bluuebunny/crossref_metadata_2025_split huggingface dataset:
Stabilization of a Time‐Dependent Discrete Adjoint Solver for Chaotic Incompressible Flows
Authors: Warnakulasuriya Suneth , Andre Michael , Wüchner Roland , Bletzinger Kai‐Uwe 
Published: December 2018

AbstractFollowing the growth of computational fluid dynamics in engineering applications, adjoint methods for sensitivity analysis are being applied to an increasing range of industrial problems. In recent years, growth in computational power has led to widespread use of high‐fidelity, time‐resolved techniques, such as large‐eddy simulation, for applications, including bluff‐body aerodynamics, where time‐averaged equations may be less reliable. The solution of time‐dependent adjoint equations, however, becomes unstable in time as the Reynolds number increases and the flow becomes chaotic. The source of this instability has been explained by analysis of the system's Lyapunov exponents and several solutions have been proposed in the literature based on least‐squares shadowing. Currently, such techniques impose a higher computational cost compared to classical adjoint methods and may become infeasible when the number of unstable Lyapunov exponents is large. In this work, we explore a more recently proposed approach, which balances the adjoint energy source term with added artificial dissipation. The stability of this approach is investigated for a discrete adjoint solver based on the time‐dependent incompressible Navier‐Stokes equations. The accuracy of the resulting shape sensitivities is analyzed using a linear regression technique for the flow around a circular cylinder.
Finite element investigations on the cyclic behavior of shape memory alloys
Authors: Waimann Johanna , Hackl Klaus , Junker Philipp 
Published: December 2018

AbstractShape memory alloys show the effect of functional fatigue under cyclically loading. This fatigue comes along with a decrease of the stress plateaus in the characteristic hysteresis curve which is also accompanied by an accumulated permanent strain. Cyclic experiments detected that a formation of dislocations trigger a stabilization of martensite and thus, are responsible for this kind of functional degradation. Our model takes into account the inelastic behavior by splitting the phase transformation from austenite to martensite into a reversible and an irreversible process. In addition to that, we include an orientation distribution function to account –in a numerically very advantageous way– for the shape memory alloy's polycrystalline structure, see [3]. After presenting the material model [6], we comment on the implementation into a finite element framework. Starting with the simulation of a cyclically loaded wire, we close with the geometrically more ambitious and application‐oriented example of a clamping ring as already presented in [7].
Data‐driven approach to elastic problems
Authors: Korzeniowski Tim Fabian , Weinberg Kerstin 
Published: December 2018

AbstractRecently Kirchdoerfer and Ortiz proposed a new computing paradigm, called data‐driven computing [1]. It is a new approach to overcome uncertainties in the modeling process of the material law. The aim of this work is to compare the solutions of the data‐driven method with classical finite element solutions and an analytic solution. A simple elastic problem is considered and solutions of all methods are presented.
Coupled mass interactions in plant tissues under frost conditions
Authors: Eurich Lukas , Wagner Arndt , Ehlers Wolfgang 
Published: December 2018

AbstractFrost‐hardy plant tissues have developed strategies to cope with frost events without damage. This is a very interesting feature with regard to technical applications in construction industry, where frost damage is still a major concern. This contribution introduces a biologically inspired macroscopic modelling approach based on the Theory of Porous Media (TPM) to describe the coupled thermo‐hydro‐mechanical processes in plant tissues. The focus is, in particular, on the mass interactions within the plant tissue under frost conditions, which are the ice formation in the intercellular space and the dehydration of the tissue cells, which is a crucial property for frost‐resistance. A numerical example illustrates the coupling of these mass interactions.
New biaxially loaded specimens for the material parameter identification of ductile damage
Authors: Gerke Steffen , Schmidt Marco , Brünig Michael 
Published: December 2018

AbstractThe paper deals with new biaxial experiments to study the damage and fracture behavior of ductile sheet metal. For this purpose the H‐specimen has been specially designed and is subjected to different proportional loadings within this context.The stress state dependent damage and fracture behavior can be described by a phenomenological continuum damage and fracture model which is briefly presented. In continuation results of new biaxial experiments are given which facilitate the study of different damage mechanisms in a well controlled way. The proposed H‐specimen is applied for shear–tension as well as for shear–compression load cases and can be used to evaluate the corresponding continuum damage model. Furthermore a perspective to future work is given.
Double‐stage optimization of truss and frame structures
Authors: Keller Alexander , Muench Ingo , Wagner Werner 
Published: December 2018

