Numerical Performance (numerical + performance)

Distribution by Scientific Domains

Selected Abstracts

Comparison between cohesive zone models

K. Y. Volokh
Cohesive zone models (CZMs) are widely used for numerical simulation of the fracture process. Cohesive zones are surfaces of discontinuities where displacements jump. A specific constitutive law relating the displacement jumps and proper tractions defines the cohesive zone model. Within the cohesive zone approach crack nucleation, propagation, and arrest are a natural outcome of the theory. The latter is in contrast to the traditional approach of fracture mechanics where stress analysis is separated from a description of the actual process of material failure. The common wisdom says that only cohesive strength,the maximum stress on the traction,separation curve,and the separation work,the area under the traction,separation curve,are important in setting a CZM while the shape of the traction,separation curve is subsidiary. It is shown in our note that this rule may not be correct and a specific shape of the cohesive zone model can significantly affect results of the fracture analysis. For this purpose four different cohesive zone models,bilinear, parabolic, sinusoidal, and exponential,are compared by using a block-peel test, which allows for simple analytical solutions. Numerical performance of the cohesive zone models is considered. It appears that the convergence properties of nonlinear finite element analyses are similar for all four CZMs in the case of the block-peel test. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Dual-mixed p and hp finite elements for elastic membrane problems

E. BertótiArticle first published online: 26 OCT 200
Abstract A complementary energy-based, dimensionally reduced plate model using a two-field dual,mixed variational principle of non-symmetric stresses and rotations is derived. Both the membrane and bending equilibrium equations, expressed in terms of non-symmetric mid-surface stress components, are satisfied a priori introducing first-order stress functions. It is pointed out that (i) the membrane-, shear- and bending energies of the plate written in terms of first-order stress functions are decoupled, (ii) although unmodified 3-D constitutive equations are applied, the energy parts do not contain the 1/(1-2,) term for isotropic, linearly elastic materials. These facts mean that the finite element formulation based on the present plate model should be free from shear locking when the thickness tends to zero and free from incompressibility locking when the Poisson ratio , converges to 0.5, irrespective of low-order h -, or higher-order p elements are used. Curvilinear dual-mixed hp finite elements with higher-order stress approximation and continuous surface tractions are developed and presented for the membrane (2-D elasticity) problem. In this case the formulation requires the approximation of three independent variables: two components of a first-order stress function vector and a scalar rotation. Numerical performance of three quadrilateral dual,mixed elements is presented and compared to displacement-based hp finite elements when the Poisson ratio converges to the incompressible limit of 0.5. The numerical results show that, as expected, the dual,mixed elements developed in this paper are free from locking in the energy norm as well as in the stress computations, for both h - and p -extensions. Copyright © 2001 John Wiley & Sons, Ltd. [source]

Dividing Wall Distillation Columns: Optimization and Control Properties

F. I. Gómez-Castro
Abstract The optimal design of dividing wall columns is a non-linear and multivariable problem, and the objective function used as optimization criterion is generally non-convex with several local optimums. Considering this fact, in this paper, we studied the design of dividing wall columns using as a design tool, a multi-objective genetic algorithm with restrictions, written in MatlabTM and using the process simulator Aspen PlusTM for the evaluation of the objective function. Numerical performance of this method has been tested in the design of columns with one or two dividing walls and with several mixtures to test the effect of the relative volatilities of the feed mixtures on energy consumption, second law efficiency, total annual cost, and theoretical control properties. In general, the numerical performance shows that this method appears to be robust and suitable for the design of sequences with dividing walls. [source]

Coupled HM analysis using zero-thickness interface elements with double nodes,Part II: Verification and application

J. M. Segura
Abstract In a companion Part I of this paper (Int. J. Numer. Anal. Meth. Geomech. 2008; DOI: 10.1002/nag.735), a coupled hydro-mechanical (HM) formulation for geomaterials with discontinuities based on the finite element method (FEM) with double-node, zero-thickness interface elements was developed and presented. This Part II paper includes the numerical solution of basic practical problems using both the staggered and the fully coupled approaches. A first group of simulations, based on the classical consolidation problem with an added vertical discontinuity, is used to compare both the approaches in terms of accuracy and convergence. The monolithic or fully coupled scheme is also used in an application example studying the influence of a horizontal joint in the performance of a reservoir subject to fluid extraction. Results include a comparison with other numerical solutions from the literature and a sensitivity analysis of the mechanical parameters of the discontinuity. Some simulations are also run using both a full non-symmetric and a simplified symmetric Jacobian matrix. On top of verifying the model developed and its capability to reflect the conductivity changes of the interface with aperture changes, the results presented also lead to interesting observations of the numerical performance of the methods implemented. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Generalized trapezoidal numerical integration of an advanced soil model

