Showing papers by "National Technical University of Athens published in 1986"
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TL;DR: In this paper, a unified view of the mixing processes, quantitative laws are sought, which would be generally valid in flows with mean motion in at least one of the layers, and four such governing laws, all of a power form, applicable in different ranges of the overall Richardson number, Ri 0.
Abstract: In this paper the phenomenon of turbulent mixing at a density interface is studied. Based on a unified view of the mixing processes, quantitative laws are sought, which would be generally valid in flows with mean motion in at least one of the layers. Re-examination of available experimental data from four types of stratified flow, supplemented by theoretical considerations, leads to the identification of four such governing laws, all of a power form, applicable in different ranges of the overall Richardson number, Ri 0. Two of these laws refer to supercritical and subcritical conditions and are considered as fundamental, being related to qualitatively different mechanisms of interfacial mixing. An intermediate law applies around the critical Ri 0 value of unity, while an asymptotic law corresponds to the limit of homogeneous flow as Ri 0 tends to zero. All the above laws are given by simple expressions in terms of the mean flow characteristics, allowing their straightforward application for the quantifica...
111 citations
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TL;DR: In this paper, the crumpling of thin-walled frusta, under axial compression, in the ‘concertina’ mode is studied, where the energy expended in bending at the plastic hinges and in stretching the metal between the hinges is minimized for the total decrease in height due to collapse.
106 citations
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TL;DR: The proposed iterative scheme decomposes the Optimal Load Flow problem into real and reactive subproblems and linearization of the non-linear constraints of the problem is succeeded by utilizing Z-matrix techniques and sensitivity analysis.
Abstract: The Optimal Load Flow problem is a constrained optimization problem of large size and great complexity. For on-line implementation fast execution times and minimum computer storage are required. The proposed iterative scheme, reported in this paper, decomposes the Optimal Load Flow problem into real and reactive subproblems. At each iteration the two subproblems are solved using quadratic programming. If the valve point loading is to be considered, the two subproblems are solved using linear programming. In our scheme, linearization of the non-linear constraints of the problem is succeeded by utilizing Z-matrix techniques and sensitivity analysis.
94 citations
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TL;DR: Although syntactic methods seem suitable to the analysis of waveforms, not much progress has been made in the area of ECG waveforms.
68 citations
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TL;DR: Inextensional collapse mechanisms for axial crumpling of thin-walled circular cones and frusta (truncated circular cones) are presented in this paper, where shortening of the (thin) shell height is achieved by folding in a non-symmetric diamond mode about stationary circumferential and inclined plastic hinges; collapse proceeds progressively from the narrower end of the conical shell during the passage of a travelling hinge.
65 citations
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TL;DR: A unified overview of the algorithms developed for the processing of ‘prewindowed’ signals is offered and the derivation of all algorithms using a unified approach reveals the relationships among the various variables and between fast algorithms for direct and lattice-ladder structures.
59 citations
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TL;DR: In this article, the mechanical properties of crosslinked polyethylene were studied in an extended range of test conditions in order to determine the recommended service conditions for articles made of cross-linked polymethylene.
Abstract: The mechanical properties of crosslinked polyethylene were studied in an extended range of test conditions in order to determine the recommended service conditions for articles made of crosslinked polyethylene. On the other hand, this study can clarify the advantages of crosslinked polyethylene against the unmodified polymer. In fact, the results referred in the literature concerning the changes of mechanical properties of polyethylene introduced by crosslinking are contradictory and cannot clearly distinguish the two types of polymer. The above study concluded that tensile tests at elevated temperatures (above 70°C) with low stresses (about 0.5 MPa) can clearly describe the effect of crosslinking on the mechanical properties of LDPE, i.e., an improvement at least for deformation and service life.
56 citations
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TL;DR: In this article, a macroelement method is proposed for the complete linear hydromechanic analysis of arbitrarily shaped bodies of revolution with vertical axis can be carried out, based on discretization of the flow field around the structure by means of ring-shaped macroelements, the velocity potential in each element being approximated with Fourier series.
54 citations
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TL;DR: In this paper, an inextensional collapse mechanism is used to describe a non-symmetrical diamond mode shell folding which takes into account the concept of stationary circumferential and inclinded travelling hinges.
51 citations
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TL;DR: An inextensional collapse mechanism for the crumpling of thin-walled, grooved tubes and tubes without any discontinuities (non-grooved), when subject to axial load is considered in this article.
