Showing papers by "National Technical University of Athens published in 1999"

A survey of logical models for OLAP databases

Abstract: In this paper, we present different proposals for multidimensional data cubes, which are the basic logical model for OLAP applications. We have grouped the work in the field in two categories: commercial tools (presented along with terminology and standards) and academic efforts. We further divide the academic efforts in two subcategories: the relational model extensions and the cube-oriented approaches. Finally, we attempt a comparative analysis of the various efforts.

A study on the landfill leachate and its impact on the groundwater quality of the greater area

Abstract: Characterisation of the leachate originating from the Ano Liosia landfill (situated in Attica region, Greece) as well as assessment on the quality of the local aquifer were carried out. The experimental results showed that most of the parameters examined in the leachate samples such as colour, conductivity, TS, COD, NH3–N, PO4–P, SO42−, Cl−, K+, Fe and Pb were found in high levels. The organic load was quite high since the COD concentrations were in the range of 3250–6125 mg L−1. In addition, the low BOD/COD ratio (0.096–0.195), confirmed that the majority of this organic matter is not easily biodegradable. The groundwater near the landfill site was characterised as not potable and not suitable for irrigation water, since most of the physical and chemical parameters examined – such as colour, conductivity, DS, hardness, Cl−, NH3–N, COD, K+, Na+, Ca2+, Fe, Ni and Pb exceeded the permissible limits given by EE, EPA and the Greek Ministry of Agriculture. Furthermore, this study presents the application of the hydrologic evaluation of landfill performance (HELP) model for the determination of the yearly leakage from the base of the landfill after the final capping.

Synthesis and properties of poly(lactic acid).

Abstract: Poly(D,L lactic acid) was prepared by bulk polymerization of D,L lactide, both under atmospheric pressure and in vacuum. The obtained polymeric products were characterized in terms of molecular weight, Mw, melting point, calorimetric response and swelling behaviour. All products were amorphous. Their molecular weights were determined by viscosimetry and ranged from 2x10(3) to 9x10(4). Similarly, the melting points ranged from 90 to 210 degrees C. Swelling experiments, with specimens immersed in buffer solutions, showed that hydrolytic degradation started in a few days for the low Mw material, whereas for the higher molecular weight products it took much longer and probably followed a two-stage mechanism. This study suggests that the high molecular weight material could be an interesting carrier for the preparation of controlled release products, in cases where prolonged delivery is necessary.

A consistent coupled-mode theory for the propagation of small-amplitude water waves over variable bathymetry regions

Abstract: Extended mild-slope equations for the propagation of small-amplitude water waves over variable bathymetry regions, recently proposed by Massel (1993) and Porter & Staziker (1995), are shown to exhibit an inconsistency concerning the sloping-bottom boundary condition, which renders them non-conservative with respect to wave energy. In the present work, a consistent coupled-mode theory is derived from a variational formulation of the complete linear problem, by representing the vertical distribution of the wave potential as a uniformly convergent series of local vertical modes at each horizontal position. This series consists of the vertical eigenfunctions associated with the propagating and all evanescent modes and, when the slope of the bottom is different from zero, an additional mode, carrying information about the bottom slope. The coupled-mode system obtained in this way contains an additional equation, as well as additional interaction terms in all other equations, and reduces to the previous extended mild-slope equations when the additional mode is neglected. Extensive numerical results demonstrate that the present model leads to the exact satisfaction of the bottom boundary condition and, thus, it is energy conservative. Moreover, it is numerically shown that the rate of decay of the modal-amplitude functions is improved from O(n−2), where n is the mode number, to O(n−4), when the additional sloping-bottom mode is included in the representation. This fact substantially accelerates the convergence of the modal series and ensures the uniform convergence of the velocity field up to and including the boundaries.

