bluuebunny/crossref_metadata_2025_split huggingface dataset:
On saturation effects in the Neumann boundary control of elliptic optimal control problems
Authors: Mateos Mariano , Rösch Arnd 
Published: October 2008

AbstractA Neumann boundary control problem for a linear‐quadratic elliptic optimal control problem in a polygonal domain is investigated. The main goal is to show an optimal approximation order for discretized problems after a postprocessing process. It turns out that two saturation processes occur: The regularity of the boundary data of the adjoint state is limited if the largest angle of the polygon is at least 2π /3. Moreover, piecewise linear finite elements cannot guarantee the optimal order, if the largest angle of the polygon is greater than π /2. We will derive error estimates of order hσ with σ ∈ [3/2, 2] depending on the largest angle and properties of the finite elements. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A wave‐type limit for the 3D Landau‐Lifshitz‐Gilbert equation
Authors: Melcher Christof 
Published: October 2008

AbstractWe present a general version of wave‐type limit for the micromagnetic Landau‐Lifshitz‐Gilbert equation in thin films previously encountered in joint work with A. Capella and F. Otto in the context of a reduced model for Néel wall motion. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A connection‐theoretic approach to reduction of second‐order dynamical systems with symmetry
Authors: Mestdag T. , Crampin M. 
Published: October 2008

AbstractWe deal with reduction of Lagrangian systems that are invariant under the action of the symmetry group. Unlike the bulk of the literature we do not rely on methods coming from the calculus of variations. Our method is based on the geometrical analysis of regular Lagrangian systems, where solutions of the Euler‐Lagrange equations are interpreted as integral curves of the associated second‐order differential equation field. In particular, we explain so‐called Lagrange‐Poincaré reduction [1] and Routh reduction [3] from the viewpoint of that vector field. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Derivative‐free optimization methods for surface structure determination of nanosystems
Authors: Meza J. C. , Garcia‐Lekue A. , Abramson M. A. , Dennis J. E. 
Published: October 2008

AbstractMany properties of nanostructures depend on the atomic configuration at the surface. One common technique used for determining this surface structure is based on the low energy electron diffraction (LEED) method, which uses a high‐fidelity physics model to compare experimental results with spectra computed via a computer simulation. While this approach is highly effective, the computational cost of the simulations can be prohibitive for large systems. In this work, we propose the use of a direct search method in conjunction with an additive surrogate. This surrogate is constructed from a combination of a simplified physics model and an interpolation that is based on the differences between the simplified physics model and the full fidelity model. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Unstable periodic orbits in a Faraday disk dynamo
Authors: Moroz Irene M. 
Published: October 2008

AbstractHide et al [2] introduced a system of three nonlinear coupled ordinary differential equations to model a self‐exciting Faraday disk homopolar dynamo. Two examples of chaotic behaviour were shown by them. Moroz [4] performed a more extensive analysis of this dynamo model, including bifurcation transition diagrams and unstable periodic orbits for the two chaotic examples. We now use these unstable periodic orbits to identify a possible template for the chaotic attractor, using ideas from topology [1] and results from a corresponding analysis of the Lorenz attractor. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A variational arbitrary Lagrangian‐Eulerian finite element formulation for standard dissipative solids at finite strains
Authors: Mosler J. , Ortiz M. 
Published: October 2008

AbstractA novel Arbitrary Lagrangian‐Eulerian (ALE) finite element formulation for standard dissipative media at finite strains is presented. In contrast to previously published ALE approaches accounting for dissipative phenomena, the proposed scheme is fully variational. Consequently, no error estimates are necessary and thus, linearity of the problem and the corresponding Hilbert‐space are not required. Hence, the resulting Variational Arbitrary Lagrangian‐Eulerian (VALE) finite element method can be applied to highly nonlinear phenomena as well. In case of standard dissipative solids, so‐called variational constitutive updates provide a variational principle. Based on these updates, the deformation mapping follows from minimizing an incrementally defined (pseudo) potential, i.e., energy minimization is the overriding criterion that governs every aspect of the system. Therefore, it is natural to allow the variational principle to drive mesh adaption as well. Thus, in the present paper, the discretizations of the deformed as well as the undeformed configuration are optimized jointly by minimizing the respective incremental energy of the considered mechanical system. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical linear stability on controlled solutions of Boussinesq Navier‐Stokes
Authors: Navarro María Cruz , Herrero Henar 
Published: October 2008