46 citations
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TL;DR: In this paper, the analytical method of the companion paper was used to verify the analytical results of the analysis, for circular, square and rectangular base shapes with aspect ratios up to 6, and including vertical, torsional and coupled swaying-rocking oscillations.
Abstract: Results of twenty-one free vibration tests on surface model footings on moist sand having different base shapes and about equal base area have been reported. These tests are used here to verify the analytical method of the companion paper, for circular, square and rectangular base shapes with aspect ratios up to 6, and including vertical, torsional and coupled swaying-rocking oscillations. Close agreement is found between predicted and measured frequencies in all cases, and also between damping ratios in vertical and torsional vibration. The experiments provide strong support for the vertical and torsional radiation damping graphs developed in the companion paper, including verification of the increased damping ratios predicted for the longest footings. The accord is also reasonably good for damping in those swaying-rocking tests in which rocking and material damping are dominant and the overall damping ratio is about 3%. However, the method overpredicts the measured swaying-rocking damping ratios in those tests in which swaying and radiation damping are also important, and for which larger damping ratios are predicted (4% to 9%). This discrepancy between half-space predicted swaying-rocking damping and measured values is explained by the effect of boundary wave reflections on the test results.
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TL;DR: In this article, a class of upper bounds for the positive roots of a polynomial is discussed, and it is shown that the bound (15) is nearly optimal in this class.
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TL;DR: In this paper, the problem of the stress distribution in an infinite medium which was weakened by an equilateral triangular hole under tension at infinity, was studied, and the triangular hole and its exterior was conformally mapped into the interior of a unit circle using the Schwarz-Christoffel transformation.
Abstract: The problem of the stress distribution in an infinite medium which was weakened by an equilateral triangular hole under tension at infinity, was studied. The triangular hole and its exterior was conformally mapped into the interior of a unit circle by using the Schwarz-Christoffel transformation. The stress function Φ(z) was defined by Muskhelishvili's complex-function theory and the conformal mapping technique. The Schwarz-Christoffel transformation was expressed as a truncated series with a finite numbers of terms. This function represents an equilateral triangle with rounded-off corners mapped into the unit circle. A change of the stress field around the triangular hole was investigated.
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TL;DR: A syntactic algorithm, described by an attribute grammar, is presented in this paper and it is shown that the detection of the QRS complexes is of fundamental importance in automatic electrocardiogram processing.
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TL;DR: The 5th BEER-INTERNATIONAL SEMINAR on MHD FLOWS and TURBULENCE as discussed by the authors was held in 1987 in London, UK with the theme of "MHD Flows and Turbulence".
