Automatic partitioning of unstructured meshes for the parallel solution of problems in computational mechanics

Most of the recently proposed computational methods for solving partial differential equations on multiprocessor architectures stem from the 'divide and conquer' paradigm and involve some form of domain decomposition. For those methods which also require grids of points or patches of elements, it is often necessary to explicitly partition the underlying mesh, especially when working with local memory parallel processors. In this paper, a family of cost-effective algorithms for the automatic partitioning of arbitrary two- and three-dimensional finite element and finite difference meshes is presented and discussed in view of a domain decomposed solution procedure and parallel processing. The influence of the algorithmic aspects of a solution method (implicit/explicit computations), and the architectural specifics of a multiprocessor (SIMD/MIMD, startup/transmission time), on the design of a mesh partitioning algorithm are discussed. The impact of the partitioning strategy on load balancing, operation count, operator conditioning, rate of convergence and processor mapping is also addressed. Finally, the proposed mesh decomposition algorithms are demonstrated with realistic examples of finite element, finite volume, and finite difference meshes associated with the parallel solution of solid and fluid mechanics problems on the iPSC/2 and iPSC/860 multiprocessors.     

Effect of magnesium on the composition,microstructure and mechanical properties of carbon fibres

This work was undertaken in order to provide more detailed information on the chemical and mechanical behaviour of carbon fibres during the elaboration of graphite-magnesium composite materials. For this purpose, PAN-based T300, pitch-based P55 and P100 carbon fibres were isothermally heat treated, at temperatures ranging from 450 to 700 °C, under a saturated vapour pressure of magnesium. The composition, microstructure and tensile strength of the resulting samples were characterized by chemical and electron probe microanalysis, Raman spectrometry, X-ray diffraction and mechanical test of single filaments. From the results obtained, it has been concluded that highly graphitized fibres such as pitch-based P55 or P100 are not affected by long-time annealing in the presence of magnesium vapour, whereas impure and disorded fibres such as PAN-based T300 undergo some chemical and microstructural modifications decreasing their mechanical properties.     

IDENTIFICATION OF NITROGEN COMPOUNDS AND THEIR EVOLUTIONDURINGAGEING NABSENCE ANDPRESENCE OF ADDITIVES EFFICIENCYOFHYDROTREATMENT

As established by several previous works, nitrogen compounds play a prominent role in the evolution of middle distillates containing cracked components, particularly regarding sediment formation and color evolution.

In a first part, this paper describes and compares stability properties of fuel blends using both an accelerated ageing method at 120°C (248°F ) and long term storage methods at 43°C(110°F) -ASTM 0 4625 - and at ambient temperature. Effectiveness of stabilizing additives is also evaluated. In mixtures containing LCOs, insoluble products are formed progressively during ageings, more or less rapidly according to the chemical constitution of the mixtures.

Then, it reports the complete identification of nitrogen compounds using gas chromatography equipped with a selective nitrogen detector and mass spectrometry showing that in light cycle oils, alkyl indoles and carbazoles are the main families.

Evolution of these compounds was followed kinetically during ageings in absence and presence of additives and alkyl indoles appeared as the moat evolutionary.

It appeared that some additives avoided evolutions of alkyl indoles without preventing sediment formation and color evolution. Oxidation mechanism involving nitrogen compounds should not be the only one to explain the storage evolutions of middle distillates.

Hydrotreatment converts all the alkyl indoles of LCO and prevents coloration end deposits in the storage of the mixtures of steaight-run distillates and LCOs.     

Prebiotic synthesis of sequential peptides on the Hadean beach by a molecular engine working with nitrogen oxides as energy sources

Addressing the still open question of the prebiotic origin of sequential macromolecules (peptides, nucleic acids) on the primitive Earth, we describe a molecular engine (the primary pump), which works at ambient temperature and continuously generates, elongates and complexifies sequential peptides. This new scenario is based on a cyclic reaction sequence, whose keystep is the activation of amino acids into their N‐carboxyanhydrides (NCA) through nitrosation by NOx. This process could have taken place on tidal beaches; it requires a buffered ocean, emerged land and a nitrosating atmosphere. With the help of geochemical studies and computer simulations of atmosphere photochemistry, we show that the primitive Earth during the Hadean may have satisfied all these requirements. © 2001 Society of Chemical Industry.     

How to optimize high-Tc superconductive cuprates

Different ways to optimizeT _c 's and the magnetic properties of high-T _c superconductive cuprates are described and discussed. Oxygen intercalation-desintercalation phenomena and cationic substitutions (M³⁺ for M²⁺ or vice versa) lead to a variation of the hole carrier density and can have a drastic influence onT _c 's, as shown for bismuth, thallium, and lead-based cuprates, as well as for La₂CuO₄ and “123”-type superconductors. In the former, the role of hole reservoirs for the rock salt type layers is outlined. The modification of the hybridization of orbitals can also explain the variations ofT _c 's observed in some systems like Y_1?x Ca _x Ba₂Cu_3?x :Fe _x O₇ in which the oxidation state of copper remains constant. A critical current density enhancement can be realized by irradiating high-T _c materials by high-energy heavy ions because of the peculiar columnar structure of the heavy ion-induced tracks. The effects of such columnar defects, 70 Å in diameter, on flux pinning, magnetic relaxation, and location of the irreversibility line of bismuth-based 2212 crystals irradiated by 6-GeV Pb ions are reported. We observe a strong shift of the irreversibility line toward high fields and temperatures, indicating that pinning effects must be taken into account in the vortex lattice motion. Such induced changes are accompanied by a strong enhancement of the critical current density and a significant enlargement of the irreversibility region in theH,T plane.     

Akustische Eigenschaften von Holz und holzhaltigen Bauplatten

Zusammenfassung Die gro?e Bedeutung, die dem Holz für die Errichtung von Tragwerken wie auch für die Ausstattung von R?umen zukommt, rechtfertigt es, sich einmal die akustischen Eigenschaften dieses Baustoffes zu vergegenw?rtigen. Grunds?tzlich handelt es sich dabei um zweierlei Dinge, n?mlich raumakustisch um die Einwirkung auf die H?rsamkeit, d. h. um die Beeinflussung von Schall, der in einem Raum erzeugt wird und in diesem selbst verl?uft, und dann bauakustisch um die D?mmung, d. h. um die F?higkeit, einen auftreffenden Schall zurückzuhalten, ihn nicht nach au?en, also in andere R?ume oder ins Freie dringen zu lassen.     

Signal classification through multifractal analysis and complex domain neural networks

This paper describes a system capable of classifying stochastic self-affine nonstationary signals produced by nonlinear systems. The classification and the analysis of these signals are important because these are generated by many real-world processes. The first stage of the signal classification process entails the transformation of the signal into the multifractal dimension domain, through the computation of the variance fractal dimension trajectory (VFDT). Features can then be extracted from the VFDT using a Kohonen self-organizing feature map. The second stage involves the use of a complex domain neural network and a probabilistic neural network to determine the class of a signal based on these extracted features. The results of this paper show that these techniques can be successful in creating a classification system which can obtain correct classification rates of about 87% when performing classification of such signals without knowing the number of classes.     

Determination of Magnetic and Nuclear Quadrupole Interactions in La@C82 from EPR Linewidth Data

From the analysis of the temperature dependence of the EPR line width, an estimate of the anisotropic parts of the dipolar and quadrupolar hyperfine couplings of lanthanum was derived. Furthermore, the rotational correlation time of the nearly spherical molecule was found to deviate considerably from that of the solvent molecules.