Showing papers by "National Technical University of Athens published in 2006"

Search for neutral MSSM Higgs bosons at LEP

Abstract: The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have searched for the neutral Higgs bosons which are predicted by the Minimal Supersymmetric standard model (MSSM). The data of the four collaborations are statistically combined and examined for their consistency with the background hypothesis and with a possible Higgs boson signal. The combined LEP data show no significant excess of events which would indicate the production of Higgs bosons. The search results are used to set upper bounds on the cross-sections of various Higgs-like event topologies. The results are interpreted within the MSSM in a number of “benchmark” models, including CP-conserving and CP-violating scenarios. These interpretations lead in all cases to large exclusions in the MSSM parameter space. Absolute limits are set on the parameter cosβ and, in some scenarios, on the masses of neutral Higgs bosons.

Competency based management: a review of systems and approaches

Abstract: Purpose – Aims to review the key concepts of competency management (CM) and to propose method for developing competency method.Design/methodology/approach – Examines the CM features of 22 CM systems and 18 learning management systems.Findings – Finds that the areas of open standard (XML, web services, RDF), semantic technologies (ontologies and the semantic web) and portals with self‐service technologies are going to play a significant part in the evolution of CM systems.Originality/value – Emphasizes the beneficial attributes of CM for private and public organizations.

Microgrids - Large Scale Integration of Microgeneration to Low Voltage Grids

Abstract: Microgrids comprise Low Voltage distribution systems with distributed energy sources, such as micro-turbines, fuel cells, PVs, etc., together with storage devices, i.e. flywheels, energy capacitors and batteries, and controllable loads, offering considerable control capabilities over the network operation. These systems are interconnected to the Medium Voltage Distribution network, but they can be also operated isolated from the main grid, in case of faults in the upstream network. From the customer point of view, Microgrids provide both thermal and electricity needs, and in addition enhance local reliability, reduce emissions, improve power quality by supporting voltage and reducing voltage dips, and potentially lower costs of energy supply. This paper outlines selected research findings of the EU funded MICROGRIDS project (Contract ENK-CT-2002-00610). These include: • Development and enhancement of Microsource controllers to support frequency and voltage based on droops. Application of software agents for secondary control. • Development of the Microgrid Central Controller (MGCC). Economic Scheduling functions have been developed and integrated in a software package able to simulate the capabilities of the MGCC to place bids to the market operator under various policies and to evaluate the resulting environmental benefits. • Analysis of the communication requirements of the Microgrids control architecture • Investigation of alternative market designs for trading energy and ancillary services within a Microgrid. Development of methods for the quantification of reliability and loss reduction. • Initial measurements from an actual LV installation.

Topological Analysis of Linear Polymer Melts: A Statistical Approach

Abstract: We introduce an algorithm for the reduction of a computer generated atomistic polymer sample to an entanglement network of primitive paths. These networks are structural representations of the topology underlying a polymer melt. By examining network ensembles of polyethylene and cis-1,4-polybutadiene melts, we provide topological measures and statistical properties of primitive paths. We present the radial distribution function of entanglements and the distribution of the number of monomers between entanglements. A renewal point process that generates entanglement events along the monomer sequence of a chain is found to describe the statistics of detected topological constraints. We discuss chain thickness effects on topological measures and provide a method for detecting persistent chain contacts in melt configurations. A suitable scaling of acquired data leads to a unifying microscopic topological description of the melts studied.

