Showing papers by "National Technical University of Athens published in 1998"

Applicability of the geological strength index (GSI) classification for very weak and sheared rock masses. The case of the Athens Schist Formation

Abstract: The Athens Schist Formation includes a wide variety of metasedimentary rocks, varying from strong or medium strong rocks such as sericite metasandstone, limestone, greywacke, sericite schist through to weak rocks such as metasiltstone, clayey and silty shale and phyllite. The overall rock mass is highly heterogeneous and anisotropic owing to the combined effect of advanced weathering and severe tectonic stressing that gave rise to intense folding and shearing followed by extensional faulting, which resulted in highly weathered rock masses and numerous shear and/or mylonite zones with distinct downgraded engineering properties. This paper is focused on the applicability of the GSI classification system to these highly heterogeneous rock masses and proposes an extension of the GSI system to account for the foliated or laminated weak rocks in the lower range of its applicability.

A conformal index (COIN) to evaluate implant quality and dose specification in brachytherapy.

Abstract: Purpose To propose a new index (COIN) that can be easily understood and computed to assess high dose rate (HDR) brachytherapy interstitial implant quality and dose specification and is an improvement on existing indexes. Methods and materials The COIN index is based on an extension of dose-volume histograms and employs an analogous concept to that of cost-benefit analysis, which has already been applied to quality-of-life assessments for two alternative treatment protocols. The COIN index calculation methodology is shown for two cases: with and without critical structures. An analysis is given of dose distributions for two planning treatment volumes (PTV) of simple geometrical shape, applying both the rules of the Paris system and that of the "Offenbach" system. 40 patients who have received interstitial implants form the clinical material. With current HDR brachytherapy technology both for dose delivery, using remote afterloaders, and for three-dimensional (3D) treatment planning, it is now possible to relatively easily plan conformal brachytherapy treatments that would have been impossible with manual afterloading techniques and two-dimensional (2D) treatment planning. Results Examples of the use of the COIN index are presented for experimental and clinical data. Conclusions The results show that COIN is a useful and practical index to improve the quality of treatment of interstitial brachytherapy implants. Further work will be undertaken with a larger population of implanted cancer patients and a subdivision of the results by treatment site.

Kinetics on color changes during drying of some fruits and vegetables

Abstract: The effect of drying conditions on color changes of apple, banana, carrot and potato during conventional and vacuum drying was investigated. The Hunder color scale parameters redness, yellowness and lightness were used to estimate color changes during vacuum and conventional drying at 50, 70 and 90°C. Air humidity during conventional drying was regulated at 15, 30 and 40%. Air temperature and humidity affected redness and yellowness, but not lightness. A first order kinetic model was fitted to experimental data adequately for both redness and yellowness. The rate of color deterioration was found to increase as temperature increased and air humidity decreased, for both drying methods and all the examined materials.

Impurity bands in photonic insulators

Abstract: A chain of impurity cells in a photonic insulator introduces impurity modes of the electromagnetic field over a narrow band of frequencies. We introduce a model of this band in the manner of a tight-binding description of impurity bands in semiconductors, and use it to describe waveguiding along the chain, and, in particular, across a corner of $90\ifmmode^\circ\else\textdegree\fi{}$. We also point out the possibility of using impurity bands in photonic insulators to study wave propagation along an effectively one-dimensional disordered chain.

