Showing papers by "University of Maribor published in 1999"

Biaxial torus around nematic point defects.

Abstract: We study the biaxial structure of both line and point defects in a nematic liquid crystal confined within a capillary tube whose lateral boundary enforces homeotropic anchoring. According to Landau--de Gennes theory the local order in the material is described by a second-order tensor $\mathbf{Q},$ which encompasses both uniaxial and biaxial states. Our study is both analytical and numerical. We show that the core of a line defect with topological charge $M=1$ is uniaxial in the axial direction. At the lateral boundary, the uniaxial ordering along the radial direction is reached in two qualitatively different ways, depending on the sign of the order parameter on the axis. The point defects with charge $M=\ifmmode\pm\else\textpm\fi{}1$ exhibit a uniaxial ring in the plane orthogonal to the cylinder axis. This ring is in turn surrounded by a torus on which the degree of biaxiality attains its maximum. The typical lengths that characterize the structure of these defects depend both on the cylinder radius and the biaxial correlation length. It seems that the core of the point defect does not depend on the far nematic director field in the bulk limit.

Multimedia: Differences in cognitive processes observed with EEG

Abstract: This study investigated the cognitive processes involved in learning information presented in multimedia and text format using electroencephalographic (EEG) measures. Thirty-eight students (19 gifted, and 19 average) learned material presented with text (text); text, sound, and picture (picture); and text, sound and video (video), while their EEG was recorded. Alpha power, which is inversely related to mental effort, was analyzed. For thetext presentation, the alpha power measures showed higher amplitudes (less mental activity) over the occipital and temporal lobes, and less alpha power (higher mental activity) over the frontal lobes. The results support the assumption that thevideo andpicture presentations induced visualization strategies, whereas thetext presentation mainly generated processes related to verbal processing. The results further showed that gifted students displayed less mental activity during all three formats of presentation. These differences were especially pronounced for thevideo format. No gender-related differences in EEG patterns related to the format of presentation were observed.

Surface characterization and adsorption abilities of cellulose fibers

Abstract: The adsorption abilities of cotton cellulose fibers are very often modified by alkaline treatments in form of alkaline purification or mercerization using high concentration of NaOH. We tried to determine the correlation between morphological modifications and the adsorption abilities of cotton fibers using several methods: the analysis of microscope images of fibers by image processing and the analysis of the electrokinetic surface properties which express the adsorption behavior of fibers. The longitudinal images and cross-sections of native and modified cotton fibers were analyzed and the parameters: form factor, wall thickness, cross-section area, fiber diameter, lumen area were calculated using image processing. The adsorption behavior of native and NaOH modified polymers was investigated by the determination of electrokinetic properties. The zeta potential (ζ) was calculated from streaming potential measurements as a function of pH and surfactant concentration in the liquid phase. The results indicate that only a correct combination between the morphological modifications and electrokinetic behavior of fibers leads to a desirable adsorption mechanism which causes a specific adsorption of components of the liquid phase.

On locally linearly dependent operators and derivations

Abstract: The first section of the paper deals with linear operators Ti : U −→ V , i = 1, . . . , n, where U and V are vector spaces over an infinite field, such that for every u ∈ U , the vectors T1u, . . . , Tnu are linearly dependent modulo a fixed finite dimensional subspace of V . In the second section, outer derivations of dense algebras of linear operators are discussed. The results of the first two sections of the paper are applied in the last section, where commuting pairs of continuous derivations d, g of a Banach algebra A such that (dg)(x) is quasi–nilpotent for every x ∈ A are characterized.

Computational fluid dynamics by boundary–domain integral method

Abstract: A boundary-domain integral method for the solution of general transport phenomena incompressible fluid motion given by the Navier-Stokes equation set is presented. Velocity-vorticity formulation of the conservations is employed. Different integral representations for conservation field functions based on different fundamental solutions are developed. Special attention is given to the use of subdomain technique and Krylov subspace iterative solvers. The computed solutions of several benchmark problems agree well with available experimental and other computational results.

