Wolfgang A. Wall

Bio: Wolfgang A. Wall is an academic researcher from Technische Universität München. The author has contributed to research in topics: Finite element method & Discretization. The author has an hindex of 57, co-authored 419 publications receiving 11355 citations. Previous affiliations of Wolfgang A. Wall include University of Stuttgart & Ludwig Maximilian University of Munich.

Fixed-point fluid-structure interaction solvers with dynamic relaxation

Abstract: A fixed-point fluid–structure interaction (FSI) solver with dynamic relaxation is revisited. New developments and insights gained in recent years motivated us to present an FSI solver with simplicity and robustness in a wide range of applications. Particular emphasis is placed on the calculation of the relaxation parameter by both Aitken’s \({\Delta^{2}}\) method and the method of steepest descent. These methods have shown to be crucial ingredients for efficient FSI simulations.

Isogeometric structural shape optimization

Abstract: An old dilemma in structural shape optimization is the needed tight link between design model or geometric description and analysis model. The intention of this paper is to show that isogeometric analysis offers a potential and promising way out of this dilemma. To this end we show a structural shape optimization framework based on the isogeometric analysis approach. With the discretization based on Non-Uniform Rational B-Splines (NURBS) the analysis model represents the structural geometry exactly. Furthermore, NURBS enable efficient geometry control together with smooth boundaries. They are the de facto standard in CAD systems, but are also widely used in a shape optimal design context to define the geometry representation and the design variables. With the presented isogeometric approach to shape optimization, the analysis model is inherently merged with the design model, omitting the typically involved interplay between both. We derive analytical sensitivities for NURBS discretizations which allow application of efficient gradient-based optimization algorithms. The present contribution is restricted to two-dimensional problems of linear elasticity, but the extension to three dimensions and other problem classes is straightforward. Some representative examples demonstrate and validate the methodology. Further, the potential of boundary continuity control within isogeometric structural shape optimization is explored to trigger smooth or less smooth (angular) designs.

Artificial added mass instabilities in sequential staggered coupling of nonlinear structures and incompressible viscous flows

Abstract: This note investigates the so-called arti cial added mass e ect which is responsible for devastating instabilities within sequentially staggered Fluid-Structure Interaction (FSI) simulations where incompressible fluids are considered. A discrete representation of the added mass operatorMA is given and `instability conditions' are evaluated for different temporal discretisation schemes. It is proven that for every sequentially staggered scheme and given spatial discretisation of a problem, a mass ratio between fluid and structural mass density can be found at which the coupled system becomes unstable. The analysis is quite general and does not depend upon the particular spatial discretisation schemes used. However here special attention is put on stabilised finite elements employed on the fluid partition. Numerical investigations further highlight the results.

Tortuosity Determination of Battery Electrodes and Separators by Impedance Spectroscopy

Abstract: Lithium ion battery performance at high charge/discharge rates is largely determined by the ionic resistivity of an electrode or separator which are filled with electrolyte. Key to understand and to model ohmic losses in porous battery components are porosity as well as tortuosity. In the first part, we use impedance spectroscopy measurements in a new experimental setup to obtain the tortuosities and MacMullin numbers of some commonly separators, demonstrating experimental errors of < 8%. In the second part, we present impedance measurements of electrodes in symmetric cells using a blocking electrode configuration, which is obtained by using a non-intercalating electrolyte. The effective ionic resistivity of the electrode can be fit with a transmission-line model, allowing us to quantify the porosity dependent MacMullin numbers and tortuosities of electrodes with different active materials and different conductive carbon content. Best agreement between the transmission-line model and the impedance data is found when constant-phase elements rather than simple capacitors are used.

An eXtended Finite Element Method/Lagrange multiplier based approach for fluid-structure interaction

Abstract: This paper presents a new fixed grid fluid–structure interaction scheme that can be applied to the interaction of most general structures with incompressible flow. It is based on an eXtended Finite Element Method (XFEM) based strategy. The extended Eulerian fluid field and the Lagrangian structural field are partitioned and iteratively coupled using Lagrange multiplier techniques for non-matching grids. The approach allows the simulation of the interaction of thin and bulky structures exhibiting large deformations. Finally, qualitative examples and a benchmark computation demonstrate key features and accuracy of the method.

