Showing papers by "University of Patras published in 1979"

Free Convection Effects on the Stokes Problem for an Infinite Vertical Limiting Surface with Constant Suction

Abstract: This paper studies the effects of free convection currents on the flow field of an incompressible viscous fluid past an impulsively started infinite vertical porous limiting surface when the fluid is subjected to a suction with uniform velocity. An exact analysis of the problem is obtained with the aid of the Laplace transform technique and the expressions for the velocity field, temperature field and for their related quantities of skin friction and rate of heat transfer are given. The results thus obtained are discussed quantitatively for air and water.

Three-dimensional periodic oscillations generating from plane periodic ones around the collinear Lagrangian points

Abstract: The three families of three-dimensional periodic oscillations which include the infinitesimal periodic oscillations about the Lagrangian equilibrium pointsL1,L2 andL3 are computed for the value μ=0.00095 (Sun-Jupiter case) of the mass parameter. From the first two vertically critical (|a v |=1) members of the familiesa, b andc, six families of periodic orbits in three dimensions are found to bifurcate. These families are presented here together with their stability characteristics. The orbits of the nine families computed are of all types of symmetryA, B andC. Finally, examples of bifurcations between families of three-dimensional periodic solutions of different type of symmetry are given.

Asymptotic distribution of the log-likelihood function for stochastic processes

Abstract: Let X 0, X 1,⋯, X nbe r.v.'s coming from a stochastic process whose finite dimensional distributions are of known functional form except that they involve a k-dimensional parameter. From the viewpoint of statistical inference, it is of interest to obtain the asymptotic distributions of the log-likelihood function and also of certain other r.v.'s closely associated with the likelihood function. The probability measures employed for this purpose depend, in general, on the sample size n. These problems are resolved provided the process satisfies some quite general regularity conditions. The results presented herein generalize previously obtained results for the case of Markovian processes, and also for i.n.n.i.d. r.v.'s. The concept of contiguity plays a key role in the various derivations.

A second-order perturbation theory for differentially rotating gaseous polytropes

Abstract: The aim of the present investigation has been to specify the structure of a differentially rotating gaseous polytrope, by extending Chandrasekhar's method to include second-order terms in the perturbation parameter. The formal results obtained permit the determination of the polytropic structure for all types of differential rotation of cylindrical symmetry. The formalism developed works efficiently in the limiting case of uniform rotation and the results obtained are compared with those of previous investigations of uniformly rotating polytropes.

Effects of extrusion on the structure and properties of high‐impact polystyrene

Abstract: A commercial heat-resistant polystyrene (Mn = 7 × 104, Mw = 3 × 105), containing 9 percent cis-1, 4-polybutadiene, was extruded either repeatedly (2 to 8 times) at 220°C, or else only once at a higher temperature (up to 290°C). Neither treatment significantly altered the melt rheology at 220°C (pseudoplastic, with n = 0.39), or the tensile modulus (1.5 GPa) and yield stress (20 MPa), or the material's rubber content, determined by both infrared spectrophotonietry and Wijs iodometry. Other properties, only slightly affected by recycling at 220°C, were changed after one extrusion at 290°C: elongation at tensile failure was reduced by 57 percent; in impact testing the strength was 29 percent less, and the mode of fracture (revealed by scanning electron microscopy of the surfaces) became brittle instead of ductile; the rubber particles seen in the transmission electron microscope had agglomerated and lost sphericity; and the ratio of weight-to-number-average molecular weight of the polystyrene component, calculated from gel permeation chromatograms, increased by 93 percent. Mechanical spectra (Rheovibron), from −120 to + 120°C at 110 Hz, changed gradually with increasingly harsh treatment of the material, a peak emerging at ∼50°C due to a beta relaxation of the polystyrene. Thus, good properties were retained after normal processing, but were lost after shearing at too high a temperature, probably because of destruction of entanglements and of the bonds between polystyrene and rubber.

Effects of free convection on the hydromagnetic accelerated flow past a vertical porous limiting surface

Abstract: The effects of free convection on the accelerated flow of a viscous, incompressible and electrically conducting fluid (e.g. of a stellar atmosphere) past a vertical, infinite, porous limiting surface (e.g. of a star) in the presence of a transverse magnetic field, is considered. The magnetic Reynolds number of the flow is taken to be small enough, so that the induced magnetic field is negligible. Expressions for velocity and skin-friction are obtained by using Laplace transform, when the Prandtl number is equal to one (P=1). Graphs showing variations of velocity and skin-friction, for different values ofG (Grashof number) andM (magnetic parameter) are plotted, and the results of them are discussed.

Damping of Power Swings in AC Tie Lines Using a Parallel DC Link Operating at Constant Reactive Power Control

Abstract: This paper presents a method for the damping of power swings in the ac lines interconnecting two power systems, utilising the fast electronic power flow control characteristics of a parallel dc link. The basic principle of the method is that the dc link is operated in order to maintain constant active power flow in the ac tie lines and constant reactive power consumption at the converter terminals. The proposed control strategy was tested using dynamic simulation techniques and the results obtained show that, under certain conditions, the contribution of a parallel dc link to the stability the ac system can be significant.

