Showing papers by "National Technical University of Athens published in 1987"
TL;DR: Thermo-economie functional analysis is a method for optimal design or improvement of complex thermal systems that minimizes the total cost of owning and operating the system, subject to constraints revealed by the functional diagram and analysis.
219 citations
TL;DR: In this paper, the results of an extensive analytical investigation on the mechanisms involved in filtration of biological sludges by studying the changes in the bound water content of activated and mixed digested sludge when they were subjected to several physical, and chemical processes.
189 citations
TL;DR: In this article, a new method was developed for numerical integration of the one dimensional radial Schrodinger equation, which involves using different integration formulae in different parts of the range of integration rather than using the same integration formula throughout.
122 citations
TL;DR: In this article, an algorithm for the calculation of the hydrogen sulphide conversion at equilibrium, which also computes all the concentrations of the components present in the gas system H 2 S/H 2 /S i (i = 1,…,8) was developed, using the available thermodynamic data.
89 citations
TL;DR: In this paper, the physico-chemical changes occurring during electro-discharge machining of steel (structural, medium carbon and alloyed steels) surfaces are reported and correlated with overall process parameters and metal removal rates.
80 citations
TL;DR: In this article, the static and dynamic horizontal stiffness of arbitrarily shaped rigid foundations embedded in a reasonably deep and homogeneous soil deposit is investigated using a boundary element formulation, and the results are presented in the form of simple and versatile analytical expressions and dimensionless graphs.
Abstract: Using a boundary element formulation, a systematic parametric study is performed on the static and dynamic horizontal stiffness of arbitrarily shaped rigid foundations embedded in a reasonably deep and homogeneous soil deposit. Results are presented for rectangular base shapes of aspect ratios up to 10, a wide range of embedment depths, three values of the Poisson ratio, and an ample range of frequencies. The height of the foundation sidewalls is varied parametrically between the two extremes of no sidewall and complete sidewall with height equal to embedment depth and in perfect contact with the surrounding soil. Results have been obtained for circular and triangular plan shapes as well, including cases of non‐uniform contact along the foundation perimeter. The results are presented in the form of simple and versatile analytical expressions and dimensionless graphs. Two examples illustrate their direct practical applicability.
70 citations
01 Jun 1987
TL;DR: The degree to which queries are amenable to rapid parallel evaluation by the cooperation of many processors is examined, and the proposed algorithm for it is proposed.
Abstract: They are written m a notation called DATALOG, that IS, PROLOG without function symbols and other lmpurities (an orthogonal way of vlewmg DATALOG 1s as Relatlonal Calculus with the additional power of recurslon) For more on DATALOG see, for example, [UV] Both queries above define a view S m terms of the database relations a and b We are Interested m the parallel complexJtyof these and similar queries, that is, the degree to which such queries are amenable to rapid parallel evaluation by the cooperation of many processors Recently, m view of the projected avallablhty of multlprocessmg systems with a very large number of processors, there has been much Interest m such a classlficatlon of computational problems In particular, it has been proposed that a problem be considered satlsfactorlly solved m parallel if there IS an algorithm for it which can be rendered as a circuit with a polynomial number of gates, andpoiyiogarlthmlc depth (that IS, of depth O(logk n), where n IS the length of the input) The class of all problems thus solvable 1s called NC [Co] Obviously, NC 1s a subset of P, the class of all problems solvable m polynomial sequentd time (It IS perhaps amusing that, m response to a sequence of lmpresslve breakthroughs m computmg technology,
60 citations
TL;DR: In this paper, a method for the numerical calculation of eigenvalues of the Schrodinger equation was developed for the transmission line theory known as the impedance, where the eigen values are computed directly as the roots of a function known in transmission line theories as impedance.
50 citations
TL;DR: In this article, the efficiency of benzoyl peroxide (BPO) as a crosslinking agent for polyethylene (PE) was studied as a function of concentration and temperature in relation with the resin melt flow index and particle size.