Characterization of the vertical structure of Saharan dust export to the Mediterranean basin

Abstract: We present the results of our investigations into the vertical structure of several North African dust plumes exported to the Mediterranean in 1997. Two backscatter lidar systems were operated in the western and eastern parts of the Mediterranean basin during dust events identified using Meteosat visible images. Dust transport soundings have shown that dust particles are trapped and transported inside well-defined layers in the free troposphere. In general, the dust transport appeared to be multilayered, with several distinct layers at different altitudes between 1.5 and 5 km. The analysis of Meteosat IR images, the Total Ozone Mapping Spectrometer aerosol index, and back-trajectories clearly shows that these layers have different origins in Africa. Finally, in addition to the free troposphere transport, the presence of dust particles inside the planetary boundary layer has been assessed and quantified for two particular events with aerosol optical thickness of 0.3–0.4. using simultaneous lidar and Sun photometer measurements. In one case only, significant dust load (dust optical thickness of ∼0.1) occurred in the boundary layer.

Water sorption and diffusion studies in an epoxy resin system

Abstract: The kinetics of water absorption in epoxide materials was studied by the aid of a diglycidyl ether of bisphenol-A–triethylenetetramine (DGEBA–TETA) epoxy–resin system containing various amounts of the plasticizer THIOCOL (0–40 phr). The presence of plasticizer permits the formation of products with different crosslinking densities and hydrophilic characters. Dynamic water absorption experiments were carried out at 15, 40, and 70°C. For the fitting of the experimental results, a new model was used, based on a model proposed earlier by Jacobs and Jones. This model considers epoxide product as a two-phase system consisting of a master phase (where the major part of the water is absorbed), which is homogeneous and nonpolar (phase 1), and of a second phase with different density and/or hydrophilic character (phase 2). By making the assumption that water diffusion can take place independently in the different phases of the material in accordance with Fick's second law, we can calculate the diffusion coefficient D and the water content at saturation M∞ for each phase separately. Equilibrium water sorption measurements were performed at 40°C, and the data were analyzed and discussed based on the Guggenheim–Anderson–de Boer (GAB) equation, the results being in support of the two-phase model used in the analysis of absorption kinetics. The linear expansion coefficient and the glass transition temperature of the materials, employed in the discussion of the results, were measured by thermomechanical analysis. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1165–1182, 1999

A study on the parameters affecting the properties of Portland limestone cements

Abstract: In this paper the parameters affecting the properties of Portland limestone cements are investigated. Portland limestone cements of different fineness and limestone content have been produced by intergrinding clinker, gypsum and limestone. Two kinds of clinker of different chemical composition, mineralogical composition and strength development and three limestones, with different contents of calcite, dolomite, quartz and clay, have been used. It is concluded that the appropriate choice of the clinker quality, limestone quality, % limestone content and cement fineness can lead to the production of a limestone cement with the desired properties. Limestone cements, having up to 10% limestone content and fineness up to a limit value, develop almost the same compressive strength, as the corresponding pure cements. The limestone cements, generally have lower paste water demand than the relative pure cements and the water demand decrease is mainly affected by the clinker type and limestone quality. In any case the properties of the limestone cements are affected by the interaction of the two components rather than their individual properties.

Effect of osmotic agent on osmotic dehydration of fruits

Abstract: Mass transfer phenomena were investigated during osmotic dehydration of apple, banana and kiwi in glucose and sucrose osmotic solution. A complete set of experiments was performed for a wide range of temperature, sample size, speed of agitation, osmotic agent concentration and immersion time. An empirical model, based on a first order kinetic equation, was fitted satisfactorily to experimental data. Furthermore, the effect of solute molecular weight on mass transfer phenomena during the osmotic treatment was evaluated. The results showed that low molecular weight solute (glucose) leads to higher water loss and solids uptake than high molecular weight solute (sucrose), of osmodehydrated fruits under the same solution concentration.

