bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantification of the degree of irreversibility in terms of material parameters by using Ziegler's non‐linear constitutive relation for the stress‐velocity gradient relationship
Authors: Müller Wolfgang H. , Emek Abali B.
Published: December 2011

The non‐trivial task of an engineer is to derive or at least to use a constitutive relation which leads to a solvable equation but still expresses the system behavior adequately, so that the solution for the motion is accurate. Often the system has solid‐like or fluid‐like behavior, and an appropriate relation is chosen. In the so‐called Bingham‐Ilyushin matter both regimes occur simultaneously. This can be expressed by using a tensor‐linear constitutive relation with a nonlinear (viscosity) term and even be solved in a specific case of material parameters (b = 0). The analytical solution for the velocity distribution in a channel with moving walls is presented here. The velocity gradients lead to an increase in temperature. The internal friction due to the (nonlinear) viscosity contributes to the entropy production and accounts for the irreversibility of the process. The aim is to analyze the role of each material parameter in the entropy production which might be used as a measure of irreversibility of the process. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Modelling the material behaviour of magnetorheological fluids under shear deformation
Authors: Obst Martin , Kästner Markus , Adam Frank , Brummund Jörg , Ulbricht Volker
Published: December 2011

In recent years the interest in materials with specific adjustable properties has increased due to higher requirements on the material performance. Here a smart composite material is to be developed, whose stiffness can be varied subjected to a magnetic field. To realise this aim a magnetorheological fluid (MRF) embedded in a polymeric matrix material is considered. To model the material behaviour of the composite a homogenisation method will be applied. Amongst others this requires the knowledge of the multiaxial material behaviour of each constituent. The modelling of the material behaviour of MRF under shear deformation, which is the aim of this work, represents the first step in this process. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Interaction of phase‐transformations and plasticity – a multi‐phase micro‐sphere approach
Authors: Ostwald Richard , Bartel Thorsten , Menzel Andreas
Published: December 2011

We present an efficient model for the simulation of solid to solid phase‐transformations in polycrystalline materials. As a basis, we implement a scalar‐valued Gibbs‐energy‐barrier‐based phase‐transformation model making use of statistical physics. In this work, we particularly adopt the model for the simulation of phase‐transformations between an austenitic parent phase and a martensitic tension and compression phase. The incorporation of plasticity phenomena is established by enhancing the Helmholtz free energy functions of the material phases considered, where the plastic driving forces acting in each phase are derived from the overall free energy potential. The coupled model is embedded into a micro‐sphere formulation in order to simulate three‐dimensional boundary value problems—a technique well‐established in the context of computational inelasticity at small strains. It is shown that the model is capable of reflecting experimentally observed behaviour. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Modeling the hydrogen material interaction as parametric instability of the continuum
Authors: Polyanskiy Vladimir , Yakovlev Yuriy
Published: December 2011

The hydrogen is contained in any material. Its concentration inside the materials leads to mechanical properties degradation. The two‐continuum model of solid allows one to describe the influence of small concentration of hydrogen on the mechanical properties of materials in terms of changing the bonding energy of the second continuum, the latter being responsible for the hydrogen concentration. The application of this model to fatigue task give the hydrogen concentration that are critical for material destruction. Such fatigue destruction has a nature of parametric instability during the cyclic redistribution of the hydrogen under the cyclic loading. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Phase Field Modeling of Crack Propagation at Large Strains with Application to Rubbery Polymers
Authors: Schänzel Lisa , Hofacker Martina , Miehe Christian
Published: December 2011

The computational modeling of failure mechanisms in solids due to fracture based on sharp crack discontinuities exhibits drawbacks in situations with complex crack topologies. This drawback is overcome by diffusive crack modeling based on the introduction of a fracture phase field characterizing via an auxiliary variable the crack topology. In the following we extend recent advances in phase‐field‐type fracture based on operator split techniques, suggested in Miehe et al. [1], to the modeling of crack propagation in geometrically large deforming solids e.g. rubber‐like materials. An extremely robust algorithmic treatment based on an operator split scheme is introduced consisting of three steps. Updating i) a local history‐field containing the maximum reference energy, ii) the fracture phase field, and iii) the displacement field. We demonstrate the performance of proposed phase field formulation for largely deforming solids by means of a representative numerical example. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the Analysis of Soils using an Abaqus‐PANDAS Interface
Authors: Schenke Maik , Ehlers Wolfgang
Published: December 2011

The foundation of buildings proceed from a detailed analysis of the interaction between the building and its environment in order to reduce the costs and ensure safety of the construction. A detailed and reliable analysis of the building‐foundation interaction requires suitable material definitions, an accurate description of the underground, and a model describing the geometry. Following this, a fluid‐saturated soil model, based on the the Theory of Porous Media, will be implemented into the finite‐element research‐code PANDAS, which will be connected to commercial FE package Abaqus. In this regard, one combines the powerful graphical user interface (GUI) for pre‐ and post‐processing and the algorithms of Abaqus with the convenient scripting environment for material definitions of PANDAS. Moreover, the linkage allows for a straight‐forward transfer of PANDAS material models into a well‐known and widely used commercial FE package. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Thermodynamics in material space‐time
Authors: Schnepp Johannes
Published: December 2011

The concept of material forces is well established in the continuum theory of defects. These forces are quantities in a three‐dimensional material manifold. The manifold can be augmented to a four‐dimensional material space‐time by a time‐like coordinate.