AbstractA numerical linear stability analysis on optimally controlled solutions in a thermoconvective problem is presented. The thermoconvective problem is a stationary Boussinesq Navier‐Stokes with a gaussian heating at the bottom with a large horizontal temperature gradient. The numerical method is Chebyshev collocation. The optimal control permits to find a boundary condition such that the vorticity is minimized. For some values of the penalizing parameter the controlled solution is more stable than the corresponding uncontrolled one. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical simulation of the endolymph flow in a semicircular canal
Authors: Obrist Dominik 
Published: October 2008

AbstractWe present a concept for the simulation of the fluid flow in the semicircular canals of the inner ear. Based on the temporal dynamics of an idealized model configuration we formulate the governing equations and devise a strategy for their numerical solution. We analyze the proposed method and find a numerical instability. This instability is characterized by an error bound which can serve as a guideline for tuning the numerical method to the specific boundary conditions. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On Wachspress pentagonal patches
Authors: Choubey Nidhi , Dikshit H.P. , Ojha Aparajita 
Published: October 2008

AbstractWachspress quadrilateral patches have been recently studied from the point of view of applications to surface modelling in CAGD [1], [3], [4]. Some more applications for defining barycentric coordinates for arbitrary polygons have also been presented in [5] [9]. The purpose of the present paper is to introduce non‐negative Wachspress rational basis functions for surface modelling on pentagonal partitions. Interpolation formula for function values and directional derivatives at the vertices of pentagon has been presented. Conditions for C1– continuity of the composite surface have also been studied in the paper. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Non‐local Dirichlet forms generated by pseudodifferential operators on compact abelian groups
Authors: Popescu Emil 
Published: October 2008

AbstractLet Gi, 1 ≤ i ≤ n, be compact abelian groups and let Γi , 1 ≤ i ≤ n, be countable dual groups. We consider G = G1 ⊕ G2 ⊕ … ⊕ Gn and Γ = Γ1 ⊕ Γ2 ⊕ … ⊕ Γn . For 1 ≤ j ≤ n, let aj be a negative definite function on Γj and a (γ) = . For φ ∈ S (G), the set of all generalized trigonometrical polynomials on G, we define , where (γ) = aj (γj) (γ), 1 ≤ j ≤ n. Then is a Dirichlet form with the domain on L2 (G). (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Nonlinearity management in optics
Authors: Porter Mason A. , Centurion Martin , Pu Ye , Kevrekidis P. G. , Frantezeskakis D. J. , Psaltis Demetri 
Published: October 2008

AbstractWe study nonlinearity management in optics by investigating the propagation of localized pulses and plane waves in a layered, cubically nonlinear (Kerr) medium that consists of alternating layers of glass and air. We show that such nonlinearity management delays the blow‐up/collapse of pulses and leads to a band structure of modulationally unstable regions for plane waves. We find excellent agreement between experiments, numerical simulations, and theory. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Can one hear shape?
Authors: Reuter Martin 
Published: October 2008

AbstractThe question “Can one hear the shape of a drum” has been asked in several contexts before (e.g., by Bers and Kac). It is a pictorial way of asking if the eigenvalues of the Laplacian on a given domain completely characterize its shape, in other words, if the spectrum is a complete shape descriptor (which it is not in general). Since the spectrum contains geometrical information and since it is an isometry invariant and therefore independent of the object's representation, parametrization, spatial position, and optionally of its size, it is well suited to be used as a fingerprint (Shape‐DNA) in contemporary computer graphics applications like database retrieval, quality assessment, and shape matching in fields like CAD, engineering or medicine. We will explain why isometry invariance is so important and point out future directions of research. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Instability mechanisms in buoyant‐thermocapillary liquid pools
Authors: Schoisswohl U. , Kuhlmann H. C. 
Published: October 2008

