Showing papers by "University of Stuttgart published in 1992"

A theory of focus interpretation

Abstract: According to the alternative semantics for focus, the semantic reflec of intonational focus is a second semantic value, which in the case of a sentence is a set of propositions. We examine a range of semantic and pragmatic applications of the theory, and extract a unitary principle specifying how the focus semantic value interacts with semantic and pragmatic processes. A strong version of the theory has the effect of making lexical or construction-specific stipulation of a focus-related effect in association-with-focus constructions impossible. Furthermore, while focus has a uniform import, the sources of meaning differences in association with focus are various.

Correlation energy of diamond.

Abstract: The correlation energy of diamond is determined by means of increments obtained in ab initio calculations for localized C-C bond orbitals and for pairs and triples of such bonds. The resulting correlation contribution to the cohesive energy is -0.129 a.u., which is approximately 85% of the experimental value.

Synthesis of dense silicon-based ceramics at low temperatures

Abstract: THE conventional preparation of advanced ceramic parts based on silicon carbide or nitride involves pressureless sintering, hot pressing or hot isostatic pressing of appropriate ceramic starting powders1. Owing to the covalent nature of the Si–C and Si–N bonds and hence the low diffusion coefficients in SiC and Si3N4, high sintering temperatures and the addition of sintering aids are normally used to enhance densification. During densification, the sintering additives form second phases located at grain boundaries, which commonly impair the mechanical and physical properties of the material, especially at higher temperatures. New processing routes that overcome these problems are therefore desirable. Here we report the direct transformation of a metallorganic precursor into non-oxide silicon-based ceramics with relative densities of up to 93%. This process can be used to make ceramic components and matrix composites at unusually low temperatures (1,000 °C) and without the addition of sintering aids.

Implementation of simplified chemical kinetics based on intrinsic low-dimensional manifolds

Abstract: A general procedure for simplifying chemical kinetics and its use in reacting flow models is developed, which is based on the dynamical systems approach. In contrast to conventional reduced mechanisms no information is required concerning which reactions are to be assumed to be in partial equilibrium nor which species are assumed to be in steady state. Based on a local eigenvector analysis, the method identifies the fast time scales of the chemical reaction systems, which differ typically by orders of magnitude. Assuming that the fastest relaxation processes in chemical reactions proceed infinitely fast (i.e., are in local equilibrium), it is then possible to reduce the state space globally, such that it can be described by means of only a small number of reaction progress variables. The only “inputs” to the procedure are the detailed kinetics mechanism and the number of degrees of freedom required in the simplified scheme. Then the state properties given by the simplified scheme are automatically determined as functions of the coordinates associated with the degrees of freedom. A tabulation procedure allows an efficient use of the results in CFD codes. Furthermore a general procedure for coupling the reduced mechanism with other than chemical processes like flow and molecular transport is discussed. Results are presented for the CO/H2/air system both for a simple homogeneous closed system and a flow reactor.

Some aspects of the genesis of structures

Abstract: Bendsoe and Kikuchi (1988) introduced a novel approach of distributing mass within a specified design domain utilizing a stiffness-density relation obtained by homogenization of a cellular microstructure This approach was extended to multiple loading cases and three-dimensional applications by the author and his associates (Mlejnek and Schirrmacher 1989; Mlejnek 1990) Moreover the well-known concept of explicit convex behaviour approximation together with a dual solution scheme (Fleury and Smaoui 1988) was successfully introduced to this problem Further objectives such as eigenvalues and displacements generalized the range of application It is the aim of this paper, to develop a simplified procedure that can be easily integrated into a FEM-analysis package Its application requires essentially not much more than the usual FEM-technology Nevertheless a traceable mathematical base is still maintained A series of examples demonstrates the suitability of this approach to the preliminary design of minimal compliance structures made of isotropic materials

The validity of modulation equations for extended systems with cubic nonlinearities

Abstract: Modulation equations play an essential role in the understanding of complicated systems near the threshold of instability. Here we show that the modulation equation dominates the dynamics of the full problem locally, at least over a long time-scale. For systems with no quadratic interaction term, we develop a method which is much simpler than previous ones. It involves a careful bookkeeping of errors and an estimate of Gronwall type.As an example for the dissipative case, we find that the Ginzburg–Landau equation is the modulation equation for the Swift–Hohenberg problem. Moreover, the method also enables us to handle hyperbolic problems: the nonlinear Schrodinger equation is shown to describe the modulation of wave packets in the Sine–Gordon equation.

