing instability is found. For moderate values of the inhibitor diffusion these two instabilities occur simultaneously at a codimension-2 wave-Turing instability. We perform a weakly nonlinear analysis of the model in the neighbourhood of this codimension-2 instability. The resulting amplitude equations consist in a set of coupled Ginzburg-Landau equations. These equations predict that the model exhibits bistability between travelling waves and Turing patterns. We present a study of interfaces separating wave and Turing patterns arising from the codimension-2 instability. We study theoretically and numerically the dynamics of such interfaces in the framework of the amplitude equations and compare these results with numerical simulations of the model near and far away from the codimension-2 instability. Near the instability, the dynamics of interfaces separating small amplitude Turing patterns and travelling waves is well described by the amplitude equations, while, far from the codimension-2 instability, we observe a locking of the interface velocities. This locking mechanism is imposed by the absence of defects near the interfaces and is responsible for the formation of drifting pattern domains, i.e. moving localised patches of travelling waves embedded in a Turing pattern background and vice versa.
82

Selbstorganisierte Nanostrukturen in katalytischen Oberflächenreaktionen

Hildebrand, Michael 25 June 1999 (has links)
In der vorliegenden Arbeit werden Musterbildungsphänomene auf Submikrometerskalen in reaktiven Adsorbaten auf einkristallinen Katalysatoroberflächen theoretisch untersucht. Da auf solch kleinen Skalen Fluktuationen nicht mehr vernachlässigt werden können, wird eine mesoskopische Theorie entwickelt, die zwischen mikroskopischen Gittermodellen und Reaktions-Diffusions-Systemen vermittelt. Sie beschreibt die Dynamik lokal gemittelter Adsorbatbedeckungen im Rahmen eines Kontinuumsmodells unter Berücksichtigung interner Fluktuationen. Dieser Ansatz wird auf verschiedene Systeme angewendet, in denen sich Muster auf Längenskalen ausbilden, die kleiner als die charakterist ische Diffusionslänge sind, die typischerweise im Mikrometerbereich liegt. Wie beispielsweise in kürzlich durchgefh hrten Experimenten mit einem vergleichsweise schnellen Rastertunnelmikroskop beobachtet wurde, können attraktive Adsorbat-Adsorbat-Wech sel wirkungen zu verschiedenen Mustern auf Nanometerskalen führen. Hier wird zunächst eine einzelne Adsorbatspezies betrachtet. In Abwesenheit von Nichtgleichgewichtsreaktionen können hinreichend starke attraktive laterale Adsorbatwechselwirkungen einen Phasenh bergang erster Ordnung in der Adsorbatbedeckung induzieren. Die mesoskopische Entwicklungsgleichung wird auf die Modellierung der Kinetik dieses Phasenh bergangs angewendet. Berücksichtigt man zusätzlich eine Nichtgleichgewichtsreakti on, so können sich stationäre räumlich periodische Mikrostrukturen aufgrund der Konkurrenz zwischen dem Phasenh bergang und der Reaktion ausbilden. Die Vorraussetzungen für deren Auftreten und ihre charakteristischen Eigenschaften werden hier detailliert analysiert. Unter anderem werden alternierende Wechselwirkungen diskutiert und der Einfluß globaler Kopplung durch die Gasphase auf die Musterbildung wird betrachtet. Außerdem wird gezeigt, da8 die Mikrostrukturen auch durch vergleichsweise