Showing papers by "National Technical University of Athens published in 1997"

Search for neutral heavy leptons produced in Z decays

Abstract: Weak isosinglet Neutral Heavy Leptons (m) have been searched for using data collected by the DELPHI detector corresponding to 3:3 106 hadronic Z0 decays at LEP1. Four separate searches have been performed, for short-lived m production giving monojet or acollinear jet topologies, and for long-lived m giving detectable secondary vertices or calorimeter clusters. No indication of the existence of these particles has been found, leading to an upper limit for the branching ratio BR(Z0 ! m) of about 1:310−6 at 95% confidence level for m masses between 3.5 and 50 GeV/c2. Outside this range the limit weakens rapidly with the m mass. The results are also interpreted in terms of limits for the single production of excited neutrinos.

Effect of drying method on shrinkage and porosity

Abstract: The effect of drying method on bulk density, particle density, specific volume and porosity of banana, apple, carrot and potato at various moisture contents was investigated, using a large set of experimental measurements. Samples were dehydrated with five different drying methods: conventional, vacuum, microwave, freeze and osmotic drying. A simple mathematical model was used In order to correlate the above properties with the material moisture content. Four parameters with physical meaning were incorporated in the model: the enclosed water density pw, the dry solid density ps, the dry solid bulk density pbo and the volume shrinkage coefficient β'. The effect of drying method on the examined properties was taken into account through its effect on the corresponding parameters. Only, dry solid bulk density was dependent on both material and drying method. Freeze dried materials developed the highest porosity, whereas the lowest one was obtained using conventional air drying.

Data Warehouse Configuration

Abstract: In the data warehousing approach to the integration of data from multiple information sources, selected information is extracted in advance and stored in a repository. A data warehouse (DW) can therefore be seen as a set of materialized views defined over the sources. When a query is posed, it is evaluated locally, using the materialized views, without accessing the original information sources. The applications using DWs require high query performance. This requirement is in conflict with the need to maintain in the DW updated information. The DW configuration problem is the problem of selecting a set of views to materialize in the DW that answers all the queries of interest while minimizing the total query evaluation and view maintenance cost. In this paper we provide a theoretical framework for this problem in terms of the relational model. We develop a method for dealing with it by formulating it as a state space optimization problem and then solving it using an exhaustive incremental algorithm as well as a *Research supported by the European Commission under the ESPRIT Program LTR project No 22469 “DWQ: Foundations of Data Warehouse Quality” Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the VLDB copyright notice and the title of the publication and its date appear, and notice is given that copying is by permission of the Very Large Data Base Endowment. To copy otherwise, or to republish, requires a fee and/or special permission from the Endowment. Proceedings of the 23rd VLDB Conference Athens, Greece, 1997 heuristic one. We extend this method by considering the case where auxiliary views are stored in the DW solely for reducing the view maintenance cost.

Greek: A Comprehensive Grammar of the Modern Language

Abstract: This reference guide to the important aspects of modern Greek, presents a description of the language, focusing on the real patterns of use in today's Greek. It is sutiable for either independent study for students in schools, colleges and universities and adult classes of all types. Throughout the emphasis is on Greek as used by native speakers today. A selection of examples are included to illustrate language points. The book features coverage of traditional grammar and language functions.

The Ms = 6.2, June 15, 1995 Aigion earthquake (Greece): evidence for low angle normal faulting in the Corinth rift