Water waves for small surface tension : an approach via normal form

Abstract: In this paper we determine the possible crest-forms of permanent waves of small amplitude which exist on the free surface of a two-dimensional fluid layer under the influence of gravity and surface tension when the Froude number is close to 1. The Bond number b, measuring surface tension, is assumed to satisfy b < ⅓. We find one-parameter families of periodic waves of two different types, quasiperiodic waves and solitary waves with oscillations at infinity. The existence of true solitary waves is established for a sequence of systems approximating the full Euler equations in every algebraic order of − 1.

Expression of odorant receptors in spatially restricted subsets of chemosensory neurones.

Abstract: From a rat olfactory library a cDNA clone (OR37) which is supposed to encode an odorant receptor protein has been isolated and characterized. Specific antisense RNA and in situ hybridization techniques have been employed to monitor the olfactory epithelium for the distribution of olfactory neurones expressing the OR37-gene. The OR37-transcripts were detected only in a subset of receptor cells segregated in two restricted areas of the olfactory epithelium. The clusters of reactive cells appear symmetrically in both nasal cavities. Within a reactive region only a subset of the cells expressed the receptor. The segregation of neurones expressing a distinct receptor supports the notion that a spatial component may be involved in coding odour quality.

On numerical cubatures of singular surface integrals in boundary element methods

Abstract: We present and analyze methods for the accurate and efficient evaluation of weakly, Cauchy and hypersingular integrals over piecewise analytic curved surfaces in ?3. The class of admissible integrands includes all kernels arising in the numerical solution of elliptic boundary value problems in three-dimensional domains by the boundary integral equation method. The possibly not absolutely integrable kernels of boundary integral operators in local coordinates are pseudohomogeneous with analytic characteristics depending on the local geometry of the surface at the source point. This rules out weighted quadrature approaches with a fixed singular weight. For weakly singular integrals it is shown that Duffy's triangular coordinates leadalways to a removal of the kernel singularity. Also asymptotic estimates of the integration error are provided as the size of the boundary element patch tends to zero. These are based on the Rabinowitz-Richter estimates in connection with an asymptotic estimate of domains of analyticity in ?2. It is further shown that the modified extrapolation approach due to Lyness is in the weakly singular case always applicable. Corresponding error and asymptotic work estimates are presented. For the weakly singular as well as for Cauchy and hypersingular integrals which e.g. arise in the study of crack problems we analyze a family of product integration rules in local polar coordinates. These rules are hierarchically constructed from "finite part" integration formulas in radial and Gaussian formulas in angular direction. Again, we show how the Rabinowitz-Richter estimates can be applied providing asymptotic error estimates in terms of orders of the boundary element size.

Termination of second messenger signaling in olfaction.

Abstract: By using isolated rat olfactory cilia and a fast kinetics methodology, it has been demonstrated that odorant-induced second messenger signaling in the millisecond time range is terminated via phosphorylation reactions catalyzed by specific protein kinases. The cyclic adenosine nucleotide pathway is turned off by kinase A activity, whereas the inositol trisphosphate cascade is terminated by kinase C. The data support the concept that desensitization of odorant responses involves phosphorylation of key elements in the transduction cascade.

Acyl- und Alkylidenphosphane. XXXIII. Lithoxy-methylidenphosphan · DME und -methylidinphosphan · 2 DME — Synthese und Struktur

