Showing papers by "National Technical University of Athens published in 2007"

Regional Drought Assessment Based on the Reconnaissance Drought Index (RDI)

Abstract: Regional drought assessment is conventionally based on drought indices for the identification of drought intensity, duration and areal extent. In this study, a new index, the Reconnaissance Drought Index (RDI) is proposed together with the well known Standardized Precipitation Index (SPI) and the method of deciles. The new index exhibits significant advantages over the other indices by including apart from precipitation, an additional meteorological parameter, the potential evapotranspiration. The drought assessment is achieved using the above indices in two river basins, namely Mornos and Nestos basins in Greece. It is concluded that although the RDI generally responds in a similar fashion to the SPI (and to a lesser extent to the deciles), it is more sensitive and suitable in cases of a changing environment.

Real-World MicroGrids-An Overview

Abstract: Microgrids are networks of small, distributed electrical power generators operated as a collective unit - a system of energy systems. The range of hardware and control options for Microgrid operation are reviewed. The paper summarizes and highlights the operating principles and key conclusions of research and field trials to-date. An overview is given on demonstration projects for Microgrids which have been, and are being, constructed.

Dijet cross sections and parton densities in diffractive DIS at HERA

Abstract: Differential dijet cross sections in diffractive deep-inelastic scattering are measured with the H1 detector at HERA using an integrated luminosity of 51.5 pb−1. The selected events are of the type ep → eXY , where the system X contains at least two jets and is well separated in rapidity from the low mass proton dissociation system Y . The dijet data are compared with QCD predictions at next-to-leading order based on diffractive parton distribution functions previously extracted from measurements of inclusive diffractive deepinelastic scattering. The prediction describes the dijet data well at low and intermediate zIP (the fraction of the momentum of the diffractive exchange carried by the parton entering the hard interaction) where the gluon density is well determined from the inclusive diffractive data, supporting QCD factorisation. A new set of diffractive parton distribution functions is obtained through a simultaneous fit to the diffractive inclusive and dijet cross sections. This allows for a precise determination of both the diffractive quark and gluon distributions in the range 0.05 < zIP < 0.9. In particular, the precision on the gluon density at high momentum fractions is improved compared to previous extractions.

Statistical analysis of hydroclimatic time series: Uncertainty and insights

Abstract: [1] Today, hydrologic research and modeling depends largely on climatological inputs, whose physical and statistical behavior are the subject of many debates in the scientific community. A relevant ongoing discussion is focused on long-term persistence (LTP), a natural behavior identified in several studies of instrumental and proxy hydroclimatic time series, which, nevertheless, is neglected in some climatological studies. LTP may reflect a long-term variability of several factors and thus can support a more complete physical understanding and uncertainty characterization of climate. The implications of LTP in hydroclimatic research, especially in statistical questions and problems, may be substantial but appear to be not fully understood or recognized. To offer insights on these implications, we demonstrate by using analytical methods that the characteristics of temperature series, which appear to be compatible with the LTP hypothesis, imply a dramatic increase of uncertainty in statistical estimation and reduction of significance in statistical testing, in comparison with classical statistics. Therefore we maintain that statistical analysis in hydroclimatic research should be revisited in order not to derive misleading results and simultaneously that merely statistical arguments do not suffice to verify or falsify the LTP (or another) climatic hypothesis.

Multi-criteria decision analysis and cost–benefit analysis of alternative scenarios for the power generation sector in Greece

Abstract: This paper examines four mutually exclusive scenarios for the expansion of the Greek electricity system developed by official authorities and representing alternative views on meeting electricity demand. The aim is to encompass all positive and negative side-effects characterizing the electricity generation technologies assumed to participate in each scenario and emphasis is given to the particular role of renewable energy sources which represent a major differentiating factor between them. The calculation of economic, technical and environmental performances of the examined scenarios for the year 2010 shows that electricity planning is a complicated task since improvements in one policy target are accompanied by losses in others. In order to resolve this conflict, the scenarios are comparatively evaluated with two decision support techniques, multicriteria decision analysis and cost–benefit analysis, which are capable of broadening the strict boundaries of a financial analysis while avoiding intuitive solutions that are often applied in practice. Following the two completely different evaluation approaches, it is confirmed that the scenario assuming the highest penetration of renewable energy sources is the best compromise configuration for the Greek power generation sector.

Extended Kalman filtering for fuzzy modelling and multi-sensor fusion

Abstract: Extended Kalman Filtering (EKF) is proposed for: (i) the extraction of a fuzzy model from numerical data; and (ii) the localization of an autonomous vehicle. In the first case, the EKF algorithm is...

