bluuebunny/crossref_metadata_2025_split huggingface dataset:
Higher‐order surface FEM for incompr. Navier‐Stokes flows on manifolds
Authors: Fries Thomas‐Peter 
Published: December 2018

AbstractStationary and instationary Stokes and Navier‐Stokes flows are considered on curved surfaces in three dimensions. A stabilized, mixed, higher‐order surface FEM with individual element orders is used for the approximation of the geometry, velocities, pressure, and Lagrange multiplier to enforce tangential velocities. Benchmark test cases are proposed and higher‐order convergence rates are confirmed.
Damage and failure mechanisms of ductile metals in biaxial experiments with non‐proportional loading paths
Authors: Zistl Moritz , Gerke Steffen , Brünig Michael 
Published: December 2018

AbstractThe paper deals with an experimental series of the biaxial X0‐specimen to study the loading path dependence of ductile damage and fracture under non‐proportional loading. The damage and fracture behavior of ductile metals depend on the stress state, consequently the material deterioration has to be studied experimentally to develop and validate the corresponding material model. Experimentally the specimen surfaces have been monitored by digital image correlation (DIC) while the fracture surfaces have been analyzed by scanning electron microscopy (SEM). The experimental results based on proportional and non‐proportional loading paths clearly show that damage and fracture processes are affected by the loading history.
Optimal parameters for a family of flux‐correction methods strongly reducing overshoot in unsteady flow computation
Authors: Niessner Herbert 
Published: December 2018

AbstractFlux‐correction allows to reduce or even remove overshoot occuring when Lax‐Wendroff methods are used to calculate one‐dimensional flow. There is a whole family of methods [3] depending on two parameters, whose optimal values are derived. One controls the amount of diffusion to reduce overshoot, the other determines removal of this diffusion as far as overshoot does not reappear, thus largely retaining accuracy of the employed Lax‐Wendroff method.
Stability of a second‐order method for phase change in porous media flow
Authors: Alvarez Mario , Gomez‐Vargas Bryan , Ruiz Baier Ricardo , Woodfield James 
Published: December 2018

AbstractWe analyse the stability of a second‐order finite element scheme for the primal formulation of a Brinkman‐Boussinesq model where the solidification process influences the drag and the viscosity. The problem is written in terms of velocity, temperature, and pressure, and we produce numerical approximations to the flow observed in heated cavities and near ice sheets.
Using magnetic fluids to model convection of planetary or stellar interiors in laboratory scale
Authors: Szabo Peter S. B. , Früh Wolf‐Gerrit 
Published: December 2018

AbstractBaroclininc flow induced via thermomagnetic convection is investigated as a potential model of convection of planetary interiors at a laboratory scale. To reproduce a central force field, a permanent magnet is used that acts on a magnetic fluid contained in a spherical shell. In addition to maintaining the inner sphere and outer shell at different temperatures, the system is rotating around its central axis. Results suggest that magnetic fluids reproduce the essential qualitative character of the baroclinic flows and may be a useful tool to investigate convective flow in planetary interiors. To quantify the effect of magnetic forcing, a parameter ΘM was developed which represents the equivalent to the thermal Rossby number.
Coupled Simulations Involving Light‐weight Structures within Turbulent Flows: A Complementary Experimental and Numerical Application
Authors: De Nayer Guillaume , Wood Jens Nikolas , Breuer Michael , Apostolatos Andreas , Wüchner Roland 
Published: December 2018

AbstractDue to their easy‐to‐shape and light‐weight characteristics, membranous structures are common in modern civil engineering. As all outside buildings they are exposed to wind loads and consequently undergo fluid‐structure interactions (FSI). In order to dimension these membranous constructions and to avoid the damage of the material by FSI, numerical and experimental investigations are necessary. The current paper presents the simulation part of complementary experimental and numerical investigations relying on an advanced numerical simulation methodology.
Robust Blind Image Fusion for Misaligned Hyperspectral Imaging Data
Authors: Bungert Leon , Ehrhardt Matthias J. , Reisenhofer Rafael 
Published: December 2018

AbstractThe low spatial resolution of hyperspectral imaging can be significantly improved by fusing the hyperspectral image with a high resolution photograph. In most practical cases, however, the exact alignment between the fused images is not known a priori. In this work, we study how including a blind deconvolution approach in the mathematical model can help resolve translational misalignments. In particular, we investigate the influence of different initialization strategies. The efficiency of the proposed model is validated by numerical experiments using both simulated and real remote sensing data.
Noncooperative Model Predictive Control for Affine‐Quadratic Games
Authors: Stieler Marleen , Baumann Michael H. , Grüne Lars 
Published: December 2018

