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Wed, 21 Jul 2010 14:00:03 +0200Wed, 21 Jul 2010 14:00:03 +0200On adjoint-based optimization of a free surface Stokes flow
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/2209
This work deals with the optimal control of a free surface Stokes flow which responds to an applied outer pressure. Typical applications are fiber spinning or thin film manufacturing. We present and discuss two adjoint-based optimization approaches that differ in the treatment of the free boundary as either state or control variable. In both cases the free boundary is modeled as the graph of a function. The PDE-constrained optimization problems are numerically solved by the BFGS method, where the gradient of the reduced cost function is expressed in terms of adjoint variables. Numerical results for both strategies are finally compared with respect to accuracy and efficiency.S. Repke; N. Marheineke; R. Pinnaureporthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/2209Wed, 21 Jul 2010 14:00:03 +0200Model reduction of nonlinear problems in structural mechanics
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/2198
This contribution presents a model reduction method for nonlinear problems in structural mechanics. Emanating from a Finite Element model of the structure, a subspace and a lookup table are generated which do not require a linearisation of the equations. The method is applied to a model created with commercial FEM software. In this case, the terms describing geometrical and material nonlinearities are not explicitly known.S. Herkt; K. Dreßler; R. Pinnaureporthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/2198Wed, 21 Jul 2010 13:23:04 +0200Adjoint based optimal control using mesh-less discretizations
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/2013
An easy numerical handling of time-dependent problems with complicated geometries, free moving boundaries and interfaces, or oscillating solutions is of great importance for many applications, e.g., in fluid dynamics (free surface and multiphase flows, fluid-structure interactions [22, 18, 24]), failure mechanics (crack growth and propagation [4]), magnetohydrodynamics (accretion disks, jets and cloud simulation [6]), biophysics and -chemistry. Appropriate discretizations, so-called mesh-less methods, have been developed during the last decades to meet these challenging demands and to relieve the burden of remeshing and successive mesh generation being faced by the conventional mesh-based methods, [16, 10, 3]. The prearranged mesh is an artificial constraint to ensure compatibility of the mesh-based interpolant schemes, that often conflicts with the real physical conditions of the continuum model. Then, remeshing becomes inevitable, which is not only extremely time- and storage consuming but also the source for numerical errors and hence the gradual loss of computational accuracy. Apart from this advantage, mesh-less methods also lead to fundamentally better approximations regarding aspects, such as smoothness, nonlocal interpolation character, flexible connectivity, refinement and enrichment procedures, [16]. The common idea of mesh-less methods is the discretization of the domain of interest by a finite set of independent, randomly distributed particles moving with a characteristic velocity of the problem. Location and distribution of the particles then account for the time-dependent description of the geometry, data and solution. Thereby, the global solution is linearly superposed from the local information carried by the particles. In classical particle methods [20, 21], the respective weight functions are Dirac distributions which yield solutions in a distributional sense.J. Marburger; N. Marheineke; R. Pinnaureporthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/2013Wed, 23 Jul 2008 15:16:09 +0200Regularized Fixed-Point Iterations for Nonlinear Inverse Problems
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1653
In this paper we introduce a derivative-free, iterative method for solving nonlinear ill-posed problems \(Fx=y\), where instead of \(y\) noisy data \(y_\delta\) with \(|| y-y_\delta ||\leq \delta\) are given and \(F:D(F)\subseteq X \rightarrow Y\) is a nonlinear operator between Hilbert spaces \(X\) and \(Y\). This method is defined by splitting the operator \(F\) into a linear part \(A\) and a nonlinear part \(G\), such that \(F=A+G\). Then iterations are organized as \(A u_{k+1}=y_\delta-Gu_k\). In the context of ill-posed problems we consider the situation when \(A\) does not have a bounded inverse, thus each iteration needs to be regularized. Under some conditions on the operators \(A\) and \(G\) we study the behavior of the iteration error. We obtain its stability with respect to the iteration number \(k\) as well as the optimal convergence rate with respect to the noise level \(\delta\), provided that the solution satisfies a generalized source condition. As an example, we consider an inverse problem of initial temperature reconstruction for a nonlinear heat equation, where the nonlinearity appears due to radiation effects. The obtained iteration error in the numerical results has the theoretically expected behavior. The theoretical assumptions are illustrated by a computational experiment.S.S. Pereverzyev; R. Pinnau; N. Siedowpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1653Wed, 10 Aug 2005 17:10:20 +0200The Stationary Current-Voltage Characteristics of the Quantum Drift Diffusion Model
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/650
This paper is concerned with numerical algorithms for the bipolar quantum drift diffusion model. For the thermal equilibrium case a quasi-gradient method minimizing the energy functional is introduced and strong convergence is proven. The computation of current - voltage characteristics is performed by means of an extended emph{Gummel - iteration}. It is shown that the involved fixed point mapping is a contraction for small applied voltages. In this case the model equations are uniquely solvable and convergence of the proposed iteration scheme follows. Numerical simulations of a one dimensional resonant tunneling diode are presented. The computed current - voltage characteristics are in good qualitative agreement with experimental measurements. The appearance of negative differential resistances is verified for the first time in a Quantum Drift Diffusion model.R. Pinnau; A. Unterreiterpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/650Mon, 03 Apr 2000 00:00:00 +0200