AbstractA cylindrical volume of fluid, with a free surface on top, is heated by a parabolic heat flux from above. Two physical effects drive a flow: thermocapillary effects due to free‐surface temperature gradients introduced by the non‐uniform heat flux and buoyancy forces due to gravity. The basic axisymmetric flow is computed by finite volumes and its stability is investigated by a linear‐stability analysis. It is found that the critical stability boundaries and modes are similar to those known from the half‐zone model of crystal growth. For low Prandtl numbers the critical mode is steady and three‐dimensional. We find an asymptotic critical value in the limit of vanishing Prandtl number. For increasing Prandtl number the critical Reynolds number increases. Near unit Prandtl number no threshold could be found with the present computational limitations. For Prandtl numbers larger than unity, the critical mode is oscillatory and the critical Reynolds number decreases with the Prandtl number. We present evidence that the low‐ and high‐Prandtl‐number instabilities are essentially centrifugal respectively due to the hydrothermal‐wave mechanism. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Multiscale methods for stiff ODE: closure and consistency
Authors: Sharp Richard , Tsai Richard , Engquist Björn 
Published: October 2008

AbstractWe apply a Heterogeneous Multiscale Method style algorithm to highly oscillatory ODEs. The algorithm tracks the slow evolution of a system in a manner consistent with, but more efficient than methods that resolve the fast oscillations. An example is given where local time averages of the original system variables form a closed set of macroscopic variables. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On adaptive step‐size selections in gradient methods
Authors: Frassoldati Giacomo , Zanni Luca , Zanghirati Gaetano 
Published: October 2008

AbstractIn recent years several proposals for the step‐size selection have largely improved the gradient methods, in the case of both constrained and unconstrained nonlinear optimization. We introduce a new step‐size rule with some crucial properties. We design step‐size selection strategies where the new rule and a standard Barzilai‐Borwein (BB) rule can be adaptively alternated to get meaningful convergence rate improvements in comparison with other BB‐like gradient schemes. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Multi‐way decomposition of projected spectra obtained in protein NMR
Authors: Billeter Martin , Staykova Doroteya K. , Fredriksson Jonas , Bermel Wolfgang 
Published: October 2008

AbstractMultidimensional nuclear magnetic resonance spectroscopy is a highly versatile tool in protein research. Experimental limitations prohibit spectra with ≥ 5 dimensions. However, similar information can be obtained by recording 2‐dimensional projections. A decomposition‐based approach for analyzing projections is presented and applied to a protein. The resulting resonance identification is a prerequisite for further characterization of structure, dynamics and interactions of proteins. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A nested dissection approach for sparse matrix partitioning
Authors: Boman Erik G. 
Published: October 2008

AbstractWe consider how to partition and distribute sparse matrices among processors to reduce communication cost in sparse matrix computations, in particular, sparse matrix‐vector multiplication. We consider 2d distributions, where the distribution is not constrained to just rows or columns. We present a new model and an algorithm based on vertex separators and nested dissection. Preliminary numerical results for sparse matrices from real applications indicate the new method performs consistently better than traditional 1d partitioning and is often also better than current 2d methods. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Asymmetric dynamic price competition with uncertainty
Authors: Ferreira Fernanda A. , Pinto Alberto A. 
Published: October 2008

AbstractWe consider a dynamic setting‐price duopoly model in which a dominant (leader) firm moves first and a subordinate (follower) firm moves second. We suppose that each firm has two different technologies, and uses one of them according to a certain probability distribution. The use of either one or the other technology affects the unitary production cost. We analyse the effect of the production costs uncertainty on the profits of the firms, for different values of the intercept demand parameters. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
An efficient algorithm for perturbed shortest path problems
Authors: Grüne Lars , Junge Oliver 
Published: October 2008

AbstractWe present an efficient algorithm for perturbed shortest paths problems that arise, e.g., in dynamic games. Via a minmax version of Dijkstra's algorithm we compute piecewise constant upper and lower bounds on the upper value function of the game. Convergence of the bounds in the limit of vanishing cell diameter can be proved. The method is numerically demonstrated on a simple 2d dynamic game, the homicidal chauffeur. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Embedding quasi laminar 1D flame profiles to model turbulent premixed combustion with a joint PDF method
Authors: Hack Mathias Leander , Jenny Patrick 
Published: October 2008

