Showing papers by "National Technical University of Athens published in 2000"

On the Stability of Functional Equations and a Problem of Ulam

Abstract: In this paper, we study the stability of functional equations that has its origins with S. M. Ulam, who posed the fundamental problem 60 years ago and with D. H. Hyers, who gave the first significant partial solution in 1941. In particular, during the last two decades, the notion of stability of functional equations has evolved into an area of continuing research from both pure and applied viewpoints. Both classical results and current research are presented in a unified and self-contained fashion. In addition, related problems are investigated. Some of the applications deal with nonlinear equations in Banach spaces and complementarity theory.

Gsi: A Geologically Friendly Tool For Rock Mass Strength Estimation

Abstract: This paper presents a review of the estimation of rock mass strength properties through the use of GSI The GSI classification system greatly respects the geological constraints that occur in nature and are reflected in the geological information A discussion is given regarding the ranges of the Geological Strength Index for typical rock masses with specific emphasis to heterogeneous rock masses

Active deformation of the Corinth rift, Greece: Results from repeated Global Positioning System surveys between 1990 and 1995

Abstract: Between 1990 and 1995, we carried out seven Global Positioning System (GPS) campaigns in the Corinth rift area in order to constrain the spatial and temporal crustal deformation of this active zone. The network, 193 points over ∼10,000 km2, samples most of the active faults. In order to estimate the deformation over a longer period, 159 of those points are also Greek triangulation pillars previously measured between 1966 and 1972. Two earthquakes of magnitude 6.2 and 5.9 have occurred in the network since it was installed. The extension rate deduced from the analysis of the different GPS data sets is 14±2 mm/yr oriented N9° in the west, 13±3 mm/yr oriented S-N in the center, and 10±4 mm/yr oriented N19°W in the east of the gulf. The comparison between GPS and triangulation gives higher rates and less angular divergence (25±7 mm/yr, N4°E; 22±7 mm/yr, S-N; 20±7 mm/yr, N15°W, respectively). Both sets of data indicate that the deforming zone is very narrow (10–15 km) in the west, might be wider in the center (15–20 km), and is more diffuse in the east. The analysis of the displacements observed after the Ms = 6.2, June 15, 1995, and the Ms = S.9, November 18, 1992, earthquakes, both located in the west of the gulf, together with seismological and tectonic observations shows that these two earthquakes occurred on low-angle (≤35°) north dipping normal faults located between 4.5 and 10 km depth in the inner part of the rift. Assuming that the deformation is concentrated in relatively narrow deforming zones, we use a simple model of a dislocation in an elastic half-space to study the implication of the localization. Using the geometry of the known seismogenic faults, our observations imply continuous aseismic deformation in the uppermost crust of the inner rift. This model predicts geodetic strain rates close to seismic strain rates in opposition to previous estimates. This is because our model takes into account the activity on low-angle normal faults in the inner rift and an effective seismogenic layer of 6–7 km, about half that usually assumed.

Scattering of elastic waves by periodic arrays of spherical bodies

Abstract: We develop a formalism for the calculation of the frequency band structure of a phononic crystal consisting of nonoverlapping elastic spheres, characterized by Lam\'e coefficients which may be complex and frequency dependent, arranged periodically in a host medium with different mass density and Lam\'e coefficients. We view the crystal as a sequence of planes of spheres, parallel to and having the two-dimensional periodicity of a given crystallographic plane, and obtain the complex band structure of the infinite crystal associated with this plane. The method allows one to calculate, also, the transmission, reflection, and absorption coefficients for an elastic wave (longitudinal or transverse) incident, at any angle, on a slab of the crystal of finite thickness. We demonstrate the efficiency of the method by applying it to a specific example.