The inelastic phenomena caused by moving defects are highly dissipative. So a thermodynamic theory in material space‐time seems to be indispensable. Some first steps towards such a theory are developed here. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A Three‐dimensional Continuum Theory of Dislocation Plasticity ‐ Modelling and Application to a Composite
Authors: Schulz Katrin , Sandfeld Stefan , Gumbsch Peter
Published: December 2011

The increasing demand for materials with well defined microstructure, which is accompanied by the advancing miniaturization of devices calls for physically motivated, dislocation‐based continuum theories of plasticity. Only recently rigorous techniques have been developed for performing meaningful averages over systems of moving, curved dislocations, yielding evolution equations for a higher order dislocation density tensor. Our continuum dislocation theory allows for generalizing the planar system towards a three‐dimensional system, where dislocations may have arbitrary orientation and curvature. With the inclusion of curvature, the theory naturally takes into account a deformation‐induced increase in the overall dislocation density without having to invoke ad‐hoc assumptions about dislocation sources. A numerical implementation and some benchmark tests of this continuum theory for dislocation dynamics has already been discussed in the literature. In this paper, we apply this continuum theory to composite materials, where we analyze a plastically deforming matrix with an elastic inclusion. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Nonlocal damage‐viscoplastic model for high temperature creep of single crystal superalloys
Authors: Trinh Bach Tuyet , Hackl Klaus
Published: December 2011

A new nonlocal damage‐viscoplastic model for high temperature creep of single crystal superalloys is developed. It is based on the variational formulation consisting of free energy, plastic and damage dissipation potentials. Evolution equations for plastic strain and damage variables are derived from the minimum principle for dissipation potentials [1]. The model is capable of describing different stages of creep in a unified way. The evolution of dislocation densities of gamma and gamma prime phases in superalloys incorporates plastic deformation. It results in the time‐dependence of the creep rate in primary and secondary creep. Tertiary creep is taken into account by introducing local and nonlocal damage variables. Herein the nonlocal one is considered as numerical treatment to remove mesh‐dependence. Numerical results and comparisons with experimental data of the single crystal superalloy LEK94 are shown. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A Geometrically Exact Incremental Variational Formulation for Phase Field Models in Micromagnetics
Authors: Ethiraj Gautam , Miehe Christian
Published: December 2011

Magnetic materials have been finding increasingly wide areas of application. We focus here on the continuum modeling of such materials and present an incremental variational principle for a dissipative micro‐magneto‐elastic model. It describes the quasi‐static evolution of both magnetically as well as mechanically driven magnetic domains, which also incorporates the surrounding free space. Furthermore, the algorithmic preservation of the geometrical nature of the variables is an important challenge from the numerical perspective and to this end we present a novel FE discretization whereby the geometric property of the magnetization director is pointwise exactly preserved by nonlinear rotational updates at the nodes. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Modelling of cracks on different scales in ferroelectric materials
Authors: Gellmann Roman , Ricoeur Andreas
Published: December 2011

In this work we study contributions to the effective fracture toughness of ferroelectric materials arising from effects on macroscopic and mesoscopic scales of the system. On the macroscopic scale, the crack in a ferroelectric material is modeled taking into account an extended theory of stresses at interfaces in dielectric solids [1‐3]. We predict several new effects, such as the “poling effect”, “collinear effect” and the coupling of a Mode‐II shear loading and the Mode‐I SIF. Further, on the mesoscopic scale, we study the influence of polarization switching limited to the fracture process zone (small scale switching) on the fracture toughness. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A ferroelectric and ferroelastic microscopic switching criterion for tetragonal ferroelectrics
Authors: Keip Marc‐André , Schröder Jörg
Published: December 2011

The present contribution addresses the continuum mechanical modeling of ferroelectric/ferroelastic material behavior from a microscopic point of view. An electric enthalpy function for the representation of tetragonal ferroelectrics will be introduced and combined with an energy based ferroelectric/ferroelastic switching criterion. The result is a microscopically motivated material model for the description of ferroelectric and ferroelastic switching effects in electro‐mechanically coupled solids. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Suppression of Electromagnetically Induced Free‐Surface Instabilities in Liquid Metals
Authors: Minchenya Vitaly , Karcher Christian , Chigarev Anatoly
Published: December 2011