Effect of microwave drying on some quality properties of dehydrated products

Abstract: The effect of microwave and microwave-vacuum drying on some quality properties such as density, porosity, color and viscoelastic behavior of dehydrated apple, banana, carrot and potato was investigated. It is concluded that microwave drying and moreover microwave-vacuum drying tends to increase the product porosity and to prevent the color damages during drying. Microwave drying seems to decrease the maximum stress and maximum strain of dehydrated products, while it increases their elasticity and decreases their viscous nature

Removal of Heavy Metals from Calcareous Contaminated Soils by EDTA Leaching

Abstract: The performance of EDTA for the treatment of calcareous soils contaminated with heavy metals from mining and smelting activities was evaluated in this study. Soil samples containing variable levels of contamination, from 500 to 35 000 mg kg-1 Pb and 700 to 20 000 mg kg-1 Zn, were subjected to EDTA treatment and the extraction of heavy metals was found to vary, ranging between 50 and 98% for Pb and 50 to 100% for Zn. Total residual concentrations were above the limits set by regulatory authorities; leachable metals, however, were efficiently removed and treated soils were all acceptable in terms of toxicity. The effect of EDTA concentration and pulp density was studied on a soil sample containing 12 000 mg kg-1 Pb and 10 000 mg kg-1 Zn. Heavy metals removal was improved at low pulp densities and when EDTA concentration was increased from 0.025 to 0.25 M. The tetrasodium salt Na4-EDTA was found to be less effective for metals removal compared to the disodium salt Na2-EDTA, though applied at higher concentrations. This experimental work has also demonstrated the great importance of soil matrix for the overall evaluation of the EDTA leaching as a cost effective remedial option. The simultaneous dissolution of calcite was found to consume approximately 90% of the available EDTA. It was thus concluded that for the treatment of calcareous soils the design criteria and cost estimations should be based on the calcite content of the soil.

Studies on the Enzymatic Synthesis of Lipophilic Derivatives of Natural Antioxidants

Abstract: The esterification of some natural antioxidants such as cinnamic acid derivatives and ascorbic acid in non-aqueous media, catalyzed by immobilized lipases from Candida antarctica and Rhizomucor miehei, was investigated. The alcohol chain length affected the rate of esterification of cinnamic acids by both lipases. Higher reaction rates were observed when the esterification was carried out with medium- or long-chain alcohols. The rate also depended on aromatic acid structure. The reactivity of the carboxylic function of the cinnamic acids was affected by electron-donating substituents in the aromatic ring. Higher yields were observed for the esterification of p-hydroxyphenylacetic acid (97%) catalyzed by C. antarctica lipase and for the esterification of cinnamic acid (59%) catalyzed by R. miehei lipase. Candida antarctica lipase was more suitable for producing ascorbic acid fatty esters, catalyzing with a relatively high yield (up to 65% within 24 h) the regioselective esterification of ascorbic acid with various fatty acids in 2-methyl-2-propanol. The reaction rate and yield depended on the fatty acid chain length and on the molar ratio of reactants. All ascorbic acid fatty esters produced by this procedure exhibited a significant antioxidant activity in a micellar substrate composed of linoleic acid.

A probabilistic view of hershfield's method for estimating probable maximum precipitation

Abstract: A simple alternative formulation of Hershfield's statistical method for estimating probable maximum precipitation (PMP) is proposed. Specifically, it is shown that the published Hershfield data do not support the hypothesis that there exists a PMP as a physical upper limit, and therefore a purely probabilistic treatment of the data is more consistent. In addition, using the same data set, it is shown that Hershfield's estimate of PMP may be obtained using the generalized extreme value (GEV) distribution with shape parameter given as a specified linear function of the average value of annual maximum precipitation series and for return period of about 60,000 years. This formulation substitutes completely the standard empirical nomograph that is used for the application of the method. The application of the method can be improved when long series of local rainfall data are available that support an accurate estimation of the shape parameter of the GEV distribution.

Fractal dimensions of speech sounds: computation and application to automatic speech recognition.