Yunming Yang
Abstract This paper investigates the numerical performance of the generalized trapezoidal integration rule by using an advanced soil model. The generalized trapezoidal integration rule can include many other integration algorithms by adjusting a single parameter , ranging from 1 to 0. The soil model used is the recently developed middle surface concept (MSC) sand model which simulates different soil response characteristics by using different pseudo-yield functions. The generalized trapezoidal rule and MSC sand model are used to simulate the responses of sand samples with different relative densities under various initial and loading conditions. Instead of a single step, multiple loading steps bring the sample to the vicinity of failure. These comprehensive investigations examine and compare the influences of various values of , on the numerical solution of integrated constitutive equations, the convergence of Newton's iterative scheme, and the integration accuracy. Copyright © 2007 John Wiley & Sons, Ltd. [source]

A parallel implicit/explicit hybrid time domain method for computational electromagnetics

Z. Q. Xie
Abstract The numerical solution of Maxwell's curl equations in the time domain is achieved by combining an unstructured mesh finite element algorithm with a cartesian finite difference method. The practical problem area selected to illustrate the application of the approach is the simulation of three-dimensional electromagnetic wave scattering. The scattering obstacle and the free space region immediately adjacent to it are discretized using an unstructured mesh of linear tetrahedral elements. The remainder of the computational domain is filled with a regular cartesian mesh. These two meshes are overlapped to create a hybrid mesh for the numerical solution. On the cartesian mesh, an explicit finite difference method is adopted and an implicit/explicit finite element formulation is employed on the unstructured mesh. This approach ensures that computational efficiency is maintained if, for any reason, the generated unstructured mesh contains elements of a size much smaller than that required for accurate wave propagation. A perfectly matched layer is added at the artificial far field boundary, created by the truncation of the physical domain prior to the numerical solution. The complete solution approach is parallelized, to enable large-scale simulations to be effectively performed. Examples are included to demonstrate the numerical performance that can be achieved. Copyright © 2009 John Wiley & Sons, Ltd. [source]

A study on the lumped preconditioner and memory requirements of FETI and related primal domain decomposition methods

Yannis Fragakis
Abstract In recent years, domain decomposition methods (DDMs) have emerged as advanced solvers in several areas of computational mechanics. In particular, during the last decade, in the area of solid and structural mechanics, they reached a considerable level of advancement and were shown to be more efficient than popular solvers, like advanced sparse direct solvers. The present contribution follows the lines of a series of recent publications on the relationship between primal and dual formulations of DDMs. In some of these papers, the effort to unify primal and dual methods led to a family of DDMs that was shown to be more efficient than the previous methods. The present paper extends this work, presenting a new family of related DDMs, thus enriching the theory of the relations between primal and dual methods, with the primal methods, which correspond to the dual DDM that uses the lumped preconditioner. The paper also compares the numerical performance of the new methods with that of the previous ones and focuses particularly on memory requirement issues related to the use of the lumped preconditioner, suggesting a particularly memory-efficient formulation. Copyright © 2007 John Wiley & Sons, Ltd. [source]

A parallel Galerkin boundary element method for surface radiation and mixed heat transfer calculations in complex 3-D geometries

X. Cui
Abstract This paper presents a parallel Galerkin boundary element method for the solution of surface radiation exchange problems and its coupling with the finite element method for mixed mode heat transfer computations in general 3-D geometries. The computational algorithm for surface radiation calculations is enhanced with the implementation of ideas used for 3-D computer graphics applications and with data structure management involving creating and updating various element lists optimized for numerical performance. The algorithm for detecting the internal third party blockages of thermal rays is presented, which involves a four-step procedure, i.e. the primary clip, secondary clip and adaptive integration with checking. Case studies of surface radiation and mixed heat transfer in both simple and complex 3-D geometric configurations are presented. It is found that a majority of computational time is spent on the detection of foreign element blockages and parallel computing is ideally suited for surface radiation calculations. Results show that the decrease of the CPU time approaches asymptotically to an inverse rule for parallel computing of surface radiation exchanges. For large-scale computations involving complex 3-D geometries, an iterative procedure is a preferred approach for the coupling of the Galerkin boundary and finite elements for mixed mode heat transfer calculations. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Multi-material incompressible flow simulation using the moment-of-fluid method,

Samuel P. Schofield
Abstract This paper compares the numerical performance of the moment-of-fluid (MOF) interface reconstruction technique with Youngs, LVIRA, power diagram (PD), and Swartz interface reconstruction techniques in the context of a volume-of-fluid (VOF) based finite element projection method for the numerical simulation of variable-density incompressible viscous flows. In pure advection tests with multiple materials MOF shows dramatic improvements in accuracy compared with the other methods. In incompressible flows where density differences determine the flow evolution, all the methods perform similarly for two material flows on structured grids. On unstructured grids, the second-order MOF, LVIRA, and Swartz methods perform similarly and show improvement over the first-order Youngs' and PD methods. For flow simulations with more than two materials, MOF shows increased accuracy in interface positions on coarse meshes. In most cases, the convergence and accuracy of the computed flow solution was not strongly affected by interface reconstruction method. Published in 2009 by John Wiley & Sons, Ltd. [source]

