Home About us Contact
|
Home About us Contact | ||
|
|
||
Method Leads (method + lead)
Selected AbstractsHigh-Zirconium-Content Nano-Sized Bimodal Mesoporous SilicasEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 13 2006David Ortiz de Zárate Abstract Silica-based nanoparticulated bimodal mesoporous materials with high Zr content (43 , Si/Zr , 4) have been synthesized by a one-pot surfactant-assisted procedure from a hydroalcoholic medium using a cationic surfactant (CTMABr = cetyltrimethylammonium bromide) as structure-directing agent, and starting from molecular atrane complexes of Zr and Si as hydrolytic inorganic precursors. This preparative technique allows optimization of the dispersion of the Zr guest species in the silica walls. The bimodal mesoporous nature of the final high surface area nano-sized materials is confirmed by XRD, TEM, and N2 adsorption,desorption isotherms. The small intraparticle mesopore system (with pore sizes around 2,3 nm) is due to the supramolecular templating effect of the surfactant, while the large mesopores (around 12,24 nm) have their origin in the packing voids generated by aggregation of the primary nanometric mesoporous particles. The basicity of the reaction medium seems to be a key parameter in the definition of this last pore system. The effects induced by the progressive incorporation of Zr atoms on the mesostructure have been examined, and the local environment of the Zr sites in the framework has been investigated by UV/Vis spectroscopy. Observations based on the consequences of post-treatments of the as-synthesized materials with HCl/ethanol mixtures corroborate that the atrane method leads to Zr-rich materials showing enhanced site accessibility and high chemical homogeneity throughout the pore walls. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Dual analysis by a meshless methodINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 9 2002Marc Duflot Abstract A meshless method to solve elastostatic problems based on an equilibrium model is presented. This means that the equilibrium and constitutive equations are satisfied a priori and that the approximation only concerns the compatibility equations. The application of this method together with the classical displacement meshless method leads to upper and lower bounds on the energy. The difference between these bounds gives a global error estimation on the solution. Copyright © 2002 John Wiley & Sons, Ltd. [source] Lower bound limit analysis of cohesive-frictional materials using second-order cone programmingINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 4 2006A. Makrodimopoulos Abstract The formulation of limit analysis by means of the finite element method leads to an optimization problem with a large number of variables and constraints. Here we present a method for obtaining strict lower bound solutions using second-order cone programming (SOCP), for which efficient primal-dual interior-point algorithms have recently been developed. Following a review of previous work, we provide a brief introduction to SOCP and describe how lower bound limit analysis can be formulated in this way. Some methods for exploiting the data structure of the problem are also described, including an efficient strategy for detecting and removing linearly dependent constraints at the assembly stage. The benefits of employing SOCP are then illustrated with numerical examples. Through the use of an effective algorithm/software, very large optimization problems with up to 700 000 variables are solved in minutes on a desktop machine. The numerical examples concern plane strain conditions and the Mohr,Coulomb criterion, however we show that SOCP can also be applied to any other problem of lower bound limit analysis involving a yield function with a conic quadratic form (notable examples being the Drucker,Prager criterion in 2D or 3D, and Nielsen's criterion for plates). Copyright © 2005 John Wiley & Sons, Ltd. [source] Singularity extraction technique for integral equation methods with higher order basis functions on plane triangles and tetrahedraINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 8 2003Seppo Järvenpää Abstract A numerical solution of integral equations typically requires calculation of integrals with singular kernels. The integration of singular terms can be considered either by purely numerical techniques, e.g. Duffy's method, polar co-ordinate transformation, or by singularity extraction. In the latter method the extracted singular integral is calculated in closed form and the remaining integral is calculated numerically. This method has been well established for linear and constant shape functions. In this paper we extend the method for polynomial shape functions of arbitrary order. We present recursive formulas by which we can extract any number of terms from the singular kernel defined by the fundamental solution of the Helmholtz equation, or its gradient, and integrate the extracted terms times a polynomial shape function in closed form over plane triangles or tetrahedra. The presented formulas generalize the singularity extraction technique for surface and volume integral equation methods with high-order basis functions. Numerical experiments show that the developed method leads to a more accurate and robust integration scheme, and in many cases also a faster method than, for example, Duffy's transformation. Copyright © 2003 John Wiley & Sons, Ltd. [source] Frequency/time-domain modelling of 3D waveguide structures by a BI-RME approachINTERNATIONAL JOURNAL OF NUMERICAL MODELLING: ELECTRONIC NETWORKS, DEVICES AND FIELDS, Issue 1 2002P. Arcioni This paper presents a full wave method for the determination of the mathematical model of a 3D waveguide structure in the form of the pole expansion in the s -plane of its generalized admittance matrix. The method is based on a boundary integral-resonant mode expansion approach. By the introduction of appropriate state-variables, the method leads to the pole expansion by solving a linear generalized eigenvalue problem, like in the well-known techniques used up to now in frequency/time domain modelling based on finite difference or finite element methods. With respect to these methods we have the advantage of a significant reduction in both memory allocation and computing time. Two examples show the accuracy of the results and the efficiency of the method. Copyright © 2002 John Wiley & Sons, Ltd. [source] Explicitly correlated SCF study of anharmonic vibrations in (H2O)2INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2002Donald D. Shillady Abstract Modeling solvation in high-pressure liquid