bluuebunny/crossref_metadata_2025 huggingface dataset:
Multi‐scale modelling of fibre reinforced composites exhibiting elastoplastic deformation
Authors: Bedzra Rex , Reese Stefanie , Simon Jaan‐Willem
Published: March 2018

In this paper, a scale bridging approach for modelling unidirectional tungsten fibre reinforced copper (W/Cu) composite by making use of an anisotropic elastoplastic constitutive model is described. To identify the yield surface of the composite and to further validate the constitutive model, a finite element model of a unit cell (UC) of the composite is set up. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A physically motivated model based on the strain amplification in filled elastomers
Authors: Darabi Ehsan , Itskov Mikhail , Klüppel Manfred
Published: March 2018

A constitutive model for filled elastomers based on a combination of the Dynamic Flocculation Model (DFM) [1] framework and the continuum damage model [2] is proposed. This contribution represents an extension of the previously proposed micro‐mechanical model explaining simultaneously induced filler breakage and polymer‐filler network damage [3]. These effects are attributed to the hydrodynamic strain amplification which is the main topic of the current work. Deformation causes damage in both the network rubbery matrix and inside the filler aggregates. As a result, the probability density function of the number of segments and the filler size distribution change with the strain in all spatial directions which leads to stress softening and the Mullins effect. The model also describes the deformation induced anisotropy and permanent set. A small number of physically motivated material constants describing the average filler cluster dimensions, filler‐filler and filler‐matrix interaction properties are included in the model. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On a relaxation approach to modeling the stochastic behavior of elastic materials
Authors: Junker Philipp , Nagel Jan
Published: March 2018

The inclusion of information on the stochastic behavior of microstructures is a key issue for modeling materials with even higher precision. To this end, stochastic series expansions offer an elegant for accounting for randomness. A prominent example is the usage of the so‐called Chaos Polynomial Expansion in the context of the Stochastic Finite Element. Although providing the desired information, the numerical effort is highly increased even for the simple linear elastic case. We aim at improving the numerical efficiency by proposing a novel approach to modeling the stochastic behavior of microstructures: we divide the material (= integration) point into a subset of domains which can be interpreted, e.g. as crystal grains. Afterwards, we employ a stochastic series expansion for the elastic constants and strains in each domain. Appropriate energy relaxation results into effective (homogenized) elastic constants and strains which are stochastic. Calculation of expectation and variance is only a technical issue yielding analytical formulas which are basically free of any additional computational effort as compared to regular deterministic linear elastic calculations. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Experimental Investigation and Modeling of Volume Shrinkage in Curing Epoxy Resins
Authors: Leistner Chris , Wittrock Jörg , Abliz Dilmurat , Ziegmann Gerhard , Hartmann Stefan
Published: March 2018

The exothermic curing reaction of epoxy resins comes along with chemical shrinkage of the material. There do not exist established standard methods for determining the chemical shrinkage during the curing process. To this end, shrinkage measurements based on Archimedes' principle have been carried out for pure epoxy resin at different curing temperatures. In order to model the shrinkage behavior, these results can be related to the curing kinetics. For this purpose, additional DSC measurements have been carried out. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A large‐strain phase‐field model for nematic elastomers based on Landau‐de‐Gennes theory
Authors: Keip Marc‐André , Nadgir Omkar
Published: March 2018

Liquid crystal elastomers bring together the orientational properties of liquid crystals as well as elastic properties of soft rubber.Nematicelastomers are a subclass of these materials which exhibit a phase transition from high‐symmetry isotropic state to low‐symmetry nematic state, when cooled down below a material specific temperature. We present a phase–field model based onLandau‐de‐Gennestheory for the modeling of nematic elastomers. The model is implemented within finite element framework and an initial boundary value problem of isotropic‐nematic phase transition is solved. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A stereologic approach for the spherical void size distribution in Polyurea
Authors: Reppel Thomas , Weinberg Kerstin
Published: March 2018

Polyurea is a high performance elastomer usually used as a protective coating on steel or concrete. Various elastic properties of Polyurea have been identified in the past, cf. [2] and references therein, where in uniaxial tension tests a Young's modulus of approximately 65 MPa was measured together with an elongation up to 650%. After the long elastic range however, Polyurea fails like a porous material.

