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Particular Choice (particular + choice)
Selected AbstractsOptimal design of supplemental viscous dampers for irregular shear-frames in the presence of yieldingEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 8 2005Oren Lavan Abstract A methodology for the optimal design of supplemental viscous dampers for regular as well as irregular yielding shear-frames is presented. It addresses the problem of minimizing the added damping subject to a constraint on an energy-based global damage index (GDI) for an ensemble of realistic ground motion records. The applicability of the methodology for irregular structures is achieved by choosing an appropriate GDI. For a particular choice of the parameters comprising the GDI, a design for the elastic behavior of the frame or equal damage for all stories is achieved. The use of a gradient-based optimization algorithm for the solution of the optimization problem is enabled by first deriving an expression for the gradient of the constraint. The optimization process is started for one ,active' ground motion record which is efficiently selected from the given ensemble. If the resulting optimal design fails to satisfy the constraints for other records from the original ensemble, additional ground motions (loading conditions) are added one by one to the ,active' set until the optimum is reached. Two examples for the optimal designs of supplemental dampers are given: a 2-story shear frame with varying strength distribution and a 10-story shear frame. The 2-story shear frame is designed for one given ground motion whereas the 10-story frame is designed for an ensemble of twenty ground motions. Copyright © 2005 John Wiley & Sons, Ltd. [source] THE CONDITIONS FOR SPECIATION THROUGH INTRASPECIFIC COMPETITIONEVOLUTION, Issue 11 2006Reinhard Bürger Abstract It has been shown theoretically that sympatric speciation can occur if intraspecific competition is strong enough to induce disruptive selection. However, the plausibility of the involved processes is under debate, and many questions on the conditions for speciation remain unresolved. For instance, is strong disruptive selection sufficient for speciation? Which roles do genetic architecture and initial composition of the population play? How strong must assortative mating be before a population can split in two? These are some of the issues we address here. We investigate a diploid multilocus model of a quantitative trait that is under frequency-dependent selection caused by a balance of intraspecific competition and frequency-independent stabilizing selection. This trait also acts as mating character for assortment. It has been established previously that speciation can occur only if competition is strong enough to induce disruptive selection. We find that speciation becomes more difficult for very strong competition, because then extremely strong assortment is required. Thus, speciation is most likely for intermediate strengths of competition, where it requires strong, but not extremely strong, assortment. For this range of parameters, however, it is not obvious how assortment can evolve from low to high levels, because with moderately strong assortment less genetic variation is maintained than under weak or strong assortment sometimes none at all. In addition to the strength of frequency-dependent competition and assortative mating, the roles of the number of loci, the distribution of allelic effects, the initial conditions, costs to being choosy, the strength of stabilizing selection, and the particular choice of the fitness function are explored. A multitude of possible evolutionary outcomes is observed, including loss of all genetic variation, splitting in two to five species, as well as very short and extremely long stable limit cycles. On the methodological side, we propose quantitative measures for deciding whether a given distribution reflects two (or more) reproductively isolated clusters. [source] An optimally convergent discontinuous Galerkin-based extended finite element method for fracture mechanicsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 6 2010Yongxing Shen Abstract The extended finite element method (XFEM) enables the representation of cracks in arbitrary locations of a mesh. We introduce here a variant of the XFEM rendering an optimally convergent scheme. Its distinguishing features are as follows: (a) the introduction of singular asymptotic crack tip fields with support on only a small region around the crack tip (the enrichment region), (b) only one and two enrichment functions are added for anti-plane shear and planar problems, respectively and (c) the relaxation of the continuity between the enrichment region and the rest of the domain, and the adoption of a discontinuous Galerkin (DG) method therein. The method is provably stable for any positive value of a stabilization parameter, and by weakly enforcing the continuity between the two regions it eliminates ,blending elements' partly responsible for the suboptimal convergence of some early XFEMs. Moreover, the particular choice of enrichment functions results in a surprisingly sparse stiffness matrix that remains reasonably conditioned as the mesh is refined. More importantly, the stress intensity factors can be extracted with a satisfactory accuracy as primary unknowns. Quadrature strategies required for the optimal convergence are also discussed. Finally, the DG method was modified to retain stability based on an inf-sup condition. Copyright © 2009 John Wiley & Sons, Ltd. [source] Trade Credit Management and the Decision to Use Factoring: An Empirical StudyJOURNAL OF BUSINESS FINANCE & ACCOUNTING, Issue 1-2 2000Barbara Summers This paper examines the firm's decision to use factoring amongst a cross-sectional sample of 655 manufacturing companies using a rich firm-level database. The paper develops and tests hypotheses that explain this particular choice of credit and financial management policy. We find strong evidence of a ,financing demand' explanation for the use