bluuebunny/crossref_metadata_2025_split huggingface dataset:
Direct Evaluation of Nonlinear Gradient Elasticity in 2D with C1 Continuous Discretization Methods
Authors: Fischer Paul , Mergheim Julia , Steinmann Paul 
Published: November 2010

AbstractIn this contribution, we present a study on the performance of four different subparametric triangular finite element formulations for linear and nonlinear strain gradient elastic continua. Fully C1 and nonconforming C* finite elements are considered for a numerical analysis. The results are demonstrated by means of two computational examples together with the L2, H1 and H2 error norms. An interesting aspect concerning the linear geometry approximation is revealed. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Use of Halley's Method in the Nonlinear Finite Element Analysis
Authors: Plate Carolin , Papadopoulos Panayiotis , Müller Ralf 
Published: November 2010

AbstractHalley's method is a higher order iteration method for the solution of nonlinear systems of equations. Unlike Newton's method, which converges quadratically in the vicinity of the solution, Halley's method can exhibit a cubic order of convergence. The equations of Halley's method for multiple dimensions are derived using Padé approximants and inverse one‐point interpolation, as proposed by Cuyt. The investigation of the performance of Halley's method concentrates on eight‐node volume elements for nonlinear deformations using Staint Venant‐Kirchhoff's constitutive law, as well as a geometric linear theory of von Mises plasticity. The comparison with Newton's method reveals the sensibility of Halley's method, in view of the radius of attraction but also demonstrates the advantages of Halley's method considering simulation costs and the order of convergence. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
State Trajectory Compression in Optimal Control
Authors: Götschel Sebastian , Weiser Martin 
Published: November 2010

AbstractIn optimal control problems with nonlinear time‐dependent 3D PDEs, the computation of the reduced gradient by adjoint methods requires one solve of the state equation forward in time, and one backward solve of the adjoint equation. Since the state enters into the adjoint equation, the storage of a 4D discretization is necessary. We propose a lossy compression algorithm using a cheap predictor for the state data, with additional entropy coding of prediction errors. Analytical and numerical results indicate that compression factors around 30 can be obtained without exceeding the FE discretization error. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Computation of derivatives for structure preserving optimal control using automatic differentiation
Authors: Ober‐Blöbaum Sina , Walther Andrea 
Published: November 2010

AbstractIn this work we combine a recently developed method, Discrete Mechanics and Optimal Control (DMOC), with the well established Automatic Differentiation package ADOL‐C. DMOC is based on the discretization of the variational structure of the mechanical system which leads to structure (symplectic‐momentum) preserving time‐stepping equations. The discretized equations provide equality constraints for the resulting finite dimensional nonlinear optimization problem. For the solution of this problem standard nonlinear optimization techniques like sequential quadratic programming (SQP) are used. To ensure robustness of these techniques the computation of derivatives of the objective function and the constraints is of great importance. The concept of Automatic Differentiation (AD) is used to improve the performance of the SQP method. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Krylov Subspace Methods for Model Order Reduction of Bilinear Discrete‐Time Control Systems
Authors: Benner Peter , Breiten Tobias , Damm Tobias 
Published: November 2010

AbstractIn this paper, we propose a projection technique for model order reduction of discrete‐time bilinear control systems based on the concept of so‐called multimoments. We will make use of an explicit solution formula of the system and consider its Z‐transform which allows us to characterize the system output by generalized transfer functions. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Invariant tracking control design via integrator backstepping
Authors: Collon Carsten , Rudolph Joachim 
Published: November 2010

AbstractA tracking control problem for systems in state representation possessing a Lie symmetry group G is considered. By designing an invariant feedback control law based on invariant tracking errors the symmetry group G can be preserved under feedback. An extension of this approach to the popular integrator backstepping design method is presented. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Eddy current measurements of surface roughness
Authors: Dziczkowski Leszek 
Published: November 2010