AbstractNash strategies are a natural solution concept in noncooperative game theory because of their ‘stable’ nature: If the other players stick to the Nash strategy it is never beneficial for one player to unilaterally change his or her strategy. In this sense, Nash strategies are the only reliable strategies.The idea to perform and analyze Model Predictive Control (MPC) based on Nash strategies instead of optimal control sequences is appealing because it allows for a systematic handling of noncooperative games, which are played in a receding horizon manner. In this paper we extend existence and uniqueness results on Nash equilibria for affine‐quadratic games. For this class of games we moreover state sufficient conditions that guarantee trajectory convergence of the MPC closed loop.
An analytical approach to the accurate modeling of visco‐elastic materials with stochastic microstructure
Authors: Junker Philipp , Nagel Jan 
Published: December 2018

AbstractThis paper presents a mathematically accurate and numerically very fast procedure to compute both expectation and variance (and standard deviation) of the stresses in a visco‐elastic material with stochastically fluctuating elastic properties. The stochastic material properties turn the evolution equation for the viscous strain components into a stochastic ordinary differential equation (ODE). We solve this ODE analytically for arbitrary loading cases which allows for a separation of deterministic and stochastic contributions. As a consequence, we are able to present precise formulas for the expectation and variance of the stresses. Material point simulations for various loading cases and varying viscosities are compared to Monte‐Carlo results which prove the reliability of our approach.
Efficient analysis of stress concentrations in laminated composite structures with geometric discontinuities
Authors: Schnabel Jan Eike , Mittelstedt Christian 
Published: December 2018

AbstractA semi‐analytical analysis method for displacements, strains and stresses in laminated composite plates with local reinforcement by patches/doublers is presented. The analysis approach is based on a “global” solution provided by the Classical Laminate Plate Theory (CLPT). Further, a “local” model is introduced which discretizes each laminate ply into several mathematical layers. For each of these mathematical layers an additional displacement field is postulated. These additional displacement fields capture the localized phenomena close to the free edge of the patch and in the plate‐doubler junction but decay rapidly with increasing distance from these locations, i.e. the CLPT solution is recovered. The model shows excellent agreement with FE analyses for different plate‐doubler lay‐ups with the benefit of highly reduced computational costs.
An anisotropic continuum damage model for concrete based on biaxial experiments
Authors: Michalski Alexander , Brünig Michael 
Published: December 2018

AbstractThe paper deals with an anisotropic continuum damage model to describe the brittle damage and fracture behavior of plain concrete. Due to the stress state dependence of damage and fracture, multiaxial experiments have to be analyzed to make sure the model is able to simulate the observed physical phenomena. Therefore, simulations with a two‐dimensional version of the proposed model have been carried out based on the experimental data from existing literature. These simulations show a dependence of the model parameters on the stress state.
Validation of Bredt's formulas by the method of asymptotic splitting
Authors: Schmidrathner Christian 
Published: December 2018

AbstractThe equations describing torsion of prismatic bars with thin‐walled closed cross sections, known as Bredt's formulas, are verified using the method of asymptotic splitting. In particular, the strong formulation of the Saint Venant problem of a straight beam is expanded asymptotically. At first, well‐known technical assumptions of the shear stress distribution are validated. Further, the influence of a transverse force acting on the beam is considered. This shear force causes a deformation of the cross section and therefore an adaption of Bredt's formulas. Two distinct formulations of the shear center, called the kinematic and the energetic shear center, are obtained. The latter are verified in numerical experiments.
Enhanced Assumed Strain Methods for Implicit Gradient‐Enhanced Damage‐Plasticity
Authors: Neuner Matthias , Schreter Magdalena , Hofstetter Günter 
Published: December 2018

AbstractThe combination of plasticity theory and continuum damage mechanics facilitates the description of hardening and softening material behavior, accumulation of inelastic strains, and stiffness degradation due to damage. For regularizing softening material behavior in finite element simulations, the enrichment of the constitutive relations by means of a nonlocal counterpart of an internal strain‐like thermodynamic state variable, implicitly defined as the solution of a higher order PDE, is a suitable approach in order to obtain mesh‐insensitive results [1]. However, for plasticity models assuming dilatant plastic flow, such as commonly employed for cohesive‐frictional materials like concrete or rock, spurious locking phenomena due to kinematic constraints can arise in conjunction with low order finite elements. This issue was observed and discussed in [2] for simple (local) linear‐elastic perfectly plastic material models. As a remedy, specific enhanced assumed strain (EAS) methods to enrich the normal strain rate field were proposed and assessed. The present contribution demonstrates the formulation of these EAS methods combined with the fully coupled displacement‐nonlocal damage framework, and shows an application to mode II failure employing a gradient‐enhanced damage‐plasticity model for concrete. The insensitivity of the presented approach with respect to the finite element mesh is discussed.
On the rate‐depentent regularization for damage modeling
Authors: Langenfeld Kai , Junker Philipp , Mosler Jörn 
Published: December 2018