Characterization of Flavonoid Subgroups and Hydroxy Substitution by HPLC-MS/MS

Abstract: HPLC-DAD coupled with mass spectrometry in the positive ionization mode was applied to study the fragmentation of twelve selected flavonoids. Compounds belonging to all the major subgroups found in common plants, i.e. flavonols, flavones, dihydroflavonols, flavanones and flavanols were studied. Compound standards were injected into the spectrometer and produced characteristic mass spectra. The fragmentation of each compound was studied and it was shown that the dehydration and carbon monoxide losses from the [M+H]+ ion by the members of each subgroup produced specific fragments, thus allowing the characterization of the flavonoid subgroups. Moreover, fragments resulting from fission of the C-rings are specific of each subgroup and revealed the substitution pattern of A- and B-rings. In order to verify the identifying efficiency of the positive ionization mode through these characteristic fragmentations, the unknown flavonoids of an Origanum vulgare diethyl ether extract were separated with the HPLC system and the major peaks were successfully identified with the mass spectrometer.

Toroidal Momentum Pinch Velocity due to the Coriolis Drift Effect on Small Scale Instabilities in a Toroidal Plasma

Abstract: In this Letter, the influence of the "Coriolis drift" on small scale instabilities in toroidal plasmas is shown to generate a toroidal momentum pinch velocity. Such a pinch results because the Coriolis drift generates a coupling between the density and temperature perturbations on the one hand and the perturbed parallel flow velocity on the other. A simple fluid model is used to highlight the physics mechanism and gyro-kinetic calculations are performed to accurately assess the magnitude of the pinch. The derived pinch velocity leads to a radial gradient of the toroidal velocity profile even in the absence of a torque on the plasma and is predicted to generate a peaking of the toroidal velocity profile similar to the peaking of the density profile. Finally, the pinch also affects the interpretation of current experiments.

Dissolution of aluminosilicate minerals and by-products in alkaline media

Abstract: In this work, the leaching behaviour of six aluminosilicate industrial minerals and by-products (kaolin, metakaolin, fly ash, natural pozzolana from Milos, zeolite and furnace slag) in alkaline solutions is investigated The variables studied are the kind of alkali metal (K, Na), the concentration of the alkaline solution (25, 5 and 10 M) and the time of dissolution (5, 10 and 24 h) The solid residue after the leaching was examined by means of XRD and FTIR As it is concluded, the leaching ability of Al and Si is according to the following descending order: Metakaolin > Zeolite > Slag > Fly Ash > Pozzolana > Kaolin The extent of dissolution is higher in NaOH than in KOH solutions, especially when the most reactive materials are concerned Finally, Si and Al seem to have a synchronized leaching behaviour in both alkaline solutions

Glass transition and segmental dynamics in poly(dimethylsiloxane)/silica nanocomposites studied by various techniques

Abstract: We report new results on segmental dynamics and glass transition in a series of poly(dimethylsiloxane) networks filled with silica nanoparticles prepared by sol-gel techniques, obtained by differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC), broadband dielectric relaxation spectroscopy (DRS) and dynamic mechanical analysis (DMA). The nanocomposites are characterized by a fine dispersion of 10 nm silica particles and hydrogen bonding polymer/filler interactions. The first three techniques indicate, in agreement with each other, that a fraction of polymer in an interfacial layer around the silica particles with a thickness of 2–3 nm shows modified dynamics. The DSC data, in particular measurements of heat capacity jump at Tg, are analyzed in terms of immobilized polymer in the interfacial layer. The dielectric TSDC and DRS data are analyzed in terms of slower dynamics in the interfacial layer as compared to bulk dynamics. We employ a special version of TSDC, the so-called thermal sampling (TS) technique, and provide experimental evidence for a continuous distribution of glass transition temperatures (Tg) and molecular mobility of the polymer in the interfacial layer, which is consistent with the DRS data. Finally, DMA results show a moderate slowing down of segmental dynamics of the whole polymer matrix (increase of glass transition temperature by about 10 K as compared to the pure matrix).

Coarse-Grained and Reverse-Mapped United-Atom Simulations of Long-Chain Atactic Polystyrene Melts: Structure, Thermodynamic Properties, Chain Conformation, and Entanglements

Abstract: A coarse-grained model of atactic polystyrene, in which meso and racemo diads are represented as single “superatoms,” parametrized using Iterative Boltzmann Inversion, has been subjected to connectivity-altering Monte Carlo simulations in order to simulate monodisperse atactic polystyrene melts of molar mass up to 210000 g mol-1 at 500 or 413 K and 1 bar. Analysis of the Monte Carlo results reveals excellent equilibration of chain conformations at all length scales. Chain dimensions, as determined from the mean square end-to-end distance, the mean square radius of gyration, and simulated Kratky plots of the single-chain scattering function, are in excellent agreement with experiment. The equilibrated long-chain configurations are reduced to entanglement networks via topological analysis with the CReTA algorithm. The resulting Kuhn length of primitive paths provides an excellent estimate of the molar mass between entanglements and of the entanglement tube diameter extracted from plateau modulus measurement...