chromatography (HPLC) requires calculation of anharmonic vibrational frequencies of solvent clusters for a statistical partition function. An efficient computational method that includes electron correlation is highly desirable for large clusters. A modified version of the "soft Coulomb hole" method of Chakravorty and Clementi has recently been implemented in a Gaussian-lobe-orbital (GLO) program (PCLOBE) to include explicit electron,electron correlation in molecules. The soft Coulomb hole is based on a modified form of Coulomb's law: An algorithm has been developed to obtain the parameter "w" from a polynomial in the effective scaling of each primitive Gaussian orbital relative to the best single Gaussian of the H1s orbital. This method yields over 90% of the correlation energy for molecules of low symmetry for which the original formula of Chakravorty and Clementi does not apply. In this work, all the vibrations of the water dimer are treated anharmonically. A quartic perturbation of the harmonic vibrational modes is constrained to be equal to the exact Morse potential eigenvalue based on a three-point fit. This work evaluates the usefulness of fitting a Morse potential to a hydrogen bond vibrational mode and finds it to be slightly better than using MP2 vibrational analysis for this important dimer. A three-point estimate of the depth, De, of a Morse potential leads to a correction formula for anharmonicity in terms of the perturbed harmonic frequency: When scaled by 0.9141, the harmonic Morse method leads to essentially the same results as scaling the BPW91 local density method by 0.9827. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 [source] A new method for the gradient-based optimization of molecular complexesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 9 2009Jan Fuhrmann Abstract We present a novel method for the local optimization of molecular complexes. This new approach is especially suited for usage in molecular docking. In molecular modeling, molecules are often described employing a compact representation to reduce the number of degrees of freedom. This compact representation is realized by fixing bond lengths and angles while permitting changes in translation, orientation, and selected dihedral angles. Gradient-based energy minimization of molecular complexes using this representation suffers from well-known singularities arising during the optimization process. We suggest an approach new in the field of structure optimization that allows to employ gradient-based optimization algorithms for such a compact representation. We propose to use exponential mapping to define the molecular orientation which facilitates calculating the orientational gradient. To avoid singularities of this parametrization, the local minimization algorithm is modified to change efficiently the orientational parameters while preserving the molecular orientation, i.e. we perform well-defined jumps on the objective function. Our approach is applicable to continuous, but not necessarily differentiable objective functions. We evaluated our new method by optimizing several ligands with an increasing number of internal degrees of freedom in the presence of large receptors. In comparison to the method of Solis and Wets in the challenging case of a non-differentiable scoring function, our proposed method leads to substantially improved results in all test cases, i.e. we obtain better scores in fewer steps for all complexes. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2009 [source] Multicomponent thermally coupled systems of distillation columns at minimum refluxAICHE JOURNAL, Issue 12 2001Zbigniew T. Fidkowski A method for calculating the minimum vapor flow in a ternary fully thermally coupled system of distillation columns was developed earlier by Fidkowski and Krolikowski. This method was developed further for fully thermally coupled systems separating four or more components. For multicomponent mixtures having constant relative volatilities and equal latent heats, the method leads to the identification of all of the minimum vapor flow rates in each distillation column that provide the same overall minimum heat duty in the reboiler. The quaternary fully coupled system of columns requires less heat energy than conventional configurations. These savings are often on the order of 20,50%. [source] Synthesis of Poly(lauryl acrylate) by Single-Electron Transfer/Degenerative Chain Transfer Living Radical Polymerization Catalyzed by Na2S2O4 in WaterMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 11 2007Jorge F. J. Coelho Abstract Living radical polymerization of lauryl acrylate was achieved by SET/DTLRP in water catalyzed by sodium dithionite. The work describes the synthesis of a highly hydrophobic and polar monomer in aqueous medium. The plots of versus conversion and ln[M]0/[M] versus time are linear, indicating a controlled polymerization. This method leads to ,,, -diiodopoly(lauryl acrylate)s that can be further functionalized. The MWDs were determined using a combination of three detectors: RALLS, DV, and RI. The method studied in this work represents a possible route to prepare well-tailored macromolecules made of LA in environment friendly reaction medium. The syndiotactic content is 75%. [source] Amplitude,shape approximation as an extension of separation of variablesMATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 3 2008N. Parumasur Abstract Separation of variables is a well-known technique for solving differential equations. However, it is seldom used in practical applications since it is impossible to carry out a separation of variables in most cases. In this paper, we propose the amplitude,shape approximation (ASA) which may be considered as an extension of the separation of variables method for ordinary differential equations. The main idea of the ASA is to write the solution as a product of an amplitude function and a shape function, both depending on time, and may be viewed as an incomplete separation of variables. In fact, it will be seen that such a separation exists naturally when the method of lines is used to solve certain classes of coupled partial differential equations. We derive new conditions which may be used to solve the shape equations directly and present a numerical algorithm for solving the resulting system of ordinary differential equations for the amplitude functions. Alternatively, we propose a numerical method, similar to the well-established exponential time differencing method, for solving the shape equations. We consider stability conditions for the specific case corresponding to the explicit Euler