AbstractThe paper proposes a method for simultaneous measurements of the material conductivity and its surface roughness for non‐ferromagnetic materials. The method takes advantage of the effect exercised by eddy currents onto impedance of a contact coil. The essence of the method is to eliminate the influence of the surface unevenness onto measurements of conductivity by the calculation of a correcting factor that serves as a specific equivalent of the surface roughness. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Tracking of Mechanical System Parameters by Phase‐Locked Loops
Authors: Kern Dominik , Seemann Wolfgang 
Published: November 2010

AbstractThe eigenfrequencies of a mechanical system depend on several parameters such as mass‐, damping‐, stiffness‐distribution and boundary conditions. Variation of these parameters results in variation of the Frequency Response Function (FRF). An eigenfrequency can be tracked by resonant excitation with a Phase‐Locked Loop (PLL) in order to monitor a certain parameter. As an example, a cantilever beam with a time‐variant mass at its end is analyzed. In this contribution, the FRF of this beam will be derived, which is the prerequisite for the following resonant excitation. The modified PLL‐design for the resonant excitation of higher modes will be explained and finally tested at the state‐space model of the beam. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Sliding Mode Control of an Underactuated Two‐Link Manipulator
Authors: Knoll Carsten , Röbenack Klaus 
Published: November 2010

AbstractWe consider a horizontally mounted two‐link manipulator of which only the first joint is actuated and the friction in the second joint is neglected. As a consequence, predefined equilibrium points are difficult to reach and point‐to‐point control is a challenging task. Starting from a partially feedback linearized model, a transformation to Byrnes‐Isidori normal form is presented. Based on this decomposition, the projection into a two‐dimensional subspace reveals certain similarities to a double integrator. Modifying an existing sliding mode approach for that system, a control law which steers the manipulator to an equilibrium point can be deduced. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Some Remarks on Affine Derivative Systems concerning Flatness
Authors: Schöberl Markus , Schlacher Kurt 
Published: November 2010

AbstractWe consider nonlinear implicit control systems and discuss the construction of a system parametrization in the context of flatness. To this end, quasilinear systems will play a prominent role since they are affine in the derivative variables which alleviates the proposed reduction and elimination strategy. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Flux‐corrected‐transport methods for modelling of gas discharge plasmas
Authors: Becker Markus M. , Loffhagen Detlef 
Published: November 2010

AbstractThe hydrodynamic description of gas discharge plasmas leads to a nonlinear coupled system of balance equations for the particle, current and energy densities of electrons and heavy particles as well as Poisson's equation for the electric potential. New implicit flux‐corrected‐transport methods for the numerical solution of the discharge model are proposed and compared with established flux‐corrected‐transport schemes based on finite‐difference as well as finite‐element methods. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A posteriori control of modelling errors in linear elasticity
Authors: Große‐Wöhrmann André , Blum Heribert , Stiemer Marcus 
Published: November 2010

AbstractThe concept of adaptive error control for finite element Galerkin discretizations has more recently been extended from the pure treatment of the discretization errors [1], [2] also to the control of modelling errors [4], [5]. These techniques can be employed for a rigorous justification of the local choice of the model out of a given hierarchy with increasing complexity. In the present paper the concept is exemplified by a hierarchy of linear‐elastic models, consisting of a basic model with constant E modulus and an improved model with oscillating E modulus. Significant reduction of the computational complexity can be achieved by a proper choice of the model in different subdomains, automatically chosen by the error estimators. Several error indicators are investigated in the context of goal oriented error estimation. Their efficiency is compared by means of finite element simulations [3] of an elementary example. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A characterization of the eigenvalues of Schrödinger operators with Dirichlet and Neumann boundary conditions
Authors: Behrndt Jussi , Rohleder Jonathan 
Published: November 2010

AbstractThe eigenvalues of the self‐adjoint Schrödinger operators on a bounded domain with Dirichlet and Neumann boundary conditions are characterized by the singularities of an associated Dirichlet‐to‐Neumann map and its inverse, respectively. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Composite Finite Elements for Elliptic Interface Problems
Authors: Peterseim Daniel 
Published: November 2010