AbstractA new model for turbulent premixed combustion is presented which is based on a joint velocity composition probability density function (JPDF) method. The key idea is a scale separation approach. The method combines the model by Bray, Moss and Libby [1] (BML) for premixed combustion with the flamelet approach for nonpremixed combustion. Here, a Lagrangian formulation of the BML model is considered. The progress variable used by the BML model becomes a computational particle property and its value is triggered by the arrival of the flame front at the particle's position. Similar as in the flamelet approach we assume that the smallest eddies are not small enough to disturb the reactive diffusive flame structure. To resolve the (embedded) quasi laminar flame structure, a flame residence time is introduced. With that residence time, the evolution of the particle composition, including enthalpy, can be determined from precomputed laminar 1D flames. The main challenge with this approach is to model the probability that an embedded flamefront arrives at the particle location, which is necessary to close the chemical source term. Numerical experiments of a turbulent premixed flame show good agreement with experimental data. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Accessibility of the matrix Riccati differential equation
Authors: Dirr G. , Helmke U. 
Published: October 2008

AbstractA known result on the classification of transitive Lie group actions on complex Grassmann manifolds is exploited to derive a necessary and sufficient accessibility criterion for the complex matrix differential Riccati equation. We treat both cases, the symmetric as well as the non‐symmetric Riccati equation. Corresponding accessibility results for the real Riccati equation are also available, but not stated here. An application to the accessibility of generalized double bracket flows is given. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Modelling of displacement washing of pulp fibers using orthogonal collocation on finite elements
Authors: Arora Shelly , Dhaliwal S.S. , Kukreja V.K. 
Published: October 2008

AbstractFlow of fluid through packed bed of porous particles is modelled with the help of Peclet number (Pe) and Biot number (Bi). Packed bed is divided into three zones, flowing liquor, intrapore solute present in pores of particles and solute adsorbed on particle surface. Langmuir isotherm is used to describe the relationship between intrapore solute concentration and concentration of solute adsorbed on particle surface, whereas the bulk fluid concentration and the intrapore solute concentration are related by linear adsorption isotherm. Model predicted values are also compared with the experimental values. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Fluctuation measurements in the boundary layer of a supersonic flow
Authors: Lenz B. , Gaisbauer U. , Krämer E. 
Published: October 2008

AbstractFluctuation measurements were carried out via constant‐temperature hot‐wire anemometry inside the turbulent boundary layer of a flat plate and a compression ramp (deflexion angle: 11.5). The free‐stream Mach number was M = 2.54. Fluctuation profiles of the alternating quantities temperature, density and velocity were obtained under the assumption of low pressure fluctuations. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Reassignment of the Stockwell spectrogram
Authors: Liu Cheng , Zhu Hongmei 
Published: October 2008

AbstractThe recently developed Stockwell spectrogram provides a multi‐scale time‐frequency representation of a signal. Due to the Heisenberg's inequality, however, the unavoidable blurring of time and frequency information may conceal important signal structures. Here, we propose to use the reassignment method to improve the readability of the Stockwell representation. An numerical example is presented to illustrate its effectiveness in practice. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Statistic analysis of nonwoven properties based on a stochastic fiber lay‐down model
Authors: Marheineke Nicole 
Published: October 2008

AbstractA quality measure for a nonwoven material is the relative variance of the fiber mass distribution. Using a stochastic fiber lay‐down model for the Monte‐Carlo simulations of nonwoven webs, the influence of the parameters of the nonwoven production process on the web quality is investigated via a design of experiments. The statistic results are confirmed by the asymptotic analysis (Chapman‐Enskog method) of the associated Fokker‐Planck equation for the hydrodynamic limit and the computation of the corresponding process functionals. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Explicit solutions of the equation –div z = f under the constraint ∥z ∥∞ = 1
Authors: Milbers Zoja 
Published: October 2008

AbstractWe consider the eigenvalue problem for the 1‐Laplace operator and the corresponding variational problem. Since the underlying functionals are not differentiable, the derivation of the Euler‐Lagrange equation is highly nontrivial and involves methods of nonsmooth analysis. A remarkable fact is, that there are infinitely many functions f for which the equation –div z = f has to be satisfied. The purpose of the present work is the investigation of this equation. We show that in a square 2‐dimensional domain for each right hand side f there are infinitely many vector fields z satisfying the Euler‐Lagrange equation. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A numerical study of 2‐D periodically‐forced internal flows
Authors: Olah Corina A. 
Published: October 2008