This work aims at the approximation of the spherical void size distribution. To avoid the high costs for creating micro‐CT images of Polyurea specimens, stereologic methods are used to estimate the size distribution of the spherical void radii. This is done with the usage of the classical Wicksell [4] approach by calculation of the void section radii distribution from micrographs of sliced specimens. Several parametric distributions for the void section radii are considered. Results of Monte‐Carlo simulations are compared with the possible void radii distributions,which are obtained and examined in Wicksell's model. Also the deviation that occurs by chosing the same distribution for both the sphere and the section radii is investigated. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Material plasticity to track the elastic anisotropy at finite strains
Authors: Weber Martin , Glüge Rainer , Bertram Albrecht
Published: March 2018

We present a phenomenological finite elasto‐plasticity theory that includes the evolution of the elastic anisotropy. The theory is called Material Plasticity after [1] and [2], assuming that the axes of anisotropy deform as material line elements. It is applicable to fiber‐reinforced materials. The main feature is the different evolutions of the stiffness tetrad and of the stress‐free placement. For this purpose we extend the well known isomorphy concept (see [3]).

To identify and compare the stiffness tetrads before and after large plastic deformations a representative volume element (RVE) [4] with a fiberous microstructure is used. Uni‐, bi‐ and and tridirectionally reinforced samples are considered. On the microscale a standard elastic‐plastic material model is used. After calculating the effective stiffnesses of the different material samples we investigate their evolution during different deformations. It turns out that it is possible to denote the change of the stiffness with sufficient accuracy using one additional second order tensor. After investigating the change in the stiffness tetrads we finally propose an analytical evolution equation for this tensor. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Simulation of self‐organization processes in filled rubber
Authors: Wulf Hans , Ihlemann Jörn
Published: March 2018

A typical property of filled rubber material, known as Mullins effect, is that prestraining leads to a reduction of stresses at strain levels smaller than the maximum strain in the loading history. This softening is related to the direction of the prestrain and therefore induces a material anisotropy. The microstructural process responsible for this effect is still subject of discussion. Here, a mechanism based on the self‐organization of weak physical links is proposed. The central idea states that these links organize into a pattern of different linkage densities leading to the observed properties. The theory is tested by a simulation program. In the simulation, several typical rubber properties can be reproduced, while clearly observing self‐organization of the model elements. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On Variationally‐Consistent Homogenization Approaches in Multi‐Phase Magnetic Solids
Authors: Kiefer Björn , Bartel Thorsten
Published: March 2018

It is well known that classical homogenization schemes, such as the Taylor/Voigt and Reuss/Sachs assumptions, can also be interpreted as energetic bounds. Furthermore, energy relaxation concepts have been established that determine stable effective material responses based on appropriate (convex, quasi‐convex, rank‐one) energy hulls for non‐convex energy landscapes associated with multi‐phase materials, see [1–3] and references therein. Our goal is to propose analogous relaxation based homogenization schemes for magnetizable solids. More specifically, we propose a magnetic potential perturbation scheme which yields relaxed effective free energy densities that simultaneously satisfy magnetic induction and magnetic field strength compatibility requirements—i.e. the magnetostatic Maxwell equations—at the phase boundary. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Coupled SPH and Phase Field method for hydraulic fracturing
Authors: Osorno Maria , Steeb Holger
Published: March 2018

Simulation of fracture initiation and propagation using classical mesh‐based methods involves computationally expensive operations for pre‐processing and (adaptive) remeshing of complex geometries. To overcome the difficulties we propose to use a Smoothed Particle Hydrodynamics (SPH) method to model hydraulic fracturing. SPH is a meshless Lagrangian method highly suitable for large deformations [1].

