• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 140
  • 45
  • 33
  • 27
  • 19
  • 3
  • 3
  • 2
  • 2
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • Tagged with
  • 343
  • 343
  • 69
  • 51
  • 45
  • 44
  • 42
  • 39
  • 33
  • 32
  • 32
  • 29
  • 29
  • 28
  • 28
  • 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.
111

Studium procesů s účastí kvarku b v experimentu Belle / Studium procesů s účastí kvarku b v experimentu Belle

Červenkov, Daniel January 2012 (has links)
Title: Study of b-quark processes in the Belle experiment Author: Daniel Červenkov Department: Institute of Particle and Nuclear Physics Supervisor: Zdeněk Doležal Co-Supervisor: Karim Trabelsi Abstract: A study regarding the plausability of using the B0 → D∗ ρ± decay channel and the data set of the Belle experiment to measure sin(2φ1+φ3) has been carried out. Two fitters have been created. A time-independent one, which ex- hibits high stability and reports gratifying fit uncertainties, and a time-dependent one. During the testing of the latter we have discovered an EvtGen flaw and pro- posed a temporary workaround. Nevertheless, the time-dependent fitter fails to recover the correct values. More work will be needed to reach conclusive results. Keywords: B meson, CP violation, angular analysis, time-dependent, fitter
112

Modelos de riscos aditivo-multiplicativos em análise de sobrevivência / Additive-multiplicative hazard models in survival analysis

Tsujimoto, Tamy Harumy Moraes 30 April 2015 (has links)
Este trabalho visa estudar o modelo de Cox-Aalen, que incorpora efeitos fixos e variantes no tempo por meio das estruturas aditiva e multiplicativa. Os principais modelos aditivos e multiplicativos que englobam efeitos fixos e variantes no tempo são revisados com enfoque de processos de contagem e um teste do tipo escore para auxiliar na escolha entre a estrutura aditiva e multiplicativa é proposto e avaliado por meio de estudos de simulação. Aplicação e comparação dos principais modelos discutidos em dados do Instituto do Câncer do Estado de São Paulo (ICESP) sugerem que o modelo estudado pode ser uma opção interessante na análise de dados de sobrevivência em que as ferramentas usuais não são adequadas. / This dissertation aims to study the Cox-Aalen regression model, which incorporates fixed and time-varying effects through the additive and multiplicative structures. The primary additive and multiplicative risk models that deal with fixed and time-dependent effects are reviewed with the counting process framework and a score test to assist the decision between the additive and multiplicative structures is proposed and evaluated with a simulation study. The application of the models to the Instituto do Câncer do Estado de São Paulo (ICESP) dataset suggests that the model under study can be considered as an interesting option to analyse survival data when the usual techniques are not aproppriate.
113

Multiple Time Scales and Longitudinal Measurements in Event History Analysis

Danardono, January 2005 (has links)
<p>A general time-to-event data analysis known as event history analysis is considered. The focus is on the analysis of time-to-event data using Cox's regression model when the time to the event may be measured from different origins giving several observable time scales and when longitudinal measurements are involved. For the multiple time scales problem, procedures to choose a basic time scale in Cox's regression model are proposed. The connections between piecewise constant hazards, time-dependent covariates and time-dependent strata in the dual time scales are discussed. For the longitudinal measurements problem, four methods known in the literature together with two proposed methods are compared. All quantitative comparisons are performed by means of simulations. Applications to the analysis of infant mortality, morbidity, and growth are provided.</p>
114

Time-dependent occurrence rates of large earthquakes in the Dead Sea fault zone and applications to probabilistic seismic hazard assessments