Abstract: The efficiency of benzoyl peroxide (BPO) as a crosslinking agent for polyethylene (PE) was studied as a function of concentration and temperature in relation with the resin melt flow index and particle size. The investigation can lead to the selection of suitable materials and operating conditions for a proper crosslinking process. The decomposition characteristics of the initiator studied can be regulated in order to allow faster production rates when crosslinked PE are prepared by injection molding.
50 citations
TL;DR: In this article, a simple method involving algebraic formulas and dimensionless parametric charts is developed for estimating the lateral radiation damping coefficients of arbitrarily shaped rigid foundations embedded in an elastic halfspace.
Abstract: Using numerical results obtained by the boundary element method, the paper develops a simple method involving algebraic formulas and dimensionless parametric charts for estimating the lateral radiation damping coefficients of arbitrarily shaped rigid foundations embedded in an elastic halfspace. Complete as well as partial, symmetric and nonsymmetric types of contact are considered between the vertical sidewalls and the surrounding soil. Valuable insight is gained into the mechanics of radiation damping and conclusions of practical significance are drawn regarding the effects of potential separation/slippage between the walls and the soil near the ground surface. Two examples illustrate the direct applicability of the method, while its versatility is shown through an extension to treat a more realistic inhomogeneous soil profile.
46 citations
TL;DR: An O(n2) algorithm for finding the point on the plane which, if added to a given set of n points, will result in the shortest possible spanning tree.
TL;DR: In this article, the reaction kinetics for the leaching of iron oxides from diasporic bauxite by hydrochloric acid have been studied at dilute solid phase concentrations in a magnetically stirred reactor.
TL;DR: The results of both the present and previous investigations on two units burning lignite from the Ptolemais deposit show that electricity generation from Greek lignites is associated with radioactivity discharges which are rather high relative to corresponding discharge data from the literature as discussed by the authors.
TL;DR: In this article, a solvent technique is studied for the recovery of polystyrene (PS) foam waste, which ensures removal of any impurities present and comprises dissolution of the starting material, filtering, reprecipitation, thorough washing of the polymer grains obtained and drying.
TL;DR: In this article, a triaxial cyclic test on concrete is presented and compared with corresponding ones for dense rocks (mainly marbles) obtained under the same laboratory conditions, and common basic characteristics are confirmed regarding the essential effect of intermediate stress, the cataclastic mode of deformation under low confinement, coupling between volumetric strain and deviatoric stress, dependence of the equivalent modulus Eq and the hysteresis loops.
TL;DR: In this paper, the stationary value of the third stress invariant as a local fracture parameter is studied and a condition of initiation of the crack is proposed as Det-criterion of fracture.
TL;DR: Pissis et al. as discussed by the authors reported on thermally stimulated depolarization current (TSDC) measurements on plant stems of six different species in the temperature range of 77-300 K and over a wide range of water contents, in an attempt to determine the binding modes of water molecules.
Abstract: Pissis, P., Anagnostopoulou-Konsta, A. and Apekis, L. 1987. A dielectric study of the state of water in plant stems.—J. exp. Bot. 38: 1528-1540. We report on thermally stimulated depolarization current (TSDC) measurements on plant stems of six different species in the temperature range of 77-300 K and over a wide range of water contents, in an attempt to determine the binding modes of water molecules. The measurements revealed the existence of three dielectric dispersions. The first dispersion at low temperatures is attributed to the reorientation of loosely bound water molecules. The origin of the second dispersion at intermediate temperatures is not yet clear. It is either due to the reorientation of tightly bound water molecules or to the relaxation of some botanical materials rather than water. Finally, the third dispersion, at high temperatures, is attributed to water-assisted space charge polarization connected with dc ionic conductivity. The amount of tightly bound water is estimated to be about 0-2 g g~1 of dry material or about 30% of the total water content, the rest being mainly loosely bound water. Little or none of the water behaves dielectrically like pure water. Key words—Free and bound water, dielectric properties, water content. Correspondence to: National Technical University of Athens, Physics Department, Zografou Campus, 15773 Athens, Greece.