A GIS based component-oriented integrated system for estimation, visualization and analysis of road traffic air pollution

Abstract: Road traffic is a dominant source of air pollution. Therefore it is necessary to provide decision-makers with up-to-date emission information in an easily understandable form. To achieve this goal we have integrated existing emission calculation software with a graphical user interface, which includes a GIS (geographical information system) component. The paper first gives a summary of the basic road traffic emission model and then focuses on the design and implementation of the computer application with the emphasis on the used component and GIS technology. The integrated emission evaluation system offers entirely new ways of using the emission model and gives additional visualization and analysis possibilities.

Compatibilization effects of styrenic/rubber block copolymers in polypropylene/polystyrene blends

Abstract: Compatibilizing effects of styrene/rubber block copolymers poly(styrene-b-butadiene-b-styrene) (SBS), poly(styrene-b-ethylene-co-propylene) (SEP), and two types of poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS), which differ in their molecular weights on morphology and selected mechanical properties of immiscible polypropylene/polystyrene (PP/PS) 70/30 blend were investigated. Three different concentrations of styrene/rubber block copolymers were used (2.5, 5, and 10 wt %). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to examine the phase morphology of blends. The SEM analysis revealed that the size of the dispersed particles decreases as the content of the compatibilizer increases. Reduction of the dispersed particles sizes of blends compatibilized with SEP, SBS, and low-molecular weight SEBS agrees well with the theoretical predictions based on interaction energy densities determined by the binary interaction model of Paul and Barlow. The SEM analysis confirmed improved interfacial adhesion between matrix and dispersed phase. The TEM micrographs showed that SBS, SEP, and low-molecular weight SEBS enveloped and joined pure PS particles into complex dispersed aggregates. Bimodal particle size distribution was observed in the case of SEP and low-molecular weight SEBS addition. Notched impact strength (ak), elongation at yield (ey), and Young's modulus (E) were measured as a function of weight percent of different types of styrene/rubber block copolymers. The ak and ey were improved whereas E gradually decreased with increasing amount of the compatibilizer. The ak was improved significantly by the addition of SEP. It was found that the compatibilizing efficiency of block copolymer used is strongly dependent on the chemical structure of rubber block, molecular weight of block copolymer molecule, and its concentration. The SEP diblock copolymer proved to be a superior compatibilizer over SBS and SEBS triblock copolymers. Low-molecular weight SEBS appeared to be a more efficient compatibilizer in PP/PS blend than high-molecular weight SEBS. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 291–307, 1999

Reflexivity of the group of surjective isometries on some Banach spaces

Abstract: In this paper we study the problem of algebraic reflexivity of the isometry group of some important Banach spaces. Because of the previous work in similar topics, our main interest lies in the von Neumann – Schatten p-classes of compact operators. The ideas developed there can be used in lp-spaces, Banach spaces of continuous functions and spin factors as well. Moreover, we attempt to attract the attention to this problem from general Banach spaces geometry view-point. This study, we believe, would provide nice geometrical results.

Implementation of multiple attribute grammar inheritance in the tool LISA

Abstract: Multiple attribute grammar inheritance is a structural organization of attribute grammars where the attribute grammar inherits the specifications from ancestor attribute grammars, may add new specifications or may override some specifications from ancestor specifications. In the paper the implementation of multiple attribute grammar inheritance is described. The proposed approach is successfully implemented in the compiler/interpreter generator tool LISA ver. 2.0.

Study of regular and irregular states in generic systems

Abstract: In this work we present the results of a numerical and semiclassical analysis of high-lying states in a Hamiltonian system, whose classical mechanics is of a generic, mixed type, where the energy surface is split into regions of regular and chaotic motion. As predicted by the principle of uniform semiclassical condensation, when the effective tends to zero, each state can be classified as regular or irregular. We were able to semiclassically reproduce individual regular states by the Einstein-Brillouin-Keller torus quantization, for which we devise a new approach, while for the irregular ones we found the semiclassical prediction of their autocorrelation function, in a good agreement with numerics. We also looked at the low-lying states to get a better understanding of the onset of semiclassical behaviour.