The work presented is devoted to studies of foundations of the suppression techniques for electromagnetically induced instabilities of the free liquid metal surfaces. Such instabilities occur, for instance, in a number of technologies for requested refinement of materials like electron beam evaporation or levitation melting, in which the electromagnetic shaping technique is applied. The instability phenomenon in our case has following magnetohydrodynamic background. High‐frequency magnetic field causes Lorentz force density generation within so‐called skin‐layer in the liquid metal, which works as a surface pressure in turn. With increase of the electromagnetic pressure a destabilizing mechanism progresses within the liquid metal domain, which causes a bifurcation. In this paper we discuss application alternating magnetic fields to increase the bifurcation threshold by passive and active redistribution of the Lorentz force density in the near‐surface layers of the liquid metal domain. A novel stabilization system on the basis of a local electromagnetic influence on the liquid metal surface in the high frequency magnetic field is presented. The experimental results are discussed. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Fluid‐structure interaction in porous media for loaded filter pleats
Authors: Andrä Heiko , Iliev Oleg , Kabel Matthias , Kirsch Ralf , Lakdawala Zahra , Ricker Sarah
Published: December 2011

Currently, the known simulation efforts with respect to fluid‐structure interaction (FSI) are mainly restricted to the study of flow interacting with deformable solid bodies. On the other hand, there is extensive literature on simulation of flow through porous media. In particular, algorithms and software for CFD simulations of filtration processes in the case of rigid filtration medium were presented earlier by Fraunhofer ITWM, see, e.g., [1, 2]. In these papers the deformation of the solid skeleton is neglected. In many cases of water filtration, filtration for hydraulic applications, and even in certain air filtration regimes, the fluid pressure can be quite high, and the deformation of the pleats can be an issue. The deflection of pleats and its effect on the filtration process merits examination because under operating conditions (and especially after a partial loading of the media) the pleats often cannot be anymore considered as rigid porous media. Therefore, in this paper, the deflection is considered for clean media, as well as for partially loaded media. A new model describing the coupled flow and deformation, and corresponding numerical results are presented. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the Stability Analysis of Decoupled Solution Schemes
Authors: Zinatbakhsh Seyedmohammad , Markert Bernd , Ehlers Wolfgang
Published: December 2011

Many problems in engineering, physics or other disciplines require an integrated treatment of coupled fields. These problems are characterised by a dynamic interaction among two or more physically or computationally distinct components, where the undergoing mathematical model commonly consists of a system of coupled PDE. Considerable progress has been made in the development of appropriate computational schemes to solve such coupled PDE systems. These attempts have resulted in various monolithic and decoupled numerical solution approaches. Despite the unconditional stability offered by implicit monolithic solution strategies, their use is not always recommended. The reason mainly lies in the complexity of the resulting system of equations and the limited flexibility in choosing appropriate time integrators for individual components. This has motivated the elaboration of tailored decoupled solution schemes, which follow the idea of splitting the problem into several sub‐problems. But selection of the way of splitting can have a direct influence on the stability of the resulting solution algorithm. This necessitates the stability analysis of such an algorithm.

Here, we introduce a general framework for the stability analysis of decoupled solution schemes. The approach is then used to study the stability behaviour of established decoupling strategies applied to typical volume‐ and surface‐coupled problems, namely, coupled problems of thermoelasticity, porous media dynamics and structure‐structure interaction. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Material properties of anisotropic materials with pores
Authors: Drach Borys , Tsukrov Igor , Piat Romana , Dietrich Stefan
Published: December 2011

An approach based on cavity compliance contribution tensor is utilized for evaluation of effective stiffness of anisotropic materials with irregularly shaped pores. The shapes considered in the present work were extracted from the X‐ray computed microtomography of a carbon/carbon composite sample. The cavity contribution tensors of individual pores were calculated numerically and then employed in the micromechanical modeling procedure. It was shown that presence of pores with total porosity of p = 9.13% results in approximately 20% reduction in the material's Young's moduli. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Modeling and homogenization of textile reinforced composites based on the isogeometric analysis
Authors: Haasemann Georg , Kerber Thomas , Ulbricht Volker
Published: December 2011

In this contribution, the isogeometric analysis is used to compute the effective material properties of textile reinforced composites. The isogeometric analysis based on non‐uniform rational B‐splines (NURBS) provides an efficient approach for numerical modeling because there is no need for a mesh generation. There are further advantages such as the availability of a geometry representation based on NURBS in computer‐aided design software and the possibility to apply different refinement methods which do not change the geometry of the numerical model. These properties motivate the combination of the isogeometric analysis with the homogenization method. Therefor, the unit cell model representing the inner architecture of a textile reinforced composite is defined using NURBS. In order to compute the effective mechanical properties of the heterogeneous material, the homogenization method with periodic boundary conditions is applied. Finally, two examples demonstrate the advantages of this approach. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Probabilistic homogenization of solid foams
Authors: Hohe Jörg , Beckmann Carla
Published: December 2011