starke interne Fluktuationen nicht zerstört werden. Im nächsten Schritt wird ein hypothetisches Modell für zwei verschiedene Adsorbatspezies untersucht, in dem ein ähnlicher Mechanismus zur Bildung von laufenden und stehenden Wellenmustern auf der Nanoskala führt. Werden vergleichsweise starke interne Fluktuationen berücksichtigt, so brechen diese Wellenmuster auf und man beobachtet eine komplexe Dynamik miteinander wechselwirkender Wellenfragmente. Im letzten Beispiel wird anhand der Analyse eines einfachen Modells gezeigt, da8 sich auf Skalen unterhalb der Diffusionslänge selbstorganisierte Mikroreaktoren in einer einzelnen reaktiven Adsorbatspezies ausbilden können, ohne daß die Teilchen miteinander wechselwirken. Sie entsprechen lokalisierten Strukturen, die aufgrund des Zusammenspiels einer Nichtgleichgewichtsreaktion, der Diffusion und eines adsorbatinduzierten strukturellen Phasenh bergangs in der Substratoberfläche entstehen. / Nanoscale pattern formation in reactive adsorbates on single crystal surfaces is investigated theoretically. Because on such small scales fluctuations become important, a mesoscopic theory for the adsorbate coverage is developed, which aims at providing a link between microscopic lattice models and reaction-diffusion equations. It describes the dynamics for the locally averaged adsorbate coverages in a continuum model taking into account internal fluctuations. This approach is applied to several systems, where patterns on scales smaller than the characteristic diffusion length, which typically lies in the micrometer range, can be formed. As has been observed e.g. in recent experiments with fast scanning tunneling microscopy, a variety of nanoscale patterns can result from the presence of attractive adsorbate-adsorbate interactions. Here, at first a single species of such an adsorbate is considered. In the absence of nonequilibrium reactions, strong enough attractive lateral interactions can induce a first-order phase transition in the adsorbate coverage. The mesoscopic evolution equation is applied to model the kinetics of this phase transition. If additionally a nonequilibrium reaction is present, stationary spatially periodic microstructures may arise as a result of the competition of the attractive lateral interactions and the reactions. The conditions for their appearance and their properties are investigated in detail, e.g. alternating lateral interactions are discussed and the influence of global coupling through the gas phase is analyzed. Furthermore, it is shown that they are not destroyed by relatively strong internal fluctuations. In the next step, a hypothetical model for two different reactive adsorbate species is investigated, where a similar mechanism leads to the formation of nanoscale traveling and standing waves. In the presence of relatively strong internal fluctuations these waves break up and a complex dynamics of interacting wave fragments is observed. In the last example, it is shown in the analysis of a simple model that self-organized nonequilibrium microreactors with submicrometer sizes may spontaneously develop in a single reactive adsorbate species without attractive lateral interactions. They represent localized structures resulting from the interplay between reaction, diffusion and an adsorbate-induced structural transformation of the surface.
83

