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排序方式: 共有271条查询结果,搜索用时 15 毫秒
41.
We study theoretically the effects of secondary (transverse) flows on the Taylor-Aris dispersion of pressure-driven, open column flow in a conduit with a rectangular cross section and account for the interaction of solutes with the retentive coating on the conduit's surface. A few plausible means of inducing secondary flows (that are independent of the primary, pressure-driven, axial flow) are described. The Taylor-Aris dispersion coefficient is computed as a function of the secondary flow's pattern and intensity. By inducing secondary flows, one can significantly reduce dispersion. 相似文献
42.
43.
Luc Berger‐Vergiat Haim Waisman Badri Hiriyur Ray Tuminaro David Keyes 《International journal for numerical methods in engineering》2012,90(3):311-328
Traditional algebraic multigrid (AMG) preconditioners are not well suited for crack problems modeled by extended finite element methods (XFEM). This is mainly because of the unique XFEM formulations, which embed discontinuous fields in the linear system by addition of special degrees of freedom. These degrees of freedom are not properly handled by the AMG coarsening process and lead to slow convergence. In this paper, we proposed a simple domain decomposition approach that retains the AMG advantages on well‐behaved domains by avoiding the coarsening of enriched degrees of freedom. The idea was to employ a multiplicative Schwarz preconditioner where the physical domain was partitioned into “healthy” (or unfractured) and “cracked” subdomains. First, the “healthy” subdomain containing only standard degrees of freedom, was solved approximately by one AMG V‐cycle, followed by concurrent direct solves of “cracked” subdomains. This strategy alleviated the need to redesign special AMG coarsening strategies that can handle XFEM discretizations. Numerical examples on various crack problems clearly illustrated the superior performance of this approach over a brute force AMG preconditioner applied to the linear system. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
44.
Fridman M Suchowski H Nixon M Friesem AA Davidson N 《Journal of the Optical Society of America. A, Optics, image science, and vision》2012,29(4):541-544
The dynamics of modes and their states of polarizations in multimode fibers as a function of time, space, and wavelength are experimentally and theoretically investigated. The results reveal that the states of polarizations are displaced in Poincaré sphere representation when varying the angular orientations of the polarization at the incident light. Such displacements, which complicate the interpretation of the results, are overcome by resorting to modified Poincaré sphere representation. With such modification it should be possible to predict the output modes and their state of polarization when the input mode and state of polarization are known. 相似文献
45.
Haim Waisman 《Engineering Fracture Mechanics》2010,77(16):3204-3215
An analytical approach combined with the extended finite element method (XFEM) is proposed to extract the Strain Energy Release Rates within the classical stiffness derivative technique. The proposed idea hinges on the following two XFEM properties: (i) the crack is mesh independent, i.e. there is no need for mesh perturbations in the vicinity of the crack and (ii) the asymptotic crack tip field is embedded in the mathematical formulation of the stiffness matrix. By employing these properties we show that the derivative of the stiffness matrix with respect to the crack extension can be computed in a closed form and on the fly during the analysis. Thus the virtual crack extension, and the error inherent in the finite difference scheme of the classical stiffness derivative technique is completely avoided. Numerical results on few benchmark problems show that this method is comparable to the J-integral method. 相似文献
46.
Hao Sun Haim Waisman Raimondo Betti 《International journal for numerical methods in engineering》2013,95(10):871-900
We present a novel algorithm based on the extended finite element method (XFEM) and an enhanced artificial bee colony (EABC) algorithm to detect and quantify multiple flaws in structures. The concept is based on recent work that have shown the excellent synergy between XFEM, used to model the forward problem, and a genetic‐type algorithm to solve an inverse identification problem and converge to the ‘best’ flaw parameters. In this paper, an adaptive algorithm that can detect multiple flaws without any knowledge on the number of flaws beforehand is proposed. The algorithm is based on the introduction of topological variables into the search space, used to adaptively activate/deactivate flaws during run time until convergence is reached. The identification is based on a limited number of strain sensors assumed to be attached to the structure surface boundaries. Each flaw is approximated by a circular void with the following three variables: center coordinates (xc, yc) and radius (rc), within the XFEM framework. In addition, the proposed EABC scheme is improved by a guided‐to‐best solution updating strategy and a local search (LS) operator of the Nelder–Mead simplex type that show fast convergence and superior global/LS abilities compared with the standard ABC or classic genetic algorithms. Several numerical examples, with increasing level of difficulty, are studied in order to evaluate the proposed algorithm. In particular, we consider identification of multiple flaws with unknown a priori information on the number of flaws (which makes the inverse problem harder), the proximity of flaws, flaws having irregular shapes (similar to artificial noise), and the effect of structured/unstructured meshes. The results show that the proposed XFEM–EABC algorithm is able to converge on all test problems and accurately identify flaws. Hence, this methodology is found to be robust and efficient for nondestructive detection and quantification of multiple flaws in structures. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
47.
