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INTERNATIONAL JOURNAL OF PLASTICITY 2023年第1期163卷

作者： Riyad, Iftekhar A. McWilliams, Brandon A. Pramanik, Brahmananda Knezevic, Marko Univ New Hampshire Dept Mech Engn Durham NH 03824 USA DEVCOM Army Res Lab Army Res Directorate Aberdeen Proving Ground MD 21005 USA Montana Technol Univ Dept Mech Engn Butte MT 59701 USA

This paper presents a multi-level viscoplastic self-consistent (VPSC) model for correlated structures (CS-VPSC). A polycrystalline aggregate response is homogenized over inclusions, which are linearized using the generalized affine scheme. Response of the inclusions is that of either a single grain or a correlated structure of multiple single crystals. The single crystal response is a strain rate adjusted power-law viscoplastic formulation, which considers not only the driving force in the glide direction but also the two orthogonal shear and three normal components of the driving force. The resistance to slip is grain size sensitive and based on the evolution of dislocation density. Furthermore, the resistances of prismatic and basal slip systems are adjusted based on the geometry of slip transfer between adjacent correlated structures. The overall model linking the single crystal micro-scale to a correlated structure meso-scale to an aggregate macro-scale response is also coupled with the implicit finite elements (FE-CS-VPSC). The model is used for modeling strain rate sensitive mechanical response and microstructural evolution of additively manufactured Ti-6Al-4 V (Ti64) alloy in two conditions. As-built and stress-relived (SR) microstructure consisted of correlated alpha-lath/lamellar structures, while thermo-hydrogen refined microstructure (THRM) consisted primarily of alpha- and beta-globular structures and some correlated alpha-lath/lamellar structures. Electron backscatter diffraction data, neutron diffraction data, and mechanical data were used to initialize, calibrate, and validate the model. To this end, the flow stress and texture evolution of the materials in tension and compression under quasi-static and high strain rate deformations were predicted using CS-VPSC, while the texture evolution during rolling and impact was simulated using FE-CS-VPSC. Good predictions of the texture evolution and mechanical response including the tension/compression asymme

关键词： Microstructures Correlated structures Crystal plasticity numerical algorithms Ti64

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Investigation of Selected Reconstruction algorithms Used for Determination of Complex Ratio of AC Voltages

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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT 2021年 70卷 1页

作者： Augustyn, Jerzy Kampik, Marian Musiol, Krzysztof Kielce Univ Technol Dept Comp Sci Elect & Elect Engn PL-25314 Kielce Poland Silesian Tech Univ Dept Measurement Sci Elect & Control PL-44100 Gliwice Poland

Issues related to measurements of the complex ratio of two sinusoidal voltages based on digital sampling are considered in this article. Several algorithms for determination of the complex voltage ratio are discussed. Results of five algorithms are compared and minimizing their errors related to differences between the two sampling channels is examined. For verification, a sampling system with National Instruments PXI 4461 data acquisition module (DAQm) was used.

关键词： Channel swapping complex voltage ratio (CVR) data acquisition (DAQ) channel errors discrete Fourier transform (DFT) ellipse fitting (EF) numerical algorithms sampling sine-fitting (SF)

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Adjoint-Based Optimization for Thrust Performance of Three-Dimensional Pitching-Rolling Plate

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AIAA JOURNAL 2019年第9期57卷 3716-3727页

作者： Xu, Min Wei, Mingjun Li, Chengyu Dong, Haibo New Mexico State Univ Dept Mech & Aerosp Engn Las Cruces NM 88001 USA Kansas State Univ Dept Mech & Nucl Engn Manhattan KS 66506 USA Villanova Univ Dept Mech Engn Villanova PA 19085 USA Univ Virginia Dept Mech & Aerosp Engn Charlottesville VA 22904 USA

An adjoint-based optimization is applied to study the thrust performance of a pitching-rolling ellipsoidal plate in a uniform stream at Reynolds number 100. To achieve the highest thrust, the optimal kinematics of pitching-rolling motion is sought in a control space including the pitching amplitude, the rolling amplitude, and the phase delay between the pitching and rolling motions. A continuous adjoint approach with boundary motion being handled by noncylindrical calculus is developed as a computationally efficient optimization algorithm to deal with large control space with a morphing domain. The comparison between the optimal motion and other reference motions shows a significant improvement of thrust from the increase of the rolling amplitude and an optimal phase delay of 122.6 deg between the pitching and rolling motions. The combination of these two factors impacts the overall thrust performance through their strong effects on the angle of attack, the leading-edge vortex circulation, and the pressure distribution on the plate. Further wake structure analysis suggests that the optimal motion improves its propulsive performance by generating a stronger leading-edge vortex and straightening the wake deflection.

