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Discrete Level (discrete + level)
Selected AbstractsPressure segregation methods based on a discrete pressure Poisson equation.INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 4 2008An algebraic approach Abstract In this paper, we introduce some pressure segregation methods obtained from a non-standard version of the discrete monolithic system, where the continuity equation has been replaced by a pressure Poisson equation obtained at the discrete level. In these methods it is the velocity instead of the pressure the extrapolated unknown. Moreover, predictor,corrector schemes are suggested, again motivated by the new monolithic system. Key implementation aspects are discussed, and a complete stability analysis is performed. We end with a set of numerical examples in order to compare these methods with classical pressure-correction schemes. Copyright © 2007 John Wiley & Sons, Ltd. [source] Finite-element simulation of incompressible fluid flow in an elastic vesselINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 2 2003Harry Y. H. Chen Abstract Finite-element simulation was performed to predict the incompressible Navier,Stokes flow in a domain, partly bounded by an elastic vessel, which is allowed to vary with time. Besides satisfying the physical conservation laws, both surface and the volume conservation laws are satisfied at the discrete level for ensuring the balance between physical and geometrical variables. Several problems which are amenable to analytical solutions were tested for validating the method. The simulated results are observed to agree favourably with analytical solutions. Having verified the applicability of the finite-element code to problems involving moving grids, we consider an incompressible fluid flow bounded by rigid and elastic vessel walls. Our emphasis was placed on the validation of the formulation developed within the moving-grid framework. Copyright © 2003 John Wiley & Sons, Ltd. [source] Some preconditioners for the CFIE equation of electromagnetismMATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 17 2008David P. Levadoux Abstract We present three weak parametrices of the operator of the combined field integral equation (CFIE). An interesting feature of these parametrices is that they are compatible with different discretization strategies and hence allow for the construction of efficient preconditioners dedicated to the CFIE. Their numerical analysis shows that a regularization process acting at the continuous level of the equation is also effective at the discrete level if the mesh size tends to zero. First numerical tests confirm this effect and preconditioning is observed indeed. Furthermore, we show that the underlying operator of CFIE satisfies a uniform discrete Inf,Sup condition that allows one to predict an original convergence result for the numerical solution of CFIE to the exact one. Copyright © 2008 John Wiley & Sons, Ltd. [source] Unified multipliers-free theory of dual-primal domain decomposition methodsNUMERICAL METHODS FOR PARTIAL DIFFERENTIAL EQUATIONS, Issue 3 2009Ismael Herrera Abstract The concept of dual-primal methods can be formulated in a manner that incorporates, as a subclass, the non preconditioned case. Using such a generalized concept, in this article without recourse to "Lagrange multipliers," we introduce an all-inclusive unified theory of nonoverlapping domain decomposition methods (DDMs). One-level methods, such as Schur-complement and one-level FETI, as well as two-level methods, such as Neumann-Neumann and preconditioned FETI, are incorporated in a unified manner. Different choices of the dual subspaces yield the different dual-primal preconditioners reported in the literature. In this unified theory, the procedures are carried out directly on the matrices, independently of the differential equations that originated them. This feature reduces considerably the code-development effort required for their implementation and permit, for example, transforming 2D codes into 3D codes easily. Another source of this simplification is the introduction of two projection-matrices, generalizations of the average and jump of a function, which possess superior computational properties. In particular, on the basis of numerical results reported there, we claim that our jump matrix is the optimal choice of the B operator of the FETI methods. A new formula for the Steklov-Poincaré operator, at the discrete level, is also introduced. © 2008 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2009 [source] Transport-equilibrium schemes for computing nonclassical shocks.NUMERICAL METHODS FOR PARTIAL DIFFERENTIAL EQUATIONS, Issue 4 2008Scalar conservation laws Abstract This paper presents a new numerical strategy for computing the nonclassical weak solutions of scalar conservation laws which fail to be genuinely nonlinear. We concentrate on the typical situation of concave,convex and convex,concave flux functions. In such situations the so-called nonclassical shocks, violating the classical Oleinik entropy criterion and selected by a prescribed kinetic relation, naturally arise in the resolution of the Riemann problem. Enforcing the kinetic relation from a numerical point of view is known to be a crucial but challenging issue. By means of an algorithm made of two steps, namely an Equilibrium step and a Transport step, we show how to force the validity of the kinetic relation at the discrete level. The proposed strategy is based on the use of a numerical flux function and random numbers. We prove that the resulting scheme enjoys important consistency properties. Numerous numerical evidences illustrate the validity of our approach. © 2007 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2008 [source] Band structure of a harmonically confined electron with an impenetrable boundaryPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2004W. Hai Abstract We study finite-size effects of the spatially bounded quantum systems exemplified by a single-electron quantum dot with a harmonic potential and an impenetrable boundary. A general solution of the corresponding Schrödinger equation is obtained and the unique special solution for any energy is derived from the normalization and boundary conditions. The classical-mechanically allowable eigenenergies form the continuous spectrum or piecewise continuous bands with the minimum value being much less than the zero point energy of a free harmonic oscillator. As the increase of the confining size, the band widths reduce and the energies finally close to the discrete level of the free oscillator. