Self-Evaluation Applied Mathematics 2003-2008 University of Twente

This report contains the self-study for the research assessment of the Department of Applied Mathematics (AM) of the Faculty of Electrical Engineering, Mathematics and Computer Science (EEMCS) at the University of Twente (UT). The report provides the information for the Research Assessment Committee for Applied Mathematics, dealing with mathematical sciences at the three universities of technology in the Netherlands. It describes the state of affairs pertaining to the period 1 January 2003 to 31 December 2008

TOW ARDS NEW TECHNIQUES IN TELECOMMUNICATIONS TO SERVE LARGE USER POPULATIONS

An account is given in this paper of the industry oriented research at the Faculty of Electrical Engineering in the past five years in the field of Public Telecommunications and Telematics. Actual and realistically anticipated needs of the users are briefly surveyed at the outset. Facts and views, concerning specific projects and underlying methodologies, are considered. The paper was presented at a symposium, held at the Faculty of Electrical Engineering, April 19 and 20, 1983 as part of the bicentennary events at the Technical University of Budapest

Periodic patterns in human mobility

The recent rise of services and networks that rely on human mobility has prompted the need for tools that detect our patterns of visits to locations and encounters with other individuals. The widespread popularity of location- and encounter-aware mobile phones has given us a wealth of empirical mobility data and enabled many novel applications that benefit from automated detection of an individual’s mobility patterns. This thesis explores the presence and character of periodic patterns in the visits and encounters of human individuals. Novel tools for extracting and analysing periodic mobility patterns are proposed and evaluated on real-world data. We investigate these patterns in a range of datasets, including visits to public transport stations on a metropolitan scale, university campus WLAN access point transitions, online location-sharing service checkins, and Bluetooth encounters among university students. The methods developed in this thesis are designed for decentralised implementation to enable their real-world deployment. Analysing an individual’s visit and encounter events is a challenging problem since the data are often highly sparse. In order to study visit patterns we propose a novel inter-event interval (IEI) analysis approach, which is inspired by neural coding techniques. The resulting measure, IEI-irregularity, quantifies the weekly periodic patterns of an individual’s visits to a location. To detect encounter patterns we propose and compare methods based on IEI analysis and periodic subgraph mining. In particular, we introduce the novel concept of a periodic encounter community; that is, a collection of individuals that share the same periodic encounter pattern. The decentralised algorithms we develop for periodic encounter community detection are of particular relevance to human-based opportunistic communication networks. We explore these communities in terms of their opportunistic content sharing performance. Our findings show that periodic patterns are a prominent feature of human mobility and that these patterns are algorithmically detectabl

Thermal characterisation and reliability analysis of power electronic devices in wind and solar energy systems

Power electronic converters (PECs) are used for conditioning the flow of energy between renewable energy applications and grid or stand-alone connected loads. Insulated gate bipolar transistors (IGBTs) are critical components used as switching devices in PECs. IGBTs are multi-layered devices made of different coefficient of thermal expansion (CTE) based materials. In wind and solar energy applications, IGBT’s reliability is highly influenced by the operating conditions such as variable wind speed and solar irradiance. Power losses occur in switching transient of high current/voltage which causes temperature fluctuations among the layers of the IGBTs. This is the main stress mechanism which accelerates deterioration and eventual failures among IGBT layers due to the dissimilar CTEs. Therefore, proper thermal monitoring is essential for accurate estimation of PECs reliability and end lifetime. Several thermal models have been proposed in literature, which are not capable of representing accurate temperature profiles among multichip IGBTs. These models are mostly derived from offline modelling approaches which cannot take operating conditions and control mechanisms of PECs into account and unable to represent actual heat path among each chip. This research offers an accurate and powerful electro thermal and reliability monitoring tool for such devices. Three-dimensional finite element (FE) IGBT models are implemented using COMSOL, by considering complex heat interactions among each layer. Based on the obtained thermal characteristics, electro thermal and thermo mechanical models were developed in SIMULINK to determine the thermal behaviour of each layer and provide total lifetime consumption analysis. The developed models were verified by real-time (RT) experiments using dSPACE environment. New materials, such as silicon carbide (SiC) devices, were found to exhibit approximately 20°C less thermal profile compared to conventional silicon IGBTs. For PECs used within wind energy systems, PECs driving algorithms were derived within the proposed models and by adjusting switching frequency PECs cycling temperatures were reduced by 12°C which led to a significant reduction in thermal stress; approximately 27 MPa. Total life consumption for the proposed method was calculated as 3.26x10-5 which is approximately 1x10-5 less compared to the other both methods. Effects of maximum power tracking algorithms, used in photovoltaic solar systems, on thermal stress were also explored. The converter’s thermal cycling was found approximately 3 °C higher with the IC algorithm. The steady state temperature was 52.7°C for the IC while it was 42.6 °C for P&O. In conclusion, IC algorithm offers more accurate tracking accuracy; however, this is on the expense of harsher thermal stress which has led to approximately 1.4 times of life consumption compared to P&O under specific operating conditions

Electron recombination studies of rotationally cold CH⁺ ions at the Cryogenic Storage Ring

The present work is concerned with an experimental study of the Dissociative Recombination (DR) process for rotationally cold CH+ ions. Experiments were conducted at the electron cooler of the Cryogenic Storage Ring (CSR) facility, a merged-electronion- beams setup. By storing CH+ ions up to 100 s in the T < 10K CSR environment, an ensemble of the lowest three rotational states was produced and time-dependent populations were additionally characterized via photodissociation. With the methods developed in this work, the DR cross section is obtained in the collision energy range of 0 − 1.3 eV as a merged-beams rate coefficient and converted into a plasma rate coefficient, which can be used for modeling the chemistry in interstellar clouds. A confinement for the pure J = 0 plasma rate is provided for this purpose, resulting from time-dependent measurements on the continually cooling rotational ensemble in CSR. Imaging and arrival time detection of DR fragments are used for characterizing final atomic carbon and hydrogen states and angular fragment distributions. Here, collision energies of 0 meV, 8 meV, 70 meV and 320 meV are investigated. As a result, conclusions about the symmetry of intermediate dissociative states of the CH molecule, participating in the DR reaction, can be drawn