Abstract: We present the results of a multidisciplinary study of the Ms = 6.2, 1995, June 15, Aigion earthquake (Gulf of Corinth, Greece). In order to constrain the rupture geometry, we used all available data from seismology (local, regional and teleseismic records of the mainshock and of aftershocks), geodesy (GPS and SAR interferometry), and tectonics. Part of these data were obtained during a postseismic field study consisting of the surveying of 24 GPS points, the temporary installation of 20 digital seismometers, and a detailed field investigation for surface fault break. The Aigion fault was the only fault onland which showed detectable breaks (< 4 cm). We relocated the mainshock hypocenter at 10 km in depth, 38 ° 21.7 ′ N, 22 ° 12.0 ′ E, about 15 km NNE to the damaged city of Aigion. The modeling of teleseismic P and SH waves provides a seismic moment Mo = 3.4 1018 N.m, a well constrained focal mechanism (strike 277 °, dip 33 °, rake − 77°), at a centroidal depth of 7.2 km, consistent with the NEIC and the revised Harvard determinations. It thus involved almost pure normal faulting in agreement with the tectonics of the Gulf. The horizontal GPS displacements corrected for the opening of the gulf (1.5 cm/year) show a well-resolved 7 cm northward motion above the hypocenter, which eliminates the possibility of a steep, south-dipping fault plane. Fitting the S-wave polarization at SERG, 10 km from the epicenter, with a 33° northward dipping plane implies a hypocentral depth greater than 10 km. The north dipping fault plane provides a poor fit to the GPS data at the southern points when a homogeneous elastic half-space is considered: the best fit geodetic model is obtained for a fault shallower by 2 km, assuming the same dip. We show with a two-dimensional model that this depth difference is probably due to the distorting effect of the shallow, low-rigidity sediments of the gulf and of its edges. The best-fit fault model, with dimensions 9 km E–W and 15 km along dip, and a 0.87 m uniform slip, fits InSAR data covering the time of the earthquake. The fault is located about 10 km east-northeast to the Aigion fault, whose surface breaks thus appears as secondary features. The rupture lasted 4 to 5 s, propagating southward and upward on a fault probably outcropping offshore, near the southern edge of the gulf. In the shallowest 4 km, the slip – if any – has not exceeded about 30 cm. This geometry implies a large directivity effect in Aigion, in agreement with the accelerogram aig which shows a short duration (2 s) and a large amplitude (0.5 g) of the direct S acceleration. This unusual low-angle normal faulting may have been favoured by a low-friction, high pore pressure fault zone, or by a rotation of the stress directions due to the possible dip towards the south of the brittle-ductile transition zone. This fault cannot be responsible for the long term topography of the rift, which is controlled by larger normal faults with larger dip angles, implying either a seldom, or a more recently started activity of such low angle faults in the central part of the rift.

Scalable processors in the billion-transistor era: IRAM

Abstract: Members of the University of California, Berkeley, argue that the memory system will be the greatest inhibitor of performance gains in future architectures. Thus, they propose the intelligent RAM or IRAM. This approach greatly increases the on-chip memory capacity by using DRAM technology instead of much less dense SRAM memory cells. The resultant on-chip memory capacity coupled with the high bandwidths available on chip should allow cost-effective vector processors to reach performance levels much higher than those of traditional architectures. Although vector processors require explicit compilation, the authors claim that vector compilation technology is mature (having been used for decades in supercomputers), and furthermore, that future workloads will contain more heavily vectorizable components.

Soil-pile-bridge seismic interaction : Kinematic and inertial effects. Part I: Soft soil

Abstract: SUMMARY A substructuring method has been implemented for the seismic analysis of bridge piers founded on vertical piles and pile groups in multi-layered soil. The method reproduces semi-analytically both the kinematic and inertial soil—structure interaction, in a simple realistic way. Vertical S-wave propagation and the pile-to-pile interplay are treated with suƒcient rigor, within the realm of equivalent-linear soil behaviour, while a variety of support conditions of the bridge deck on the pier can be studied with the method. Analyses are performed in both frequency and time domains, with the excitation specified at the surface of the outcropping (‘elastic’) rock. A parameter study explores the role of soil—structure interaction by elucidating, for typical bridge piers founded on soft soil, the key phenomena and parameters associated with the interplay between seismic excitation, soil profile, pile—foundation, and superstructure. Results illustrate the potential errors from ignoring: (i) the radiation damping generated from the oscillating piles, and (ii) the rotational component of motion at the head of the single pile or the pile-group cap. Results are obtained for accelerations of bridge deck and foundation points, as well as for bending moments along the piles.