The present study is concerned with a numerical procedure for prediction of uncertainties in the effective properties of solid foams. The approach is based on the multiple homogenization analysis of small‐scale testing volume elements. Their microstructure is defined in terms of random variables with known probability distribution. Using a discretization of the space of the random variables, the probability distribution for the effective properties can easily be determined from the homogenization results and the probability for occurrence of the underlying microstructures of the testing volume elements. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Nonlinear homogenization of metal‐ceramic composites with lamellar microstructure
Authors: Piat Romana , Sinchuk Yuriy
Published: December 2011

Using nonlinear homogenization methods for the computation of the material response of metal‐ceramic composites with lamellar microstructure is a power approach to do computation less costly in comparison to finite elements modeling. A modified secant homogenization method is utilized in this study for simulation of inelastic behaviors of the composite micro‐constituents. A nonlinear homogenization method is based on a linear homogenization scheme for multilayer composites. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Meso‐Macro Modelling of Coated Forming Tools under Thermal Shock Conditions
Authors: Sauerland Kim‐Henning , Mahnken Rolf
Published: December 2011

In hybrid‐forming processes workpieces are heated up before forming in order to reduce the forming forces. They are innovative methods for the production of components with graded properties, particularly with regard to tailored material properties and geometrical shape. During service life the forming tools are subjected to cyclic thermal shock loading conditions which can result into damage and failure. For improvement of the tool durability in the hybrid‐forming process coated forming tools with multilayered coating systems are considered to be applied in future.

This contribution shows the actual state of work for the development of a twoscale FE model for the simulation of the multilayered coated forming tool. Within this model the three‐dimensional model of the forming tool builds the macromodel. On the macrolevel the multilayered coating is discretized with one element over the coating thickness. The mesomodel of the coating considers the actual layer design with metallic and ceramic layers. The macro‐meso transition is realized with a Taylor‐assumption. As the microscale is not considered in our model, the constitutive equations are formulated on the mesoscale. The meso‐macro transition is done using volume averaging procedures. Furthermore, a damage model is included for particular layers. The scalar damage variable is used in a thermo‐mechanical coupled model for simulation of a reduced heat transfer through a partially damaged layer. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Stall‐delay modelling of wind turbine blades
Authors: Dumitrescu Horia , Cardos Vladimir
Published: December 2011

Most aerodynamic design tools for horizontal‐axial wind turbines are based on the blade‐element momentum theory (BEM). Due to the nature of this theory, the design tools need 2‐D steady sectional lift and drag curves as an input. In practice, flow over a wind turbine rotor blade is neither two‐dimensional nor steady, and is affected by rotation. Pioneering experiments have identified a consequence: at inboard rotor blade sections stall is delayed. This so‐called Himmelskamp effect [1] gives a larger lift than predicted and, as a result, a higher power and loading than expected. Consequently, an aerodynamic model is needed to explain and predict sectional lift and drag under rotating conditions. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Experimentelle Untersuchung elektroosmotischer Strömungen in Mikrokanälen mit internen Elektroden
Authors: Gizewski Carsten , Moeller Jan , Ehrhard Peter
Published: December 2011

In dieser Arbeit werden elektroosmotische Strömung in einem Mikrokanal mit internen Elektroden untersucht. Es wird das Konzept den Mikrokanals vorgestellt. Die experimentellen Untersuchungen basieren auf dem μPIV‐Vefahren, mit welchem zwei Komponenten des Geschwindigkeitsfeldes im kompletten Kanal gemessen werden können. Unter Verwendung der Kontinuitätsgleichung wird dann die dritte Geschwindigkeitskomponente, und damit das vollständige Geschwindigkeitsfeld, durch numerische Integration rekonstruiert. Die Messungen können damit, ohne Einschränkung, zur Validierung von theoretischen Ergebnissen dienen. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Transitional intermittency in a rough wall boundary layer
Authors: Jonáš Pavel , Hladík Ondřej , Mazur Oton , Uruba Václav
Published: December 2011

The transitional intermittency factor distributions were evaluated from the wall friction records acquired in the rough wall boundary layer under free stream turbulence. Important characteristics of by‐pass transition were derived from the factor distributions. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical simulation of particles movement in cellular flows under the influence of magnetic forces
Authors: Hriberšek Matjaž , Steinmann Paul , Ravnik Jure , Vogel Franziska
Published: December 2011