The present contribution discusses a numerical approach to model fractures initiation and propagation, by coupling SPH with the Phase Field method [2]. The proposed hybrid method overcomes the instability problems that can present SPH due to kernel incompleteness. We first validate the proposed model with a stationary elastic fracture and compare the results with the classical SPH to the SPH‐Phase Field approach. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optimal control of a slot car racer
Authors: Penner Johann , Schlögl Tristan , Leyendecker Sigrid
Published: March 2018

In this contribution, we present a simulation method for the optimal control of a mechatronic system that is based on discrete variational calculus and apply it to compute the time‐minimal path of a slot car racer. Here, the DMOC (Discrete Mechanics and Optimal Control [4]) method is used to generate offline optimal trajectories for the electro‐mechanically coupled system, i.e. sequences of discrete configurations and sequences of driving voltages. The time‐minimal path is achieved by the choice of different cost functions, the sum of time steps, the negative sum of the quadratic momenta and the negative sum of the quadratic velocities. Simulation results show that the momentum formulation yields the lowest number of iterations. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A phase‐field model of hydraulic fracture in saturated binary porous media
Authors: Pise Mangesh , Bluhm Jochim , Schröder Jörg
Published: March 2018

In many fields of engineering, especially in geo sciences and rock mechanics, the theoretical and numerical modeling of hydraulic fracturing of porous materials plays an important role. Hydraulic fracturing is a well stimulation technology in which porous materials are fractured by a pressurized liquid. The process involves the pressure injection of the fracking fluid (primarily water, often enriched with filling materials and thickening agents) and accompanied by crack nucleation and propagation as well as mass transport. In this contribution, a macroscopic model based on the Theory of Porous Media (TPM) is presented in view of the description of the aforementioned phenomena. For simplification an incompressible binary model consisting of the phases solid and liquid is used. Attention is paid to the description of the damage of the elastic solid phase. The development of the damage parameter is controlled by an evolution equation, which corresponds to known diffusive phase field models within a continuum mechanical framework. A numerical example shows that the simplified model is indeed capable of simulating hydraulic fracturing of porous media. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical investigation of the effective Skempton coefficient in fluid‐saturated fractured rock
Authors: Pollmann Nele , Jänicke Ralf , Renner Jörg , Steeb Holger
Published: March 2018

Characterization of hydro‐mechanical processes in reservoir rocks is an essential issue for many geo investigations such as characterization of subsurface fluid flow or geothermal exploitation. For geothermal applications, the role of fractures as storage and transport components of a hydraulic system are highly important. In the present contribution we focus on investigating the effective Skempton coefficient

of a damaged porous rock analyzing a modified Cryer problem, which provides a simple model of a porous rock containing a storage and transport pat. The effective Skempton coefficient is defined as the ratio of the increase in mean pore pressure induced by change in confining pressure for undrained boundary conditions. Using approaches from computational homogenization, we evaluate the confining pressure as the negative volume average of the total mean stress. Similarly, we compute the effective fluid pressure in terms of the volume‐averaged fluid pressure in the rocks. We compare the numerical results to those from typical experiments and highlight the problems with the latter. The proposed concept for determination of an effective Skempton coefficient based on numerically evaluated volume averages helps to generate a better understanding of the process‐inherent constituents. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Modeling the solidification process in mortar due to internal mass exchange
Authors: Sauerwein Malte , Steeb Holger
Published: March 2018

Aim of the investigation is to model phase transition processes induced by mass transfer in multi‐phase and multi‐species mixtures. More specific, the innovative concept of using Superabsorbent Polymers (SAPs) as additives in grouting mortars represents a promising practical application, e. g. in mechanized tunneling. In this contribution, we focus on both the driving force for mass transfer as well as the kinetics for water migration, whereas an extension to the classical Theory of Porous Media (TPM) provides a reasonable modeling framework. As a result, the gelation process of the mixture can be described with respect to time. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A temperature‐based GENERIC approach for the thermodynamically consistent integration of thermoelastic solids
Authors: Schiebl Mark , Betsch Peter , Hesch Christian
Published: March 2018

This work deals with the thermodynamically consistent (TC) time integration of thermoelastic systems with polyconvex density functions using the notion of the tensor‐cross‐product. While energy‐momentum preserving integrators are well‐known for conservative (isothermal) mechanical systems, Romero introduced in [7, 8] the new class of TC integrators. While [8] dealt with the sample application of thermo‐elastodynamics, the scope of application was extended in [2] to coupled thermo‐viscoelastodynamics in temperature form. A first step towards the systematic design of a TC integrator is to cast the evolution equations into the GENERIC (General Equation for Non‐Equilibrium Reversible‐Irreversible Coupling) framework [6] which reveals additional underlying physical structures of the system. Relying on a polyconvex density function and using the notion of the tensor‐cross‐product [1] we arrive at a polyconvex version of the GENERIC framework. Further applying the notion of a discrete gradient leads to a TC integrator.

