Showing papers by "National Technical University of Athens published in 1984"

On Concurrency Control by Multiple Versions

Abstract: We examine the problem of concurrency control when the database management system supports multiple versions of the data. We characterize the limit of the parallelism achievable by the multiversion approach and demonstrate the resulting space-parallelism trade-off.

The even-path problem for graphs and digraphs

Abstract: On donne un simple algorithme en temps lineaire pour trouver des chemins de meme longueur entre deux nœuds specifie d'un graphe donne. On montre que le meme probleme pour des graphes orientes est NP complet

On laminar steady flow in sinusoidal channels

Abstract: In the present paper a perturbation method is developed in order to study viscous laminar flows through wavy-walled channels. The stream function of the flow is expanded in a series thereby the wall amplitude being the perturbation parameter. The walls of the channel are transformed into parallel straight lines in order to simplify the boundary conditions of the problem on the wall. Flow field and wall-shear stresses are calculated numerically up to the first perturbation order. The position of the beginning separation on the channel walls and the associated critical Reynolds number are determined, as well as the extension of the region of the separated flow. The position of separation and reattachment points are given as functions of Reynolds numbers lying above the critical Reynolds number. The results are discussed and compared with the experimental results of other papers and further theoretical analysis.

Scattering from conducting cylinders embedded in a lossy medium

Abstract: Using a Green's function approach, similar to that used to treat scattering from underground tunnels, the problem is considered of electromagnetic scattering from conducting wires buried in the earth. The method leads to an integral equation for the unknown surface current J(ř) of the conducting cylinder. The expansion of J(ř) in terms of surface cylindrical wavefunctions and substitution in the integral equation results in an infinite set of linear equations for the unknown expansion coefficients. An effective truncation algorithm has been employed to evaluate these coefficients and, afterwards, the scattered field. Numerical results for the far-field region of the buried scatterer are plotted and some useful conclusions based on them are discussed.

Stage Discharge Relationships for Compound Channels

Abstract: Rivers and land drainage channels periodically overtop their banks and flow in the ‘out of bank’ condition. Under these circumstances, the interaction between the generally slower moving water on the flood plains and the faster moving water in the main channel, affects the flow structure in a number of ways and makes the prediction of the stage discharge relationship difficult.

The Adhesion Quality and the Extent of the Mesophase in Particulates

Abstract: The three-term and two-term unfolding models, which were developed to describe mesophase phenomena in fiber-reinforced composites, are extended in this paper for particulate composites. It has been already shown that the thin boundary layer, developed during casting and curing of the matrix around the inclusions in the composite, influences considerably the overall properties of the composite working as a crust between the two main phases of a bimaterial composite. However, in a fiber-composite the elastic modulus of the composite (Ec) consists of three components contributing parallelly in a meridional section, that is the modulus of the fiber, mesophase and matrix, which then contribute additively to the E,-modulus, multiplied each one by a weight parameter equal to the particular volume content of each phase. In a particulate the situation is inversed. Here the compliances of the typical phases contribute in an equatorial plane superimposing their contribution and therefore the respective compliances are additive. Based on this fact the three-term and two-term unfolding models, introduced for the fiber-composites, were established also for the particulates. It was shown that, by measuring the heat capacity jumps of the matrix and the composites, the thickness of the mesophase layer was experimentally established. The experimental value of D~ compliance, combined together with the Df-filler compliance and the D.-matrix compliance could define completely the model and gave, furthermore, important indications about the adhesion quality of the phases, by defining the so-called adhesion coefficient of the composite.

The elastic moduli of particulate-filled polymers

Abstract: The static and dynamic elastic moduli of particulate composites, consisting of two phases, one of which has isotropic–elastic and the other linear viscoelastic properties, were studied. For this purpose a model defining the approximate equations for determining the elastic modulus of a composite from the properties of the constituent materials was used. Classical theory of elasticity was applied to this simplified model of a composite-unit cell. The following assumptions are made: (i) filler particles are spherical; (ii) fillers are completely dispersed; and (iii) the volume fraction of fillers is sufficiently small, so that any interaction among fillers may be neglected. A class of iron-filled epoxy composites was subjected to tests in order to compare the theoretical values with the experimental results. The elastic modulus calculated by the expression derived in this study seems to corroborate with the experimental results fairly well. Finally, by applying the correspondence principle to this expression, theoretical relationships for the dynamic storage and loss moduli were also derived.

On the evaluation of adhesion between phases in fiber composites

Abstract: A theoretical model was introduced for the evaluation of the boundary layer developed between the main phases during the preparation of unidirectional fiber composites. It has been shown that this thin layer influences considerably the physical properties of the composite. It was assumed that the physical properties of themesophase unfold from those of the hard-core fibers to those of the softer matrix. Thus, a multicylinder model was assumed improving the classical two-cylinder model introduced by Hashin and Rosen for the representative volume element of the composite.

Analysis of longitudinal discontinuities in dielectric slab waveguides

Abstract: The interaction of waves propagating in dielectric slab waveguides with longitudinal discontinuities is treated analytically. An integral-equation approach is employed to formulate the corresponding boundary-value problem. Transverse electric (TE) and transverse magnetic (TM) waves are considered in parallel. A Fourier-series-type expansion is utilized to expand the induced field inside the discontinuity region. The unknown coefficients of this expansion are determined by solving an integral equation. Scattered wave amplitudes in the forward and back-ward directions are computed by applying a steepest-descent integration technique. Radiation-field patterns are also obtained for the region outside the slab waveguide. Numerical results are presented for several cases. Resonance scattering, mode mixing, and several other interesting physical phenomena are examined.

