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86

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Introduction

Additional affiliations

July 2009 - September 2011

## Publications

Publications (86)

We present a general numerical solution method for control problems with state variables defined by a linear PDE over a finite set of binary or continuous control variables. We show empirically that a naive approach that applies a numerical discretization scheme to the PDEs to derive constraints for a mixed-integer linear program (MILP) leads to sy...

More than ever before, people around the world are frequently exposed to different sections of the electromagnetic spectrum, mainly emitted from wireless modern communication technologies. Especially, the level of knowledge on non-thermal biological EMF effects remains controversial. New technologies allow for a more detailed detection of non-codin...

Purpose
The purpose of this paper is to present the implementation of a balanced domain decomposition approach for the numerical simulation of large electro-quasistatic (EQS) systems in biology. The numerical scheme is analyzed and first applications are discussed.
Design/methodology/approach
The scheme is based on a finite element discretization...

We present a general numerical solution method for control problems with PDE-defined state variables over a finite set of binary or continuous control variables. We show empirically that a naive approach that applies a numerical discretization scheme to the PDEs (and if necessary a linearization scheme) to derive constraints for a mixed-integer lin...

We introduce oFEM, an efficient and flexible finite element environment for Matlab. The proposed code is fast due to vectorization and scalable in terms of computation time. It further offers interfaces to pre and post processing tools, to import meshes and export computed data. A strict object oriented approach makes it flexible, easily extensible...

Radiofrequency electromagnetic fields (RF-EMF) are a basic requirement of modern wireless communication technology. Statutory thresholds of RF-EMF are established to limit relevant additional heat supply in human tissue. Nevertheless, to date, questions concerning nonthermal biological effects have yet to be fully addressed. New versions of microar...

We derive an optimization problem for a mission planning problem of a firefighting department challenged by a wildfire. Here the fire is modeled using partial differential equations (PDEs), and the response from the firefighters is modeled as a dynamic network flow. The firefighters influence the spread of the wildfire, and vice versa, the fire res...

The specific absorption rate of tissue samples in during in vitro or ex vivo exposure test is one of the key parameters if one investigates possible damages to cells or their components by electromagnetic waves. While many other interactions were investigated, no non-thermal damage was ever verified by reproducible and scientifically valid experime...

Sheet metal forming is an inherent part of todays production industry. A major goal is to increase the forming limits of classical deep-drawing processes. One possibility to achieve that is to combine the conventional quasi-static (QS) forming process with electromagnetic high-speed (HS) post-forming. This work focuses on the finite element analysi...

High Altitude Nuclear Electromagnetic Pulses (HEMP) are among most extreme external signals that can interfere with an electric device. To ensure the survival of vital components even in case of a HEMP event, several procedures have been designed to test DUTs in a lab environment. As a consequence, field tests with HEMP simulators are mandatory. A...

The formability of deep drawing can be extended by combining it with a subsequent high-speed forming method such as electromagnetic forming. However, up to now, no sufficient systematic understanding of the underlying principles or of a succesful design of such coupled processes has been gained. Hence, in this work, a methodology for the analysis a...

In this work, a framework for virtual process design for coupled processes including
electromagnetic impulse forming is presented. Virtual process design is here understood as
the computer based identification of suitable geometry and process parameters to reach a
predefined forming result via physically feasible process paths. Implementation of th...

The asymptotic approximation of continuous minimal s-Riesz energy by the discrete minimal energy of systems of n points on regular sets in R3 is studied. For this purpose an optimization framework for the numerical solution of the corresponding Gauß variational problem based on an interior point method is developed. Moreover, numerical results for...

Sheet metal forming processes play a key role in a vast number of manufacturing cycles. The pursuit of novel approaches and enhancements of process chains is limited by the tremendous cost of experimental investigations. Hence, virtual process design is a suitable tool to overcome this limitation and to investigate new aspects of the processes via...

A mathematical framework for the efficient numerical treatment of a class of optimization problems is presented that arise in certain technological applications, such as, e.g., process parameter identification in forming technology. In the context of an interior point method, the governing physical relations are provided via constraints rather than...