A Search in Gamma-Ray Burst Data for Nonconstancy of the Velocity of Light

Abstract: We discuss possible tests of the constancy of the velocity of light using distant astrophysical sources such as gamma-ray bursters (GRBs), active galactic nuclei (AGNs), and pulsars. This speculative quest may be motivated by some models of quantum fluctuations in the spacetime background, and we discuss explicitly how an energy-dependent variation in photon velocity δc/c ~ -E/M arises in one particular quantum-gravitational model. We then discuss how data on GRBs may be used to set limits on variations in the velocity of light, which we illustrate using BATSE and OSSE observations of the GRBs that have recently been identified optically and for which precise redshifts are available. We show how a regression analysis can be performed to look for an energy-dependent effect that should correlate with redshift. The present data yield a limit M 1015 GeV for the quantum gravity scale. We discuss the prospects for improving this analysis using future data, and how one might hope to distinguish any positive signal from astrophysical effects associated with the sources.

A generalized mathematical framework for stochastic simulation and forecast of hydrologic time series

Abstract: A generalized framework for single-variate and multivariate simulation and forecasting problems in stochastic hydrology is proposed. It is appropriate for short-term or long-term memory processes and preserves the Hurst coefficient even in multivariate processes with a different Hurst coefficient in each location. Simultaneously, it explicitly preserves the coefficients of skewness of the processes. The proposed framework incorporates short-memory (autoregressive moving average) and long-memory (fractional Gaussian noise) models, considering them as special instances of a parametrically defined generalized autocovariance function, more comprehensive than those used in these classes of models. The generalized autocovariance function is then implemented in a generalized moving average generating scheme that yields a new time-symmetric (backward-forward) representation, whose advantages are studied. Fast algorithms for computation of internal parameters of the generating scheme are developed, appropriate for problems including even thousands of such parameters. The proposed generating scheme is also adapted through a generalized methodology to perform in forecast mode, in addition to simulation mode. Finally, a specific form of the model for problems where the autocorrelation function can be defined only for a certain finite number of lags is also studied. Several illustrations are included to clarify the features and the performance of the components of the proposed framework.

Supercritical fluid extraction of celery seed oil

Abstract: The supercritical fluid extraction of oil from milled celery seeds, using CO2 as a solvent, is presented in this study. The effect of the process parameters — pressure and temperature of extraction, particle size of celery seeds and flow rate of CO2 — on the extraction rate was examined in a series of experiments. The results indicated a significant increase in extraction rate with increase of pressure or decrease of the particle size of celery seed. A similar effect was observed with the increase of the solvent flow rate and decrease of temperature. The experimental data were described by an empirical model and two mass balance models: (1) a simplified model, which takes into account only the time dependence of the extract concentration in the two phases and is coupled with various equilibrium relationships, and (2) an extended Lack's model. The correlation results were satisfactory especially for the mass balance models, which account for the double regime — solubility and diffusion controlled regimes — of the extraction.

The Force-Angle Measure of Tipover Stability Margin for Mobile Manipulators

Abstract: .SUMMARY Mobile manipulators operating in field environments will be required to apply large forces, or manipulate large loads, and to perform such tasks on uneven terrain which may cause the system to approach, or reach, a dangerous tipover instability. To avoid tipover in an automatic system, or to provide a human operator with an indication of proximity to tipover, it is necessary to define a measure of available stability margin. This work presents a new tipover stability measure the Force-Angle . stability measure which has a simple geometric interpretation, is easily computed, and is sensitive to changes in Center of Mass height. The proposed metric is applicable to systems subject to inertial and external forces, operating over even or uneven terrains. Requirements for computation and implementation of the measure are described, and several different categories of application of the measure are presented along with useful normalizations. Performance of the Force-Angle measure is demonstrated and compared with that of other stability margin measures using a forestry vehicle simulation. Results show the importance of considering both center-of-mass height and system heaviness, and confirm the effectiveness of the Force-Angle measure in monitoring the tipover stability margin.

Effect of sugar replacement by polyols and acesulfame-K on properties of low-fat cookies.