Elastic J/psi production at HERA

Abstract: Cross sections for elastic production of J/Psi mesons in photoproduction and electroproduction are measured in electron proton collisions at HERA using an integrated luminosity of 55 pb^{-1}. Results are presented for photon virtualities Q^2 up to 80 GeV^2. The dependence on the photon-proton centre of mass energy W_{gamma p} is analysed in the range 40 < \Wgp < 305 GeV in photoproduction and 40 < \Wgp < 160 GeV in electroproduction. The \Wgp dependences of the cross sections do not change significantly with Q^2 and can be described by models based on perturbative QCD. Within such models, the data show a high sensitivity to the gluon density of the proton in the domain of low Bjorken x and low Q^2. Differential cross sections d\sigma/dt, where t is the squared four-momentum transfer at the proton vertex, are measured in the range |t|<1.2 GeV^2 as functions of \Wgp and Q^2. Effective Pomeron trajectories are determined for photoproduction and electroproduction. The J/Psi production and decay angular distributions are consistent with s-channel helicity conservation. The ratio of the cross sections for longitudinally and transversely polarised photons is measured as a function of Q^2 and is found to be described by perturbative QCD based models.

STIR: Software for Tomographic Image Reconstruction Release 2

Abstract: We present an update to STIR, an Open Source object-oriented library in C++ for 3D PET reconstruction. This library has been designed so that it can be used for many algorithms and scanner geometries, while being portable to various computing platforms. This second release enhances its flexibility and modular design, but also adds extra capabilities such as list mode reconstruction, more data formats etc.

Hardening effect induced by incorporation of SiC particles in nickel electrodeposits

Abstract: Pure Ni and nickel matrix composite electrocoatings containing micron- and nano-SiC particles (1 μm and 20 nm respectively) were produced under direct and pulse current conditions from an additive-free Watts type bath. The effect of the particle size, codeposition percentage of SiC and type of imposed current on the microhardness as well as on the microstructure of the electrodeposits were investigated. Ni/SiC composite deposits prepared under either direct or pulse current conditions exhibited a considerable strengthening effect with respect to pure Ni coatings. The improved hardness of composite coatings was associated to specific structural modifications of Ni crystallites provoked by the adsorption of H+ on the surface of SiC particles, thus leading to a (211) texture mode of Ni crystal growth. Pulse electrodeposition significantly improved the hardness of the Ni/SiC composite coatings, especially at low duty cycles, in which grain refinement and higher SiC incorporation (vol. %) was achieved. The enhanced hardness of Ni/nano-SiC deposits, as compared to Ni/micron-SiC composites, was attributed to the increasing values of the number density of embedded SiC particles in the nickel matrix with decreasing particle size. In addition, the observed hardening effects of the SiC particles might be associated to the different embedding mechanisms of the particles, which could be characterized as inter-crystalline for micron-SiC and partially intra-crystalline for nano-SiC particles.

SANICLAY : simple anisotropic clay plasticity model

Abstract: SANICLAY is a new simple anisotropic clay plasticity model that builds on a modification of an earlier model with an associated flow rule, in order to include simulations of softening response under undrained compression following Ko consolidation. Non-associativity is introduced by adopting a yield surface different than the plastic potential surface. Besides, the isotropic hardening of the yield surface both surfaces evolve according to a combined distortional and rotational hardening rule, simulating the evolving anisotropy. Although built on the general premises of critical state soil mechanics, the model induces a critical state line in the void ratio–mean effective stress space, which is a function of anisotropy. To ease interpretation, the model formulation is presented firstly in the triaxial stress space and subsequently, its multiaxial generalization is developed systematically, in a form appropriate for implementation in numerical codes. The SANICLAY is shown to provide successful simulation of both undrained and drained rate-independent behaviour of normally consolidated sensitive clays, and to a satisfactory degree of accuracy of overconsolidated clays. The new model requires merely three constants more than those of the modified Cam clay model, all of which are easily calibrated from well-established laboratory tests following a meticulously presented procedure. Copyright © 2006 John Wiley & Sons, Ltd.