Abstract: Past Due DATE : FEBRUARY 25-27, 1987 LOCATION: GOLDEN, CO TITLE: INDUSTRY-UNIVERSITY ADVANCED MATERIALS CONFERENCE INFO : DR JEROME G MORSE, ADVANCED MATERIALS INSTITUTE, COLORADO SCHOOL OF MINES, GOLDEN, CO 80401 TEL 303 273 3852 DATE : MARCH 1-5, 1987 LOCATION: HOUSTON, TX ABSTRACT: No Info TITLE: 6TH INTL SYMP ON OFFSHORE MECHANICS AND ARTIC ENGINEERING INFO : AMERICAN SOCIETY OF MECHANICAL ENGINEERS, UNITED ENGINEERING CENTER 345 E 47TH STREET, NEW YORK, NY 10017 DATE : MARCH 2-6, 1987 LOCATION: BEER-SHEVA, ISRAEL ABSTRACT: Past Due TITLE: 5TH BEER-INTERNATIONAL SEMINAR ON MHD FLOWS AND TURBULENCE INFO : PROF H BRANOVER, CENTER FOR MHD STUDIES, BEN-GURION UNIVERSITY OF THE NEGEV, BEER-SHEVA 85105, ISRAEL DATE : MARCH 18-20, 1937 LOCATION: CAMBRIDGE, UK ABSTRACT: No Info TITLE: EUROMECH 220: MIXING AND CHEMICAL REACTIONS IN TURBULENT FLOWS INFO : PROF K N C BRAY, UNIVERSITY ENGINEERING DEPARTMENT, TRUMPINGTON ST, CAMBRIDGE, CB2 1PZ UK DATE : MARCH 22-26, 1987 LOCATION: SAN ANTONIO, TX ABSTRACT: 8/1/36 TITLE: FOURTH INT. CONF. ON NUMERICAL METHODS IN FRACTURE MECHANICS INFO : M.F. KANNINEN, ENGIN. AND MAT. SCIENCE DIV., SOUTHWEST RESEARCH INST., PO DRAWER 28510, SAN ANTONIO, TX 78284 TEL: 512-522-3248 OATE : APRIL 6-8, 1987 LOCATION: GHENT, BELGIUM ABSTRACT: Past Due TITLE: INTERNATIONAL CONFERENCE ON STABILITY OF PLATE AND SHELL STRUCTURES INFO : D VANDEPITTE, GROTESTEENWEG-NORD 2, B-9710 ZWIJNAARDE, BELGIUM DATE : APRIL 6-8, 1987 LOCATION: MONTEREY, CA ABSTRACT: 8/22/86 TITLE: 28TH AIAA STRUCTURES, STRUCTURAL DYNAMICS, AND MATERIALS CONFERENCE INFO : DR. KEITH T. KEDWARD, ALCOA DEFENSE SYSTEMS, INC., 16761 VIA DEL CAMPO COURT, SAN DIEGO, 92127 TEL: 619-695-2260 DATE : APRIL 6-9, 1987 LOCATION: LONDON, ENGLAND ABSTRACT: 7/1/86 TITLE: 5TH INTERNATIONAL MODAL ANALYSIS CONFERENCE INFO : DOMINICK J. DEMICHELE, UNION COLLEGE, GRADUATE S CONTINUING STUDIES, WELLS HOUSE 1 UNION AVENUE, SCHENECTADY, NY 12308 TEL:518-370-6288 DATE : APRIL 6-10, 1987 LOCATION: BARCELONA, SPAIN ABSTRACT: No Info TITLE: INT CONF ON COMPUTATIONAL PLASTICITY INFO : D R J OWEN, DEPT OF CIVIL ENGINEERING, UNIVERSITY COLLEGE OF SWANSEA, SINGLETON PARK, SWANSEA SA2 8PP, UK DATE : APRIL 7-8, 1987 LOCATION: GLASGOW, SCOTLAND ABSTRACT: No Info TITLE: APPLIED SOLID MECHANICS CONFERENCE INFO : A S TOOTH, DEPARTMENT OF MECHANICS OF MATERIALS, UNIVERSITY OF STRATHCLYDE, 75 MONTROSE ST, GLASGOW Gl 1XJ UK ABSTRACT: 8/22/86 DATE : APRIL 9-10, 1987 LOCATION: MONTEREY, CA TITLE: AIAA DYNAMICS SPECIALISTS CONFERENCE INFO : ANTHONY F. MESSINA, DEPT. 76-12, LOCKHEED-CALIFORNIA CO., P.O. BOX 551, BURBANK, CA 91520 TEL: 818-847-4910 8/22/86 DATE : APRIL 9-10, 1987 LOCATION: MONTEREY, CA TITLE: AIAA DYNAMICS SPECIALISTS CONFERENCE INFO : ANTHONY F. MESSINA, DEPT. 76-12, LOCKHEED-CALIFORNIA CO., P.O. BOX 551, BURBANK, CA 91520 TEL: 818-847-4910 DATE : APRIL 13-15, 1987 LOCATION: UNIVERSITY PARK,PA ABSTRACT: Past Due TITLE: INTERNATIONAL CONFERENCE ON ENVIRONMENTAL DEGRADATION OF MATERIALS INFO : R P MCNITT, 227 HAMMOND BUILDING, UNIVERSITY PARK, PA 16802 OATE : APRIL 13-16, 1987 LOCATION: SAN ANTONIO, TX ABSTRACT: Past Due TITLE: IUTAM SYMPOSIUM ON ADVANCED BOUNDARY ELEMENT METHODS INFO : DR T A CRUSE, SOUTHWEST RESEARCH INSTITUTE, P 0 DRAWER 28510, SAN ANTONIO, TX 78284 TEL 512 684 5111 DATE : APRIL 26-30, 1987 LOCATION: CINCINNATI, OH ABSTRACT: 6/1/86 TITLE: SYM. ON COMPOSITE MATERIALS: FATIGUE AND FRACTURE, 9TH SYM. (ASTM) INFO : PAUL A. LAGACE, MIT, ROOM 33-313, CAMBRIDGE, MA 02139 TEL: 617-253-3628 DATE : APRIL 26-30, 1987 LOCATION: CINCINNATI, OH ABSTRACT: 6/1/86 TITLE: MECHANICAL RELAXATION OF RESIDUAL STRESSES (ASTM) INFO : LEONARD MORDFIN, NATIONAL BUREAU OF STANDARDS, B344 MATERIALS BUILDING, GAITHERSBURG, MD 20899 DATE : APRIL 28-MAY 1, 1987 LOCATION: UXBRIDGE, UK ABSTRACT: Past Due TITLE: MAFELAP 1987:C0NF ON THE MATHEMATICS OF FINITE ELEMENTS 8 APPLICATIONS INFO : SECRETARY, INSTITUTE OF COMPUTATIONAL MATHEMATICS, BRUNEL UNIVERSITY, UXBRIDGE, MIDDLESEX, UB8 3PH UK Journal of Applied Mechanics DECEMBER 1986, Vol. 53/917 Downloaded 12 Aug 2010 to 146.6.92.224. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm
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TL;DR: A new queueing model appropriate for the analysis of a buffered CSMA/CD protocol is proposed, and a two-dimensional semi-Markov chain is constructed, examining the state of the system immediately after the successful transmission of packets.