On the Genesis of Some Manganese Deposits from Eastern Greece

Abstract: In this paper the genesis of some Mn deposits in the Orthrys and Perachora areas is investigated. These deposits are characterized by high concentrations of Mn, low Fe/Mn ratios and low trace metal contents. Their chemistry and stratigraphic location suggest that they are syngenetic and have been formed by rapid precipitation from hydrothermal exhalations, related to leaching of magmatic rocks, and which have been discharged onto the sea floor during Triassic-Cretaceous. The fractionation of Fe from Mn is due to its earlier precipitation in the form of sulfides. In the Orthrys area Cu-rich pyrite forms dikes in the diabase underlain the chert horizon with the Mn deposits. The depletion of the Mn deposits in trace metals is due to their absence from the hydrothermal exhalations and to the rapid precipitation of the Mn-phases which did not allow scavenging from the normal sea water. The precipitation of the sulfides within the fractures of the diabase may acted as “filters” absorbing trace metals from the passing hydrothermal solutions. The trace metal concentrations of the Perachora deposits support a greater hydrogenous influence in their formation compared with the Orthrys deposits.

On the shape of post-welding deformations of plate panels in newly built ships

Abstract: The paper presents the results of an investigation carried out on nearly 700 plate panels in newly built ships and marine structures in order to establish the actual deformed shape of the panels due to welding. The ultimate objective was to describe the deformation by a two-dimensional Fourier series, to examine the statistical distribution of the Fourier series coefficients, and to relate the coefficients to the principal welding or geometrical parameters. In view of the influence of the plate a statistical relationship between the maximum welding distortion and the magnitude of the primary buckling mode component has been obtained, and limiting values of welding deformation, based on the primary buckling mode component, have been suggested.

An implementation of the exponential pyramid data structure with application to determination of symmetries in pictures

Abstract: A simple indexing scheme to implement the exponential pyramid data structure is considered. The scheme uses (4N 2 − 1) 3 consecutive memory locations to store the average intensities of various resolution versions of a picture having N × N pixels. A procedure is presented which accepts as input the N2 pixels produced by raster scanning and constructs the associated exponential pyramid data structure for the picture. An application of the scheme to the determination of possible symmetries present in a picture is also given. The particular symmetries considered are the horizontal, vertical, right and left diagonal, and 90 and 180° rotational.

Polytopes and Complexity

Abstract: Polyhedral combinatorics is one of the oldest and most important techniques for the algorithmic solution of combinatorial optimization problems. A host of efficient algorithms for combinatorial optimization problems have a “polyhedral interpretation”, and in fact for many the polyhedral approach was instrumental in the derivation of the algorithm and its proof. In this paper we review some recent results which relate the polyhedral theory to the theory of computational complexity. These results reveal the limitations of this approach in solving NP-complete problems. They focus on the computational complexity of certain elements of the structure (such as the adjacency relation, the facets, the supporting hyperplanes, and the interior) of the polytopes of NP-complete problems in general – and, in some cases, of the traveling salesman problem in particular. The thrust of these results is that most of these elements are extremely complex (NP-complete or worse). Since the understanding of these elements is at the root of “polyhedral” algorithms, these results seem to suggest that the polytopal approach is not likely to yield feasible algorithms for NP-complete combinatorial optimization problems. Finally, we present a class of persistent open problem, pertaining to whether a given hyperplane contains any vertex of a fixed “nice” polytope, such as the matching, or even the assignment polytope.

Computation of Mode Propagation Constants and Cut-off Frequencies of Optical Planar Layers Using the Resonance Technique

Abstract: By applying Fourier transformations to Maxwell's equations, we derive transmission-line equations representing the TE and TM modes of dielectric planar layers. The propagation constants of any mode can be found from the resonance condition of the T circuit representation of the transmission line. The cut-off frequency equivalent circuit is also derived, which is useful for the computation of the cut-off frequencies of the propagation modes. The method is developed for uniaxial materials and is applied to uniaxial planar layered structures of arbitrary refractive index. The inhomogeneous isotropic profiles are a special case in this analysis. Examples are given for comparison.

A new method for obtaining exact analytical formulae for the roots of transcendental functions

Abstract: A new method is proposed for the derivation of closed-form formulae for the zeros and poles of sectionally analytic functions in the complex plane. This method makes use of the solution of the simple discontinuity problem in the theory of analytic functions and requires the evaluation of real integrals only (for functions with discontinuity intervals along the real axis). Many transcendental equations of mathematical physics can be successfully solved by the present approach. An application to such an equation, the molecular field equation in the theory of ferromagnetism, is made and the corresponding analytical formulae are reported together with numerical results.

The dynamic behaviour of an oblique edge-crack under impact loading

Abstract: When a stress wave (tensile or compressive) impinges on a crack existing in an elastic medium, reflection, refraction and diffraction-phenomena take place. A result of diffraction is the loading of the crack. While compressive stress-waves do not create any stress concentration at the tip of an existing crack, tensile stress-waves develop stresses at the tip which may cause a propagation of the crack. If the tensile pulse is weak the crack may propagate by steps under the action only of successive tensile stress-pulses, whereas intermediate compressive-stress pulses do not have any influence. A complete study of the phenomena of incubation, initiation and propagation of cracks in thin plates, when they are subjected to a compressive pulse, which is subsequently reflected from the free boundaries of the plate and changed to complicated wave-trains, was undertaken in this paper, based on the method of caustics. Interesting results that were derived from this experimental study are presented.