AbstractComprehensive optimization of truss and frame structures is intended to optimize all aspects of a structure: the topology, shape and size of members. However, simultaneous optimization of all mentioned aspects increases the complexity of the problem and leads to high demands for the used optimization algorithm. We propose to decompose the optimization task and to use specialized algorithms for each stage of the process. First, a phase field model evolves the topology as basic design. Then, we use an interface to set up a simplified beam model, where connection topologies and nodal positions as well as cross sections are simultaneous optimization parameters of the Evolutionary Algorithm.
On a least‐squares finite element formulation for sea ice dynamics
Authors: Schröder Jörg , Nisters Carina , Ricken Tim 
Published: December 2018

AbstractIn this contribution a mixed least‐squares finite element method (LSFEM) for the modeling of sea ice motion including a viscous‐plastic (VP) sea ice rheology is investigated. The simulation of sea ice motion goes back to the findings in Hibler III [4], where a numerical model for the simulation of sea ice circulation and thickness evolution over a seasonal cycle is introduced. Both the ice thickness and ice concentration distribution are explicitly described on the basis of evolution equations. Recent research in this field is devoted to finite element formulations based on the Galerkin variational approach. Here, special focus lies on the stabilization of the numerically complex scheme. It is therefore desirable to establish a least‐squares formulation to overcome possible numerical drawbacks. The least‐squares variational approach is well established in finite element formulations in the branch of fluid mechanics, see [2], [3] and [6], for instance. A great advantage of the method is its applicability to first‐order systems, such that it results in stable and robust formulations also for not self‐adjoint operators like in the Navier‐Stokes equations, for instance. A box test case, see [7], is investigated for the least‐squares formulation.
Restrictions in phase field modeling of brittle fracture
Authors: Strobl Michael , Seelig Thomas 
Published: December 2018

AbstractA key feature of phase field approaches to fracture is the relatively simple structure of the method. However, current approaches inevitably exhibit some limitations which do not seem to be obvious and are therefore neglected in many investigations. The assumption of isotropic stiffness degradation by a scalar phase field parameter does not capture the anisotropy introduced by a crack and severely restricts the scope of application. Widely used tension‐compression splits violate the specific conditions at cracks like traction free crack surfaces and, in addition, introduce mesh orientation dependency. Accurate approximation of the surface (fracture) energy is another important requirement which is discussed for a non‐vanishing internal length ℓ.
An adaptive FE2 method for the thermomechanical behaviour of a SMA‐Fiber matrix composite
Authors: Praster Maximilian , Klinkel Sven 
Published: December 2018

AbstractThis contribution deals with the multiscale analysis of a reinforced matrix with shape‐memory‐alloys (SMA). The complicated micro‐structure permits a full discretized macro‐structure, due to that an FE2 approach is used. Unlike classic materials, the SMA has more complex behavior with a high‐temperature dependency in the loading and unloading case. The micro‐structure of the macroscopic problem consists of a linear‐elastic matrix and a random fiber distribution. The stress response of the composite depends non‐linearly on the deformation, the fiber orientation and the temperature. Due to the non‐linear behavior of the fiber a nested homogenized is employed at each integration point. This results in the so‐called FE2 method. One disadvantage of the FE2 method is the high computational effort by solving the boundary value problem (BVP) in every integration point and every iteration step. This motivates the present work to introduce an indicator, which determines whether an accompanying homogenization is needed. Due to the temperature dependency, on the macro‐scale, a coupled thermo‐mechanical problem is solved. In the first homogenization step, the BVP of the RVE is solved with Neumann Boundary conditions. This leads to an overestimation of the strains. The SMA will remain linear elastic until the phase transition condition is reached. The formulation is similar to the classical yield condition of elasto‐plastic materials, however, here a strain instead a stress creteria is used. From the phase transition condition, the temperature dependent indicator is defined and formulated as a limit strain for the linear behavior of the SMA‐fibers. The accompanying homogenization is firstly needed when the limit strain is reached.
External boundary value problems in the quasi static theory of thermoelasticity for materials with triple voids
Authors: Svanadze Merab 
Published: December 2018

AbstractIn this paper the quasi static linear theory of thermoelasticity for materials with triple voids is considered. Basic external boundary value problems (BVPs) of steady vibrations are formulated. The uniqueness and existence theorems for regular (classical) solutions of these BVPs are established.
A normalization concept for smart material actuation by the example of hydrogels
Authors: Ehrenhofer Adrian , Wallmersperger Thomas 
Published: December 2018

AbstractFor passive (classical) materials, stress and strain are used to extract the material behavior from the sample behavior in a tensile test. In analogy, the actuation behavior of active (smart) materials can be normalized. In the present research, we show the normalization using the example of hydrogels that react with a volume change (swelling and deswelling) when exposed to stimulus‐changes like temperature, chemical concentrations, pH or light intensity changes. The normalized behavior can then be implemented with the Temperature‐Expansion‐Model which is based on the analogy of active behavior with thermal expansion. This allows the simulation of arbitrary active structures and the extraction of the sensitivity measure to a stimulus.
First Steps Towards the Direct Micro‐Macro Simulation of Reinforced Concrete Under Impact Loading
Authors: Tamsen Erik , Weber Wolfang , Balzani Daniel 
Published: December 2018