AbstractA model framework for the analysis of isotropic quasi‐brittle damage, was recently presented in [1]. Within this paper, the model in [1] is significantly improved. To be more precise, and in contrast to [1], the novel model: (i) eliminates unnecessary model parameters, (ii) can be better interpreted from a physics point of view, (iii) can capture a fully softened state (zero stresses) and (iv) is characterized by a very simple evolution equation which (v) can be integrated fully implicitly and (vi) the resulting time discrete evolution equation can be solved analytically providing a numerically efficient closed form solution, cf. [2].
Funnel control for nonlinear systems with higher relative degree
Authors: Berger Thomas , Lê Huy Hoàng , Reis Timo 
Published: December 2018

AbstractWe consider tracking control for nonlinear multi‐input, multi‐output systems which have arbitrary strict relative degree and input‐to‐state stable internal dynamics. For a given reference signal, our aim is to design a controller which achieves that the tracking error evolves within a prespecified performance funnel around the reference signal. To this end, we introduce a new controller which involves the first r−1 derivatives of the tracking error, where r is the strict relative degree of the system.
Mixed Eulerian–Lagrangian description in the statics of a flexible belt with tension and bending hanging on two pulleys
Authors: Scheidl Jakob , Vetyukov Yury , Oborin Evgenii , Krommer Michael , Schmidrathner Christian 
Published: December 2018

AbstractStudying the mechanics of thin, axially moving strings, beams or plates (e.g.: belt drives, cable cars, …) at mixed Eulerian–Lagrangian description, which features the transformation of material coordinates to spatial ones, is more appropriate than the classical material (Lagrangian) one, see [1, 2]. Aiming at testing a newly proposed non‐material finite element formulation we study the statics of a looped belt as a rod hanging in contact with two pulleys. Numerical experiments demonstrate rapid mesh convergence as well as correspondence to semi‐analytical results and conventional Lagrangian finite element solutions.
Discontinuous approximation of flow in porous media with adsorption
Authors: Bürger Raimund , Kenettinkara Sudarshan Kumar , Ruiz‐Baier Ricardo , Torres Héctor 
Published: December 2018

AbstractPolymer flooding in petroleum reservoir engineering can be described by a Brinkman‐based model of flow in porous media coupled to a non‐strictly hyperbolic system of conservation laws for multiple components that form the aqueous phase. The coupled flow‐transport problem is discretized by coupling an H(div)‐conforming discontinuous Galerkin (DG) method for the Brinkman flow problem with a classical DG method for the transport equations.
Periodic solutions of measure differential inclusions
Authors: Paschkowski Manuela , Arnold Martin 
Published: December 2018

AbstractWhen modelling mechanical systems with contact and impact forces the most important characteristic is the occurrence of velocity jumps during the impacts. Even for simple applications like the bouncing ball [3] or the impact oscillator [1] this nonsmooth effect may arise infinitely often during a finite time. A suitable setting, whose solution space includes such strongly nonsmooth functions, are the measure differential inclusions (MDI) [7]. The mentioned framework is used to prove existence of periodic solutions for the impact oscillator independent of initial values or system parameters.
Numerical and Experimental Analyses of the Effect of Negative Stress Triaxialities on Ductile Damage and Failure
Authors: Schmidt Marco , Gerke Steffen , Brünig Michael 
Published: December 2018

AbstractThe paper deals with an experimental and corresponding numerical series of the biaxial Z‐specimen, which is made of an aluminum alloy, to investigate the effect of negative stress triaxialities on ductile damage and failure, which are stress‐state‐dependent. Therefore, the continuum model contains a stress‐state‐dependent damage condition and a stress‐state‐dependent damage rule to describe the damage and failure mechanisms. In the experimental evaluation, in addition to force‐displacement curves, optical investigations such as digital image correlation (DIC) and scanning electron microscopy (SEM) were used. The results showed that with compression‐shear load fracture occurs at greater negative triaxialities than previously known. Therefore a new function for the cut‐off value of stress triaxiality is proposed.
Numerical models for the prediction of the behavior of a flexible slender shaped membrane in interaction with free surface flow under the influence of a moving flotsam
Authors: Makaremi Masouleh M. , Wozniak G. 
Published: December 2018