Abstract: In this paper, we propose a new queueing model appropriate for the analysis of a buffered CSMA/CD protocol. We assume that each user has a finite buffer capacity. The system, in general, can be modeled as a multidimensional semi-Markov chain. The enormous state space for the complete description is reduced by appropriate exploitation of the features of the system, e.g., fairness towards all users, dependence of the probability of successful transmission only on the total number of busy users, etc. Using a further assumption we construct a two-dimensional semi-Markov chain, examining the state of the system immediately after the successful transmission of packets. The obtained solution is extremely accurate and exhibits excellent agreement with simulation results, which corroborates the correctness of our method.
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TL;DR: In this article, an experimental study was conducted to evaluate the use of liquefied petroleum gas (LPG) as a secondary fuel for a Ricardo E-6, naturally aspirated, four-stroke diesel engine having a turbulence combustion chamber (indirect injection).
Abstract: An experimental study is conducted to evaluate the use of liquefied petroleum gas (LPG) as a secondary fuel for a Ricardo E-6, naturally aspirated, four-stroke diesel engine having a turbulence combustion chamber (indirect injection). The gaseous LPG is introduced together with the aspirated air (fumigation) at various proportions with respect to the diesel fuel which constitutes the main part. The influence of fuel feed ratios (LPG/diesel), in a vast range of loads, on fuel consumption, pressure diagrams, exhaust smokiness and exhaust gas emissions (nitrogen oxides, hydrocarbons and carbon monoxide) is investigated, the baseline being the single diesel fuel operation. The study for this type of engine, which has not being reported in the literature, shows a promise of the present method and reveals that above 60 per cent of maximum load the whole effect is beneficial concerning specific fuel consumption and smoke reduction. The examination of gaseous pollutant levels shows an involved relation with respect to load and fuel proportions. The best results (coupled to acceptable cylinder pressure levels) is obtained at a diesel fuel substitution value of 75% of maximum load, with an LPG mass fraction in the range 10 to 15%.
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TL;DR: In this article, the shape of Griffith crack was investigated and it was shown that the deformed Griffith crack is always an ellipse in opposition to the singular and two-term solutions, which give a parabolic shape for deformed lips of the crack.
Abstract: A theoretical investigation has been performed to determine the exact geometrical features of an inclined, biaxially loaded, crack in an infinite elastic plate It has been shown that the shape of the deformed Griffith crack, as this follows from the exact linear elastic displacement field at the lips of the crack, is always an ellipse in opposition to the singular and two-term solutions which give a parabolic shape for the deformed lips of the crack The combined effect of the mode of loading, the load biaxiality and the crack orientation on the lengths of the semiaxes of the ellipse, and the orientation of its major axis with respect to the initial crack was studied in detail It is also shown that in general the positions of the initial crack tips after deformation on the curve of the ellipse do not coincide with the points of the maximum curvature of the ellipse The case, where the ellipse presents overlapping lips and where the elastic solution becomes invalid, has been examined and the configurations of the problem-parameters for which this phenomenon happens has been established These important features of the deformed shape of the loaded crack may have a considerable influence on the mode of loading and the behaviour in yielding and fracture of the cracked bodies
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TL;DR: In this article, the long-term migration data for polyvinyl chloride (PVC)/labelled dioctyl phthalate (DOP)/methanol, n-propanol, or n-butanol were presented.