Effects of mass transfer on the free convective flow in the Stokes' problem for an infinite vertical limiting surface

Abstract: An exact analysis of the mass transfer effects on the free convection flow of an incompressible viscous fluid past an impulsively started infinite vertical (wall) limiting surface (Stokes's or Rayleigh's problem) has been carried out. Expressions for the velocity, temperature, species concentration and skin friction are obtained by using the Laplace transform technique. The velocity field and the skin friction are shown graphically for air (P=0.71) and mercury (P=0.025). The effects ofG (Grashof number),Gc (the modified Grashof number) andSc (Schmidt number) are considered qualitatively during the course of discussion.

The alcohol dehydrogenase locus affects meiotic crossing-over in Drosophila melanogaster

Abstract: Although it is widely known that a variety of factors such as temperature, high or low energy radiation and chemical agents affect the recombination fraction (Broadhead & Kidwell, 1975), little information exists on the genetic control of meiotic crossing-over (GreU, 1978). It has been shown that ethanol has manifold biochemical and genetical effects in eucaryotes (Harsanyi et al., 1977) including man (Ristow & Obe, 1978). Among other effects ethanol is a recombinogen (Harsanyi et al., 1977). Furthermore, it is known that the Alcohol dehydrogenase locus (Adh; E.C.I.I.I.1.) is located on the second chromosome (map position 50.1) of Drosophila melanogaster and is polymorphic with two common alleles, Adh F (Fast) and AdhS (Slow) (Grell et al., 1965). Adh alleles extracted from different populations ofD. melanogaster exhibit different specific activities, but within a population the specific activity of AdhF (or AdhF/S) genotypes are greater than that of AdhS (AdhF >AdhF/S >AdhS; Gibson, 1970). It has been also shown that the Adh specific activity depends upon regulatory genes (Ward & Hebert, 1972; McDonald et al., 1977) and on various alcohols which are used

Transient Stability of AC-DC Systems

Abstract: A power system consisting of two control areas is simulated in this paper. The two areas are interconnected via two parallel ac tie lines and as an alternative, the Interconnection via an ac line and a parallel dc link is examined. The two system configurations are subjected to the same disturbances and faults and their responses are compared and assessed with regard to system stability.

Pumpellyite-actinolite and greenschist facies metamorphism in lesvos island (Greece)

Abstract: The southern part of Lesvos island consists of a Late Palaeozoic-Triassic marble-phyllite series with intercalations of mafic metavolcanics which exhibit mineralogies of the pumpellyite-actinolite and greenschist facies. Pumpellyite is developed best in the metabasalts of the south-eastern part and towards the northwestern part of the terrain it gives way to actinolite and/or epidote bearing assemblages. Local variations in μCO2 are suggested to explain the extensive distribution of chlorite-calcite instead of the Ca−Al-silicate bearing assemblages diagnostic of subgreenschist facies metamorphism.

Bifurcation in non-negotiable games: A paradigm for self-organisation in cognitive systems

Abstract: The interaction between two self-organising systems, each possessing two hierarchical levels, is dealt with. The communication process is pursued as a bidirectional information transfer. The criterion for the selection of the intrasystem control algorithm (s) is based on the maximisation of the ‘Joint Figure of Merit’ related to the long-term average of an appropriately weighted sum of conflicting terms: Probabilities of homeostases versus adequate matching with the partner.

Ability of the male recombination factor 31.1 MRF to be transposed to another chromosome in Drosophila melanogaster.

Abstract: By classical genetic experiments, evidence is provided that the male recombination factor, 31.1 MRF, has the ability to be transposed to another chromosome. The procedure by which the transposition occurs must be different from that of classical crossing over. It appears that transposition occurs only when the factor is active in male germ cells. Moreover, the factor appears to be able to undergo successive transpositions. Furthermore, the integration sites of the factor, when transposed into another chromosome, may not be completely random. Finally, the third chromosome of the 31.1/CyL4 strain can also induce male recombination.

Asymptotic expansion of the log-likelihood function based on stopping times defined on a Markov process

Abstract: Consider the parameter space Θ which is an open subset of ℝk,k≧1, and for each θ∈Θ, let the r.v.′sYn,n=0, 1, ... be defined on the probability space (X,A,Pθ) and take values in a Borel setS of a Euclidean space. It is assumed that the process {Yn},n≧0, is Markovian satisfying certain suitable regularity conditions. For eachn≧1, let υn be a stopping time defined on this process and have some desirable properties. For 0 < τn → ∞ asn→∞, set\(\theta _{\tau _n } = \theta + h_n \tau _n^{ - 1/2} \)hn →h ∈Rk, and consider the log-likelihood function\(\Lambda _{ u _n } (\theta )\) of the probability measure\(\tilde P_{n,\theta _{r_n } } \) with respect to the probability measure\(\tilde P_{n,\theta } \). Here\(\tilde P_{n,\theta } \) is the restriction ofPθ to the σ-field induced by the r.v.′sY0,Y1, ...,\(Y_{ u _n } \). The main purpose of this paper is to obtain an asymptotic expansion of\(\Lambda _{ u _n } (\theta )\) in the probability sense. The asymptotic distribution of\(\Lambda _{ u _n } (\theta )\), as well as that of another r.v. closely related to it, is obtained under both\(\tilde P_{n,\theta } \) and\(\tilde P_{n,\theta _{r_n } } \).