Abstract: The effect of sucrose replacement by polyols or fructose on dough rheology and properties of low-fat cookies was studied. Polydextrose was used to replace 35% of fat in low-fat cookie formulations. The rheological properties of dough were affected by sugar replacement. Maltitol and fructose resulted in dough with high values of hardness and consistency and low adhesiveness and cohesiveness, while lactitol, sorbitol and xylitol had the opposite effect. The properties of cookies prepared with maltitol and lactitol were similar to sucrose ones. Sorbitol cookies had acceptable properties too. In addition, lactitol and sorbitol improved the texture of low-fat cookies, making them softer and less brittle. Xylitol also affected texture favourably, but not flavour. Mannitol was found not suitable for cookie formulation, as it restricts spread and imparts unpleasant flavour and appearance to the cookies. Fructose was judged unacceptable too, because it led to bitter aftertaste and dark colour. Cookies with polyols or fructose were less sweet than sucrose ones, but supplementation with acesulfame-K increased sweetness and improved perceived flavour and general acceptance. # 2000 Society of Chemical Industry

Sulfur hexafluoride (SF6): global environmental effects and toxic byproduct formation.

Abstract: This work provides information concerning possible global environmental implications and personnel safety aspects that should be considered during the commercial uses of sulfur hexafluoride (SF6). SF6 is an anthropogenically produced compound, mainly used as a gaseous dielectric in gas insulated switchgear power installations. It is a potent greenhouse gas with a high global warming potential, and its concentration in the earth atmosphere is rapidly increasing. During its working cycle, SF6 decomposes under electrical stress, forming toxic byproducts that are a health threat for working personnel in the event of exposure. Several precautions are recommended to avoid personnel exposure to toxic byproducts: oxyfluoride levels or other byproduct concentrations in the operating gas matrix should be traced to predetermine the overall gas toxicity; contaminants should be systematically considered during maintenance, chamber evacuation and system opening process; small SF6 quantities leaking into air or stagnated pollutant concentrations in the operating field should be analyzed and compared to the threshold limit values and permissible exposure levels. New system design rules (i.e., hermetically sealed gas compartments, gas recycling or disposal in the field area) and different handling policies--both during maintenance and final disposal--now should be considered globally to provide for environmental and personnel safety.

Towards quality-oriented data warehouse usage and evolution

Abstract: As a decision support information system, a data warehouse must provide high level quality of data and quality of service. In the DWQ project we have proposed an architectural framework and a repository of metadata which describes all the data warehouse components in a set of metamodels to which is added a quality metamodel, defining for each data warehouse metaobject the corresponding relevant quality dimensions and quality factors. Apart from this static definition of quality, we also provide an operational complement, that is a methodology on how to use quality factors and to achieve user quality goals. This methodology is an extension of the Goal-Question-Metric (GQM) approach, which allows to capture (a) the inter-relationships between different quality factors and (b) to organize them in order to fulfil specific quality goals. After summarizing the DWQ quality model, this paper describes the methodology we propose to use this quality model, as well as its impact on the data warehouse evolution.

Water/Hydrocarbon Phase Equilibria Using the Thermodynamic Perturbation Theory

Abstract: Two equations of state, the cubic plus association (CPA) and the statistical associating fluid theory (SAFT), which account explicitly for the effect of hydrogen bonding on the thermodynamic properties of associating fluids using the perturbation theory of Wertheim (J. Stat. Phys. 1986, 42, 459, 477), are applied to predict the phase equilibrium of pure water, n-alkanes, and 1-alkenes as well as the low- and high-pressure phase equilibrium of water/hydrocarbon mixtures. The pure compound parameters for the two equations are estimated by fitting experimental vapor pressure and saturated liquid density data that cover a very wide temperature range from approximately the triple point to very close to the critical point. One temperature-independent binary interaction parameter is calculated for each of the mixtures examined. The analysis of the results shows that the increased complexity of SAFT over CPA does not offer any improvement in modeling highly nonideal fluid behavior, at least for the systems examin...