TL;DR: In this paper, the integral forms of the momentum and tracer equations were integrated on the basis of an assumption concerning a function of the spreading coefficients, and the analysis led to unified analytical expressions concerning the axial velocity and concentration distributions along the jet axis.
Abstract: Experiments on round vertical turbulent buoyant jet have been performed using tap water as the jet liquid and saltwater solution as the ambient liquid. Measurements were made of the centerline axial velocity and the centerline and radial concentrations. The integral forms of the momentum and tracer equations were integrated on the basis of an assumption concerning a function of the spreading coefficients. The validity of this assumption has been verified by the experimental results and the analysis led to unified analytical expressions concerning the axial velocity and concentration distributions along the jet axis. The present findings are discussed and compared with previously reported works.
TL;DR: The interactive environment allows the user to obtain the results of his studies against the one-line diagram of the network and effect changes in it in basically diagrammatic terms.
Abstract: This paper presents an interactive graphics package for distribution system analysis. The radial operation of the distribution networks is exploited in order to effect a fast and accurate calculation of the power flows, the voltage drops and the power losses at the various segments of a feeder for a given set of typical load curves. The algorithms developed are based on an efficient method to store the network topology and can easily accommodate changes in the infeeding nodes and variations in the network structure due to switching operations. The interactive environment allows the user to obtain the results of his studies against the one-line diagram of the network and effect changes in it in basically diagrammatic terms. The package requires very modest computer resources and provides planning and operating engineers with a powerful tool to conduct analytical distribution system studies with ease and flexibility.
TL;DR: In this article, the stress field for the three-dimensional plasticity fracture problem is formulated by reducing the formulation to a system of singular integral equations, referred to as the Singular Integral Operators Method (SIOM).
TL;DR: In this article, a new technique is proposed for the numerical evaluation of the finite-part singular integral equations of the first and the second kind with general kernels, which are frequently encountered in problems of applied mechanics and especially in plane and antiplane elasticity.
Abstract: A new technique is proposed for the numerical evaluation of the finite-part singular integral equations of the first and the second kind with general kernels, which are frequently encountered in problems of applied mechanics and especially in plane and antiplane elasticity. Some new formulas are established in order to solve this type of singular integral equations. It can be said, that these formulas are a generalization of the well known Plemelej formulas. A numerical application is given to the determination of the stress field in the neighbourhood of a crack parallel to the free boundary of an elastic isotropic semiplane.
TL;DR: In this paper, the authors used Fourier transforms in combination with the Wiener-Hopf technique to evaluate asymptotically the cleavage stress and its intensity at the crack tip.
Abstract: The stress intensity factor for a long cracked strip was determined within the context of the linear orthotropic clasticity. The body had the form of an infinite strip containing a semi-infinite crack at the middle distance of the strip faces. Fourier transforms in combination with the Wiener-Hopf technique were employed to evaluate asymptotically the cleavage stress and its intensity at the crack tip.
TL;DR: In this article, the postbuckling response of a simply supported, axially compressed, uniform bar of non-linearly elastic material is analyzed and conditions of stability of the post-buckling path depending on the degree of the material non-linearity are established.
Abstract: The post-buckling response of a simply supported, axially compressed, uniform bar of non-linearly elastic material, is presented. The buckling analysis considers large deflections of the elastica type in which the influence of bar axis shortening is also taken into account. The solution of the strongly non-linear differential equation is obtained analytically by a simple and very efficient approximate technique leading to reliable results. Conditions of stability of the post-buckling path depending on the degree of the material non-linearity are established. Moreover, the individual and coupling effect of the material non-linearity and the bar slenderness ratio on the post-buckling path are thoroughly discussed. Numerical results for both non-linearly and linearly elastic material show the efficiency and reliability of the proposed procedure.
TL;DR: In this paper, the same measurements are employed in conjunction with appropriate models for analysing total radiation in direct and diffuse components and for estimating the energy incident on tilted surfaces, which are presented in the form of algebraic correlations having as independent variable time (hour, day, month), surface azimuth and angle of inclination to the horizontal plane.