Hakimhashemi, Amir Hossein January 2009 (has links)
Die relativ hohe seismische Aktivität der Tote-Meer-Störungszone (Dead Sea Fault Zone - DSFZ) ist mit einem hohen Gefahrenpotential verbunden, welches zu einem erheblichen Erdbebenrisiko für die Ballungszentren in den Ländern Syrien, Libanon, Palästina, Jordanien und Israel führt. Eine Vielzahl massiver, zerstörerischer Erdbeben hat sich in diesem Raum in den letzten zwei Jahrtausenden ereignet. Ihre Wiederholungsrate zeigt Anzeichen für eine zeitliche Abhängigkeit, insbesondere wenn lange Zeiträume in Betracht gezogen werden. Die Berücksichtigung der zeitlichen Abhängigkeit des Auftretens von Erdbeben ist für eine realistische seismische Gefährdungseinschätzung von großer Bedeutung. Ziel der vorliegenden Arbeit ist es, anhand des zeitabhängigen Auftretens von Erdbeben eine robuste wahrscheinlichkeitstheoretische seismische Gefährdungseinschätzung am Beispiel der DSFZ zu entwickeln. Mittels dieser Methode soll die zeitliche Abhängigkeit des Auftretens von großen Erdbeben (Mw ≥ 6) untersucht und somit eine Gefährdungseinschätzung für das Untersuchungsgebiet getroffen werden. Primär gilt es zu prüfen, ob das Auftreten von großen Erdbeben tatsächlich einer zeitlichen Abhängigkeit unterliegt und wenn ja, inwiefern diese bestimmt werden kann. Zu diesem Zweck werden insgesamt vier zeitabhängige statistische Verteilungen (Weibull, Gamma, Lognormal und Brownian Passage Time (BPT)) sowie die zeitunabhängige Exponentialverteilung (Poisson-Prozess) getestet. Zur Abschätzung der jeweiligen Modellparameter wird eine modifizierte Methode der gewichteten Maximum-Likelihood-Schätzung (MLE) verwendet. Um einzuschätzen, ob die Wiederholungsrate von Erdbeben einer unimodalen oder multimodalen Form folgt, wird ein nichtparametrischer Bootstrap-Test für Multimodalität durchgeführt. Im Falle einer multimodalen Form wird neben der MLE zusätzlich eine Erwartungsmaximierungsmethode (EM) herangezogen. Zur Auswahl des am besten geeigneten Modells wird zum einem das Bayesschen Informationskriterium (BIC) und zum anderen der modifizierte Kolmogorow-Smirnow-Goodness-of-Fit-Test angewendet. Abschließend werden mittels der Bootstrap-Methode die Konfidenzintervalle der geschätzten Parameter berechnet. Als Datengrundlage werden Erdbeben mit Mw ≥ 6 seit dem Jahre 300 n. Chr. herangezogen. Das Untersuchungsgebiet erstreckt sich von 29.5° N bis 37° N und umfasst ein ca. 40 km breites Gebiet entlang der DSFZ. Aufgrund der seismotektonischen Situation im Untersuchungsgebiet wird zwischen einer südlichen, zentralen und nördlichen Subzone unterschieden. Dabei kann die südliche Subzone aus Mangel an Daten nicht für die Analysen herangezogen werden. Die Ergebnisse für die zentrale Subzone zeigen keinen signifikanten multimodalen Verlauf der Wiederholungsrate von Erdbeben. Des Weiteren ist kein signifikanter Unterschied zwischen den zeitabhängigen und dem zeitunabhängigem Modell zu verzeichnen. Da das zeitunabhängige Modell vergleichsweise einfach interpretierbar ist, wird die Wiederholungsrate von Erdbeben in dieser Subzone unter Annahme der Exponentialverteilungs-Hypothese abgeschätzt. Sie wird demnach als zeitunabhängig betrachtet und beträgt 9.72 * 10-3 Erdbeben (mit Mw ≥ 6) pro Jahr. Einen besonderen Fall stellt die nördliche Subzone dar. In diesem Gebiet tritt im Durchschnitt alle 51 Jahre ein massives Erdbeben (Mw ≥ 6) auf. Das letzte Erdbeben dieser Größe ereignete sich 1872 und liegt somit bereits 137 Jahre zurück. Somit ist in diesem Gebiet ein Erdbeben dieser Stärke überfällig. Im statistischen Mittel liegt die Zeit zwischen zwei Erdbeben zu 96% unter 137 Jahren. Zudem wird eine deutliche zeitliche Abhängigkeit der Erdbeben-Wiederauftretensrate durch die Ergebnisse der in der Arbeit neu entwickelten statistischen Verfahren bestätigt. Dabei ist festzustellen, dass die Wiederholungsrate insbesondere kurz nach einem Erdbeben eine sehr hohe zeitliche Abhängigkeit aufweist. Am besten repräsentiert werden die seismischen Bedingungen in der genannten Subzone durch ein bi-modales Weibull-Weibull-Modell. Die Wiederholungsrate ist eine glatte Zeitfunktion, welche zwei Häufungen von Datenpunkten in der Zeit nach dem Erdbeben zeigt. Dabei umfasst die erste Häufung einen Zeitraum von 80 Jahren, ausgehend vom Zeitpunkt des jeweiligen Bebens. Innerhalb dieser Zeitspanne ist die Wiederholungsrate extrem zeitabhängig. Die Wiederholungsrate direkt nach einem Beben ist sehr niedrig und steigert sich in den folgenden 10 Jahren erheblich bis zu einem Maximum von 0.024 Erdbeben/Jahr. Anschließend sinkt die Rate und erreicht ihr Minimum nach weiteren 70 Jahren mit 0.0145 Erdbeben/Jahr. An