Abstract: Das aus Lithium-dihydrogenphosphid. DME(1) und Ethylformiat im Molverhaltnis 2 : 1 mit 87 proz. Ausbeute isolierte, flussige Lithium-formylphosphid. DME liegt als Lithoxy-methylidenphosphan. DME (1) mit einem an den Sauerstoff der Carbonylgruppe gebundenen, DME-komplexierten Lithium vor. Nach Rontgenstrukturanalysen an kristallinen Derivaten [5, 6] dimerisieren Verbindungen dieses Typs unter Ausbildung eines viergliedrigen LiOLiO-Ringes; die Auswertung der NMR-Spektren ergibt fur die an der PC-Gruppe E- und Z-konfigurierten Isomeren (δ1H P: 3,87 u. 4,49; 1JHP: 150,8 u. 136,5; δ1H C: 11,4 u. 10,05; 2JHP: 6,1 u. 81,2; 3JHH: 6,6 u. 13,9; δ31P: 38,6 u. 8,8; δ13C = P: 225,0 u. 215,4 ppm; 1JCP: 41,2 u. 65,0 Hz) des in 1,2-Dimethoxyethan gelosten Acylphosphids 1 ein Molverhaltnis von 1,86 : 1. Lithium-bis(trimethylsilyl)phosphid. 1,6 THF(1) setzt sich in 1,2-Dimethoxyethan mit Dimethylcarbonat im Uberschus nach einem bisher nicht vollig geklarten Mechanismus zu Lithoxy-methylidinphosphan. 2DME (2) um. Die auch aus Lithium-dihydrogenphosphid. DME zugangliche Verbindung 2 kristallisiert monoklin in der Raumgruppe P21/n {a = 880,6(2); b = 1296,6(2); c = 1267,4(2)pm; β = 96,07(2)° bei - 100 ± 3°C; Z = 4}. Nach der Rontgenstrukturanalyse (Rw = 0,052) weist der zu 155,5 pm bestimmte PC-Abstand einen fur die Dreifachbindung typischen Wert auf, wahrend der CO-Abstand mit 119,8 pm gegenuber dem fur eine Einfachbindung gultigen Standard von 139 pm stark verkurzt ist. Mit Winkeln von 178,5° und 170,7° am Kohlenstoff-und Sauerstoffatom ist das P COLi-Fragment erwartungsgemas linear. Charakteristische NMR-Daten sind: δ31P −384,2; δ13C 166,6 ppm; 1JCP 41,5 Hz. Acyl- and Alkylidenephosphines. XXXIII Lithoxy-methylidenephosphine · DME and -methylidynephosphine · 2DME — Syntheses and Structures Lithium dihydrogenphosphide · DME(1) and ethyl formate in a molar ratio of 2 : 1 react in 1,2-dimethoxyethane to give liquid lithium formylphosphide · DME in 87% yield. Since lithium complexed by the chelate ligand DME is bound to the oxygen atom of the carbonyl group, the compound has to be considered as lithoxy-methylidenephosphine · DME (1). According to x-ray structure analyses of crystalline derivatives [5, 6], molecules of this type dimerize forming a four membered LiOLiO ring. Characteristic nmr-data show the presence of an E- and Z-isomer (δ1HP: 3.87 and 4.49; 1JHP: 150.8 and 136.5; δ1HC: 11.4 and 10.05; 2JHP: 6.1 and 81.2; 3JHH: 6.6 and 13.9; δ31P: 38.6 and 8.8; δ13CP: 225.0 and 215.4 ppm; 1JCP: 41.2 and 65.0 cps); in 1,2-dimethoxyethane an E : Z ratio of 1.86 : 1 is found. In a similar reaction of lithium bis (trimethylsilyl)phosphide · 1.6 THF(1) with excess dimethyl carbonate lithoxy-methylidynephosphine · 2DME (2) is formed via an up to now poorly understood mechanism. The compound can also be prepared from lithium dihydrogenphosphide · DME; it crystallizes in the monoclinic space group P21/n {a = 880.6(2); b = 1296.6(2); c = 1267.4(2) pm; β = 96.07(2)° at −100 ± 3°C; Z = 4}. An x-ray structure analysis (Rw = 0.052) gives a PC distance of 155.5 pm which is typical for a triple bond. The CO bond length of 119.8 pm, however, is extremely short compared to the standard value of a single bond (139 pm). Angles of 178.5° and 170.7° at the carbon and oxygen correspond with the expected linear configuration of the PCOLi backbone of the molecule, Characteristic nmr-data are as follow: δ31P-384.2; δ13C 166.6ppm; 1Jcp 41.5 cps.

Resolution of gas phase and surface combustion chemistry into elementary reactions