A numerical model of particle motion in fluid flow under the influence of hydrodynamic and magnetic forces is presented. As computational tool, a flow solver based on the Boundary Element Method is used. The Euler‐Lagrange formulation of multiphase flow is considered. In the case of a particle with a magnetic moment in a nonuniform external magnetic field, the Kelvin body force acts on a single particle. The derived Lagrangian particle tracking algorithm is used for simulation of dilute suspensions of particles in viscous flows taking into account gravity, buoyancy, drag, pressure gradient, added mass and magnetophoretic force. As a benchmark test case the magnetite particle motion in cellular flow field of water is computed with and without the action of the magnetic force. The effect of the Kelvin force on particle motion and separation from the main flow is studied for a predefined magnetic field and different values of magnetic flux density. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Morphing of Flying Configuration Using Spanwise Movable Leading Edge Flaps, in Supersonic Flow
Authors: Nastase Adriana
Published: December 2011

The computation of the flow over flattened wing‐fuselage flying configuration (FC) with movable leading edge flaps in spanwise direction is here considered. Hyperbolical potential solutions are given in closed form and are used for the determination of hybrid solutions for the Navier‐Stokes layer, over this FC with the flaps in retracted and in open position. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical Investigation of Dense Turbulent Sprays with an Eulerian Approach and DQMOM
Authors: Gumprich Werner , Sadiki Amsini , Sommerfeld Martin
Published: December 2011

The spray characteristics of the injected liquid fuel predominantly influence the combustion and emissions in IC‐engines and gas turbines. This is predetermined by a dense spray region in which droplet‐droplet interaction processes play a significant role. In order to accurately describe and control the dense spray behavior in modern engines, an appropriate numerical modeling tool is needed. This contribution aims at including droplet‐droplet interactions into an Eulerian approach coupled to the Direct Quadrature Method of Moments (DQMOM) in order to describe evaporating droplet polydispersity and analyze dense turbulent sprays phenomena. Among the advantages of the Eulerian approach are a lower computational cost through optimal parallel computing and a straightforward liquid‐gas phase coupling. To assess the designed tool, numerical results are compared to Phase Doppler Anemometry (PDA) measurements of a hollow‐cone water spray. The experiment provides comprehensive validation data that include gas velocities, droplet size distribution, droplet mass fluxes and droplet velocities. Turbulence is captured by two different k‐ε based models. Preliminary results show that the designed tool is able to capture the process under study in a satisfactory way. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Small‐scale mixing at the turbulent/non‐turbulent interface in turbulent jets
Authors: Steiner Helfried , Walchshofer Christian
Published: December 2011

The numerical simulation of the turbulent mixing between the turbulent core flow region and the non‐turbulent irrotational ambient region requires a reliable modelling of the mass and momentum transfer in the intermittent zone. The dominant role of the small‐scale whirling motion in the turbulent entrainment process particularly challenges the subgrid‐scale (SGS) modelling in Large‐Eddy Simulation (LES). The present work carried out a Direct Numerical Simulation (DNS) of a round jet to analyze the scalar mixing conditions near the turbulent/non‐turbulent interface. The obtained statistical results widely confirm existing experimental and analytical findings made in previous studies on the momentum transfer. An a priori test of the DNS data reveals the limits of the standard Smagorinsky‐type SGS‐model for the eddy diffusivity in the description of the subgrid‐scale mixing near the interface. The extension to a dynamic SGS‐model using non‐constant, flow dependent model coefficients does not lead to a substantial improvement. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical Study on the Influence of Particle Inertia in Particle‐Driven Gravity Currents
Authors: Häuselmann Oliver , Henniger Rolf , Kleiser Leonhard
Published: December 2011

We present two‐dimensional numerical simulations of particle‐driven gravity currents in a lock‐exchange configuration. The fluid is described in an Eulerian framework, whereas the particles are tracked in a Lagrangian manner. The study is restricted to dilute suspensions, allowing to neglect particle‐particle interactions. The particle forces considered are buoyancy and the Stokes drag. We study the influence of particle inertia on the flow evolution by performing simulations with different Stokes numbers. We also consider the case where particle inertia is neglected. Generally, we observe significant changes in the form and structure of the gravity current with increasing particle Stokes numbers. Particularly, the formation of Kelvin‐Helmholtz vortices is more and more suppressed. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Analysis of Lamb waves with Spectral Finite Elements
Authors: Bumb Andreas , Lammering Rolf , Hennings Bianca
Published: December 2011

For the analysis of the elastic wave propagation at high frequencies, the spectral finite element method (SFEM) is under investigation. The SFEM uses high‐order shape functions which are composed of Lagrange polynomials with nodes at the Gauss‐Lobatto quadrature points. In this way we obtain a diagonal mass matrix which makes an explicit time scheme more efficient. In a numerical example a computation by Montjoie of an elastic wave propagation within a Reissner‐Mindlin (RM) model is presented. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Increasing the scanning range of Lamb wave based SHM systems by optimizing the sensor design and excitation frequency
Authors: Duczek Sascha , Willberg Christian , Gabbert Ulrich
Published: December 2011

The objective of the paper is to investigate and to optimize parameters which effect the excitation of Lamb waves generated by adhesively bonded piezoceramic actuators. Particularly, the influence of the adhesive layer and the resonances of the actuator are examined both numerically and experimentally. Understanding these properties helps to develop energy efficient structural health monitoring (SHM) systems, meaning that less energy is needed to excite waves with the same scanning range.

