Preconditioned iterative methods for coupled discretizations of fluid flow problems

Computational fluid dynamics, where simulations require largecomputation times, is one of the areas of application of highperformance computing. Schemes such as the SIMPLE (semi-implicitmethod for pressure-linked equations) algorithm are often usedto solve the discrete Navier-Stokes equations. Generally theseschemes take a short time per iteration but require a largenumber of iterations. For simple geometries (or coarser grids)the overall CPU time is small. However, for finer grids or morecomplex geometries the increase in the number of iterationsmay be a drawback and the decoupling of the differential equationsinvolved implies a slow convergence of rotationally dominatedproblems that can be very time consuming for realistic applications.So we analyze here another approach, DIRECTO, that solves theequations in a coupled way. With recent advances in hardwaretechnology and software design, it became possible to solvecoupled Navier-Stokes systems, which are more robust but implyincreasing computational requirements (both in terms of memoryand CPU time). Two approaches are described here (band blockLU factorization and preconditioned GMRES) for the linear solverrequired by the DIRECTO algorithm that solves the fluid flowequations as a coupled system. Comparisons of the effectivenessof incomplete factorization preconditioners applied to the GMRES(generalized minimum residual) method are shown. Some numericalresults are presented showing that it is possible to minimizeconsiderably the CPU time of the coupled approach so that itcan be faster than the decoupled one.     

Direct quantification of ethyl carbamate in distilled alcoholic beverages using a cold capillary injection system and optimised selected ion monitoring

Summary An integrated procedure is described which allows direct injection quantitative screening to single figure parts per billion (g/L) levels of ethyl carbamate in the natural matrix of distilled alcoholic beverage. Injection and detection performance was studied and optimized to allow this routine, reproducible, trace analysis without any sample pre-treatment. All aspects of the analysis including autosampler run sequences and automated internal standard report generation are initiated and controlled from a single GC/MS workstation.Excerpt from the Ph. D. Thesis (in preparation) of K. MacNamara, Universität Karlsruhe     

1,3-di(methythio)-2,2,4,4-tetramethylbicyclo[1.1.0]butane

1,3-Di(methylthio)-2,2,4,4-tetramethylbicyclo[1.1.0]butane has been synthesized by two alternate routes and has been characterized by single crystal X-ray crystallography. This report corrects earlier, erroneous discussions of the title compound which have appeared in the literature.     

Reaction of CO with molecularly chemisorbed oxygen on TiO2-supported gold nanoclusters

In this study we present results of an investigation into the reactivity of molecularly chemisorbed oxygen species on a Au/TiO2 model catalyst. We have previously shown that a Au/TiO2 model catalyst sample can be populated with both atomically and molecularly chemisorbed oxygen species following exposure to a radio frequency-generated oxygen plasma-jet. To test the reactivity of the molecularly chemisorbed oxygen species, we compare the CO2 produced from a sample that is populated with both oxygen species to the CO2 produced from a sample that has been given an identical exposure but has been cleared of molecularly chemisorbed oxygen employing collision-induced desorption. We observe that samples that are populated with both oxygen species consistently result in greater CO2 production. For the data presented in this paper, we observe a difference of 41% in the CO2 production. We interpret this result to indicate that molecularly chemisorbed oxygen can react directly with CO to form CO2.     

Capillarity-Induced Surface Morphologies

Surface energetics is reviewed including expressions for the chemical potential of a curved surface element and the Legendre transform relation between the projected surface free energy as a function of orientation and the Wulff equilibrium shape. A well known equation is derived describing surface evolution by surface diffusion, assuming local equilibrium. Solutions are reviewed including a decaying sinusoid and a developing thermal groove. Breakdown of local equilibrium is considered. The structure, energetics and dynamics of steps on a vicinal surface are discussed. Facet sizes on the Wulff shape and the surface profile at the edge of a facet are related to the step self and interaction free energies respectively. Fourier analysis of step fluctuations is described, revealing the underlying transport processes. Analysis of the decay of a sinusoidal profile on a vicinal surface in terms of step behavior is given. Finally, examples are reviewed of surface evolution below the roughening temperature T _R in which case facets move by the lateral spreading of steps. Results differ greatly from those of the continuum theory applicable above T _R.     

Wolff–Kishner reduction reactions using a solar irradiation heat source and a green solvent system

Due to the recognition of the irreversible damage done to the environment through man-made materials, scientists have attempted to transform synthetic procedures into environmentally favorable procedures. Since fossil fuels are used for electrical energy in the USA, the amount of electricity required to complete an experiment has become an environmental concern. Solar parabolic reflectors have been proposed as a means for minimizing the amount of electricity needed to perform chemical reactions. The ability to use the solar reflector as the sole heat source for synthetic reactions is being considered. Another area of environmental concern is the chemical solvent systems involved in synthetic reactions that are not friendly to the environment. The ability to exchange solvent systems for greener solvents is being considered. A comparative study was conducted using an electrical and solar heat source on a series of Wolff–Kishner reduction reactions performed in a green solvent system. The following generalized chemical reaction is representative: where R is a hydrocarbon chain and R′ is a hydrocarbon chain or hydrogen.     

Preparation, structural analysis and dielectric properties of Bi x La1−x FeO3 perovskite

Rare earth element substituted bismuth ferrites (BiFeO₃) are of enormous importance as magnetoelectric materials. The polycrystalline samples of Bi _x La_1−x FeO₃ (x=0, 0.2, 0.4, 0.6, 0.8) were prepared by solid-state reaction using standard ceramic method. The single-phase formation of these compounds was confirmed by X-ray diffraction (XRD) studies. The samples with x=0, 0.2, 0.4, 0.6 are found to be orthorhombic while the sample with x=0.8 is triclinic. The dielectric constant (ε′) and dissipation factor (tan δ) were measured in the frequency range 100 Hz to 1 MHz at room temperature and as a function of temperature at certain fixed frequencies (1 kHz, 10 kHz, 100 kHz, 1 MHz). All the samples showed dielectric dispersion. The dielectric constant with temperature shows a broad peak; the peak temperature shifts with frequency which reflects the relaxor-type behavior. The peak above 600 K in the measured temperature range corresponds to antiferromagnetic ordering temperature (Néel temperature). The broadness of the peak changes with composition. The ac conductivity as well as ε′ are found to be maximum for the sample x=0.2 at room temperature.