Abstract: It is well known for many decades, that combustion chemistry is a complex phenomenon, and realistic reaction mechanisms for simple hydrocarbons have been published in the last decade. Nevertheless, for 2D-or 3D-applications in combustion science and technology nearly exclusively one-step approaches are used at the present to make solutions possible within realistic computing times or to avoid principal physical problems (e.g. closure problems in modeling turbulent combustion). However, this is paid with a huge disadvantage: No extrapolation is possible to other experimental or operational conditions, and even interpolation is dangerous. This review describes (1) how detailed reaction mechanisms can be developed for simple hydrocarbons and be verified using literature data on flame structure, flame propagation, and ignition from the literature; (2) how these mechanisms can be extended to the treatment of very large hydrocarbons by use of additivity rules and processing of these rules by computer programs generating reaction mechanisms; (3) how these large reaction mechanisms can be simplified, for use in multi-dimensional codes, e.g. by use of lumping techniques or by equilibration of fast reactions in a systematic way; (4) how treatment in terms of sets of elementary reactions can be transferred to the handling of surface reactions, e.g. for the description of catalytic combustion. Examples are given for each of these developments. The special role of sensitivity analysis is emphasized in the understanding of detailed reaction mechanisms and the identification of rate-limiting steps which should be objects of further research efforts.

An exact finite rotation shell theory, its mixed variational formulation and its finite element implementation

Abstract: A non-linear shell theory, including transverse shear strains, with exact description of the kinematical fields is developed. The strain measures are derived via the polar decomposition theorem allowing for an explicit use of a three parametric rotation tensor. Thus in-plane rotations, also called drilling degrees of freedom, are included in a natural way. Various alternatives of the theory are derived. For a special version of the theory, with altogether six kinematical fields, different mixed variational principles are given. A hybrid finite element formulation, which does not exhibit locking phenomena, is developed. Numerical examples of shell deformation at finite rotations, with excellent element performance, are presented. Comparison with results reported in the literature demonstrates the features of the theory as well as the proposed finite element formulation.

Rule-oriented method for parameterized computer-aided design

Abstract: The paper presents an implementation for computer-aided design with dimensional parameters. The approach is based on the use of an expert system to uncouple constraint equations, and to find a possible sequence for the computation of the geometric elements for given dimension values. A set of rules for the expert system is described that solves the problem for 2D designs. The method is illustrated with an example design.

Hydrocarbon ignition: Automatic generation of reaction mechanisms and applications to modeling of engine knock

Abstract: A computational technique is described which automatically develops detailed chemical kinetic reaction mechanisms for large aliphatic hydrocarbon fuel molecules. This formulation uses the LISP language to apply general rules which identify the chemical species produced, the reactions between these species, and the elementary reaction rates for each reaction step. Reaction mechanisms for cetane (n-hexadecane) and most alkane fuels C{sub 7} and smaller are developed using this automatic technique, and detailed sensitivity analyses for n-heptane and cetane are described. These reaction mechanisms are then applied to calculation of knock tendencies in internal combustion engines. The model is used to study the influence of fuel molecule size and structure on knock tendency, to examine knocking properties of fuel mixtures, and to determine the mechanisms by which pro-knock and anti-knock additives change knock properties.

Homoclinic period blow-up in reversible and conservative systems

Abstract: We show that in conservative systems each non-degenerate homoclinic orbit asymptotic to a hyperbolic equilibrium possesses an associated family of periodic orbits. The family is parametrized by the period, and the periodic orbits accumulate on the homoclinic orbit as the period tends to infinity. A similar result holds for symmetric homoclinic orbits in reversible systems. Our results extend earlier work by Devaney and Henrard, and provide a positive answer to a conjecture of Stromgren. We present a unified approach to both the conservative and the reversible case, based on a technique introduced recently by X.-B. Lin.

Initial hydrogenation during catabolism of picric acid by Rhodococcus erythropolis HL 24-2.

Abstract: Rhodococcus erythropolis HL 24-2, which was originally isolated as a 2,4-dinitrophenol-degrading bacterium, could also utilize picric acid as a nitrogen source after spontaneous mutation. During growth, the mutant HL PM-1 transiently accumulated an orange-red metabolite, which was identified as a hydride-Meisenheimer complex of picric acid. This complex was formed as the initial metabolite and further converted with concomitant liberation of nitrite. 2,4,6-Trinitrocyclohexanone was identified as a dead-end metabolite of the degradation of picric acid, indicating the addition of two hydride ions to picric acid.

Cascades of homoclinic orbits to, and chaos near, a Hamiltonian saddle-center

Abstract: We consider a class of reversible, two-degree of freedom Hamiltonian systems possessing homoclinic orbits to a saddle-center: an equilibrium having two non-zero real and two nonzero imaginary eigenvalues. Under mild nondegeneracy conditions, we construct a two-parameter unfolding and show that there is a countable infinity of “secondary” homoclinic bifurcations in any neighborhood of the original system. We also demonstrate the existence of families of periodic orbits and of shifts on two symbols (horseshoes). The lack of hyperbolicity and the presence of conserved quantities make the analysis somewhat delicate. We discuss specific examples for which the nondegeneracy conditions can be explicitly checked but indicate that this is not always possible. We illustrate our results with numerical work.