Knowing the main influence parameters a numerical optimization is proposed, which is aimed at increasing the range of the excited waves by optimizing the sensor geometry and the excitation frequency. Various tests with different test specimens have been performed. An evolutionary based algorithm is used to find the optimal configuration. It has been found that small changes in the geometry and an optimized excitation frequency elevate the amplitudes of the signal measured with a piezoelectric transducer significantly. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Inhomogeneous Plane Waves in Monoclinic Crystals subject to a Bias
Authors: Simionescu‐Panait Robin Olivian
Published: December 2011

Here we investigate the conditions of inhomogeneous plane waves propagation in monoclinic crystals subject to initial electromechanical fields. We obtain the components of the electroacoustic tensor for the class 2, resp. m, of the monoclinic system. For particular isotropic directional bivectors we derive the decomposition of the propagation condition, and we show that the specific coefficients are similar to the case of guided wave propagation in monoclinic crystals subject to a bias. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Electromagnetic flow rate measurement in turbulent liquid metal channel flow
Authors: Jian Dandan , Karcher Christian
Published: December 2011

Flow control in hot aggressive melts is a big challenge for metallurgical and glass technology applications. For instance, during the production of float glass, the glass melt solidifies on the top of a liquid tin bath. To achieve a spatially homogeneous glass thickness, both global and local control of the thermofluiddynamic transport phenomena in the tin bath are crucial. The concept a global flow control involves the coupling of the non‐contact electromagnetic flow rate measurement technique of Lorentz force velocimetry and the non‐contact flow driving technique of electromagnetic pumps based on rotating permanent magnets. The paper presents a model experiment using the low‐melting test melt GaInSn as well as numerical simulations using the commercial code FLUENT/MHD. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Electromagnetic interaction of moving conductors with magnets: kinematic solutions for infinitely long square and cylindrical geometries
Authors: Kirpo Maksims , Boeck Thomas , Thess André
Published: December 2011

The electromagnetic drag force on a point dipole near a moving conductor caused by the induced electric currents is investigated by numerical and analytical computations. Our focus is on prototypical configurations for Lorentz force velocimetry, i.e. velocity measurement from the electromagnetic drag force on the dipole. We examine the particular cases of conducting infinite bars of square or round cross‐section, which are moving with constant velocity in the field of arbitrary oriented magnetic dipole. In addition, we study the laminar liquid‐metal flow in a square duct. The motion of the conductor is prescribed. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Direct numerical simulation of transition in MHD duct flow
Authors: Krasnov Dmitry , Zikanov Oleg , Thess André , Boeck Thomas
Published: December 2011

Transition in the flow of electrically conducting fluid in a square duct with insulating walls is studied by direct numerical simulations. A uniform magnetic field is applied in the transverse direction. Moderate values of the Reynolds (Re = 5000) and Hartmann (Ha = 0 … 30) numbers are considered that correspond to the classical Hartmann & Lazarus [1] experiments. It is shown that the laminarization begins in the Hartmann layers, whereas the sidewall layers remain turbulent. Complete re‐laminarization occurs in the range of R = Re/Ha ≈︁ 220, which is in agreement with the H. & L. experiments. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the analysis of an electronic model for solar cells including energy resolved defect densities
Authors: Glitzky Annegret
Published: December 2011

In this paper we study analytic properties of electronic models for solar cells, which take into account varying, energy resolved densities of defects. We establish energy estimates and the boundedness of weak solutions and give an existence and uniqueness result for weak solutions to the model equations in two space dimensions. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Derivation of effective interface conditions for reaction‐diffusion equations
Authors: Liero Matthias
Published: December 2011

We address the derivation of effective interface conditions for reaction‐diffusion systems. The considered system is defined in a domain containing a thin layer that shrinks to the interface when its thickness ε tends to zero. The evolution of the system can be written in the form of an energy balance involving an energy and a dissipation functional. Using the Mosco convergence of the dual of the dissipation functional for ε → 0 it is possible to do a limit passage in the energy balance and obtain a limit system that describes the evolution on the interface. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Convergence of the fixed pivot technique for continuous Smoluchowski coagulation equation
Authors: Giri Ankik Kumar , Hausenblas Erika
Published: December 2011