of factoring, and also some support for theories which relate the decision to use a factor to the firm's product characteristics, to market characteristics and to the preferences of the factor (supply constraints). The motivation to use factoring, however, appears to be related more to a demand for asset-based finance from small companies than to firm-level choices about organisational structure. [source] Quantum chemical modeling of enzymatic reactions: The case of histone lysine methyltransferaseJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 8 2010Polina Georgieva Abstract Quantum chemical cluster models of enzyme active sites are today an important and powerful tool in the study of various aspects of enzymatic reactivity. This methodology has been applied to a wide spectrum of reactions and many important mechanistic problems have been solved. Herein, we report a systematic study of the reaction mechanism of the histone lysine methyltransferase (HKMT) SET7/9 enzyme, which catalyzes the methylation of the N-terminal histone tail of the chromatin structure. In this study, HKMT SET7/9 serves as a representative case to examine the modeling approach for the important class of methyl transfer enzymes. Active site models of different sizes are used to evaluate the methodology. In particular, the dependence of the calculated energies on the model size, the influence of the dielectric medium, and the particular choice of the dielectric constant are discussed. In addition, we examine the validity of some technical aspects, such as geometry optimization in solvent or with a large basis set, and the use of different density functional methods. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010 [source] Translational Parallel Manipulators: New ProposalsJOURNAL OF FIELD ROBOTICS (FORMERLY JOURNAL OF ROBOTIC SYSTEMS), Issue 12 2002Raffaele Di Gregorio Translational parallel manipulators are parallel manipulators wherein the end-effector performs only spatial translations. This paper presents a new family of translational parallel manipulators. The manipulators of this family are independent constraint manipulators. They have three limbs that are topologically identical and have no rotation singularity. The limbs of these manipulators feature five one-degree-of-freedom kinematic pairs in series. Four joints are revolute pairs and the remaining one, called T-pair, is a kinematic pair that can be manufactured in different ways. In each limb, three adjacent revolute pairs have parallel axes and the remaining revolute pair has an axis that is not parallel to the axes of the other revolute pairs. The mobility analysis of the manipulators of this new family is addressed by taking into account two different choices for the actuated pairs. One of the results of this analysis is that the geometry of a translational parallel manipulator free from singularities can be defined for a particular choice of the actuated pairs. © 2002 Wiley Periodicals, Inc. [source] Multi-dimensional inhomogeneity indicators and the force on uncharged spheres in electric fieldsMATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 7 2009Dirk Langemann Abstract Uncharged droplets on outdoor high-voltage equipment suffer a non-vanishing total force in non-homogeneous electric fields. Here, the model problem of a spherical test body is considered in arbitrary dimensions. A series expansion of inhomogeneity indicators is proven, which approximates the total force in local terms of the undisturbed electric field. The proof uses the ideas of generalized spherical harmonics without referring to the particular choice of the orthonormal system. The fast converging series expansion establishes a relationship between the solutions of two partial differential equations on different domains. Copyright © 2008 John Wiley & Sons, Ltd. [source] On the multilevel preconditioning of Crouzeix,Raviart elliptic problemsNUMERICAL LINEAR ALGEBRA WITH APPLICATIONS, Issue 5 2008J. Kraus Abstract We consider robust hierarchical splittings of finite element spaces related to non-conforming discretizations using Crouzeix,Raviart type elements. As is well known, this is the key to the construction of efficient two- and multilevel preconditioners. The main contribution of this paper is a theoretical and an experimental comparison of three such splittings. Our starting point is the standard method based on differences and aggregates (DA) as introduced in Blaheta et al. (Numer. Linear Algebra Appl. 2004; 11:309,326). On this basis we propose a more general (GDA) splitting, which can be viewed as the solution of a constraint optimization problem (based on certain symmetry assumptions). We further consider the locally optimal (ODA) splitting, which is shown to be equivalent to the first reduce (FR) method from Blaheta et al. (Numer. Linear Algebra Appl. 2004; 11:309,326). This means that both, the ODA and the FR splitting, generate the same subspaces, and thus the local constant in the strengthened Cauchy,Bunyakowski,Schwarz inequality is minimal for the FR (respectively ODA) splitting. Moreover, since the DA splitting corresponds to a particular choice in the parameter space of the GDA splitting, which itself is an element in the set of all splittings for which the ODA (or equivalently FR) splitting yields the optimum, we conclude that the chain of inequalities ,,,,,,3/4 holds independently of mesh and/or coefficient anisotropy. Apart from the theoretical considerations, the presented numerical results provide a basis for a comparison of these three approaches from a practical point of view. Copyright © 2007 John Wiley & Sons, Ltd. [source] "Barbed nanowires" from polymers via electrospinningPOLYMER ENGINEERING & SCIENCE, Issue 1 2009Andreas Holzmeister Electrospinning is a highly versatile technique that allows producing fibers with diameters down to a few nanometers not only from polymers but also from metals, metal oxides, or ceramics. Fiber formation in electrospinning differs strongly from other fiber producing methods such as extrusion in that it is basically governed by self-assembly processes induced by specific electrostatic interactions following the Earnshaw theorem of electrostatics. This allows the production of nanofibers with very peculiar shapes. Here, we report the one step fabrication of barbed nanofibers due to a particular choice of the spinning conditions. Such barbed fibers allow, among others, to control the total porosity of nanofiber nonwovens and to reduce the tendency of linear nano-objects towards aggregation. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers [source] Local Stone (A Fragment)ARCHITECTURAL DESIGN, Issue 6 2007Reinhold Martin Abstract At a time when a wide range of building materials is available around the world, the decision to use locally sourced stone in India often extends beyond the pragmatic. As Reinhold Martin explains, a particular choice of stone can potentially set off a complex string of associations with geopolitical connotations. Copyright © 2007 John Wiley & Sons, Ltd. [source] Estimating uncertainty in fish stock assessment and forecastingFISH AND FISHERIES, Issue 2 2001Kenneth Patterson Abstract A variety of tools are available to quantify uncertainty in age-structured fish stock assessments and in management forecasts. These tools are based on particular choices for the underlying population dynamics model, the aspects of the assessment considered uncertain, and the approach for assessing uncertainty (Bayes, frequentist or likelihood). The current state of the art is advancing rapidly as a consequence of the availability of increased computational power, but there remains little consistency in the choices made for assessments and forecasts. This can be explained by several factors including the specifics of the species under consideration, the purpose for which the analysis is conducted and the institutional framework within which the methods are developed and used, including the availability and customary usage of software tools. Little testing of either the methods or their assumptions has yet been done. Thus, it is not possible to argue either that the methods perform well or perform poorly or that any particular conditioning choices are more appropriate in general terms than others. Despite much recent progress, fisheries science has yet to identify a means for identifying appropriate conditioning choices such that the probability distributions which are calculated for management purposes do adequately represent the probabilities of eventual real outcomes. Therefore, we conclude that increased focus should be placed on testing and carefully examining the choices made when conducting these analyses, and that more attention must be given to examining the sensitivity to alternative assumptions and model structures. Provision of advice concerning uncertainty in stock assessments should include consideration of such sensitivities, and should use model-averaging methods, decision tables or management procedure simulations in cases where advice is strongly sensitive to model assumptions. [source] Block factorized preconditioners for high-order accurate in time approximation of the Navier-Stokes equationsNUMERICAL METHODS FOR PARTIAL DIFFERENTIAL EQUATIONS, Issue 4 2003Alessandro Veneziani Computationally efficient solution methods for the unsteady Navier-Stokes incompressible equations are mandatory in real applications of fluid dynamics. A typical strategy to reduce the computational cost is to split the original problem into subproblems involving the separate computation of velocity and pressure. The splitting can be carried out either at a differential level, like in the Chorin-Temam scheme, or in an algebraic fashion, like in the algebraic reinterpretation of the Chorin-Temam method, or in the Yosida scheme (see 1 and 19). These fractional step schemes indeed provide effective methods of solution when dealing with first order accurate time discretizations. Their extension to high order time discretization schemes is not trivial. To this end, in the present work we focus our attention on the adoption of inexact algebraic factorizations as preconditioners of the original problem. We investigate their properties and show that some particular choices of the approximate factorization lead to very effective schemes. In particular, we prove that performing a small number of preconditioned iterations is enough to obtain a time accurate solution, irrespective of the dimension of the system at hand. © 2003 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 19: 487,510, 2003 [source] The generalized Dirichlet-to-Neumann map for certain nonlinear evolution PDEsCOMMUNICATIONS ON PURE & APPLIED MATHEMATICS, Issue 5 2005A. S. Fokas Let q(x,t) satisfy a nonlinear integrable evolution PDE whose highest spatial derivative is of order n. An initial boundary value problem on the half-line for such a PDE is at least linearly well-posed if one prescribes initial conditions, as well as N boundary conditions at x = 0, where for n even N equals n/2 and for n odd, depending on the sign of the highest derivative, N equals either n,1/2 or n+1/2. For example, for the nonlinear Schrödinger (NLS) and the sine-Gordon (sG), N = 1, while for the modified Korteweg-deVries (mKdV) N = 1 or N = 2 depending on the sign of the third derivative. Constructing the generalized Dirichlet-to-Neumann map means determining those boundary values at x = 0 that are not prescribed as boundary conditions in terms of the given initial and boundary conditions. A general methodology is presented that constructs this map in terms of the solution of a system of two nonlinear ODEs. This formulation implies that for the focusing NLS, for the sG, and for the two focusing versions of the mKdV, this map is global in time. It appears that this is the first time in the literature that such a characterization for nonlinear PDEs is explicitly described. It is also shown here that for particular choices of the boundary conditions the above map can be linearized. © 2005 Wiley Periodicals, Inc. [source] | |||||||||||||