AbstractWe present a Composite Finite Element Method for the approximation of linear elliptic boundary value problems of Dirichlet type with discontinuous coefficients. The challenge is the discontinuity of the coefficient (interface) which is not necessarily resolved by the underlying finite element mesh. The method is non‐conforming in the sense that shape functions preserve continuity across the interface only in an approximative way. However, the construction allows to balance the non‐conformity and the overall discretization error. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Configurational‐Force‐Based Adaptive FE Solver for a Phase Field Model of Fracture
Authors: Welschinger Fabian , Hofacker Martina , Miehe Christian 
Published: November 2010

AbstractThe computational modeling of failure mechanisms in solids due to fracture based on sharp crack discontinuities suffers in situations with complex crack topologies. This can be overcome by diffusive crack modeling, based on the introduction of a crack phase field as outlined in [1, 2]. Following these formulations, we outline a thermodynamically consistent framework for phase field models of crack propagation in elastic solids, develop incremental variational principles and, as an extension to [1, 2], consider their numerical implementations by an efficient h‐adaptive finite element method. A key problem of the phase field formulation is the mesh density, which is required for the resolution of the diffusive crack patterns. To this end, we embed the computational framework into an adaptive mesh refinement strategy that resolves the fracture process zones. We construct a configurational‐force‐based framework for h‐adaptive finite element discretizations of the gradient‐type diffusive fracture model. We develop a staggered computational scheme for the solution of the coupled balances in physical and material space. The balance in the material space is then used to set up indicators for the quality of the finite element mesh and accounts for a subsequent h‐type mesh refinement. The capability of the proposed method is demonstrated by means of a numerical example. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Characterization of the disordered‐to‐α‐helical transition of IA3 by SDSL‐EPR spectroscopy
Authors: Pirman Natasha L. , Milshteyn Eugene , Galiano Luis , Hewlett Justin C. , Fanucci Gail E. 
Published: November 2010

AbstractElectron paramagnetic resonance (EPR) spectroscopy coupled with site‐directed spin labeling (SDSL) is a valuable tool for characterizing the mobility and conformational changes of proteins but has seldom been applied to intrinsically disordered proteins (IDPs). Here, IA3 is used as a model system demonstrating SDSL‐EPR characterization of conformational changes in small IDP systems. IA3 has 68 amino acids, is unstructured in solution, and becomes α‐helical upon addition of the secondary structural stabilizer 2,2,2‐trifluoroethanol (TFE). Two single cysteine substitutions, one in the N‐terminus (S14C) and one in the C‐terminus (N58C), were generated and labeled with three different nitroxide spin labels. The resultant EPR line shapes of each of the labels were compared and each reported changes in mobility upon addition of TFE. Specifically, the spectral line shape parameters h(+1)/h(0), the local tumbling volume (VL), and the percent change of the h(−1) intensity were utilized to quantitatively monitor TFE‐induced conformational changes. The values of h(+1)/h(0) as a function of TFE titration varied in a sigmoidal manner and were fit to a two‐state Boltzmann model that provided values for the midpoint of the transition, thus, reporting on the global conformational change of IA3. The other parameters provide site‐specific information and show that S14C‐SL undergoes a conformational change resulting in more restricted motion than N58C‐SL, which is consistent with previously published results obtained by studies using NMR and circular dichroism spectroscopy indicating a higher degree of α‐helical propensity of the N‐terminal segment of IA3. Overall, the results provide a framework for data analyzes that can be used to study induced unstructured‐to‐helical conformations in IDPs by SDSL.
Ab initio simulation of a 57‐residue protein in explicit solvent reproduces the native conformation in the lowest free‐energy cluster
Authors: Ikebe Jinzen , Standley Daron M. , Nakamura Haruki , Higo Junichi 
Published: November 2010