Strain-softening of concrete in uniaxial compression

Abstract: 0025-5432/97 © RILEM tory cast its own specimens following a prescribed recipe The pre-peak behaviour was found to be independent of specimen slenderness when low friction loading platens were used However, for all loading systems a strong increase of (post-peak) ductility was found with decreasing specimen slenderness Analysis of the results, and comparison with data from literature, showed that irrespective of the loading system used, a perfect localization of deformations occured in the post-peak regime, which was first recognised by Van Mier in a series of uniaxial compression tests on concrete between brushes in 1984 Based on the results of the Round Robin, a draft recommendation will be made for a test procedure to measure strain softening of concrete under uniaxial compression Although the post-peak stress-strain behaviour seems to be a mixture of material and structural behaviour, it appears that a test on either prismatic or cylindrical specimens of slenderness h/d = 2, loaded between low friction boundaries (for example by inserting sheets of tef lon between the steel loading platen and the specimen), yields reproducible results with relatively low scatter For normal strength concrete, the closed-loop test can be controlled by using the axial platen-to-platen deformation as a feed-back signal, whereas for high-strength concrete either a combination of axial and lateral deformation should be used, or a combination of axial deformation and axial load FOREWORD An extensive Round Robin test programme on compressive softening was carried out by the RILEM Technical Committee 148-SSC “Test methods for the Strain Softening response of Concrete” The goal was to develop a reliable standard test method for measuring strain softening of concrete under uniaxial compression The main variables in the test programme were the specimen slenderness h/d and the boundary restraint caused by the loading platen used in the experiments Both high friction and low friction loading systems were applied Besides these main variables, which are both related to the experimental environment under which softening is measured, two different concretes were tested: a normal strength concrete of approximately 45 MPa and a higher strength concrete of approximately 75 MPa In addition to the prescribed test variables, due to individual initiatives, the Round Robin also provided information on the effect of specimen shape and size The experiments revealed that under low boundary friction a constant compressive strength is measured irrespective of the specimen slenderness For high friction loading systems (plain steel loading platen), an increase of specimen strength is found with decreasing slenderness However, for slenderness greater than 2 (and up to 4), a constant strength was measured The shape of the stress-strain curves was very consistent, in spite of the fact that each laboraRILEM TC 148-SSC: TEST METHODS FOR THE STRAIN-SOFTENING RESPONSE OF CONCRETE

Dielectric relaxation spectroscopy of polyethylene terephthalate (PET) films

Abstract: The dielectric properties of biaxially stretched polyethylene terephthalate (PET) films of thickness and 68% degree of crystallinity were investigated by means of dielectric relaxation spectroscopy in the frequency range Hz and the temperature range C. Differencial scanning calorimetry (DSC), in the range C, was employed to investigate the thermal properties of the PET samples. Besides measuring the relaxation associated with the glass transition and the secondary relaxation, special attention has been paid to the investigation of DC-conductivity-related effects. They give rise to high dielectric permittivity values and dielectric losses at low frequencies and high temperatures. The results are analysed within the complex permittivity formalism and discussed in terms of interfacial Maxwell - Wagner - Sillars polarization, the peak, conductivity relaxation, space-charge polarization, electrode polarization and DC conductivity. DC conductivity values determined from frequency plots of the AC conductivity follow the Vogel - Tamman - Fulcher equation at temperatures higher than the glass transition temperature, indicating that the charge-carrier transport mechanism is governed by the motion of the polymeric chains. On the basis of the temperature dependence of the DC conductivity PET is classified as a fragile system.