Abstract: The dynamics of airflow during speech production may often result in some small or large degree of turbulence. In this paper, the geometry of speech turbulence as reflected in the fragmentation of the time signal is quantified by using fractal models. An efficient algorithm for estimating the short-time fractal dimension of speech signals based on multiscale morphological filtering is described, and its potential for speech segmentation and phonetic classification discussed. Also reported are experimental results on using the short-time fractal dimension of speech signals at multiple scales as additional features in an automatic speech-recognition system using hidden Markov models, which provide a modest improvement in speech-recognition performance.

Optical properties of metallodielectric photonic crystals

Abstract: We present a systematic examination of the optical properties of photonic crystals consisting of metallic particles (plasma spheres) arranged periodically in a host dielectric medium. We calculate exactly the transmission and absorption coefficients of light incident on a slab of the material as functions of the frequency of the incident light and analyze the results by reference to the properties of a single sphere and to the frequency band structure of the corresponding infinite crystal. We examine the dependence of the above coefficients on the fractional volume occupied by the spheres and on the thickness of the slab. Finally we compare our results with those of the Maxwell Garnett effective-medium theory and in this way we establish the limitations of the latter. We show in particular that multipole interactions which the Maxwell Garnett theory does not take into account lead to significant structure in the transmission/absorption spectra.

10 Gbit/s all-optical Boolean XOR with SOA fibre Sagnac gate

Abstract: All-optical Boolean XOR logic is demonstrated with a three terminal fibre Sagnac interferometer employing a semiconductor optical amplifier. Full duty cycle operation at 10 GHz is shown and low pattern dependence has been achieved on a pseudo-data pattern at 10 Gbit/s with low switching energy.

Lateral Vibration and Internal Forces of Grouped Piles in Layered Soil

Abstract: Simplified analytical models are developed for the lateral harmonic response of single piles and pile groups in layered soil. Pile-to-soil interplay is represented by a dynamic Winkler formulation based on frequency-dependent springs and dashpots. For pile-to-pile interaction, the wave field originating from each oscillating (“source”) pile and the diffraction of this field by the adjacent (“receiver”) piles are considered. The response of single piles and pile pairs is evaluated both numerically (through a transfer-matrix formulation) and analytically (introducing an efficient virtual-work approximation). Closed-form solutions are obtained: (1) for the impedance of single piles; (2) for the dynamic interaction factors between two piles; and (3) for the “additional” internal forces (“distress”) developing in grouped piles because of pile-to-pile interaction, a phenomenon frequently ignored in current methods of analysis. Both swaying and rocking vibrational modes are considered. The effect of pile length ...

Use of Multicriteria Methods for Assessment of Transport Projects

Abstract: This paper attempts to consider some key elements of the transport assessment process in order to present and compare the most commonly applied multicriteria methods to be used in the assessment of transport projects. This comparative analysis is based on the performance of the methods with respect to transparency, simplicity, robustness, and accountability. To assist the comparison, an application example consisting of three transport projects in Greece is used. Finally, the methods' advantages and disadvantages are systematically presented so as to allow for the decision maker to draw definite conclusions on the appropriateness of each method for the different fields of transport appraisal.

Designing data warehouses

Abstract: A Data Warehouse (DW) is a database that collects and stores data from multiple remote and heterogeneous information sources. When a query is posed, it is evaluated locally, without accessing the original information sources. In this paper we deal with the issue of designing a DW, in the context of the relational model, by selecting a set of views to materialize in the DW. First, we briefly present a theoretical framework for the DW design problem, which concerns the selection of a set of views that (a) fit in the space allocated to the DW, (b) answer all the queries of interest, and (c) minimize the total query evaluation and view maintenance cost. We then formalize the DW design problem as a state space search problem by taking into account multiquery optimization over the maintenance queries (i.e., queries that compute changes to the materialized views) and the use of auxiliary views for reducing the view maintenance cost. Finally, incremental algorithms and heuristics for pruning the search space are presented.