AbstractAn enhanced conformational sampling method, multicanonical molecular dynamics (McMD), was applied to the ab intio folding of the 57‐residue first repeat of human glutamyl‐ prolyl‐tRNA synthetase (EPRS‐R1) in explicit solvent. The simulation started from a fully extended structure of EPRS‐R1 and did not utilize prior structural knowledge. A canonical ensemble, which is a conformational ensemble thermodynamically probable at an arbitrary temperature, was constructed by reweighting the sampled structures. Conformational clusters were obtained from the canonical ensemble at 300 K, and the largest cluster (i.e., the lowest free‐energy cluster), which contained 34% of the structures in the ensemble, was characterized by the highest similarity to the NMR structure relative to all alternative clusters. This lowest free‐energy cluster included native‐like structures composed of two anti‐parallel α‐helices. The canonical ensemble at 300 K also showed that a short Gly‐containing segment, which adopts an α‐helix in the native structure, has a tendency to be structurally disordered. Atomic‐level analyses demonstrated clearly that inter‐residue hydrophobic interactions drive the helix formation of the Gly‐containing segment, and that increasing the hydrophobic contacts accompanies exclusion of water molecules from the vicinity of this segment. This study has shown, for the first time, that the free‐energy landscape of a structurally well‐ordered protein of about 60 residues is obtainable with an all atom model in explicit water without prior structural knowledge.
Modeling of the Toll‐like receptor 3 and a putative Toll‐like receptor 3 antagonist encoded by the African swine fever virus
Authors: Henriques Elsa S. , Brito Rui M. M. , Soares Hugo , Ventura Sónia , de Oliveira Vivian L. , Parkhouse R. Michael E. 
Published: November 2010

AbstractAfrican swine fever virus (ASFV) is a large double‐stranded DNA virus responsible for a lethal pig disease, to which no vaccine has ever been obtained. Its genome encodes a number of proteins involved in virus survival and transmission in its hosts, in particular proteins that inhibit signaling pathways in infected macrophages and, thus, interfere with the host's innate immune response. A recently identified novel ASFV viral protein (pI329L) was found to inhibit the Toll‐like receptor 3 (TLR3) signaling pathway, TLR3 being a crucial “danger detector.” pI329L has been predicted to be a transmembrane protein containing extracellular putative leucine‐rich repeats similar to TLR3, suggesting that pI329L might act as a TLR3 decoy. To explore this idea, we used comparative modeling and other structure prediction protocols to propose (a) a model for the TLR3–Toll‐interleukin‐1 receptor homodimer and (b) a structural fold for pI329L, detailed at atomistic level for its cytoplasmic domain. As this later domain shares only remote sequence relationships with the available TLR3 templates, a more complex modeling strategy was employed that combines the iterative implementation of (multi)threading/assembly/refinement (I‐TASSER) structural prediction with expertise‐guided posterior refinement. The final pI329L model presents a plausible fold, good structural quality, is consistent with the available experimental data, and it corroborates our hypothesis of pI329L being a TLR3 antagonist.
NMR determination of pKa values in α‐synuclein
Authors: Croke Robyn L. , Patil Sharadrao M. , Quevreaux Jason , Kendall Debra A. , Alexandrescu Andrei T. 
Published: November 2010

AbstractThe intrinsically unfolded protein α‐synuclein has an N‐terminal domain with seven imperfect KTKEGV sequence repeats and a C‐terminal domain with a large proportion of acidic residues. We characterized pKa values for all 26 sites in the protein that ionize below pH 7 using 2D 1H‐15N HSQC and 3D C(CO)NH NMR experiments. The N‐terminal domain shows systematically lowered pKa values, suggesting weak electrostatic interactions between acidic and basic residues in the KTKEGV repeats. By contrast, the C‐terminal domain shows elevated pKa values due to electrostatic repulsion between like charges. The effects are smaller but persist at physiological salt concentrations. For α‐synuclein in the membrane‐like environment of sodium dodecylsulfate (SDS) micelles, we characterized the pKa of His50, a residue of particular interest since it is flanked within one turn of the α‐helix structure by the Parkinson's disease‐linked mutants E46K and A53T. The pKa of His50 is raised by 1.4 pH units in the micelle‐bound state. Titrations of His50 in the micelle‐bound states of the E46K and A53T mutants show that the pKa shift is primarily due to interactions between the histidine and the sulfate groups of SDS, with electrostatic interactions between His50 and Glu46 playing a much smaller role. Our results indicate that the pKa values of uncomplexed α‐synuclein differ significantly from random coil model peptides even though the protein is intrinsically unfolded. Due to the long‐range nature of electrostatic interactions, charged residues in the α‐synuclein sequence may help nucleate the folding of the protein into an α‐helical structure and confer protection from misfolding.
Charge transport in organic crystals: Theory and modelling
Authors: Ortmann Frank , Bechstedt Friedhelm , Hannewald Karsten 
Published: November 2010