A major issue related to the use of fuel cells to convert electrical energy in chemical energy in modern power supply concepts are their bad dynamical properties. To overcome these problems, it seems promising to introduce a suitable mechanism to control the ionic flow inside the fuel cell. The purpose of this work is to estimate the potential of c...

The electromagnetic fields in an electromagnetic reverberation chamber (ERC) with a rotating mode stirrer are numerically determined for each stirrer position via a finite element (FE) simulation. With these results both a correlation analysis according to the present engineering standard is performed and the spatial variation of the field averaged...

The quality of a real open area test site is affected by many factors, e.g., constructive elements carrying antennas, their power supply, the electric properties of the ground, or surrounding scattering sources. To quantitatively determine the influence of all factors that make a real test site differ from an ideal one, numerical simulation is empl...

A method for the virtual process design of combined quasi-static and electromagnetic forming processes based on a thorough process simulation is developed. Its flexibility is demonstrated by means of an identification problem for process parameters yielding a minimum bottom edge radius in round cup forming. Particularly, an optimum double exponenti...

A major issue related to the use of fuel cells to convert electrical energy in chemical energy in modern power supply concepts are their bad dynamical properties. To overcome these problems, it seems promising to introduce a suitable mechanism to control the ionic flow inside the fuel cell. The purpose of this work is to estimate the potential of c...

The SAND formulation known from structural and topology optimization is applied to a forming problem arising from certain technological forming processes. The SAND approach is compared to a well known and widely used method on a model problem within the scope of linear elasticity. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)

Risk analysis of complex electronic systems against radiated interferences is an evident part of the design phase, especially in modern military environments, and is vital for later operation. Since affordable high power electromagnetic (HPEM) sources become more and more available, risk analysis against such threats has to keep pace with these cha...

S i n c e high power electromagnetic (HPEM) threats to electric devices had been emerging, various attempts were made to classify the threat parameters and to settle on a standard for testing devices. Today, additional sources of high power interference signals are available, with a wide range of bandwidths and frequency coverage. This paper will p...

The asymptotic approximation of continuous minimal s-Riesz energy (0 ≤ s ≤ d − 2) by the discrete minimal energy of systems of n-points on compact Ahlfors-David d-regular sets in ℝd, d ≥ 2, is analyzed. In addition, numerical examples are presented, computed via an interior point method for constrained optimization.

Mathematical optimisation of real technological forming processes requires expensive simulations to evaluate objective function and constraint values as well as their derivative and Hessian information. The costs of these evaluations are directly correlated to the refinement depth of the finite-element discretisation. This work investigates the eff...

Mathematical optimisation of technological forming processes requires expensive simulations to evaluate objective function and constraint values as well as their derivative and Hessian informations. Therefore numerical linearisation methods like the finite difference method or the BFGS-update are necessary. In this work the effects of these approxi...

Investigations of the reaction of biologic tissue to external electromagnetic fields is becoming more and more important in modern day signal environment with its eveqirowing number of electromagnetic emissions, both intended (i.e. cellphones, WLAN-routers, microwave transmission lines) and unintended (i.e. power leakage from apertures in computer...

In this paper, a method is presented for the virtual process design of combinations of deep
drawing and electromagnetic forming. With suitably chosen parameters, such process
combinations of a quasi-static and an impulse forming process extend forming limits of
classical, purely quasi-static forming. To determine parameters leading to the desired
f...

Subject of this work is the incorporation of forming limits in the numerical optimization of technological forming processes for sheet metal. Forming processes with non-linear load paths and strongly varying strain-rate, such as, e.g., combinations of deep drawing and electromagnetic forming are of particular interest. While in the latter impulse f...

Numerical optimization methods are utilized to determine process parameters for process chains which combine high speed processes such as electromagnetic forming with suitable quasi-static processes, e.g., deep drawing. Such process combinations allow for an extension of classical forming limits (FL). Both processes involved are simulated using the...