AbstractWe use a stabilized FEM with Gear time‐stepping to study periodically forced internal flows. The time‐periodically forced Navier‐Stokes equations always admit a time‐periodic solution but we show that this time‐periodic solution might not be stable, even in 2‐D. Numerical results are presented for a time‐periodic lid driven cavity. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optische Uhren. Zeitmessung mit kalten Atomen und Laserlicht
Authors: Peik Ekkehard , Sterr Uwe 
Published: October 2008

AbstractOptische Uhren nutzen die Anregung optischer Übergänge in neutralen Atomen oder Ionen. Deren Übergangsfrequenz liegt bei etwa 1015 Hz, also fünf Größenordnungen oberhalb der Gigahertz‐Schwingungen heutiger Atomuhren. Das erlaubt eine bis zu tausendmal präzisere Zeitmessung. Für die Genauigkeit ist es entscheidend, dass der Uhrenübergang eine schmale Linienbreite besitzt und seine Frequenz nicht verschoben wird. Dazu müssen die Atome oder Ionen möglichst ungestört und genau lokalisiert sein. Deshalb basiert ein Uhrentyp auf einem kalten Ion in einer Paul‐Falle, ein anderer Uhrentyp arbeitet mit vielen kalten Neutralatomen in optischen Gittern. Der Metrologie sollen optische Uhren zunächst als sekundäre Realisierung der Sekunde dienen. Derzeit ermöglichen sie bereits Frequenzmessungen mit 17 Stellen Genauigkeit.
Die Spintronik‐Jagd. Elektronenspins in Halbleiterbauelementen
Authors: Steinmüller Stephan , Lee Kiyoung , Bland Tony 
Published: October 2008

AbstractMikroelektronische Bauelemente der Zukunft könnten nicht mehr allein die Ladung des Elektrons als Informationsträger nutzen, sondern auch seinen Spin. Eine solche Spintronik würde die näher rückende Grenze der Miniaturisierung durchbrechen, die der konventionellen Mikroelektronik bei sehr kleinen Strukturen wegen unerwünschter Quanteneffekte und Wärmeleitungsproblemen droht. Die Spintronik ermöglicht zudem Bauelemente mit völlig neuen Funktionen. Dazu zählt die Spin‐Leuchtdiode, die zirkular polarisiertes Licht emittiert. Die große Herausforderung ist allerdings eine effektive Injektion spinpolarisierter Elektronen in etablierte Halbleitermaterialien. Dazu können diese mit verdünnten magnetischen Halbleitern kombiniert werden. Die Alternative ist die Kombination eines magnetischen Metalls mit einem Halbleitermaterial. Dieser Weg scheint derzeit schneller zur technisch realisierten Spintronik zu führen.
Die vielen Väter des Transistors. Meilensteine in der Geschichte des Festkörperverstärkers
Authors: Hillmer Hartmut 
Published: October 2008

AbstractVor hundert Jahren wurde der zweifache Physik‐Nobelpreisträger John Bardeen geboren, der zusammen mit William Shockley und Walter Brattain als Vater des Transistors gilt. Die Vorgeschichte des Festkörperverstärkers ist jedoch komplex. Zum Konzept des Feldeffekt‐Transistors leisteten deutsche Physiker zuvor in den zwanziger und dreißiger Jahren entscheidende Beiträge: Julius Edgar Lilienfeld, Oskar Heil und Heinrich Welker meldeten wichtige Patente an. Robert Pohl und Rudolf Hilsch bauten in Göttingen wahrscheinlich den ersten funktionsfähigen Festkörperverstärker auf Basis eines Kaliumbromid‐Kristalls. Welker und Herbert Franz Mataré arbeiteten 1946 im Auftrag von Westinghouse in Paris an Transistoren. Den ersten funktionsfähigen Germaniumtransistor realisierten jedoch Shockley, Bardeen und Brattain 1947 an den Bell Labs.