Investigation of high-harmonic generation from xenon atom clusters

Abstract: We report on the generation of harmonic radiation (in the 70 - 90 nm range) from clusters of Xe atoms formed in a gas jet. We find that the harmonic yield from the clusters exhibits an anomalous cubic scaling with backing pressure to the gas jet. This scaling is consistent with a cluster dipole moment resulting from collective oscillations of electrons around the central ions of the cluster. Using a nanosecond ultraviolet prepulse to dissociate the clusters, we have also attempted to compare harmonic yields from clusters with those produced from monatomic Xe, under otherwise identical conditions. Our results suggest that yields from clusters might exceed those from monomers by up to a factor of five.

Geodetic estimate of seismic hazard in the Gulf of Korinthos

Abstract: The recent 15 June 1995, M0 = 6.0 × 1018 N m, Aigion earthquake in the western Gulf of Korinthos has focussed attention on the seismic hazard of the region. Although there have been few large earthquakes in the region during this century, the historical record suggests that there may have been many large earthquakes there in the interval 1750–1900. We present geodetic data that give estimates of the rate of extension of the Gulf of Korinthos during this century and which suggest that less than half of the elastic strain in the central and western Gulf of Korinthos has been released by earthquakes during this century. In contrast, the seismic and geodetic strains in the eastern Gulf of Korinthos are in agreement with each other. If the discrepancy between seismic and geodetic strains in the western Gulf of Korinthos that has accumulated during this century is removed in earthquakes, the moment release will be equivalent to several Ms > 6.5 earthquakes.

The static and dynamic axial crumbling of thin-walled fibreglass composite square tubes

Abstract: In the present paper we report on the behaviour and crashworthiness characteristics of square composite tubes subjected to static and dynamic axial compression exerted by a hydraulic press and a drop-hammer, respectively. The effect of specimen geometry, i.e. of thickness and axial length, and of the loading rate on the energy absorbing capability are studied in detail. Attention is directed towards the mechanics of the axial crumbling process from macroscopic and microscopic point of view for facilitating engineering design calculations of the amount of energy dissipated and for a somewhat more complete aspect on the actual fracture mechanism during the failure of the composite material tested. A theoretical analysis of the collapse mechanism of the components tested under axial compression is proposed, leading to a good approximation of the energy absorbed during crushing.

Recognizing D shape features using a neural network and heuristics

Abstract: This work presents a Feature Recognition system developed using a previously trained Artificial Neural Network. The part description is taken from a B-rep solid modeller's data base. This description refers only to topological information about the faces in the part in the form of an Attributed Adjacency Graph. A set of heuristics is used for breaking down this compound feature graph into subgraphs, that correspond to simple features. Special representation patterns are then constructed for each of these subgraphs. These patterns are presented to a Neural Network which classifies them into feature classes: pockets, slots, passages, protrusions, steps, blind slots, corner pockets, and holes. The scope of instances/ variations of these features that can be recognised is very wide. A commercially available neural network modelling tool was used for training. The user interface to the neural network recogniser has been written in Pascal. The program can handle parts with up to 200 planar or curved faces. The performance of the recogniser in terms of speed is far better than that of any other rule-based system due to the Neural Network approach employed. The basic limitation is that of the heuristics used to break down compound features into simple ones which are fed to the ANN, but this is still a step ahead compared to other approaches.

Sh surface Waves in a Homogeneous Gradient-Elastic Half-Space with Surface Energy

Abstract: The existence of SH surface waves in a half-space homogeneous material (i.e. anti-plane shear wave motions which decay exponentially with the distance from the free surface) is shown to be possible within the framework of the generalized linear continuum theory of gradient elasticity with surface energy. As is well-known such waves cannot be predicted by the classical theory of linear elasticity for a homogeneous half-space, although there is experimental evidence supporting their existence. Indeed, this is a drawback of the classical theory which is only circumvented by modelling the half-space as a layered structure (Love waves) or as having non-homogeneous material properties. On the contrary, the present study reveals that SH surface waves may exist in a homogeneous half-space if the problem is analyzed by a continuum theory with appropriate microstructure. This theory, which was recently introduced by Vardoulakis and co-workers, assumes a strain-energy density expression containing, besides the classical terms, volume strain-gradient and surface-energy gradient terms.