Accuracy of pavement thicknesses estimation using different ground penetrating radar analysis approaches

Abstract: The Ground Penetrating Radar (GPR) is a Non-destructive Testing (NDT) technique, which is able to capture continuous pavement layer thicknesses data. The radar pulse propagation is a major factor for gathering the dielectric constants of pavement materials accurately. The present research focuses on the accuracy of pavement asphalt layer thicknesses estimation using GPR data analysis by employing different estimation approaches based on material dielectric properties. A comprehensive comparative analysis of GPR and asphalt-drilled cores data incorporating a variety of pavements is performed. The major findings of the detailed comparative analysis including the effectiveness of the approaches used are reported and discussed in the present paper.

Multimodal emotion recognition from expressive faces, body gestures and speech

Abstract: In this paper we present a multimodal approach for the recognition of eight emotions that integrates information from facial expressions, body movement and gestures and speech. We trained and tested a model with a Bayesian classifier, using a multimodal corpus with eight emotions and ten subjects. First individual classifiers were trained for each modality. Then data were fused at the feature level and the decision level. Fusing multimodal data increased very much the recognition rates in comparison with the unimodal systems: the multimodal approach gave an improvement of more than 10% with respect to the most successful unimodal system. Further, the fusion performed at the feature level showed better results than the one performed at the decision level.

Using the Hough transform to segment ultrasound images of longitudinal and transverse sections of the carotid artery.

Abstract: Automatic segmentation of the arterial lumen from ultrasound images is an important task in clinical diagnosis. In this paper, the Hough transform (HT) was used to automatically extract straight lines and circles from sequences of B-mode ultrasound images of longitudinal and transverse sections, respectively, of the carotid artery. In 10 normal subjects, the specificity and accuracy of HT-based segmentation were on average higher than 0.96 for both sections, whereas the sensitivity was higher than 0.96 in longitudinal and higher than 0.82 in transverse sections. The intima-media thickness (IMT) was also estimated from images of longitudinal sections; the corresponding validation parameters were generally higher than 0.90. To further validate the results, arterial distension waveforms (ADW) were estimated from sequences of images using the HT technique as well as motion analysis using block matching (BM). In longitudinal sections, diastolic and systolic diameters and relative diameter changes using HT and BM were not significantly different. In transverse sections, diastolic and systolic diameters were significantly lower using the HT technique; the differences were <7%. Relative diameter changes in transverse sections were not significantly different from BM-estimated ones. The HT technique was also applied to four subjects with atherosclerosis, in which sensitivity, specificity and accuracy were comparable to those of normal subjects; the low values of sensitivity in transverse sections may reflect departure from the circular model because of the presence of plaque. In conclusion, the HT technique provides a reliable way to segment ultrasound images of the carotid artery and can be used in clinical practice to estimate indices of arterial wall physiology, such as the IMT and the ADW.

Targeted energy transfers in vibro-impact oscillators for seismic mitigation

Abstract: In the field of seismic protection of structures, it is crucial to be able to diminish ‘as much as possible’ and dissipate ‘as fast as possible’ the load induced by seismic (vibration-shock) energy imparted to a structure by an earthquake. In this context, the concept of passive nonlinear energy pumping appears to be natural for application to seismic mitigation. Hence, the overall problem discussed in this paper can be formulated as follows: Design a set of nonlinear energy sinks (NESs) that are locally attached to a main structure, with the purpose of passively absorbing a significant part of the applied seismic energy, locally confining it and then dissipating it in the smallest possible time. Alternatively, the overall goal will be to demonstrate that it is feasible to passively divert the applied seismic energy from the main structure (to be protected) to a set of preferential nonlinear substructures (the set of NESs), where this energy is locally dissipated at a time scale fast enough to be of practical use for seismic mitigation. It is the aim of this work to show that the concept of nonlinear energy pumping is feasible for seismic mitigation. We consider a two degree-of-freedom (DOF) primary linear system (the structure to be protected) and study seismic-induced vibration control through the use of Vibro-Impact NESs (VI NESs). Also, we account for the possibility of attaching to the primary structure additional alternative NES configurations possessing essential but smooth nonlinearities (e.g., with no discontinuities). We study the performance of the NESs through a set of evaluation criteria. The damped nonlinear transitions that occur during the operation of the VI NESs are then studied by superimposing wavelet spectra of the nonlinear responses to appropriately defined frequency – energy plots (FEPs) of branches of periodic orbits of underlying Conservative systems.