Abstract: Long-term migration data, by radioactivity measurements, are presented for the systems poly(vinyl chloride) (PVC)/labelled dioctyl phthalate (DOP)/methanol, n-propanol, or n-butanol. At equilibrium, migration levels determined exceed 85% for methanol and reveal complete removal for the higher alcohols. Migration kinetics failed to be described by the long-term Fickian approximation with a diffusion coefficient independent of concentration. Nevertheless, qt/q∫, where q∫ the amount migrated at infinite time and qt the amount lost up to time t, increases linearly with (time)1/2/(thickness) during the initial stages of the process and then a linear also relationship appears, but with severely reduced slope. Similar data concerning the systems PVC/DOP/petroleum oils confirm the behavior observed.
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TL;DR: In this paper, an econometric model, utilizing the translog cost function, is estimated for three major sectors of the economy - agriculture, industry, and transport, using time-series data over the period 1960-1980.
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TL;DR: In this paper, the effect of drying on the macro-and partly meso-pore structure of Greek lignite lumps was investigated under vacuum (∼0.2 Torr) in a constant-temperature (100 − 250 °C) environment.
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TL;DR: In this paper, a theoretical model for the evaluation of the elastic modulus in particulate composites has been developed, taking into account the existence of a mesophase between main phases, which constitutes an important parameter influencing the behaviour of a composite material.
Abstract: A theoretical model for the evaluation of the elastic modulus in particulate composites has been developed. The method takes into account the existence of a mesophase between main phases, which constitutes an important parameter influencing the behaviour of a composite material. This layer between the matrix and filler develops different physico-chemical properties from those of the constituent phases and variable ones along its thickness. The effect of the progressive variation of the elastic modulus of the mesophase on the modulus of the composite was estimated by applying various simple laws of variation. Convenient laws of variation were introduced, varying from a simple one, assuming a linear law, to a more refined one using a parabolic law. Experimental results with particulates, based on iron-filled epoxy composites, compared satisfactorily with other models. However, the model based on a parabolic law was superior to all others on physical grounds.
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TL;DR: In this article, the authors studied the deformation and fracture between a pair of holes in an infinite plate submitted to uniaxial tension in order to gain insight for the mechanism of ductile fracture.
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TL;DR: In this article, a short review of available computer-aided methods for system reliability assessment, the DYLAM technique is proved to be a more adequate tool than the normally used fault trees/event trees.
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TL;DR: A method which recognizes the shape of the ST segment and gives the equation of a straight line or of a parabola that best approximates this ST segment.
Abstract: A method which recognizes the shape of the ST segment is presented in this paper. This method when applied to an ST segment gives as results: 1) the onset and the end of the ST segment and 2) the equation of a straight line or of a parabola that best approximates this ST segment.
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TL;DR: In this paper, the influence of the hard fiber and the mesophase layers in a soft-hard-soft composite of a biphase plate subjected to a dynamic tensile load, on the fracture mode and bifurcation process in both phases was investigated.
Abstract: The influence of the hard fiber and the mesophase layers in a soft-hard-soft com bination of a biphase plate subjected to a dynamic tensile load, on the fracture mode and bifurcation process in both phases was investigated. It was assumed that the soft matrix envelops the hard-fiber layer of the plate, so that this plate constitutes a merid ianal section of a representative volume element of a unidirectional fiber com posite.
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TL;DR: It is shown how the total-step and single-step iterative methods, as well as their improvements, for the simultaneous determination of simple zeros of polynomials can be used for the determination of basic functions or sectionally analytic functions.
Abstract: It is shown how the total-step and single-step iterative methods, as well as their improvements, for the simultaneous determination of simple zeros of polynomials can be used (with one slight modification) for the determination of simple zeros of analytic functions (inside or outside a simple smooth closed contour in the complex plane) or sectionally analytic functions (outside their arcs of discontinuity). Numerical results, obtained by the single-step method, are also presented.
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TL;DR: A new notion of multiversion serializability (MVSR) based on conflicts (MMVCSR) is introduced, and its relation with the well known single version conflict serialIZability (CSR).
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TL;DR: In this paper, an analysis of the displacement field along the lips of an internal crack in an infinite elastic plate, submitted to pure shear, was undertaken, based on the exact solution.
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TL;DR: The potential use of main-frame computers to carry out algebraic operations by means of the ALTRAN language and system is discussed with reference to the Levy-Nadai method for plate bending to generate a truly closed-form solution.