关键词： Reynolds Number Vortex Structure Angle of Attack Flapping Frequency numerical algorithms Optimal Kinematics Shape Optimization Flow Velocity Navier Stokes Equations Immersed Boundary Method

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A parallel algorithm to produce long polymer chains in molecular dynamics

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JOURNAL OF CHEMICAL PHYSICS 2019年第22期150卷 224902-224902页

作者： Lemarchand, C. A. Bousquet, D. Schnell, B. Pineau, N. CEA DAM DIF F-91297 Arpajon France MICHELIN 23 Pl Carmes Dechaux F-63040 Clermont Ferrand France

Generating initial configurations of polymer melts above the entanglement molecular weight is a challenge in molecular dynamics and Monte Carlo simulations. In this work, we adapt an algorithm mimicking a chemical polymerization to all-atom force fields. The principle of this algorithm is to start from a bath of monomers between which bonds are created and relaxed sequentially. Our implementation is parallel and efficient. The parallelization is that of a classical molecular dynamics code and enables the user to generate large systems, up to 7 x 10(6) atoms. The efficiency of the algorithm comes from the linear scaling between the simulation time and the chain length in the limit of very long chains. The implementation is able to produce long polymer chains, up to approximate to 2000 carbon atoms, with thermodynamic and local structural properties in good agreement with their experimental and numerical counterparts. Moreover, the chain conformations are close to being equilibrated right after the end of the polymerization process, corresponding to only a few hundred of picoseconds of simulation, despite a systematical drift from Gaussian-like behavior when the density of reactively available monomers decreases. Finally, the algorithm proposed in this work is versatile in nature because the bond creation can be easily modified to create copolymers, block copolymers, and mixtures of polymer melts with other material.

关键词： Composite materials Polymers Equilibrium thermodynamics Molecular dynamics Probability theory numerical algorithms

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A mesoscale model for the micromechanical study of gels

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JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS 2022年 167卷

作者： Wagner, Robert J. Dai, Jinyue Su, Xinfu Vernerey, Franck J. Univ Colorado Boulder Dept Mech Engn Boulder CO 80309 USA Univ Colorado Boulder Mat Sci & Engn Program Boulder CO 80309 USA

Gels are comprised of polymer networks swelled by some interstitial solvent. They are under wide investigation by material scientists and engineers for their broad applicability in fields ranging from adhesives to tissue engineering. Gels' mechanical properties greatly influence their efficacy in such applications and are largely dictated by their underlying microstructures and constituent scale properties. Yet predictively mapping the local-to-global property functions of gels remains difficult due in part to the complexity introduced by solute-solvent interactions. We here introduce a novel, discrete mesoscale modeling method that preserves local solute concentration dependent gradients in osmotic pressure through the Flory-Huggins mixing parameter, chi. The iteration of the model used here replicates gels fabricated from telechelically crosslinked star shaped polymers and intakes chi, macromer molecular weight (Mw), crosslink functionality (f), and as-prepared solute concentration (phi*) as its inputs, all of which are analogues to the control parameters of experimentalists. Here we demonstrate how this method captures solvent dependent homogenization (chi <= 0.5) or phase separation (chi > 0.5) of polymer suspensions in the absence of phenomenological pairwise potentials. We then demonstrate its accurate, ab initio prediction of gel topology, isotropic swelling mechanics, and uniaxial tensile stress for a 10k tetra-PEG gel. Finally, we use the model to predict trends in the mechanical response and failure of multi-functional PEG-based gels over a range of Mw and f, while investigating said trends' micromechanical origins. The model predicts that increased crosslink functionality results in higher initial chain stretch (as measured at the equilibrated swollen state) for gels of the same underlying chain length, which improves modulus and failure stress but decreases failure strain and toughness.

关键词： Gels Microstructures Fracture mechanisms Inhomogeneous material numerical algorithms

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Immersed boundary-simplified lattice Boltzmann method for incompressible viscous flows

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PHYSICS OF FLUIDS 2018年第5期30卷 053601-053601页

作者： Chen, Z. Shu, C. Tan, D. Natl Univ Singapore Dept Mech Engn 10 Kent Ridge Crescent Singapore 119260 Singapore

An immersed boundary-simplified lattice Boltzmann method is developed in this paper for simulations of two-dimensional incompressible viscous flows with immersed objects. Assisted by the fractional step technique, the problem is resolved in a predictor-corrector scheme. The predictor step solves the flow field without considering immersed objects, and the corrector step imposes the effect of immersed boundaries on the velocity field. Different from the previous immersed boundary-lattice Boltzmann method which adopts the standard lattice Boltzmann method (LBM) as the flow solver in the predictor step, a recently developed simplified lattice Boltzmann method (SLBM) is applied in the present method to evaluate intermediate flow variables. Compared to the standard LBM, SLBM requires lower virtual memories, facilitates the implementation of physical boundary conditions, and shows better numerical stability. The boundary condition-enforced immersed boundary method, which accurately ensures no-slip boundary conditions, is implemented as the boundary solver in the corrector step. Four typical numerical examples are presented to demonstrate the stability, the flexibility, and the accuracy of the present method. Published by AIP Publishing.