TL;DR: In this article, the authors hypothesised that these features reproduce a new mode of modern Greek capitalism such as the widespread pattern of multiple employment, the growing role of informal activities, the new function of the household in relation to outworking, and the gender dimension.
Abstract: The period 1970–73 was a turning point after which certain intermediate regions and cities in Greece stopped declining and began to develop new productive activities After 1974, the slowdown of economic growth rates coincided with the increasing economic, social, and political dynamism of these intermediate areas These phenomena are poorly explained by many liberal and Marxist regional development approaches Their almost exclusive attention to industrial production, to capital–labour movements or to the ‘basic’ formal, full-time, skilled, male industrial employment in an area, reproduces a production determinism which seems inadequate for the present period of crisis and deep restructuring These approaches neglect certain key features of modern Greek capitalism such as the widespread pattern of multiple employment, the growing role of informal activities, the new function of the household in relation to outworking, and the gender dimension It is hypothesised that these features reproduce a new mode o
TL;DR: In this paper, the scattering of propagating waves from an abruptly terminated singlemode-fiber waveguide is investigated analytically by means of an integral equation, where weak-guidance conditions on the fiber guide are assumed to formulate the corresponding boundary value problem, and then a Neumann-series iterative solution is developed to solve the basic integral equation.
Abstract: The scattering of propagating waves from an abruptly terminated single-mode-fiber waveguide is investigated analytically by means of an integral equation. Weak-guidance conditions on the fiber guide are assumed to formulate the corresponding boundary value problem, and then a Neumann-series iterative solution is developed to solve the basic integral equation. The reflection coefficients of the propagating waves and the radiation patterns of the diffracted waves are also computed. Numerical results are presented for several abruptly terminated fiber guides.
TL;DR: In this paper, three real gas isentropic exponentskTv,krv,kpT are introduced, which when used in place of the classical exponentk=cp/c in the ideal gas isENTropic change equations, the latter may describe very accurately the isentropy change of real gases.
Abstract: Three real gas isentropic exponentskTv,krv,kpT are introduced, which when used in place of the classical isentropic exponentk=cp/c in the ideal gas isentropic change equations, the latter may describe very accurately the isentropic change of real gases. The usual practice of employing exponentk may lead to considerably incorrect results even when the value ofk corresponds to the correct local value ofcp/cv of the real gas under examination. The numerical values of the new exponents are calculated in the case of real air for temperatures from 150 K to 450 K and pressures from 1 bar to 1000 bar. It is seen that at low temperatures and high pressures the values of the new exponents differ considerably from the values of the classical exponentk. Therefore, the error resulting by approximating, as is usually the case, the behaviour of real gases by the ideal gas isentropic change equations in a stepwise fashion with exponentk instead of the new exponents, is considerable. It follows that exponentk, which appears in various relations in thermodynamics, fluid mechanics, gasdynamics, heat transfer etc., should be suitably replaced by combinations of the three exponents. Related numerical examples, made in the case of real air, showed that the use ofk leads (in the temperature and pressure ranges examined) to a 5% error in the calculation of blowby rate in internal combustion enginers, high pressure compressors or steam turbines and to a 50% error in the calculation of the isentropic expansion or compression.
TL;DR: In this paper, the problem of finding an approximation to the conformal map of a simply-connected domain onto a rectangle R, where four specified points on ∂Ω are mapped respectively onto the four vertices of R, is studied.
TL;DR: In this article, the authors investigated the formulae of the stress field in the neighbourhood of a plane crack of arbitrary shape in an anisotropic material for the two-dimensional case.
TL;DR: In this paper, the performance of NH3/H2O absorption units, operating in conjunction with high and intermediate temperature solar collectors in Athens, is predicted along the typical year, in the cases (a) of absorption refrigeration units working as refrigerators, and (b) of reversed absorption devices working as heat transformers.