Fracture behaviour of specimens with surface notch tip in the heat affected zone (HAZ) of strength mis-matched welded joints

Abstract: The generally accepted conditions for the strength overmatched welded joints of high strength steel are not clearly defined. In this paper, the fracture mechanics analysis of specimens, with surface notch tips completely embedded in the heat affected zones was conducted. The results showed that the strength of mismatching of a welded joint caused a redirection of the crack propagation towards the low strength region of the welded joint. This redirection of the crack propagation affected the values of the critical CTOD. In the cases of the overmatched welded joints containing a soft root layer it is possible to achieve a comparable fracture behaviour related to the homogeneous overmatched welded joint if the impact toughness of the soft root layer is higher than the impact toughness of the overmatched weld metal. Such a type of welded joint is therefore preferable for the welding of high strength low alloy steels, because it enables the manufacturing of a welded joint without preheating.

An FTIR Study of Anhydride Formation on Heating Butane Tetracarboxylic Acid in the Presence of Various Catalysts

Abstract: Polycarboxylic acids containing more than three carboxylic acid groups react with hydroxyl groups in cotton cellulose and crosslink cotton through stable ester bonds. The aim of this research is to...

Deriving formulas for domination numbers of fasciagraphs and rotagraphs

Abstract: Recently, an algebraic approach which can be used to compute distance-based graph invariants on fasciagraphs and rotagraphs was given in [Mohar, Juvan, Žerovnik, Discrete Appl. Math. 80 (1997) 57–71]. Here we give an analogous method which can be employed for deriving formulas for the domination number of fasciagraphs and rotagraphs. In other words, it computes the domination numbers of these graphs in constant time, i.e. in time which depends only on the size and structure of a monograph and is independent of the number of monographs. Some further generalizations of the method are discussed, in particular the computation of the independent number and the k-coloring decision problem. Examples of fasciagraphs and rotagraphs include complete grid graphs. Grid graphs are one of the most frequently used model of processor interconnections in multiprocessor VLSI systems.

Mixed-Integer Nonlinear Programming Problem Process Synthesis under Uncertainty by Reduced Dimensional Stochastic Optimization

Abstract: This paper presents the developments of a novel simultaneous method for the optimization and the synthesis of complex chemical problems under uncertainty with a fixed degree of flexibility. The approach approximates the stochastic method in using a weighted objective function calculated over a reduced set of the extreme points (vertices). The feasibility of the design is ensured simultaneously by the feasibility constraints at critical vertices. The main part of the proposed method which was called the method for reduced dimensional stochastic optimization (the RDS method) is a special setup procedure for determining the reduced set of vertices and their weights for the approximation of the expected value of the objective function. A very attractive feature of this method is that the sizes of mathematical models and the computational times are reduced by 1 or 2 orders of magnitude when compared to those of the stochastic methods. On the basis of the RDS method, a robust strategy for the synthesis of compl...

Mixed boundary elements for laminar flows

Abstract: We present a mixed boundary element formulation of the boundary domain integral method (BDIM) for solving diffusion-convective transport problems. The basic idea of mixed elements is the use of a continuous interpolation polynomial for conservative field function approximation and a discontinuous interpolation polynomial for its normal derivative along the boundary element. In this way, the advantages of continuous field function approximation are retained and its conservation is preserved while the normal flux values are approximated by interpolation nodal points with a uniquely defined normal direction. Due to the use of mixed boundary elements, the final discretized matrix system is overdetermined and a special solver based on the least squares method is applied. Driven cavity, natural and forced convection in a closed cavity are studied. Driven cavity results at Re = 100, 400 and 1000 agree better with the benchmark solution than Finite Element Method or Finite Volume Method results for the same grid density with 21 × 21 degrees of freedom

Intermediate statistics in the regime of mixed classical dynamics

Abstract: We explain the arguments in support of the Berry-Robnik (1984 17 2413) picture of the energy-level statistics in the asymptotic strict (far) semiclassical limit of sufficiently small , where the entire energy spectrum can be represented as a statistically independent superposition of regular and irregular level sequences, the regular ones obeying the Poissonian statistics and the irregular ones the RMT statistics (GOE or GUE). We generalize the results to describe not only the level spacing distribution, the number variance and the delta statistics, but also arbitrary statistics (= the probability to find k levels inside an interval of length L - after unfolding). Very useful and effective approximations for are described. We demonstrate very clearly that this regime is excellently described by this picture for k as high as , the outer energy scale, even when taking into account the regular component and only one (the dominant) chaotic component. This we show numerically for the compactified standard map and for the quartic 2D billiard.