The 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...

In this article boundary value problems for partial differential equations of mixed elliptic–parabolic type are considered.
To ensure that the considered problems possess a unique solution, the usual variational existence proof for parabolic problems
is extended to the mixed situation. Further, the convergence of approximations computed by a time-s...

In this work auto-adaptive finite-element techniques are presented that allow for quantitatively reliable numerical computation of phase field models. These techniques are based on goal oriented error estimation for finite elements with dual weighted residuals and algorithms for auto-adaptive mesh adaptation in space and in time. It is discussed fo...

Despite the comprehensive understanding of the modeling and numerical simulation of electromagnetic metal forming that has recently been gained, the simulation of real forming situations is still a challenging task due to the large computational resources required. A bottleneck is the computation of the electromagnetic fields, since 100.000 up to s...

Electromagnetic metal forming is a contact-free high-speed forming process in which strain rates of more than 103s−1 are achieved. The deformation of the workpiece is driven by the Lorentz force, a material body force, that results from the interaction of a pulsed magnetic field with eddy currents induced in the workpiece by the magnetic field itse...

The purpose of this work is the formulation and application of a continuum thermodynamic approach to the phenomenological modeling of a class of engineering materials which can be formed using strong magnetic fields. This is carried out in the framework of a thermodynamic, internal-variable-based formulation in which the deformation, temperature an...

The purpose of this work is the formulation and application of a continuum field approach to the phenomenological modeling of the behavior of technological alloys undergoing phase transitions and attendant inelastic deformation. To describe the phase transition, a phase–field approach is utilized. For the fully coupled system, an algorithmic formul...

The purpose of this work is the formulation of a thermo-magneto-mechanical multifield model and presentation of the numerical
strategies applied. In particular, this model is used to simulate electromagnetic sheet metal forming processes (EMF). In
this process, deformation of the workpiece is driven by the interaction of a current generated in the...

The purpose of this work is the development and application of strategies to identify material model parameters for metals at high strain-rates using data obtained from high-speed electromagnetic metal forming. To this end, a staggered algorithm for the finite-element-based numerical solution of the coupled electromagnetic-mechanical boundary-value...

The purpose of this work is to develop time-space adaptive techniques for phase field models for liquid-solid phase transition.
These models are based on coupled systems of the Cahn-Hilliard and the Allen-Cahn equation. The adaptive techniques under
consideration allow for mesh adaptation with respect to a functional of interest. They are based on...

A recent interest in potential industrial applications of electromagnetic forming processes has inspired a demand for adequate simulation tools. Aiming at the virtual design of industrial applications, the purpose of this work is to develop algorithmic formulations particularly suitable to reduce the enormous computational cost inherent to 3D simul...

We consider mixed boundary value problems for partial differential equations which are parabolic in one part of the underlying domain and elliptic in another part. Such problems arise in eddy current simulations in electromagnetics. A fully discrete scheme for the numerical solution of such problems is presented. Particularly, adaptivity in space a...

Periodic components of the Fatou set of a rational map are either simply, doubly or infinitely connected. However, preimages of these may possibly possess a higher finite connectivity. The purpose of this article is to study certain classes of generalized Blaschke products that possess Fatou components of a given arbitrary finite connectivity numbe...

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 9 (2 0 0 8) 341–362 j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j m a t p r o t e c Finite-element method 3D modeling of electromagnetic forming a b s t r a c t Although electromagnetic forming is a technology known for a few decades,...

Electromagnetic metal forming is a contact-free high-speed forming process in which strain rates of more than 10 3 s −1 are achieved. The deformation of the workpiece is driven by a material body force, the Lorentz force, that results from the interaction of a pulsed magnetic field with eddy currents induced in the workpiece by the magnetic field i...

Electromagnetic forming is a contact-free high-speed forming process. The deformation of the work piece is driven by the Lorentz force which results from the interaction of a pulsed magnetic field with eddy currents induced in the work piece by the field itself. The purpose of this work is to present a fully-coupled three-dimensional simulation of...