Free convection effects on the oscillatory flow in the Stokes's problem past an infinite porous vertical limiting surface with constant suction, II

Abstract: In this work we present the two-dimensional free convection flow of an incompressible viscous fluid past an infinite vertical limiting surface (porous wall) for the Stokes's problem when the fluid is subjected to a constant suction velocity. The flow is normal to the porous wall and the free stream oscillates about a mean value. As the mean steady flow has been presented in Part I, only the solutions for the transient velocity profiles, transient temperature profiles, the amplitude and the phase of the skin friction and the rate of heat transfer are presented in this work. As in the case of mean steady flow, the influence of the Grashof numberG and Eckert numberE on the unsteady flow field is discussed for air (P=0.71) and water (P=7) and for the cases of externally heating and cooling the porous limiting surface by free convection currents.

Modified procedures for the determination of hexuronic acids, hexoses and proteins using the Auto Analyzer II system.

Abstract: Previous automated procedures for the determination of hexuronic acids, hexoses and proteins have been modified to suit the Technicon Auto Analyzer II system. The present methods were highly reproducable and the detection limits showed to be in the order of 10 μg/1 (hexuronic acids and hexoses) and 60 μg/1 (protein) when 0.13–0.34 ml of the sample was used.

Mechanical properties of SBS block copolymers. II. Effect of structure and selective solvent casting on stress–strain properties and mechanical hysteresis

Abstract: Near-equilibrium stress–strain properties were obtained for poly(styrene–b–butadiene) copolymer films cast from different solvents at 35°C. The solvents used were methyl ethyl ketone and cyclohexane, selective for PS and PB, respectively, and toluene, a common solvent for both copolymer components. Tensile properties were studied at two successive loading cycles up to a maximum elongation ratio of λmax = 7.0. At constant composition, the results were interpreted on the basis of the available morphology for these systems. The effect of hard block content (35% to 45% styrene) and at constant composition (39% styrene) of block length was also examined on such properties as elasticity and mechanical hysteresis. The results indicate that at constant composition the PB block length influences elasticity and mechanical hysteresis, also that films cast from a common solvent have higher tensile strength and increased mechanical hysteresis presumably because of a more effective load transfer between phases.

Free Convection Effects on the Unsteady Laminar Boundary-Layer Flow Past a Porous Limiting Surface with Uniform Suction in Stellar Atmospheres

Abstract: Unsteady two-dimensional free convection flow of a viscous fluid (e.g., of a stellar atmosphere) past a porous limiting surface (e.g., of a star) through which suction with uniform velocity occurs is considered when the free-stream velocity and the temperature of the limiting surface are arbitrary functions of time. General solution of the equations governing the flow is obtained in closed form with the help of two-sided Laplace transform technique under the assumption that there exists a mean steady flow to which is superimposed the unsteady flow. Further, in order to demonstrate the applications of the results of the general theory, four particular cases have been considered by prescribing physically acceptable different time-dependent forms to the temperature of the limiting surface and to the free-stream velocity. The results thus obtained for these four cases are discussed quantitatively.

Mechanical properties of SBS block copolymers. I. Effect of structure and selective solvent casting on dynamic viscoelasticity

Abstract: Thermomechanical spectra of poly(styrene-b-butadiene) copolymers were obtained at 110 Hz and between −120 and 120°C. The molecular parameters studied were the styrene content (35%–45%) and, at constant composition (39% styrene), the length of the copolymer blocks. The films studied were cast from selective solvents, methyl ethyl ketone (MEK), cyclohexane, and toluene, a common solvent. MEK- and toluene-cast films were quite similar in their dynamic viscoelasticity and more rigid than the corresponding cyclohexane-cast films. Solvent differentiation was more evident at the higher styrene content. No evidence of a relaxation due to a mixed interphase was obtained. At constant composition a morphological transformation takes place when the length of the blocks exceeds a certain limit; this affected the dynamic modulus of the cyclohexane-cast films. The results were analyzed using two-phase models proposed by the composite theory. For some samples results on the effect of annealing and prestretching are also reported.

Cellular two's complement serial-pipeline multipliers

Abstract: Serial two's complement pipeline multipliers are the basic module in the serial arithmetic implementation of digital signal processing algorithms. These multipliers accept the data serially in two's complement notation and generate a serial output product in two's complement notation as well. The designs, however, presented up to now lack in modularity; in addition, they have the problem of the internal overflow. In this paper a new two's complement multiplication algorithm is worked out, that allows cellular implementation of serial-pipeline multipliers and does not limit the dynamic range of the data input. In addition, by using the proposed cell, useful functions for digital signal processing can be performed. Methods are proposed for overflow detection, and comparison is made with previously proposed realizations.