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Bayesian hierarchical models in ecological studies of health,environment effectsENVIRONMETRICS, Issue 2 2003Sylvia Richardson Abstract We describe Bayesian hierarchical models and illustrate their use in epidemiological studies of the effects of environment on health. The framework of Bayesian hierarchical models refers to a generic model building strategy in which unobserved quantities (e.g. statistical parameters, missing or mismeasured data, random effects, etc.) are organized into a small number of discrete levels with logically distinct and scientifically interpretable functions, and probabilistic relationships between them that capture inherent features of the data. It has proved to be successful for analysing many types of complex epidemiological and biomedical data. The general applicability of Bayesian hierarchical models has been enhanced by advances in computational algorithms, notably those belonging to the family of stochastic algorithms based on Markov chain Monte Carlo techniques. In this article, we review different types of design commonly used in studies of environment and health, give details on how to incorporate the hierarchical structure into the different components of the model (baseline risk, exposure) and discuss the model specification at the different levels of the hierarchy with particular attention to the problem of aggregation (ecological) bias. Copyright © 2003 John Wiley & Sons, Ltd. [source] Evolutionarily stable investment in secondary defencesFUNCTIONAL ECOLOGY, Issue 5 2005M. BROOM Summary 1Previous workers have suggested that the evolutionarily stable strategy (ESS) for investment in antipredator defences, such as toxins, will critically depend on the nature of expression of the defence. Specifically, it has been suggested that if the different levels of a defence are best described as a continuous variable, then this will lead to pure ESSs with all individuals in a population adopting similar defence levels; whereas defences that can only take on discrete levels will lead to mixed ESSs (featuring variation in defence within the population). 2Our principal aim is to determine the validity of these viewpoints, and examine how the pure and mixed strategies predicted by the two types of defences can be reconciled with practical and philosophical difficulties in defining any given defence unambiguously as continuous or discrete. 3We present the first model of a continuously varying defence that is solved explicitly for evolutionarily stable strategies. 4We are able to demonstrate analytically, that the model always has a unique ESS, which is always pure. This strategy may involve all members of the population adopting no defence, or all members of the population making the same non-zero investment in defence. 5We then modify our model to restrict the defence to a number of discrete levels and demonstrate that the unique ESS in this case can be either pure or mixed. We further argue that the mixed ESS can be a combination of no more than two defence levels, and the two levels in a mixed ESS must be nearest neighbour levels in an ordered list of the levels that the defence can take. 6This, in turn, means that the mixed ESS will be practically identical to a pure ESS if the discrete defence is fine-grained. [source] Times of sand: Sedimentary history and archaeology at the Sigatoka Dunes, FijiGEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 2 2006Atholl Anderson The orthodox archaeological sequence at the Sigatoka Dunes site (VL 16/1) in Fiji proposes three phases of occupation spanning Fijian prehistory, each associated with a period of dune stability. It has been taken as the standard model of Fijian prehistory for more than 30 years. Recently, however, it has been argued that there is no stratigraphic support for three discrete levels and that the occupation history was fragmented, complex, and continuous within a volatile dune system. We present new data, from optical and radiocarbon dating, to argue that a three-phase model, although somewhat more complex in detail, remains the most robust interpretation of site history. The longest stable phase (Level 2) began 2500,2300 cal yr B.P. and is possibly associated with relatively low ENSO frequency. Substantial sand dune accumulation began after ,1300 cal yr B.P. © 2006 Wiley Periodicals, Inc. [source] Electronic transport through large quantum dots in the Kondo regimePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003P. Stefa Abstract Conductance through a large two-level quantum dot is investigated theoretically in the strong coupling regime. In large quantum dots the separation between discrete levels becomes smaller than the level width due to strong hybridization with electrodes. In such circumstances, apart from strong electronic correlations in the quantum dot, the indirect interaction between both the spatial levels comes into play. It takes place in lateral quantum dots, where the spatial level index is not conserved during the hybridization process with electrodes. This interaction shifts the Kondo resonance peak in the density of states out of the Fermi surface and alters its intensity. This feature can be observed in the differential conductance dependence vs. bias voltage. The virtual inter-level mixing is suppressed for temperatures above the Kondo temperature of the system. The results of theoretical predictions are compared with the results of experimental conductance measurements performed on large quantum dots and some non-typical conductance features are clarified. [source] The influence of level and polarity of figure-ground contrast on visionACTA OPHTHALMOLOGICA, Issue 4 2001Jonathan S. Pointer ABSTRACT. Purpose: To document the effect upon human foveal vision of changes in the level and polarity of figure-ground contrast under photopic controlled test conditions, with particular emphasis on performance at low contrast levels. Methods: Using a forced-choice psychophysical paradigm, threshold acuity estimates were derived at 9 discrete levels over a near-3 octave contrast range for Landolt ring-type stimuli of either positive or negative polarity. Data were obtained under binocular conditions from 10 young adults, each wearing their optimum low myopic spectacle correction. Results: Visual acuity declined linearly with reducing stimulus contrast, the deterioration increasing substantially at <10% figure-ground contrast regardless of stimulus polarity. Performance was slightly (but not statistically significantly) better for positive contrast stimuli. Conclusion: Irrespective of contrast polarity, a reduction in stimulus figure-ground contrast <10% produces an accelerated decrement in photopic foveal vision compared to the performance at levels >10%. Some clinical and practical implications of this outcome are considered with regard to the examination of patients with normal and compromised visual function. [source] | ||||||||||