Crustal strain in central Greece from repeated GPS measurements in the interval 1989–1997

Abstract: Summary A 66-station GPS network spanning central Greece, first observed in 1989, has been occupied fully on three occasions: June 1989, October 1991 and May 1993. Subsets of this network bounding the Gulf of Korinthos have also been occupied in June 1995, October 1995, May 1996 and September/October 1997. The first three occupations were processed using a fiducial GPS methodology, whereas later surveys were processed using CODE precise orbits. Combination of data from different surveys to yield smooth site velocities requires global network translations at each epoch to compensate for errors in the realization of the reference frame. This method provides a posteriori estimates of the relative coordinate errors and reference frame noise. Only one earthquake, the 1995 June 15 Egion event, has caused significant local coseismic displacement, and its effects on the interseismic velocity field are removed using an elastic dislocation model. We constrain the orientation of the 100 yr triangulation–GPS velocity estimates of Davies et al. (1997) using 14 sites common to the two networks. The goodness of fit of this transformation indicates that the short-term and 100 yr geodetic estimates of deformation are highly compatible. We infer that short-term geodetic studies are capable of determining longer-term deformation rates provided that transient, local effects can be modelled. From the combined velocity field, we estimate principal strains and rigid-body rotation rates at points on a regular grid using data from neighbouring sites. Strain rates are high within the Gulf of Korinthos and much lower elsewhere. The extension rate across the Gulf of Korinthos increases from east to west. Comparison of the extension rate with historical and recent rates of seismic release of strain reveals significant medium-term seismic hazard in the western Gulf of Korinthos, and may also indicate long-term aseismic strain.

Feedback Control of an Omnidirectional Autonomous Platform for Mobile Service Robots

Abstract: This paper proposes a feedback control scheme for an omnidirectional holonomic autonomous platform, which is equipped with three lateral orthogonal-wheel assemblies. Firstly, the dynamic properties of the platform are studied, and a dynamic model suitable for the application of control is derived. The control scheme constructed is of the resolved-acceleration type, with PI and PD feedback. The control scheme was experimentally applied to an actual mobile robotic platform. The results obtained show that full omnidirectionality can be achieved with decoupled rotational and translational motions. Omnidirectionality is one of the principal requirements for mobile robots designed for health-care and other general-hospital services.

Engineering a Molecular Model for Water Phase Equilibrium over a Wide Temperature Range

Abstract: The pure water phase equilibrium is calculated over a wide temperature range using the Gibbs ensemble Monte Carlo method with simple two-body molecular models. The Ewald summation method is used to account for the long-range Coulombic interactions. Coexisting liquid and vapor densities and vapor pressure at different temperatures are calculated explicitly. A new expression is developed for the direct calculation of pressure suitable for systems where the Ewald method is used. To improve agreement with experimental data, a simple scaling procedure is proposed that allows reparametrization of the molecular models without the need for additional calculations. Critical constants, second virial coefficient, and heat of vaporization are calculated from the different models. Finally, water structure is examined at low and high temperature. In all cases, comparison with experimental data is shown.

Spatio-temporal composition and indexing for large multimedia applications

Abstract: Multimedia applications usually involve a large number of multimedia objects (texts, images, sounds, etc.). An important issue in this context is the specification of spatial and temporal relationships among these objects. In this paper we define such a model, based on a set of spatial and temporal relationships between objects participating in multimedia applications. Our work exploits existing approaches for spatial and temporal relationships. We extend these relationships in order to cover the specific requirements of multimedia applications and we integrate the results in a uniform framework for spatio-temporal composition representation. Another issue is the efficient handling of queries related to the spatio-temporal relationships among the objects during the authoring process. Such queries may be very costly and appropriate indexing schemes are needed so as to handle them efficiently. We propose efficient such schemes, based on multidimensional (spatial) data structures, for large multimedia applications that involve thousands of objects. Evaluation models of the proposed schemes are also presented, as well as hints for the selection of the most appropriate one, according to the multimedia author's requirements.

Glass Transition in Liquids: Two versus Three-Dimensional Confinement

Abstract: The molecular dynamics of glass-forming liquids is strongly affected by nm-scale confinements. Our dielectric data provide strong evidence both for the ``cooperativity'' concept and for the existence of two states (interfacial layer and volume liquid). With decreasing size of confinement the orientational relaxation associated with the glass transition becomes faster, and the glass transition temperature is lowered. These effects are stronger for 3D confined droplets than for 2D confined pores. However, the radius at which size effects cease, i.e., the cooperativity length, does not depend on the dimensionality of the confinement.