method. We also consider a generalization of the method for solving systems of coupled partial differential equations. Finally, we consider the simple reaction diffusion equation and a numerical example from chemical kinetics to demonstrate the effectiveness of the method. The ASA results in far superior numerical results when the relative errors are compared to the separation of variables method. Furthermore, the method leads to a reduction in CPU time as compared to using the Rosenbrock semi-implicit method for solving a stiff system of ordinary differential equations resulting from a method of lines solution of a coupled pair of partial differential equations. The present amplitude,shape method is a simplified version of previous ones due to the use of a linear approximation to the time dependence of the shape function. Copyright © 2007 John Wiley & Sons, Ltd. [source] A limiting absorption principle for scattering problems with unbounded obstaclesMATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 14 2001Anne-Sophie Bonnet-Bendhia Abstract A generalized mode matching method that applies to a wide class of scattering problems is developed in the time harmonic two-dimensional Helmholtz case. This method leads by variational means to an integro-differential formulation whose unknown is the trace of the field on an unbounded one-dimensional interface. The well-posedness is proved after a careful study of the rather original functional framework. Owing to a fundamental density result,based upon some properties of a singular integral operator similar to the Hilbert transform,the limiting absorption principle related to this original formulation is established. Finally, two other situations are emphasized. Copyright © 2001 John Wiley & Sons, Ltd. [source] Probabilistic forecasting from ensemble prediction systems: Improving upon the best-member method by using a different weight and dressing kernel for each memberTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 617 2006Vincent Fortin Abstract Ensembles of meteorological forecasts can both provide more accurate long-term forecasts and help assess the uncertainty of these forecasts. No single method has however emerged to obtain large numbers of equiprobable scenarios from such ensembles. A simple resampling scheme, the ,best member' method, has recently been proposed to this effect: individual members of an ensemble are ,dressed' with error patterns drawn from a database of past errors made by the ,best' member of the ensemble at each time step. It has been shown that the best-member method can lead to both underdispersive and overdispersive ensembles. The error patterns can be rescaled so as to obtain ensembles which display the desired variance. However, this approach fails in cases where the undressed ensemble members are already overdispersive. Furthermore, we show in this paper that it can also lead to an overestimation of the probability of extreme events. We propose to overcome both difficulties by dressing and weighting each member differently, using a different error distribution for each order statistic of the ensemble. We show on a synthetic example and using an operational ensemble prediction system that this new method leads to improved probabilistic forecasts, when the undressed ensemble members are both underdispersive and overdispersive. Copyright © 2006 Royal Meteorological Society. [source] Exploiting Gene-Environment Independence for Analysis of Case,Control Studies: An Empirical Bayes-Type Shrinkage Estimator to Trade-Off between Bias and EfficiencyBIOMETRICS, Issue 3 2008Bhramar Mukherjee Summary Standard prospective logistic regression analysis of case,control data often leads to very imprecise estimates of gene-environment interactions due to small numbers of cases or controls in cells of crossing genotype and exposure. In contrast, under the assumption of gene-environment independence, modern "retrospective" methods, including the "case-only" approach, can estimate the interaction parameters much more precisely, but they can be seriously biased when the underlying assumption of gene-environment independence is violated. In this article, we propose a novel empirical Bayes-type shrinkage estimator to analyze case,control data that can relax the gene-environment independence assumption in a data-adaptive fashion. In the special case, involving a binary gene and a binary exposure, the method leads to an estimator of the interaction log odds ratio parameter in a simple closed form that corresponds to an weighted average of the standard case-only and case,control estimators. We also describe a general approach for deriving the new shrinkage estimator and its variance within the retrospective maximum-likelihood framework developed by Chatterjee and Carroll (2005, Biometrika92, 399,418). Both simulated and real data examples suggest that the proposed estimator strikes a balance between bias and efficiency depending on the true nature of the gene-environment association and the sample size for a given study. [source] Deterministic and Probabilistic Estimation of Appropriate Distances: Motivation for Considering the Consequences for Industrial SitesCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 2 2009M. Gawlowski Abstract Accidents and disruptions in chemical process installations can, in principle, lead to the rare events in which the release of flammable and/or toxic substances occurs, and which at particular distances from the installation can result in a hazard potential due to thermal radiation, blast wave effects or the concentration of toxic substances. The possibilities and limits of deterministic and probabilistic estimation methods for appropriate distances from hazardous installations, based on the example of an ammonia release and a large surface fire, are shown. In this, it is demonstrated that the deterministic and probabilistic approaches are in no way conflicting or unnecessary, but rather that they are complementary. The use of a deterministic estimation method leads to a maximum set radius of effects which only take account of the damage impact. Depending on the selection of the appropriate and suitable consequence models, critical distances are calculated which are in some cases much larger than the current standardised distances, as is shown by the example of large-scale fires. The use of a probabilistic estimation method leads to a range of distances for which the individual risk can be given in addition. In principle, iso-contours joining points of same risk or areas of same risk may be defined through the use of such estimations. [source] | ||||||||||||||||