In this work, the fixed pivot technique (FPT) [2] is analyzed for nonlinear continuous Smoluchowski coagulation equation on four different types of grids. More importantly, the FPT gives the accuracy of second order for uniform and geometric grids while it reduces the order of accuracy by one on a locally uniform grid. At the end, the scheme is unfortunately zero order accurate on random grids. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A Deterministic Filter for non‐Gaussian State Estimation
Authors: Pajonk Oliver , Rosić Bojana V. , Litvinenko Alexander , Matthies Hermann G.
Published: December 2011

The usual mathematical method to represent uncertain quantities, for example the state of a dynamical system with uncertain initial conditions, are random variables (RVs). In many problems the space of elementary events Ω, on which the RVs are defined as functions of these events, is not concretely accessible, so that the usual idea of a function (e.g. given as a formula) loses much of its meaning. The representation of RVs is therefore often strikingly different from what is used for “normal” functions. With the help of RVs one can formulate Bayesian estimators for the uncertain quantity when additional information (usually noisy, incomplete measurements) becomes available. A common way to derive such an estimator is to use an instance of the projection theorem for Hilbert spaces.

In this work we present a linear Bayesian estimation method which results from using a recently popular representation of an RV, the polynomial chaos expansion (PCE), also known as “white noise analysis”. The resulting method is completely deterministic, as well as computationally efficient. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optimization of Optimal Power Flow Problems
Authors: Brune Maria , Castaing Laure , Walther Andrea
Published: December 2011

Optimal power flow problems arise in the context of the optimization and secure exploitation of electrical power in alternating current (AC) networks. This optimization problem evaluates the flow on each line and to ensure that they are under line thermal limits. To improve the reliability of the power supply, a secure network is necessary, i.e., it has to be able to cope with some contingencies. Nowadays high performance solution methods, that are based on nonlinear programming algorithms, search for an optimal state while considering certain contingencies. According to the number of contingencies the problem size increases linearly. As the base case can already be large‐scaled, the optimization time computation increases quickly. Parallelization seems to be a good way to solve quickly this kind of problem. This paper considers the minimization of an objective function and at least two constraints at each node. This optimization problem is solved by IPOPT, an interior point method, coupled with ADOL‐C, an algorithmic differentiation tool, and MA27, a linear solver. Several results on employed parallelizing strategies will be discussed. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical Methods for Parameter Estimation Using Krylov‐Type Techniques
Authors: Kriwet Gregor , Kostina Ekaterina
Published: December 2011

In the paper we consider constrained nonlinear parameter estimation problems. The method of choice to solve such problems is the generalized Gauss‐Newton method. At each iteration of the Gauss‐Newton we solve the linearized parameter estimation problem and compute covariance matrix, necessary for the error assessment of the estimates, using an iterative linear algebra technique, namely LSQR algorithm. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Control of hydrothermal waves in a thermocapillary flow using a gradient‐based control strategy
Authors: Muldoon Frank , Kuhlmann Hendrik
Published: December 2011

The problem of controlling the hydrothermal waves in a thermocapillary flow is addressed using a gradient based control strategy. The control is a spatially and temporally varying heat flux boundary condition at the free surface. The control which minimizes the hydrothermal waves is found using a conjugate gradient method, where the gradient of the objective function with respect to the control variables is obtained from solving a set of adjoint equations. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Solution of inverse problems to detect permissible design spaces
Authors: Pannier Stephan , Grossenbacher Kathrin , Liebscher Martin , Graf Wolfgang , Kaliske Michael
Published: December 2011

In many engineering applications, a structural design is determined by way of a multi‐stage design process. Thereby, the information of previous simulations, e.g. sensitivity analysis, should be used for further investigations. In special, the aim is to improve the reliability as early as possible. Therefore, an approach is presented to determine alternative permissible design spaces on the basis of arbitrary point sets. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Two‐sided harmonic subspace extractions for the generalized eigenvalue problem
Authors: Benner Peter , Hochstenbach Michiel , Kürschner Patrick
Published: December 2011

One crucial step of the solution of large‐scale generalized eigenvalue problems with iterative subspace methods, e.g. Arnoldi, Jacobi‐Davidson, is a projection of the original large‐scale problem onto a low dimensional subspaces. Here we investigate two‐sided methods, where approximate eigenvalues together with their right and left eigenvectors of the full‐size problem are extracted from the resulting small eigenproblem. The two‐sided Ritz‐Galerkin projection can be seen as the most basic form of this approach. It usually provides a good convergence towards the extremal eigenvalues of the spectrum. For improving the convergence towards interior eigenvalues, we investigate two approaches based on harmonic subspace extractions for the generalized eigenvalue problem. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The FEAST algorithm for large eigenvalue problems
Authors: Galgon Martin , Krämer Lukas , Lang Bruno
Published: December 2011