On the asymptoticity aspect of hyers-ulam stability of mappings

Abstract: The object of the present paper is to prove an asymptotic analogue of Th. M. Rassias’ theorem obtained in 1978 for the Hyers-Ulam stability of mappings.

Mass transfer modelling of the osmotic dehydration of some fruits

Abstract: An empirical model was developed to predict the water loss and solid gain during osmotic dehydration of apple, banana and kiwi fruit. The model is based on a first-order kinetic equation, in which the rate constant is a function of the main process variables (speed of agitation, solute concentration, size of fruit and process temperature). This model was applied to a wide range of experimental data on the osmotic dehydration of apple, banana and kiwi fruit, and its parameters were estimated using non-linear regression analysis. The results showed that all of the above process variables have a significant effect on the mass transfer phenomena during osmotic dehydration.

Molecular Simulation of Phase Equilibria for Water−Methane and Water−Ethane Mixtures

Abstract: Monte Carlo simulations were used to calculate water−methane and water−ethane phase equilibria over a wide range of temperatures and pressures. Simulations were performed from room temperature up to near the critical temperature of water and from subatmospheric pressure to 3000 bar. The Henry's law constants of the hydrocarbons in water were calculated from Widom test particle insertions. The Gibbs ensemble Monte Carlo method was used for simulation of the water-rich and hydrocarbon-rich phases at higher pressures. Two recently proposed pairwise additive intermolecular potentials that describe accurately the pure component phase equilibria were used in the calculations. Equations of state for associating fluids were also used to predict the phase behavior. In all cases, calculations were compared with experimental data. For the highly nonideal hydrogen bonding mixtures studied here, molecular simulation-based predictions of the mutual solubilities are accurate within a factor of 2, which is comparable wit...

Prediction of Phase Equilibria in Binary Aqueous Systems Containing Alkanes, Cycloalkanes, and Alkenes with the Cubic-plus-Association Equation of State

Abstract: The cubic-plus-association (CPA) equation of state (EoS) is applied in this study to binary aqueous mixtures containing hydrocarbons. The CPA EoS combines the Soave−Redlich−Kwong (SRK) cubic equation of state for the physical part and perturbation theory for the chemical (association) part. Rigorous expressions for the contribution of the association term to the pressure and to the chemical potential, which do not include any derivatives of the mole fraction of molecules i not bonded at site A (XAi), are presented. Three different association models for water have been considered depending on the number of hydrogen bonding sites per water molecule: the two-, three-, and four-site models. Successful correlation of both vapor pressures and saturated liquid volumes is obtained with all three models. However, satisfactory correlation results of the mutual solubilities of water/aliphatic hydrocarbon systems are obtained only with the four-site model using a single interaction parameter (kij) in the attractive...

Coupled wellbore erosion and stability analysis

Abstract: This paper extends earlier work on sand erosion and presents an attempt to couple sand erosion to mechanical damage of rock around a wellbore. Porosity which evolves in time and space as surface erosion progresses, is chosen as the coupling parameter. Both rock elasticity and strength (cohesion) are assumed to depend on porosity in such a way that the material becomes weaker with increasing porosity. The mathematical model, consists of erosion equations, mixture flow equations and stress equilibrium equations, is solved numerically by Galerkin finite element method. Numerical results suggest that erosion, resulting in sand production, is high close to the free surface. Erosion is accompained by changes in porosity and a significant permeability increase. Erosion in the vicinity of the wellbore induces alterations in the mechanical behaviour of the medium. Weakening of rock stiffness leads to severe alteration of both effective stresses and pore pressure near the cavity. Since cohesion decreases with increasing porosity, one can also identify the time instant at which rock mechanical failure starts. © 1998 John Wiley & Sons, Ltd.