AbstractThe motivation for this work is the great need of applying light structures for the protection of coastal areas from detrimental effects of rapid water runoff. In case of windy or stormy weather conditions, light structures are extremely vulnerable to be attacked by floating debris and even tree trunks. To meet the safety criteria, speculation of this impulsive force on light structures gains in importance on account of the massive deformation of membrane. This matter provides the incentive to investigate an impact phenomenon numerically. The analysis was first performed within a standalone Abaqus standard environment, where the effect of fluid domain on the flexible structure was executed on structure by user subroutine of DLOAD, which was updated in each iteration based on new deformed shape. This model was extended by a partitioned strategy which makes it possible for fluid and structure domains to be interconnected powerfully. The impact of free surface water flow on the membrane was executed using the finite volume solver of Star CCM+, which behaves in response to displacements of the structures. On account of added mass effects, the implicit coupling algorithm between fluid and solid regions was implemented.
Examining errors and correction techniques for SPH
Authors: Fürstenau Jan‐Philipp , Weißenfels Christian , Wriggers Peter 
Published: December 2018

AbstractDespite its popularity, it is well known that the SPH method in its original form is not even zeroth order consistent [5]. This inconsistency results from the SPH‐ansatz function, the so‐called kernel, only depending on the chosen smoothing length, ignoring the current particle distribution. For the examination the heat conduction equation was chosen. Heat conduction test cases have the advantage, that their particle distribution can remain constant during the whole simulation. In this work the resulting errors are highlighted and the influence of common correction techniques of zeroth order are compared.
Determining angular velocities of fast rotating objects based on motion blur to control an optomechanical derotator
Authors: Altmann Bettina , Pape Christian , Reithmeier Eduard 
Published: December 2018

AbstractInvestigations of machine components are important for a faultless functioning of the machine. Ideally, the components are investigated during operation. Thus, a realistic performance of the machine can be analyzed. However, measurements of fast rotating machine components can be quite challenging especially under bad illumination conditions leading to the necessity of high exposure times. In case high‐speed cameras reach their limits due to motion blur resulting of high exposure times or high velocities, an optomechanical image derotator can still lead to sufficient results. This is due to its operating principle consisting of a rotating reflector assembly inspired by a Abbe‐Koenig‐prism which optically eliminates the rotational movement. To generate a stationary image, the optomechanical image derotator containing the reflector assembly has to rotate exactly half as fast as the rotating object. Therefore, the angular velocity of the object has to be determined. A common way for this is the use of rotary encoders although not every machine can be equipped with such sensors. A flexible solution for this is to identify the angular position and thereby the velocity with a high speed camera capturing images of the rotating object. Using image processing algorithms the marker can be detected and thus the velocity can be calculated from the image data. However, due to high rotational velocities and long exposure times motion blur of the marker can occur. Generally, motion blur can be treated as an artifact since it impedes the image analysis. However, motion blur can be used to determine the velocity of the rotating objects in knowing the shutter speed as well as the length of the streaking of a blurred object. This work presents an approach which determines the rotational velocity from image blur for different velocities. The resulting information will be used as the input source for a velocity control of the derotator. Furthermore it will be discussed whether the velocity information is precise enough to control the derotator leading to a resting image of the rotating object.
Sequential solution of parameter identification and optimal control problems for robotic systems
Authors: Runge Margareta , Flaßkamp Kathrin , Büskens Christof 
Published: December 2018

AbstractNowadays Parameter Identification Problems (PIP) are one of the main tasks in many engineering applications. For example, a model for the dynamic behavior of a robotic system has to be found. Before we can work with a system, e.g. solve an optimal control problem (OCP) to optimally perform an assigned task, we have to identify unknown parameters within the system model. In general, system identification is a one‐time job before system operation, but often there remain inaccuracies or the system changes over time, e.g. due to wear. Thus, the model has to be adapted during the process. This paper presents an algorithm where PIPs and OCPs are sequentially solved in order to obtain online parameter updates. First numerical results for an idealized two‐link robot are presented.
An Overview of Isogeometric Boundary Element Methods for Acoustic and Electromagnetic Scattering Problems
Authors: Dölz Jürgen , Kurz Stefan , Schöps Sebastian , Wolf Felix 
Published: December 2018

AbstractWe introduce the concept of isogeometric boundary element methods, by first reviewing the required trace spaces and then constructing a conforming spline discretisation. We highlight how to use this approach to solve Dirichlet problems induced by the Helmholtz‐ and electric wave equation, and conclude by presenting numerical examples.
A Numerical Study of the Robustness of Transcription Methods for Parameter Identification Problems
Authors: Schäfer Kai , Flaßkamp Kathrin , Büskens Christof 
Published: December 2018

AbstractIn this work, we apply transcription methods known from optimal control to nonlinear dynamic parameter identification problems. We analyze and compare the methods with the help of an application example from robotics. More specifically, we investigate their robustness against varying initial parameters and show numerically, that an increasing number of variables leads to a more robust optimization behavior and qualitatively better solutions.