Showing papers by "École Polytechnique Fédérale de Lausanne published in 1986"

Estimating the Voltage Stability of a Power System

Abstract: A method for the online testing a power system is proposed which is aimed at the detection of voltage instabilities. Thereby an indicator L is defined which varies in the range between 0 (noload of system) and 1 (voltage collapse). Based on the basic concept of such an indicator various models are derived which allow to predict a voltage instability or the proximity of a collapse. The indicator uses information of a normal load flow. The advantage of the method lies in the simplicity of the numerical calculation and the expressiveness of the result.

Growth of Saccharomyces cerevisiae is controlled by its limited respiratory capacity: Formulation and verification of a hypothesis

Abstract: A novel mechanistic model for the growth of baker's yeast on glucoseis presented. It is based on the fact that glucose degradation proceeds via two pathways under conditions of aerobic ethanol formation. Part is metabolized oxidatively and part reductively, with ethanol being the end product of reductive energy metabolism. The corresponding metabolic state is designated oxidoreductive. Ethanol can be used oxidatively only. Maximum rates of oxidative glucose and ethanol degradation are governed by the respiratory capacity of the cells. The model is formulated by using the stoichiometric growth equations for pure oxidative and reductive (fermentative) glucose and ethanol metabolism. Together with the experimentally determinable yield coefficients (Y(X/S)) for the respective metabolic pathways, the resulting equation system is sufficiently determined. The superiority of the presented model over hitherto published ones is based on two essential novelities. (1) The model was developed on experimentally easily accessible parameters only. (2) For the modeling of aerobic ethanol formation, the substrate flow was split into two simultaneously operating (i.e., in parallel) metabolic pathways that exhibit different but constant energy-generating efficiencies (respiration and fermentation) and consequently different and constant biomass yields (Y(X/S)). The model allows the prediction of experimental data without parameter adaption in a biologically dubious manner.

Sum and Difference Histograms for Texture Classification

Abstract: The sum and difference of two random variables with same variances are decorrelated and define the principal axes of their associated joint probability function. Therefore, sum and difference histograms are introduced as an alternative to the usual co-occurrence matrices used for texture analysis. Two maximum likelihood texture classifiers are presented depending on the type of object used for texture characterization (sum and difference histograms or some associated global measures). Experimental results indicate that sum and difference histograms used conjointly are nearly as powerful as cooccurrence matrices for texture discrimination. The advantage of the proposed texture analysis method over the conventional spatial gray level dependence method is the decrease in computation time and memory storage.

Self-consistent Hartree-Fock and screened-exchange calculations in solids: Application to silicon.

Abstract: A simple procedure to perform Hartree-Fock calculations in solids using extended basis functions is proposed. The method also applies when correlation is added with the self-consistent Coulomb-hole-plus-screened-exchange (COHSEX) scheme. Application to silicon in the pseudopotential approach yields excitation energies which agree with recent results by Hybertsen and Louie. We find that the simple diagonal COHSEX approximation does not provide satisfactory indirect excitation energies in silicon but is quite accurate for the direct ones.

Regulation of carbon metabolism in Saccharomyces cerevisiae and related yeasts.

Abstract: Publisher Summary This chapter describes the regulation of glucose metabolism in Saccharomyces cerevisiae and related yeasts in view of recent data. Metabolism of growing yeasts depends on the yeast strain, the carbon source, and the physicochemical factors in the environment. Glucose breakdown in Saccharomyces cerevisiae and related yeast proceeds via different pathways. Principally respirative, fermentative (anaerobic), and respiro-fermentative glucose metabolism have to be considered, but growth on ethanol is also considered due to the diauxic growth pattern observed in batch cultures. Ethanol accumulated during the first growth phase with respiro-fermentative glucose catabolism is used for subsequent growth. The type of glucose metabolism depends on cultivation conditions. Respirative glucose catabolism is possible in feed-controlled systems only. The shift to respiro-fermentative glucose breakdown is governed by the respirative capacity of cells, and therefore occurs above a certain glucose-feed rate. Respiro-fermentative glucose metabolism is primarily the consequence of an overflow reaction at the level of pyruvate when respiration is saturated.

The hybrid-Trefftz finite element model and its application to plate bending

Abstract: This paper presents a new hybrid element approach and applies it to plate bending. In contrast to more conventional models, the formulation is based on displacement fields which fulfil a priori the non-homogeneous Lagrange equation (Trefftz method). The interelement continuity is enforced by using a stationary principle together with an independent interelement displacement. The final unknowns are the nodal displacements and the elements may be implemented without any difficulty in finite element libraries of standard finite element programs. The formulation only calls for integration along the element boundaries which enables arbitrary polygonal or even curve-sided elements to be generated. Where relevant, known local solutions in the vicinity of a singularity or stress concentration may be used as an optional expansion basis to obtain, for example, particular singular corner elements, elements presenting circular holes, etc. Thus a high degree of accuracy may be achieved without a troublesome mesh refinement. Another important advantage of the formulation is the possibility of generating by a single element subroutine a large number of various elements (triangles, quadrilaterals, etc.), presenting an increasing degree of accuracy. The paper summarizes the results of numerical studies and shows the excellent accuracy and efficiency of the new elements. The conclusions present some ideas concerning the adaptive version of the new elements, extension to nonlinear problems and some other developments.