Multiplicative vs. arbitrary gene action in heterosis.

Abstract: In this article we investigate multiplicative effects between genes in relation to heterosis. The extensive literature on heterosis due to multiplicative effects between characters is reviewed, as is earlier work on the genetic description of heterosis. A two-locus diallelic model of arbitrary gene action is used to derive linear parameters for two multiplicative models. With multiplicative action between loci, epistatic effects are nonlinear functions of one-locus effects and the mean. With completely multiplicative action, the mean and additive effects form similar restrictions for all the rest of the effects. Extensions to more than two loci are indicated. The linear parameters of various models are then used to describe heterosis, which is taken as the difference between respective averages of a cross (F1) and its two parent populations (P). The difference (F2 - P) is also discussed. Two parts of heterosis are distinguished: part I arising from dominance, and part II due to additive x additive (a x a)-epistasis. Heterosis with multiplicative action between loci implies multiplicative accumulation of heterosis present at individual loci in part I, in addition to multiplicative (a x a)-interaction in part II. Heterosis with completely multiplicative action can only be negative (i.e., the F1 values must be less than the midparent), but the difference (F2 - P) can be positive under certain conditions. Heterosis without dominance can arise from multiplicative as well as any other nonadditive action between loci, as is exemplified by diminishing return interaction. The discussion enlarges the scope in various directions: the genetic significance of multiplicative models is considered.(ABSTRACT TRUNCATED AT 250 WORDS)

Analyzing and Optimizing Multibody Systems

Abstract: Optimization of holonomic as well as non-holonomic multibody systems is presented as a nonlinear programming problem that can be solved with general-purpose optimization codes. The adjoint variable approach is used for calculating design derivatives of a rather general integral type performance measure with respect to design parameters. The resulting equations are solved by numerical integration backward in time. A multi-step integration algorithm with order and step-size control is adapted for this application by including an interpolation scheme. Numerical experiments and a comparison to the common approach of approximating the gradient of the performance measure by finite differences show that high efficiency, accuracy, and reliability are achievable.

Video-STM, LEED and X-ray diffraction investigations of PTCDA on graphite

Abstract: Thin films of the organic molecule perylene-3,4,9,10-tetracarboxylic-dianhydride (“PTCDA”) on graphite (0001) have been investigated from the mono- to the multilayer regime with low energy electron diffraction (LEED), X-ray-diffraction in Bragg-Brentano geometry, and high resolution scanning tunneling microscopy (STM). These different methods proved epitaxial growth in a coincident superstructure and yielded congruent results concerning details of the crystallographic structure of the epilayer. In addition it was possible to resolve submolecular structures in high resolution STM images; a comparison of the 10 resolved maxima of the tunneling current with the molecular structure leads us to question the conventional model description of tunneling.

Degradation of 2,4-dinitrophenol by two Rhodococcus erythropolis strains, HL 24-1 and HL 24-2.

Abstract: Two Rhodococcus erythropolis strains, HL 24-1 and HL 24-2, were isolated from soil and river water by their abilities to utilize 2,4-dinitrophenol (0.5 mM) as the sole source of nitrogen. Although succinate was supplied as a carbon and energy source during selection, both isolates could utilize 2,4-dinitrophenol also as the sole source of carbon. Both strains metabolized 2,4-dinitrophenol under concomitant liberation of stoichiometric amounts of nitrite and 4,6-dinitrohexanoate as a minor dead-end metabolite.

Determination of diffusion mechanisms in amorphous silicon.

Abstract: We have established experimentally and theoretically that transition metals in amorphous Si undergo direct interstitial diffusion that is retarded by temporary trapping at the defects intrinsic to the amorphous structure. The diffusion of Cu, Zn, Pd, Ag, Pt, and Au has been investigated by means of Rutherford-backscattering spectrometry and that of Au tracer atoms by neutron-activation and sputter-sectioning analysis. The data can be fitted using the foreign-atom interstitial diffusion coefficients in crystalline Si modified due to the presence of traps with concentrations between 0.2 and 1 at.% and trapping enthalpies of about 0.9 eV.