Haim Waisman Jacob Fish 《International journal for numerical methods in engineering》2008,73(3):407-426
A heterogeneous space–time full approximation storage (HFAS) multilevel formulation for molecular dynamics simulations is developed. The method consists of a waveform Newton smoothing that produces initial space–time iterates and a coarse model correction. The formulation is coined as heterogeneous since it permits different interatomic potentials to be applied at different physical scales. This results in a flexible framework for physics coupling. Time integration is performed in windows using the implicit Newmark predictor–corrector method that permits larger time integration steps than the explicit method. The size of the time steps is governed by accuracy rather than by stability considerations of the algorithm. We study three different variants of the method: the Picard iteration, constrained dynamics and force splitting. Numerical examples show that FAS based on force splitting provides significant time savings compared to standard explicit methods and alternative implicit space–time schemes. Parallel studies of the Picard iteration on harmonic problems illustrate the time parallelization effect that leads to a superior parallel performance compared to explicit methods. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
48.
Haim Waisman Ted Belytschko 《International journal for numerical methods in engineering》2008,73(12):1671-1692
An adaptive method within the extended finite element method (XFEM) framework which adapts the enrichment function locally to the physics of a problem, as opposed to polynomial or mesh refinement, is presented. The method minimizes a local residual and determines the parameters of the enrichment function. We consider an energy form and a ‘strong’ form of the residual as error measures to drive the algorithm. Numerical examples for boundary layers and solid mechanics problems illustrate that the procedure converges. Moreover, when only the character of the solution is known, a good approximation is obtained in the area of interest. It is also shown that the method can be used to determine the order of singularities in solutions. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
49.
From time immemorial people dealt with size reduction processes (mill, mineral liberation, etc.). As time has passed industrial units for comminution processes have become larger and more sophisticated, but still they perform with low efficiencies [1], [2] and [3]. The strength of a particle is one of its most crucial characteristics due to the mechanical stresses experienced by each particle within an industrial unit. This is because the final size of particles is mostly dependant on the strength distribution of the raw material [4]. In this present study, the ability of a number of statistical formulations to accurately describe the strength distribution of particles was examined. Additionally, selected equations were analyzed and a general expression including the effect of the material and particle size was developed. A number of approaches to define particle strength were considered, and strength in terms of crushing force was chosen. Particle strength in terms of force and in terms of energy was also compared and found to be size independent. Finally, particle strength in terms of stress was examined and compared to the particle strength in terms of force.The ability to describe the compression strength distribution will significantly improve the accuracy of the comminution processes simulation, design and optimization. 相似文献
50.
Direct evaluation of stress intensity factors for curved cracks using Irwin's integral and XFEM with high‐order enrichment functions 下载免费PDF全文
Yongxiang Wang Haim Waisman Isaac Harari 《International journal for numerical methods in engineering》2017,112(7):629-654
This paper presents a comprehensive study on the use of Irwin's crack closure integral for direct evaluation of mixed‐mode stress intensity factors (SIFs) in curved crack problems, within the extended finite element method. The approach employs high‐order enrichment functions derived from the standard Williams asymptotic solution, and SIFs are computed in closed form without any special post‐processing requirements. Linear triangular elements are used to discretize the domain, and the crack curvature within an element is represented explicitly. An improved quadrature scheme using high‐order isoparametric mapping together with a generalized Duffy transformation is proposed to integrate singular fields in tip elements with curved cracks. Furthermore, because the Williams asymptotic solution is derived for straight cracks, an appropriate definition of the angle in the enrichment functions is presented and discussed. This contribution is an important extension of our previous work on straight cracks and illustrates the applicability of the SIF extraction method to curved cracks. The performance of the method is studied on several circular and parabolic arc crack benchmark examples. With two layers of elements enriched in the vicinity of the crack tip, striking accuracy, even on relatively coarse meshes, is obtained, and the method converges to the reference SIFs for the circular arc crack problem with mesh refinement. Furthermore, while the popular interaction integral (a variant of the J‐integral method) requires special auxiliary fields for curved cracks and also needs cracks to be sufficiently apart from each other in multicracks systems, the proposed approach shows none of those limitations. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献