AbstractThis contribution presents a micro‐macro formulation for impact loading based on kinematic admissibility and the principle of multiscale virtual power, which allows for an energetically consistent scale‐bridging taking into account inertia forces at the microscale. Necessary constraints are discussed, a consistent formulation for the macroscopic tangent modulus for dynamic loading is presented and applied to a simple numerical example.
On the Construction of Diagonal Mass Matrices for Serendipity Elements
Authors: Duczek Sascha , Gravenkamp Hauke 
Published: December 2018

AbstractThe diagonalization of the mass matrix is an important prerequisite for the implementation of explicit time integration schemes. However, in the context of higher order serendipity finite elements (p ≥ 2) established mass lumping approaches, such as the row‐sum method or the nodal quadrature technique cannot be applied. Due to the existence of negative/zero lumped masses explicit time integrators might diverge and therefore different methods are needed. In this communication, we investigate the performance of the newly introduced manifold based mass lumping technique in comparison to the standard formulation using a consistent mass matrix. By means of a modal analysis of a slender rod it is shown that only sub‐optimal rates of convergence are achieved if a lumped mass matrix is employed, whereas the conventional formulation yields optimal convergence. That is to say, when using diagonal mass matrices we observe a significant gain in computational efficiency by sacrificing the accuracy of the approach to a certain extent.
Parametric model order reduction for district heating networks
Authors: Rein Markus , Mohring Jan , Damm Tobias , Klar Axel 
Published: December 2018

AbstractThis contribution focuses on the model order reduction of index 1 hyperbolic differential algebraic equations which are quadratic in state, at the example of district heating networks. We demonstrate that an appropriate splitting of the state space variables in the two categories massflow and temperature leads to a linear parameter varying system, allowing the use of tools from linear model order reduction. We measure a significant decrease of computational effort to simulate the underlying network dynamics while preserving stability.
Similarity of Field Quantities in Scaled Radiation Transfer Lattice Boltzmann Simulations (RT‐LBM)
Authors: McHardy Christopher , Becker Deborah , Rauh Cornelia 
Published: December 2018

AbstractThe lattice Boltzmann framework provides a powerful tool to solve radiation transfer problems in participating media. Thereby, tuning of the collision rate to the numerical time step is necessary to ensure accurate solutions. One way of doing this is to scale the radiation characteristics of the medium by means of similarity relations. In the present contribution radiation characteristics of participating media are scaled to the numerical grid by means of the similarity relations to tune the accuracy of a recently proposed RT‐LBM. A free parameter was introduced and its effect on the solution is investigated. Also, the similarity of higher order angular moments is studied. The results show that conservation of the transport mean free path ensures the similarity of solutions.
Modeling and Simulation of Solid‐State Lasers: From Mathematics to Industrial Application
Authors: Pflaum Christoph 
Published: December 2018

AbstractSolid‐state lasers are widely used for different industrial applications ranging from material processing to medical applications. For designing and optimization of a solid‐state laser an accurate simulation tool is need. Such a simulation tool has to be able to simulate different kind of physical effects like thermal effects, depolarization caused by stress inside a laser crystal, non‐linear gain, and mode competition. In order to calculate output power and beam quality, we apply an extension of the dynamic mode analysis concept (DMA) (see [1]). This model is based on a 3‐dimensional discretization of population inversion and a system of complex rate equations. The resulting mathematical equation system is a large system of ODE's, which has to be solved by a suitable method including time step control. The difficulty in such a simulation is the extreme change of parameters. As a consequence the output signal might be a pulsed signal with high output power of pulse width 1 ns and time period 1 ms. In order to solve such a system of ordinary differential equations accurately, we present a suitable numerical method. Simulation results with industrial applications are presented.
Sensitivity of a partially learned model‐based reconstruction algorithm
Authors: Boink Yoeri E. , van Gils Stephan A. , Manohar Srirang , Brune Christoph 
Published: December 2018

AbstractWe replace part of a model‐based iterative algorithm with a convolutional neural network in order to improve the quality of tomography reconstructions. We analyse its robustness against uncertainties in the image and uncertainties in system settings. Results are presented for the application of photoacoustic tomography in a limited angle setup.
Multiscale modelling of alkali transport and ASR in concrete structures
Authors: Iskhakov Tagir , Timothy Jithender J. , Meschke Günther 
Published: December 2018