关键词： Computational fluid dynamics numerical algorithms Lattice Boltzmann methods Astrodynamics Navier Stokes equations

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Efficient solver for a special class of convection-diffusion problems

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PHYSICS OF FLUIDS 2019年第2期31卷 023606-023606页

作者： Karedla, Narain Thiele, Jan Christoph Gregor, Ingo Enderlein, Joerg Georg August Univ Inst Phys 3 D-37077 Gottingen Germany

We describe an exact and highly efficient numerical algorithm for solving a special but important class of convection-diffusion equations. These equations occur in many problems in physics, chemistry, or biology, and they are usually hard to treat due to the presence of a convection term, represented by a first order derivative in the spatial co-ordinate. Our algorithm reduces the convection-diffusion equation to a pure diffusion problem within a complex-valued potential which can be solved efficiently and accurately using conventional parabolic partial differential equation solvers. Published under license by AIP Publishing.

关键词： Stochastic processes Operator theory Capillary flows Partial differential equations numerical integration Finite difference methods numerical algorithms

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Identification of fully parameterized linear and nonlinear state-space systems by projected gradient search

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IFAC Proceedings Volumes 2003年第16期36卷 711-716页

作者： Vincent Verdult Niek Bergboer Michel Verhaegen Delft University of Technology Faculty of Information Technology and Systems Control Systems Engineering P. O. Box 5031 NL-2600 GA Delft The Netherlands Maastricht University Department of Computer Science St Jacobstraat 6 Maastricht The Netherlands

A nonlinear optimization-based identification procedure for fully parameterized multivariable state-space models is presented. The method can be used to identify linear time-invariant, linear parameter-varying, composite local linear, bilinear. Hammerstein and Wiener systems. The nonuniqueness of the full parameterization is dealt with by a projected gradient search to solve the nonlinear optimization problem. Both white and nonwhite measurement noise at the output can be dealt with in a maximum likelihood setting. It is proposed to use subspace identification methods to initialize the nonlinear optimization problem. A computationally efficient and numerically reliable implementation of the procedure is discussed in detail.

关键词： System identification state-space methods nonlinear models nonlinear programming numerical algorithms maximum likelihood bilinear systems

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Certifying Global Optimality for the L ∞ -Norm Computation of Large-Scale Descriptor Systems ⁎

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IFAC-PapersOnLine 2020年第2期53卷 4279-4284页

作者： P. Schwerdtner E. Mengi M. Voigt Technische Universität Berlin Institut für Mathematik FG Numerische Mathematik Straße des 17. Juni 136 10623 Berlin Germany Koç University Department of Mathematics Rumeli Feneri Yolu 34450 Sanyer Istanbul Turkey University of Hamburg Department of Mathematics Center for Optimization and Approximation Bundesstraβe 55 20146 Hamburg Germany

We present a method for the certification of algorithms that approximate the L ∞ - or H ∞ -norm of transfer functions of large-scale (descriptor) systems. This certification is needed because such algorithms depend heavily on user input, and may converge to a local maximizer of the related singular value function leading to an incorrect value, much lower than the actual norm. Hence, we design an algorithm that determines whether a given value is less than the L ∞ -norm of the transfer function under consideration, and that does not require user input other than the system matrices. In the algorithm, we check whether a certain structured matrix pencil has any purely imaginary eigenvalues by repeatedly applying a structure-preserving shift-and-invert Arnoldi iteration combined with an appropriate shifting strategy. Our algorithm consists of two stages. First, an interval on the imaginary axis which may contain imaginary eigenvalues is determined. Then, in the second stage, a shift is chosen on this interval and the eigenvalues closest to this shift are computed. If none of these eigenvalues is purely imaginary, then an imaginary interval around the shift of appropriate length is removed such that two subintervals remain. This second stage is then repeated on the remaining two subintervals until either a purely imaginary eigenvalue is found or no critical subintervals are left. We show the effectiveness of our method by testing it without any parameter adaptation on a benchmark collection of large-scale systems.

关键词： Computer-aided control systems design descriptor systems eigenvalue problems H ∞ control linear control systems norms numerical algorithms robust control

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EFFICIENT ALGORITHM FOR L ∞ -NORM CALCULATIONS

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IFAC Proceedings Volumes 2006年第9期39卷 519-524页

作者： Vasile Sima National Institute for Research & Development in Informatics Bd. Maresal Averescu Nr. 8-10 011455 Bucharest 1 Romania

An efficient algorithm for L ∞ -norm calculations, implemented in the SLICOT Library, is described and compared with other available algorithms. The algorithm exploits the Hamiltonian structure of the computational problem and offers generality and flexibility. Continuous- and discrete-time, standard as well as generalized systems are addressed. Extensive comparisons with other L ∞ -norm solvers show that the generalpurpose SLICOT solver is the most efficient one, especially for standard continuous-time problems.

关键词： Computer-aided control systems design Linear control systems Norms numerical algorithms Robust control

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