dieses Minimum schließt sich die zweite Häufung von Daten an, dessen Dauer abhängig von der Erdbebenwiederholungszeit ist. Innerhalb dieses Zeitfensters nimmt die Erdbeben-Wiederauftretensrate annähernd konstant um 0.015 Erdbeben/Jahr zu. Diese Ergebnisse bilden die Grundlage für eine zeitabhängige probabilistische seismische Gefährdungseinschätzung (PSHA) für die seismische Quellregion, die den nördlichen Raum der DSFZ umfasst. / The seismicity of the Dead Sea fault zone (DSFZ) during the last two millennia is characterized by a number of damaging and partly devastating earthquakes. These events pose a considerable seismic hazard and seismic risk to Syria, Lebanon, Palestine, Jordan, and Israel. The occurrence rates for large earthquakes along the DSFZ show indications to temporal changes in the long-term view. The aim of this thesis is to find out, if the occurrence rates of large earthquakes (Mw ≥ 6) in different parts of the DSFZ are time-dependent and how. The results are applied to probabilistic seismic hazard assessments (PSHA) in the DSFZ and neighboring areas. Therefore, four time-dependent statistical models (distributions), including Weibull, Gamma, Lognormal and Brownian Passage Time (BPT), are applied beside the exponential distribution (Poisson process) as the classical time-independent model. In order to make sure, if the earthquake occurrence rate follows a unimodal or a multimodal form, a nonparametric bootstrap test of multimodality has been done. A modified method of weighted Maximum Likelihood Estimation (MLE) is applied to estimate the parameters of the models. For the multimodal cases, an Expectation Maximization (EM) method is used in addition to the MLE method. The selection of the best model is done by two methods; the Bayesian Information Criterion (BIC) as well as a modified Kolmogorov-Smirnov goodness-of-fit test. Finally, the confidence intervals of the estimated parameters corresponding to the candidate models are calculated, using the bootstrap confidence sets. In this thesis, earthquakes with Mw ≥ 6 along the DSFZ, with a width of about 20 km and inside 29.5° ≤ latitude ≤ 37° are considered as the dataset. The completeness of this dataset is calculated since 300 A.D. The DSFZ has been divided into three sub zones; the southern, the central and the northern sub zone respectively. The central and the northern sub zones have been investigated but not the southern sub zone, because of the lack of sufficient data. The results of the thesis for the central part of the DSFZ show that the earthquake occurrence rate does not significantly pursue a multimodal form. There is also no considerable difference between the time-dependent and time-independent models. Since the time-independent model is easier to interpret, the earthquake occurrence rate in this sub zone has been estimated under the exponential distribution assumption (Poisson process) and will be considered as time-independent with the amount of 9.72 * 10-3 events/year. The northern part of the DSFZ is a special case, where the last earthquake has occurred in 1872 (about 137 years ago). However, the mean recurrence time of Mw ≥ 6 events in this area is about 51 years. Moreover, about 96 percent of the observed earthquake inter-event times (the time between two successive earthquakes) in the dataset regarding to this sub zone are smaller than 137 years. Therefore, it is a zone with an overdue earthquake. The results for this sub zone verify that the earthquake occurrence rate is strongly time-dependent, especially shortly after an earthquake occurrence. A bimodal Weibull-Weibull model has been selected as the best fit for this sub zone. The earthquake occurrence rate, corresponding to the selected model, is a smooth function of time and reveals two clusters within the time after an earthquake occurrence. The first cluster begins right after an earthquake occurrence, lasts about 80 years, and is explicitly time-dependent. The occurrence rate, regarding to this cluster, is considerably lower right after an earthquake occurrence, increases strongly during the following ten years and reaches its maximum about 0.024 events/year, then decreases over the next 70 years to its minimum about 0.0145 events/year. The second cluster begins 80 years after an earthquake occurrence and lasts until the next earthquake occurs. The earthquake occurrence rate, corresponding to this cluster, increases extremely slowly, such as it can be considered as an almost constant rate about 0.015 events/year. The results are applied to calculate the time-dependent PSHA in the northern part of the DSFZ and neighbouring areas.
115