We consider the eigensolver named FEAST that was introduced by Polizzi in 2009 [1]. This solver, tailored to the (partial) solution of large sparse (generalized) eigenproblems offers good potential for parallelism and showed good robustness in our experiments. We briefly introduce the algorithm, point out some problems and give two examples. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the Computation of Particular Eigenvectors of Hamiltonian Matrix Pencils
Authors: Benner Peter , Voigt Matthias
Published: December 2011

We discuss a structure‐preserving algorithm for the accurate solution of generalized eigenvalue problems for skew‐Hamiltonian/Hamiltonian matrix pencils λN − ℋ. By embedding the matrix pencil λ𝒩 − ℋ into a skew‐Hamiltonian/Hamiltonian matrix pencil of double size it is possible to avoid the problem of non‐existence of a structured Schur form. For these embedded matrix pencils we can compute a particular condensed form to accurately compute the simple, finite, purely imaginary eigenvalues of λ𝒩 − ℋ. In this paper we describe a new method to compute also the corresponding eigenvectors by using the information contained in the condensed form of the embedded matrix pencils and associated transformation matrices. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Adaptive coupling of FEM and BEM: Simple error estimators and convergence
Authors: Aurada Markus , Feischl Michael , Karkulik Michael , Praetorius Dirk
Published: December 2011

A posteriori error estimators and adaptive mesh‐refinement have themselves proven to be important tools for scientific computing. For error control in finite element methods (FEM), there is a broad variety of a posteriori error estimators available, and convergence as well as optimality of adaptive FEM is well‐studied in the literature. This is, however, in sharp contrast to the boundary element method (BEM) and to the coupling of FEM and BEM. In our contribution, we present an easy‐to‐implement error estimator for some FEM‐BEM coupling which, to the best of our knowledge, has not been proposed in the literature before. The derived mesh‐refining algorithm provides the first adaptive coupling procedure which is mathematically proven to converge. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical investigation of nanoindentation of viscoelastic polymer layers and parameters re‐identification
Authors: Chen Zhaoyu , Diebels Stefan
Published: December 2011

In the present contribution, we focus on the numerical investigation of nanoindentation of viscoelastic polymer layers using the finite element code ABAQUS®. The time‐dependent behaviour of the viscoelastic layer is modeled with Boltzman hereditary integration based on Prony series. The normal contact between the rigid indenter and the deformable layer are treated numerically as a unilateral non‐penetration constraint problem. The computer‐based numerical optimisation routine is treated by employing the Matrix Laboratory MATLAB® combined with ABAQUS®. The stochastic strategy is used based on principles of biological evolution, i. e. an evolution strategy. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Variational integrators for electric circuits
Authors: Ober‐Blöbaum Sina , Tao Molei , Owhadi Houman
Published: December 2011

Variational integrators are symplectic‐momentum preserving integrators that are based on a discrete variational formulation of the underlying system. So far, variational integrators have been mainly developed and used for a wide variety of mechanical systems. In this work, we develop a variational integrator for the simulation of electric circuits. An appropriate variational formulation is presented to model the circuit from which the equations of motion are derived. Finally, a corresponding time‐discrete variational formulation provides an iteration scheme for the simulation of the electric circuit. In this way, a variational integrator is constructed that gains several advantages. A comparison to standard integration techniques shows that even for simple LCR circuits a better long‐time energy behavior and frequency preservation can be obtained. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Sliding Mode Control of a Remotely Operated Underwater Vehicle with Adaptive Fuzzy Dead‐Zone Compensation
Authors: Bessa Wallace M. , Kreuzer Edwin
Published: December 2011

Due to the great technological improvement obtained in the last decades, it became possible to use robotic vehicles for underwater exploration. During the execution of a certain task with a remotely operated underwater vehicle, the operator needs to monitor and control a number of parameters. If some of these parameters, as for instance position and attitude, could be controlled automatically, the teleoperation of the vehicle can be enormously facilitated. Based on experimental tests, it was verified that marine thrusters can exhibit dead‐zone nonlinearities. This work describes the development of a variable structure control strategy for an underwater vehicle with a thrust system subject to dead‐zone input. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
State Reconstruction in Higher Dimensional PDEs with Spatially Varying Parameters
Authors: Jadachowski Lukas , Meurer Thomas , Kugi Andreas
Published: December 2011

The state estimation problem is considered for a diffusion‐reaction system with spatially varying parameters defined on a 3‐dimensional rectangular domain with the measured output being restricted to a single surface. For this, a backstepping‐based observer design is applied, which enables to obtain the observer gains such that the observer error dynamics decays exponentially in the L2‐norm. At first, an idealized system output restricted to a single surface is assumed as an available measurement. Secondly, in view of a practical realization of the proposed observer, the idealized system output is reconstructed from a set of finite‐dimensional measurements. The observer error convergence and the applicability of the proposed approach are evaluated by means of numerical simulations. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)