AbstractASR is a microscopic process in concrete characterized by the formation of a hydrophilic alkali‐silica gel due to the reaction of the alkali in the pore‐fluid with silica in the aggregates. In the presence of moisture, the gel swells and induces an internal pressure that leads to microcracking, expansion and overall deterioration of the material. When subject to external alkali supply, the material is susceptible to higher levels of degradation. Transport of alkali ions at the structural scale in partially saturated concrete is modelled by coupling the diffusion equation with the Richards equation for moisture transport. The overall deterioration of concrete pavements due to ASR can be modelled by a synthesis of two sub‐models: 1) the alkali concentration is coupled to a meso‐scale reaction model describing the formation and evolution of the alkali‐silica gel; 2) the gel induced microcracking in the material is estimated using a semi‐analytical multiscale micromechanics model [1]. The model capabilities are evaluated using selected numerical examples and comparisons with experimental observations.
Efficient Computation of Reachable Sets for Critical Space Mission Phases
Authors: Chan Kai Wah , Seelbinder David , Theil Stephan , Knauer Matthias , Büskens Christof 
Published: December 2018

AbstractTo achieve true autonomy during critical mission phases in deep space, it is imperative to know which states are attainable from a given starting point using only admissible control inputs. These attainable states form the reachable set. If the latter is computed in real time, this can provide a system for Guidance, Navigation and Control on‐board of an autonomous spacecraft with knowledge of all the feasible and infeasible options in order to choose the correct course of action. Though several means exist to compute reachable sets, the most promising ones are based on optimization techniques. We currently explore an approach, in which we approximate a reachable set with an altering polytope. The center of a selected facet of the polytope is the starting point from which a new feasible vertex is placed in normal direction with the largest possible step size. This approximates the set boundary successively. Simultaneously, the underlying equations of motion must be satisfied. We formulate these tasks as an optimal control problem.
Overview of Craig‐Bampton Substructuring Approaches for Model Order Reduction of Nonclassically Damped Systems
Authors: Gruber Fabian M. , Rixen Daniel J. 
Published: December 2018

AbstractThe classical Craig‐Bampton method (1968) does not take any damping effects into account for the model order reduction of damped systems. One procedure to handle nonclassically damped systems is to transform the second‐order differential equations into state‐space representation. Solving the corresponding eigenvalue problem allows the damped equations to be decoupled, but complex eigenmodes and eigenvalues occur. A comparison between different formulations for nonclassically damped systems is given and demonstrated by an example.
A time‐space FCT‐FE formulation for 1D time dependent advection‐diffusion equation
Authors: Feng Dianlei , Neuweiler Insa , Nackenhorst Udo 
Published: December 2018

AbstractWe present a time‐space flux‐corrected transport (FCT) finite element formulation for solving the linear time‐dependent advection dominated advection‐diffusion equation. Solving advection dominated transport equations with conventional finite element (FE) methods suffers from drawbacks of excessive numerical dispersion which results in non‐physical, non‐monotonic solutions. The FCT algorithm is an effective method which suppresses the non‐monotonic behavior of the solution by applying a limited anti‐diffusion operator to a first order scheme. Applying the FCT algorithm to time‐space FE formulation, such as the time‐discontinuous Galerkin (TDG) method, benefits from the advantages of both the TDG scheme and the FCT algorithm. In another word, the time‐space FCT‐FE formulation achieves arbitrary odd order accuracy in time at the discontinuous time nodes. Large time steps can be applied and the scheme ensures monotonic solution when linear interpolation is used for spatial discretization.
Tikhonov regularization for ill‐posed backward nonlinear Maxwell's equations
Authors: Chen De‐Han , Yousept Irwin 
Published: December 2018

AbstractTikhonov regularization for ill‐posed backward nonlinear Maxwell's equations is considered. We present a convergence result with rate for the Tikhonov regularization under piecewise Hs‐type Sobolev regularity assumption on the exact initial data.
Numerical approaches for the simulation of a real multiphase flow in a fermentation tank
Authors: Klembt Daniel , Meironke Heiko 
Published: December 2018

AbstractIn the context of investigations of multiphase flows, the beer production is currently being investigated, especially fermentation, maturation and storage in cooperation with the local brewery. The university has its own 350 litre fermentation tank with comprehensive acoustic flow and temperature measurement technology for the systematical investigation of the influence of the fermentation activity, distribution of yeast and occurring convection phenomena. This paper deals with the numerical simulation of the fermentation tank and the combination of a numerical approach and an experimental investigation with the two Fluid Eulerian‐Eulerian time‐averaged model [3]. The Eulerian‐Eulerian model allows two sets of governing equations, continuity and momentum equations to be solved for either phase and their interactions are modelled using interface transfer terms for interfacial heat, mass and momentum exchanges. As a result, the numerical solution of a natural convection flow which is superimposed by rising gas bubbles will be presented.