The purpose of this work is to examine the regularity of solutions to mixed elliptic-parabolic boundary value problems. Based on this analysis, estimates on the convergence rate of standard Galerkin discretizations for such problems are given. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electromagnetic sheet metal forming (EMF) is a high-speed forming process driven by the dynamics of a coupled electromagneticmechanical system. The purpose of the current work is the investigation of a possible reduction in the yield stress due to electric currents. While thermoelastic and viscoplastic effects are well-understood, the possible infl...

The purpose of this work is the formulation of a thermo-magneto-mechanical multifield model and presentation of the numerical strategies applied. In particular, this model is used to simulate electromagnetic sheet metal forming processes (EMF). In this process, deformation of the workpiece is driven by the interaction of a current generated in the...

The modeling of numerous industrial processes leads to multifield problems, which are governed by the coupled interaction
of several physical fields. As an example, consider electromagnetic forming, where the evolution of the deformation field
of a mechanical structure consisting of well conducting material is coupled with an electromagnetic field,...

Electromagnetic metal forming is a contact-free high-speed forming process in which strain rates of more than 103 s^(-1) are achieved. The deformation of the workpiece is driven by a material body force, the Lorentz force, that results from the interaction of a pulsed magnetic field with eddy currents induced in the workpiece by the magnetic field...

Electromagnetic sheet metal forming (EMF) is an example of a high-speed forming process driven by the dynamics of a coupled electromagnetic-mechanical system. Basic physical processes involved in EMF, such as e.g. inelastic and hardening behavior or inertia, have been considered in previous works [1, 2]. The purpose of the current work is the inves...

Let Γ be an analytic Jordan curve in the unit disk
We regard the hyperbolic minimal energy problem
where
(Γ) denotes the set of all probability measures on Γ. There exist several extremal point discretizations of μ*, among others introduced by M. Tsuji (Tsuji points) or by K. Menke (hyperbolic Menke points). In the present article, it is proven tha...

In recent years, inverse analysis has become a common approach to typical engineering problems such as model identification. In this contribution, the inverse problem is discussed in light of taking experimental uncertainties into account. This involves in particular the propagation of experimental errors and the analysis of the sensitivity of the...

In this note, a method is outlined to obtain a priori error estimates for the finite element discretization of coupled electromagnetic mechanical problems as arise, e.g., in electromagnetic metal forming. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Kurzfassung
Rechnerunterstützte Berechnungen von Umformprozessen finden im industriellen Betrieb immer häufiger Anwendung. Dabei spielt die Beschreibung des Materialverhaltens und die genaue Bestimmung von Materialparametern eine wichtige Rolle. In diesem Artikel werden Methoden zur Einschätzung der Bestimmbarkeit von Materialparametern und deren G...

Computer aided simulations are commonly applied in nowadays analysis of forming operations. Here, modelling of the material behavior and determination of its material parameters is crucial to the accuracy of the process simulation. In this article methods to assess the determability of material parameters and measurement errors propagated to the id...

Let be an analytic Jordan curve in the complex plane.
We formulate a discrete minimal energy problem in a suitable class of functions
whose solution provides a geometrically fast converging approximation to the
equilibrium measure of . For this purpose an extremal point system that was introduced by K. Menke in 1972
is applied. In particular, an...

The increasing need for lightweight building components has led to the development of new methods to manufacture such components.
A promising concept is the systematic application of high-speed metal forming methods. Electromagnetic forming is one such
method. Here, the deformation of the workpiece is driven by the Lorentz force which results from...

The Robin capacity δ(A) of a compact, non-empty set A ⊂∂ω with respect to a domain ω ⊂Ĉ containing ∞ is defined by
$$\delta(A) = \delta(A,\Omega) = exp \bigg(\matrix{lim \cr z\rightarrow\infty} - R(z)+ {\rm}log|z|\bigg)$$
where (R(z) = R(z), ∞) is the fundamental solution of a mixed boundary value problem with pole at ∞, where Dirichlet conditions...