Multiple Time Scales and Longitudinal Measurements in Event History Analysis

Danardono, January 2005 (has links)
A general time-to-event data analysis known as event history analysis is considered. The focus is on the analysis of time-to-event data using Cox's regression model when the time to the event may be measured from different origins giving several observable time scales and when longitudinal measurements are involved. For the multiple time scales problem, procedures to choose a basic time scale in Cox's regression model are proposed. The connections between piecewise constant hazards, time-dependent covariates and time-dependent strata in the dual time scales are discussed. For the longitudinal measurements problem, four methods known in the literature together with two proposed methods are compared. All quantitative comparisons are performed by means of simulations. Applications to the analysis of infant mortality, morbidity, and growth are provided.
116

Theoretical investigation of the first-order hyperpolarizability in the two-photon resonant region / Teoretisk undersökning av andra ordningens susceptibilitet i det tvåfotonresonanta området

Bergstedt, Mikael January 2007 (has links)
Time-dependent density functional theory calculations have been carried out to determine the complex first-order hyperpolarizability in the two-photon resonance region of the molecule IDS-Cab. Calculations show that three strongly absorbing states, in the ultraviolet region, are separated to the extent that no significant interference of the imaginary parts of the tensor elements of the first-order hyper-polarizability occurs. Consequently, and in contrast to experimental findings [27], no reduced imaginary parts of the first-order hyperpolarizability in the two-photon resonant region can be seen.
117

Time Dependent Deformations in Normal And Heavy Density Concrete

Reddy, D Harinadha 06 1900 (has links)
Time dependent deformations in concrete, both creep and shrinkage, play a critical role in prestressed concrete structures, such as bridge girders, nuclear containment vessels, etc. These strains result in lossess, through release of prestress, and thereby influence the safety of these structures. The present study comprises of an experimental and analytical program to assess the levels of creep and shrinkage in normal and heavy density concrete. The experimental program includes tests on creep using standard cylinder specimen, while shrinkage studies have been conducted using prism specimen, both under controlled environmental conditions. The experimental results suggest that creep and shrinkage strains are higher in heavy density concrete than in normal concrete. This may be attributed to the relatively smaller pore structure of heavy density concrete, that results in larger availability of free water and a relatively slower hydration process in comparison to normal concrete. While there is some scatter in the results, creep strains decrease with age of loading and both creep and shrinkage strains are smaller when the relative humidity is higher. Statistical model reported in the literature for normal concrete is able to predict the test results for both normal and heavy density concrete quite well. Long term predictions of creep and shrinkage using this model, accounting for uncertainties, is also projected and shown to predict some long term measured results not used in the model calibration. The long term predictions are sensitive to the initial data used in model calibration.
118