Modelling and simulation of surface morphology driven by ion bombardment / Modellieren und Simulation der Oberflächenmorphologie gefahren durch Ionenbombardierung

Yewande, Emmanuel Oluwole 02 May 2006 (has links)
No description available.
84

Segmentace obrazu pomocí neuronové sítě / Neural Network Based Image Segmentation

Jamborová, Soňa January 2011 (has links)
This work is about suggestion of the software for neural network based image segmentation. It defines basic terms for this topics. It is focusing mainly at preperation imaging information for image segmentation using neural network. It describes and compares different aproaches for image segmentation.
85

Framtiden är självorganiserad? : Så ett frö och få det att växa / The future is self-organised? : Set seeds and make it grow

Alfredsson, Josefine, Farrensteiner, Rebecca January 2020 (has links)
Background: Since the 1980s, society has been hit hard by economization and deprofessionalization through neoliberal abstract forms of governance. Control is achieved by measuring efficiency through simplified models. The demand for measurability has also hit the art field where administration and paperwork have increased. As an effect, a large part of cultural funding goes to non-artistic activities. Values other than those of art are focused on and art institutions are having to adapt to the logic of the market and meet commercial interests. In addition, the conditions of artistic production have deteriorated: there are no resources, space or time yet, which threatens art production. The situation is, to say the least, strained for many artists, who are more or less forced into their own corporate activities by the field's surrounding organizations and institutions. However, there are those who work against this development and act for change. Artists have collectively organized themselves outside the city that no longer meets the condition for artistic production. They are about 40 organisations spread across Sweden. Through networks, they have united under one designation; the self-organized. They could be said to constitute a micro-resistance to the current order. Purpose: The purpose of this study is to investigate what the self-organized means. Which ideas and practices make sense and distinction concerning the self-organised? We want to understand and highlight how the self-organised are motivated and how they go about trying to alter and expand the artistic field and the society at large. Methodology: Using a qualitative approach, the study has been a case study of a central actor in the self-organized fields; Art Lab Gnesta. Materials have been collected through a group interview, participant observation and through written sources produced by the study object itself. This is to better understand underlying ideas and what it is that makes sense for self-organized arts organizations. Conclusions: This study is about a possible institutionalisation process in its future. The self-organised is a kind of avant-garde of the 20th century. In this study, we conclude that the self-organised should be understood as a concept that provides alternative frameworks in a world characterized by an unsustainable order. The self-organised opens up opportunities and shows other ways to go. If many join the concept, changes can possibly be achieved. We have seen that they succeed in reaching out and are legitimized by both the authorities, the art world and the local population. When enough people act on similar ideas, they can constitute institutional pressure and possibly create change. We understand the self-organised as a predestination and a strategy for a potential future. It is about change, nothing else can be determined since we are in the initial stage of the course of event. They are a grassroots movement. / Bakgrund: Samhället har allt sedan 1980-talet drabbats av en hårt driven ekonomisering och avprofessionalisering via nyliberala abstrakta styrformer. Kontroll uppnås genom att via förenklade modeller mäta effektivitet. Kravet på mätbarhet har även drabbat konstfältet där administrationen har ökat. Således går en stor del av kulturfinansieringen åt till icke-konstnärlig verksamhet. Andra värden än den av konsten fokuseras och konstinstitutioner har i större utsträckning marknadsanpassas och möter kommersiella intressen. Utöver detta har konstnärers produktionsförhållandena försämrats: det finns inte rum, tid och resurser nog, varvid konsten hotas. Situationen är minst sagt ansträngd för många konstnärer, som mer eller mindre tvingats in i egen företagsverksamhet av fältets omgivande organisationer och institutioner. Det finns dock dem som tar spjärn mot denna utveckling och verkar för en förändring. Konstnärer har kollektivt organiserat sig utanför staden som inte längre erbjuder. De är ett fyrtiotal verksamheter spridda över Sverige. Genom nätverk har de förenat sig under en beteckning; de självorganiserade. De skulle kunna sägas utgöra ett mikro-motstånd mot rådande ordning. Syfte: Syftet med denna studie är att undersöka vad de självorganiserade betyder. Vilka idéer och vad för praktiker ger mening och distinktion gällande de självorganiserade? Vi vill förstå och belysa hur de självorganiserade motiveras och hur de går tillväga för att försöka uppnå förändring av den ordning som de tar spjärn emot på det konstnärliga fältet och i samhället i stort. Metod: Utifrån en kvalitativ ansats har studien utgjorts av en fallstudie på en central aktör på de självorganiserades fält; Art Lab Gnesta. Material har samlats in genom en gruppintervju, en deltagande observation samt genom skriftliga källor producerade av studieobjektet själv. Detta för att bättre förstå bakomliggande idéer och vad det är som ger mening för självorganiserade konstorganisationer. Slutsatser: Denna studie handlar om en eventuell institutionaliseringsprocess i sitt blivande. De självorganiserade är ett slags 2020-talets avantgarde. I studien kommer vi fram till att de självorganiserade ska förstås som ett koncept som ger alternativa ramverk i en värld präglad av ohållbar ordning. De självorganiserade öppnar upp för möjligheter och visar på andra vägar. Ansluter många till konceptet kan eventuellt förändringar uppnås. Vi har sett att de lyckas nå ut och legitimeras av både makthavare, konstvärld och lokalbefolkning. När tillräckligt många handlar utifrån liknande koncept kan de utgöra ett institutionellt tryck och eventuellt skapa förändring på fältet. Vi förstår de självorganiserade som en predistination och en strategi för en potentiell framtid. Det handlar om förändring, inget annat kan slås fast då vi är i skeendets initiala stadie. De är en gräsrotsrörelse.

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