Numerical and Experimental Analysis of an Axial Flow Left Ventricular Assist Device: The Influence of the Diffuser on Overall Pump Performance

Alexandrina Untaroiu
Abstract:, Thousands of adult cardiac failure patients may benefit from the availability of an effective, long-term ventricular assist device (VAD). We have developed a fully implantable, axial flow VAD (LEV-VAD) with a magnetically levitated impeller as a viable option for these patients. This pump's streamlined and unobstructed blood flow path provides its unique design and facilitates continuous washing of all surfaces contacting blood. One internal fluid contacting region, the diffuser, is extremely important to the pump's ability to produce adequate pressure but is challenging to manufacture, depending on the complex blade geometries. This study examines the influence of the diffuser on the overall LEV-VAD performance. A combination of theoretical analyses, computational fluid (CFD) simulations, and experimental testing was performed for three different diffuser models: six-bladed, three-bladed, and no-blade configuration. The diffuser configurations were computationally and experimentally investigated for flow rates of 2,10 L/min at rotational speeds of 5000,8000 rpm. For these operating conditions, CFD simulations predicted the LEV-VAD to deliver physiologic pressures with hydraulic efficiencies of 15,32%. These numerical performance results generally agreed within 10% of the experimental measurements over the entire range of rotational speeds tested. Maximum scalar stress levels were estimated to be 450 Pa for 6 L/min at 8000 rpm along the blade tip surface of the impeller. Streakline analysis demonstrated maximum fluid residence times of 200 ms with a majority of particles exiting the pump in 80 ms. Axial fluid forces remained well within counter force generation capabilities of the magnetic suspension design. The no-bladed configuration generated an unacceptable hydraulic performance. The six-diffuser-blade model produced a flow rate of 6 L/min against 100 mm Hg for 6000 rpm rotational speed, while the three-diffuser-blade model produced the same flow rate and pressure rise for a rotational speed of 6500 rpm. The three-bladed diffuser configuration was selected over the six-bladed, requiring only an incremental adjustment in revolution per minute to compensate for and ease manufacturing constraints. The acceptable results of the computational simulations and experimental testing encourage final prototype manufacturing for acute and chronic animal studies. [source]

Adjustment for Missingness Using Auxiliary Information in Semiparametric Regression

BIOMETRICS, Issue 1 2010
Donglin Zeng
Summary In this article, we study the estimation of mean response and regression coefficient in semiparametric regression problems when response variable is subject to nonrandom missingness. When the missingness is independent of the response conditional on high-dimensional auxiliary information, the parametric approach may misspecify the relationship between covariates and response while the nonparametric approach is infeasible because of the curse of dimensionality. To overcome this, we study a model-based approach to condense the auxiliary information and estimate the parameters of interest nonparametrically on the condensed covariate space. Our estimators possess the double robustness property, i.e., they are consistent whenever the model for the response given auxiliary covariates or the model for the missingness given auxiliary covariate is correct. We conduct a number of simulations to compare the numerical performance between our estimators and other existing estimators in the current missing data literature, including the propensity score approach and the inverse probability weighted estimating equation. A set of real data is used to illustrate our approach. [source]

Large-scale multiple testing under dependence

Wenguang Sun
Summary., The paper considers the problem of multiple testing under dependence in a compound decision theoretic framework. The observed data are assumed to be generated from an underlying two-state hidden Markov model. We propose oracle and asymptotically optimal data-driven procedures that aim to minimize the false non-discovery rate FNR subject to a constraint on the false discovery rate FDR. It is shown that the performance of a multiple-testing procedure can be substantially improved by adaptively exploiting the dependence structure among hypotheses, and hence conventional FDR procedures that ignore this structural information are inefficient. Both theoretical properties and numerical performances of the procedures proposed are investigated. It is shown that the procedures proposed control FDR at the desired level, enjoy certain optimality properties and are especially powerful in identifying clustered non-null cases. The new procedure is applied to an influenza-like illness surveillance study for detecting the timing of epidemic periods. [source]

ADI-FDTD method perturbed by the second order cross derivative terms

Ki-Bok Kong
Abstract A two-step FDTD method as a compromise of conditional stability and reduced splitting error is formulated and its numerical stability is investigated. It is the perturbed form to the ADI-FDTD method by the addition of second order cross derivative term. It is validated from the comparison of numerical anisotropy and numerical error over the ADI-FDTD that numerical performances can be improved by controlling the perturbed term within the stable region of the cross derivative term. © Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 1822,1826, 2008; Published online in Wiley InterScience ( DOI 10.1002/mop.23479 [source]