Planning Container Drayage Operations at Congested Seaports

Namboothiri, Rajeev 19 May 2006 (has links)
This dissertation considers daily operations management for a fleet of trucks providing container pickup and delivery service to a port. Truck congestion at access points for ports may lead to serious inefficiencies in drayage operations, and the resultant cost impact to the intermodal supply chain can be significant. Recognizing that port congestion is likely to continue to be a major problem for drayage operations given the growing volume of international containerized trade, this research seeks to develop optimization approaches for maximizing the productivity of drayage firms operating at congested seaports. Specifically, this dissertation addresses two daily drayage routing and scheduling problems. In the first half of this dissertation, we study the problem of managing a fleet of trucks providing container pickup and delivery service to a port facility that experiences different access wait times depending on the time of day. For this research, we assume that the wait time can be estimated by a deterministic function. We develop a time-constrained routing and scheduling model for the problem that incorporates the time-dependent congestion delay function. The model objective is to find routes and schedules for drayage vehicles with minimum total travel time, including the waiting time at the entry to the port due to congestion. We consider both exact and heuristic solution approaches for this difficult optimization problem. Finally, we use the framework to develop an understanding of the potential impact of congestion delays on drayage operations, and the value of planning with accurate delay information. In the second half of this dissertation, we study methods for managing a drayage fleet serving a port with an appointment-based access control system. Responding to growing access congestion and its resultant impacts, many U.S. port terminals have implemented appointment systems, but little is known about the impact of such systems on drayage productivity. To address this knowledge gap, we develop a drayage operations optimization approach based on a column generation integer programming heuristic that explicitly models a time-slot port access control system. The approach determines pickup and delivery sequences with minimum transportation cost. We use the framework to develop an understanding of the potential efficiency impacts of access appointment systems on drayage operations. Findings indicate that the set of feasible drayage tasks and the fleet size required to complete them can be quite sensitive to small changes in time-slot access capacities at the port.
119

Modeling Time-dependent Responses of Piezoelectric Fiber Composite

Li, Kuo-An 2009 December 1900 (has links)
The existence of polymer constituent in piezoelectric fiber composites (PFCs) could lead to significant viscoelastic behaviors, affecting overall performance of PFCs. High mechanical and electrical stimuli often generate significant amount of heat, increasing temperatures of the PFCs. At elevated temperatures, most materials, especially polymers show pronounced time-dependent behaviors. Predicting time-dependent responses of the PFCs becomes important to improve reliability in using PFCs. We study overall performance of PFCs having unidirectional piezoceramic fibers, such as PZT fibers, dispersed in viscoelastic polymer matrix. Two types of PFCs are studied, which are active fiber composites (AFCs) and macro fiber composites (MFCs). AFCs and MFCs consist of unidirectional PZT fibers dispersed in epoxy placed between two interdigitated electrode and kapton layers. The AFCs have a circular fiber cross-section while the MFCs have a square fiber cross-section. Finite element (FE) models of representative volume elements (RVEs) of active PFCs, having square and circular fiber cross-sections, are generated for composites with 20, 40, and 60 percent fiber contents. Two FE micromechanical models having one fiber embedded in epoxy matrix and five fibers placed in epoxy matrix are considered. A continuum 3D piezoelectric element in ABAQUS FE is used. A general time-integral function is applied for the mechanical, electrical, and piezoelectric properties in order to incorporate the time-dependent effect and histories of loadings. The effective properties of PZT-5A/epoxy and PZT-7A/LaRC-SI piezocomposites determined from the FE micromechanical models are compared to available experimental data and analytical solutions in the literature. Furthermore, the effect of viscoelastic behaviors of the LaRC-SI matrix at an elevated temperature on the overall electro-mechanical and piezoelectric constants are examined.
120

A coarse-mesh transport method for time-dependent reactor problems

Pounders, Justin Michael 06 April 2010 (has links)
A new solution technique is derived for the time-dependent transport equation. This approach extends the steady-state coarse-mesh transport method that is based on global-local decompositions of large (i.e. full-core) neutron transport problems. The new method is based on polynomial expansions of the space, angle and time variables in a response-based formulation of the transport equation. The local problem (coarse mesh) solutions, which are entirely decoupled from each other, are characterized by space-, angle- and time-dependent response functions. These response functions are, in turn, used to couple an arbitrary sequence of local problems to form the solution of a much larger global problem. In the current work, the local problem (response function) computations are performed using the Monte Carlo method, while the global (coupling) problem is solved deterministically. The spatial coupling is performed by orthogonal polynomial expansions of the partial currents on the local problem surfaces, and similarly, the timedependent response of the system (i.e. the time-varying flux) is computed by convolving the time-dependent surface partial currents and time-dependent volumetric sources against pre-computed time-dependent response kernels.

Page generated in 0.0527 seconds