Isogeometric analysis : CAD, finite elements, NURBS, exact geometry and mesh refinement

Abstract: The concept of isogeometric analysis is proposed. Basis functions generated from NURBS (Non-Uniform Rational B-Splines) are employed to construct an exact geometric model. For purposes of analysis, the basis is refined and/or its order elevated without changing the geometry or its parameterization. Analogues of finite element h - and p -refinement schemes are presented and a new, more efficient, higher-order concept, k -refinement, is introduced. Refinements are easily implemented and exact geometry is maintained at all levels without the necessity of subsequent communication with a CAD (Computer Aided Design) description. In the context of structural mechanics, it is established that the basis functions are complete with respect to affine transformations, meaning that all rigid body motions and constant strain states are exactly represented. Standard patch tests are likewise satisfied. Numerical examples exhibit optimal rates of convergence for linear elasticity problems and convergence to thin elastic shell solutions. A k -refinement strategy is shown to converge toward monotone solutions for advection–diffusion processes with sharp internal and boundary layers, a very surprising result. It is argued that isogeometric analysis is a viable alternative to standard, polynomial-based, finite element analysis and possesses several advantages.

Topology optimization approaches: A comparative review

Abstract: Topology optimization has undergone a tremendous development since its introduction in the seminal paper by Bendsoe and Kikuchi in 1988. By now, the concept is developing in many different directions, including “density”, “level set”, “topological derivative”, “phase field”, “evolutionary” and several others. The paper gives an overview, comparison and critical review of the different approaches, their strengths, weaknesses, similarities and dissimilarities and suggests guidelines for future research.

Gray's Anatomy

Abstract: MEDICAL LITERATURE has been deluged during the past few years with books and papers on penicillin; but a book which has been produced under the general editorship of Sir Alexander Fleming himself represents a complete and authoritative summary of penicillin therapy as it stands today.' The book contains a series of independent contributions by \"experienced and eminent men who have worked with penicillin in Great Britain\". Their opinions and practical methods differ slightly, and there is some overlapping; but these are not disadvantageous, comparison and contrast lending interest to the reading. In the first or general section of the book Fleming contributes two chapters, one on the history and development of penicillin, introducing some interesting sidelights in the romance of discovery, the other on the bacteriological control of penicillin therapy. In both chapters the information is set out in meticulous detail and with a clarity and simplicity which can be enjoyed by all readers. Fleming also gives the right perspective to the place of penicillin amongst the antibiotics and lays down the principles of treatment. Both chapters are well illustrated and are the most outstanding in the book. Included in this first section also are chapters on the chemistry and manufacture of penicillin and its pharmacy, pharmacology and methods of administration. The second section of the book is entirely clinical, giving each author's view on the use of penicillin therapy in a disease or an infection of some particular region of the body. The entire range of peniCillin-sensitive conditions is considered in twenty authoritative and clearly written chapters; these contain many references and illustrations. Dental and veterinary diseases are also given fairly full consideration. The final section is a condensed resume of much of the preceding chapters and is written for. the general practttioner. This chapter is superttuous: it does not contain enough detall to be of much practical value. The book as a Whole, however, will undoubtedly be considered a classical contribution to medical literature and is strongly recommended, not only because of the general interest of its topic, but as a reference book on penicillin therapy of hitherto unequalled excellence. The typography, although conforming to war economy standards, is clear and the paper is good. There is an excellent list of references and the index is satisfactory.

The extended/generalized finite element method: An overview of the method and its applications

Abstract: An overview of the extended/generalized finite element method (GEFM/XFEM) with emphasis on methodological issues is presented. This method enables the accurate approximation of solutions that involve jumps, kinks, singularities, and other locally non-smooth features within elements. This is achieved by enriching the polynomial approximation space of the classical finite element method. The GEFM/XFEM has shown its potential in a variety of applications that involve non-smooth solutions near interfaces: Among them are the simulation of cracks, shear bands, dislocations, solidification, and multi-field problems. Copyright © 2010 John Wiley & Sons, Ltd.

Review: Meshless methods: A review and computer implementation aspects

Abstract: The aim of this manuscript is to give a practical overview of meshless methods (for solid mechanics) based on global weak forms through a simple and well-structured MATLAB code, to illustrate our discourse. The source code is available for download on our website and should help students and researchers get started with some of the basic meshless methods; it includes intrinsic and extrinsic enrichment, point collocation methods, several boundary condition enforcement schemes and corresponding test cases. Several one and two-dimensional examples in elastostatics are given including weak and strong discontinuities and testing different ways of enforcing essential boundary conditions.