Resonances of two-dimensional particle gratings in surface-enhanced Raman scattering

Abstract: Dipolar interactions between silver particles arranged on a square two-dimensional grating give rise to characteristic features in the wavelength dependence of surface-enhanced Raman scattering from adsorbed molecules. The interactions produce two pronounced effects: first, they shift and broaden the plasmon resonance corresponding to the isolated particle, and second, they induce an additional sharp resonance feature at lower energy. The two enhancement maxima are separated by a minimum (cusp), which corresponds to the wavelength at which a grating order changes from radiating to evanescent in character. Dispersion relations describing the grating-induced resonances are presented. The consequences of the simultaneous enhancement of both the input and output electric fields are discussed, and the effects of imperfections in the particle shape are considered. The wavelength dependence of Raman scattering from p-nitrobenzoic acid adsorbed on a 380-nm-period two-dimensional array of silver spheroids was measured by means of an internal standard. Close agreement between experimental results and theoretical predictions was obtained.

Network Topology Optimization with Security Constraints

Abstract: Strategic switching can be achieved by a current injection scheme which simulates the change in topology. The injected currents are applied to the terminals of the elements of a socalled base network corresponding to those actually switched. This requires that the base network must contain all elements in the "in" state. The injected currents to be used as a compensation in the commonly employed system matrices (Y, Z) for the real change in topology can be taken as control variables in an optimization procedure for the switching problem. With the aid of a method similar to linear programming (LP) objective functions such as line current, short circuit current or even losses can be formulated. By means of a switching sequence consisting of elementary switching operations the desired objective function will be brought to its optimum value.

Polymerisation of lignins by ligninases from Phanerochaete chrysosporium

Abstract: Four major hemoproteins were purified by isoelectric focusing from an extracellular crude enzyme preparation, produced by the white rot fungus Phanerochaete chrysosporium under carbon-limited conditions. Both the crude enzyme and the purified proteins oxidised milled wood lignin, HCl-dioxane-extracted straw lignin and alkali straw lignin in the presence of hydrogen peroxide. The oxidation resulted mainly in further polymerisation of the lignins and was enhanced by addition of veratryl alcohol to the reaction mixture. Alkali straw lignin was also polymerised by horseradish peroxidase, although veratryl alcohol had no influence on this reaction.

Ergonomics in computerized offices

Abstract: "Ergonomics in Computerized Offices should be required reading for office managers, union representatives, engineers, designers, or anyone employed in implementing a computerized office or improving conditions in an already computerized office...an excellent addition to any personal library."--Human Factors Bulletin

Approximate dynamic model of embedded foundation in time domain

Abstract: A discrete model to represent the unbounded soil (halfspace) in a soil–structure interaction analysis in the time domain is developed. For each dynamic degree of freedom of the foundation node, the discrete model consists of a mass M0 which is attached to a rigid support with a spring K and with a damper C0. In addition, a free node with the mass M1 is introduced, which is connected to the foundation node with a damper C1. All coefficients are frequency-independent. The discrete model is semi-empirical. It is based on a semi-infinite truncated cone, whereby, after enforcing the static stiffness, the remaining parameters are modified to achieve an optimal fit of the dynamic-stiffness coefficient in the frequency domain. The spring K is equal to the static stiffness. The coefficients appearing in the equations for the dampers C0, C1 and the masses M0, M1 are specified (assuming a homogeneous halfspace) for the disc, the embedded cylinder, the rectangle (also embedded) and the strip. A square on a layer whose stiffness increases with depth resting on a homogeneous halfspace is also treated. For an embedded foundation, eccentricities arise. Material damping increases the damper C0 and the mass M0.

A Discrete Fourier-Cosine Transform Chip

Abstract: An 8-point Fourier-cosine transform chip designed for a data rate of 100 Mbits/s is described. The top-down design is presented step by step, including algorithm modification for VLSI suitability, architectural choices, testing overhead, internal precision assignments, mask generation, and finally, verification of the layout. A high-level language (C) design tool was developed concurrently with the layout. This tool allows mimicking exactly the different representations of the algorithm: software, mask, and chip. This provides an automatic cross-checking at all design stages. The VLSI environment created by this tool, as well as existing powerful CAD tools, made a fast design-time possible.

Horizontal sedimentation differences in a eutrophic Swiss lake

Abstract: Settling fluxes measured by means of sediment traps were 887 g dry wt m-2 yr- I, 113 g POC m-2 yr-I, and 2.6 g PP m-2 yr-I in 1982-1983 at a central station in eutrophic Lake Hallwil, Switzerland. Nearshore settling rates were higher by a factor of 1.4-3.4. Since particulate organic carbon (POC), particulate phosphorus (PP), and biomass concentration in the lake water showed vertical rather than horizontal differences, this result was attributed to permanent bottom sediment resuspension and redeposition in the littoral zone. Horizontal sediment transport (sediment focusing) and sediment resuspension in the profundal zone in winter, when wind and turbulence are increased, accounted for only about 20% of the sedimentation rates. Because 70-80% of the total sedimentation for the year occurred during summer, such processes are of minor importance in Lake Hallwil, although a “nepheloid” layer with increased POC and PP concentrations 5-8 m above the bottom was present. It is stressed that the mechanisms of sediment transport must be known to permit interpretation of trap results and to understand lake metabolism.

Comparison of Quasi-Static and Exact Electromagnetic Fields from a Horizontal Electric Dipole Above a Lossy Dielectric Backed by an Imperfect Ground Plane

Abstract: In most microstrip transmission lines, analysis is made assuming that a quasi-TEM mode exists and propagates down the line. The primary objective of this paper is to obtain the region of validity of this assumption. The second objective of this paper is to derive the expressions for the fields for a horizontal electric dipole over a Iossy dielectric medium backed by an imperfect ground plane. It is shown that, to a first approximation, fields at the air-dielectric interface are independent of the ground plane conductivity. Since we are interested in coupfing between lines, our interest is in the computation of the fields primarily at the air-dielectric interface. Finally, numerical results are presented to show where the quasi-static approximations deviate from the exact solution for a given microstrip geometry as the frequency of operation or the observation point is changed.