AbstractEfficient charge transport in organic semiconductors is the key for their application in Organic Electronics. Therefore the most important design ansatz is directed to improve carrier mobilities by means of available tools such as chemical and/or structural modifications of the organic materials. Thereby theoretical input can provide guidelines towards possible realizations of high‐mobility or, more general, highly functional materials. In particular the structure–property relationship is at the heart of theoretical studies which are extremely helpful if they achieve predictive power. In this paper we systematically review several charge‐transport approaches and illustrate their relationship in order to display their capabilities regarding this goal. Special focus is put on the transport mechanism, the mobility anisotropy and temperature dependence of charge‐carrier transport. As the central concept the inclusion of the strong coupling of the carriers with the vibrating lattice and, hence, the dressing of carriers to polarons is described. The availability of material parameters is tremendously important for transport description. We highlight their computation by ab initio theory in order to arrive at a satisfactory level beyond phenomenological approaches.
Advances in electronic structure methods for defects and impurities in solids
Authors: Van de Walle Chris G. , Janotti Anderson 
Published: November 2010

AbstractDefects and impurities are often decisive in determining the physical properties of most materials. The process of defect identification and characterization is typically difficult and indirect, usually requiring an ingenious combination of different experimental techniques. First‐principles calculations have emerged as a powerful microscopic tool that complements experiments or sometimes even serves as the sole source of atomistic information due to experimental limitations. Still, first‐principles calculations based on density functional theory in the local density or generalized gradient approximations suffer from serious limitations when describing defects in solids. Recent advances in electronic structure methods, rapid increases in computing power, and the development of efficient algorithms indicate a promising future for computational defect physics. We review recent advances in the theory of defects in solids from the perspective of first‐principles calculations. We focus in particular on methods that improve the description of band gaps, leading to results that can be directly compared to experiments on a quantitative level. We discuss the use of LDA+U in wide‐band‐gap materials, screened hybrid functionals, the quasiparticle GW method, and the use of modified pseudopotentials. Advantages and limitations of these methods are illustrated with examples.
Near‐exact supersymmetric partner potentials: Construction and exploitation
Authors: Mukherjee Neetik , Pathak Ranjit K. , Bhattacharyya Kamal 
Published: November 2010

AbstractPrecise supersymmetric (SUSY) partner potentials can be generated only for exactly solvable problems of the stationary Schrödinger equation. This is a severe restriction, as most problems are not amenable to exact solutions. We employ here a linear variational strategy to explicitly construct approximate SUSY partners of a few common, not exactly solvable potentials and subsequently examine their properties to explore the advantages in practical implementation. The efficacy of our proposed scheme is commendable. We demonstrate that, for symmetric potentials, the constructed partners may be so good that the overall recipe has the nicety of generating the whole eigenspectrum by employing only half of the full Hilbert space functions. A similar strategy is shown to work for the odd states too, with proper boundary conditions. Pilot calculations involve a number of low‐lying states of some mixed oscillator and double‐well potentials. Analysis of the results reveals a few interesting features of the problem of construction of approximate SUSY partners and their practical use. Particularly, we identify places where the operator‐level approximations are involved and how far they affect the bounding properties of energies that are obtained as eigenvalues of a matrix diagonalization problem associated with linear variations. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011