Sampling Trajectory Streams with Spatiotemporal Criteria

Abstract: Monitoring movement of high-dimensional points is essential for environmental databases, geospatial applications, and biodiversity informatics as it reveals crucial information about data evolution, provenance detection, pattern matching etc. Despite recent research interest on processing continuous queries in the context of spatiotemporal data streams, the main focus is on managing the current location of numerous moving objects. In this paper, we turn our attention onto a historical perspective of movement and examine trajectories generated by streaming positional updates. The key challenge is how to maintain a concise, yet quite reliable summary of each object’s movement, avoiding any superfluous details and saving in processing complexity and communication cost. We propose two single-pass approximation techniques based on sampling that take advantage of the spatial locality and temporal timeliness inherent in trajectory streams. As a means of reducing substantially the scale of the datasets, we utilize heuristic prediction to distinguish which locations to preserve in the compressed trajectories. A comprehensive experimental study verifies the stability and robustness of the proposed techniques and demonstrates that intelligent compression schemes are able to act as effective load shedding operators achieving remarkable results.

4 – Adsorption and Ion Exchange

Abstract: Publisher Summary This chapter deals with the phenomena of adsorption and ion exchange, and highlights their role in environmental protection. Activated carbon, silica gel, and alumina are the most popular adsorbent materials in industry due to the fact that they provide large surface areas per unit weight. Activated carbon is produced from coconut shell, wood, and bone, whereas silica gel is made of hydrated silicon dioxide. Alumina can be either mined or produced by precipitated aluminum oxide and hydroxide. There are a number of cases where carbon adsorption is preferably used in water treatment—if compounds are not compliant for biodegradation, if the molecule contains branched chairs, has a large molecular weight, and low polarity, and generally, in the case of removing organic and inorganic species, for concentrations lower than 5000 and 1000 mg/L, respectively. Besides the applications of water treatment, activated carbon is called into action for the removal of volatile organic compounds, such as solvents, toxic gases, and odors from gaseous waste streams. Ion exchange shares many characteristics with adsorption, such as mass transfer from the fluid to the solid phase; there are, however, some significant differences. In ion exchange, the ions removed from the liquid phase are replaced by ions from the solid phase. So, there actually occurs an exchange of ions and not only a removal in the latter process. This principle is the basis of use of ion exchange in removing the effects of pollution in air and water.

Sustainable construction and drivers of change in Greece: a Delphi study

Abstract: During the last decade, sustainable development issues have been gradually adopted in Greece's construction industry, changing the traditional methods and technology. The aim of this investigation is to identify potential drivers of change, to position them properly in the wider context of sustainable construction and to outline the progress to be expected in the coming decade. The methodological framework used is a Delphi technique based on a questionnaire of 20 experts (consultant engineers, construction managers and contractors). The results show that the most important influences on sustainable construction in Greece are energy conservation measures, resource conservation strategies and waste reduction. In terms of the initiatives expected in the coming decade, energy conservation measures, waste reduction measures, as well as product innovation and certification are expected to rank high. In conclusion, the results are consistent with those of previous researchers showing significant trends towards l...

Dietary patterns and their socio-demographic determinants in 10 European countries: data from the DAFNE databank

Abstract: To describe the dietary patterns of 10 European countries and their socio-demographic determinants, using the comparable between-countries DAFNE data. Analysis of standardized and postharmonized data collected through the national household budget surveys. Nationally representative surveys undertaken in 10 European countries, generally in the second half of the 1990s. The differences in the fruit and vegetable consumption previously identified between Mediterranean and Northern European countries seem to be leveling out, particularly in relation to fruit consumption. Pulses, however, still characterize the diet of the Mediterraneans. Straying from their traditional food choices, Mediterraneans recorded high availability of unprocessed red meat, while Central and Northern Europeans preferably consumed meat products. The household availability of beverages (alcoholic and non-alcoholic) is generally higher among Central and Northern European populations. Principal component (PC) analysis led to the identification of two dietary patterns in each of the 10 countries. The first was similar in all countries and indicated ‘wide-range’ food buyers. The second was slightly more varied and described ‘beverage and convenience’ food buyers. PC1 was common among households of retired and elderly members, while PC2 was common among households located in urban or semi-urban areas and among adult Scandinavians living alone. The dietary patterns identified point towards a progressive narrowing of dietary differences between North and South European countries. The comparable between-countries DAFNE data could prove useful in ecological studies, in the formulation of dietary guidelines and public health initiatives addressing specific population groups. European Commission.