Using the entropy as the thermodynamical state variable as in [5, 8] the GENERIC framework possesses an easy structure. However, this choice of thermodynamical state variable only allows to prescribe entropy Dirichlet boundary conditions directly. This drawback can be compensated by using Lagrange‐multipliers to be able to handle temperature Dirichlet boundary conditions leading to an extended system of algebraic equations to be solved, see [5]. Alternatively, the present work uses the temperature as the thermodynamical state variable, see also [2, 3] and the use of an energy‐based Newton‐Raphson termination criterion. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Efficient numerical modelling of hydro‐mechanical effects in fractures using a hybrid dimensional approach and non‐conformal mesh coupling
Authors: Schmidt Patrick , Steeb Holger
Published: March 2018

The investigation of subsurface fluid flow in porous and fractured media is of interest in particular to determine the properties of matter and heat transport below the earth surface and the characteristics of the underground storage of fluids. Classical approaches such as extended diffusion equations are lacking the possibilities to capture key phenomena such as inverse water‐level fluctuations (Noordbergum effect) obtained during pumping tests performed on aquifers [1, 2, 4]. In order to efficiently model hydro‐mechanical effects, this contribution presents an implementation of the newly introduced hybrid‐dimensional approach [3] in the DUNE‐PDELab environment. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Untersuchung des Schließvorgangs von Notfallschützen und ihres Einflusses auf die Strömung von Laufwasserkraftanlagen
Authors: Seidel Christian
Published: March 2018

Verschlussorgane wie Notfallschütze sind bei Wasserkraftanlagen sicherheitsrelevante Bauteile, die bei unvorhergesehenen Ereignissen wie Lastabwurf des Kraftwerks, Blitzschlag, Wellen‐ oder Getriebebruch sowie anderer Arten von Havarien die Trennung des Volumenstroms zum Kraftwerk bewirken. Auf Grund der hohen Empfindlichkeit der elektrischen Komponenten eines Kraftwerkes wie z.B. Generatoren oder Umrichter gegen Überdrehzahlen und der daraus resultierenden Wärmeentwicklung in den elektrischen Bauteilen ist es erforderlich, die Notfallschütze in sehr kurzer Zeit zu schließen. Der Vorgang des schnellen Schützverschlusses löst dabei instationäre Strömungsprozesse wie Schwall‐ und Sunkerscheinungen aus, deren Intensität mit der Abnahme der Verschlusszeit stark zunimmt. Gleichzeitig besitzen die instationären Strömungsprozesse einen entscheidenden Einfluss auf die Unterströmung des schließenden Notfallschützes, auf den dabei noch abfließenden Volumenstrom und auf die Strömungsgeschwindigkeiten. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Electric leakage current density in phase field simulations for nanogenerator concepts
Authors: Wöhler Franziska J. , Münch Ingo , Wagner Werner
Published: March 2018

For the purpose of energy harvesting on the nanoscale with ferroelectric barium titanate (BaTiO3), our nanogenerator concept transforms parasitic mechanical oscillations into usable electric energy. Experimental difficulties occur in sample preparation, e.g., surface roughness, bonding contact, and leakage currents. Latter can be considered in our finite element phase field model, such that the nanogenerator concept can be optimized. Leakage current density is implemented from different phenomenological approaches. First, Ohm's law represents a linear relation between leakage and electric field. Second, the Space‐Charge‐Limited Current (SCLC) relation assumes a quadratic dependency on the electric field. So far, SCLC is suitable for one‐dimensional problems, however, in two or three dimensions it is not found in literature. Therefore, we discuss a reformulation to bring into account that the electrical field E is a vector out of ℝ3 in our model. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
From particle mechanics to microcontinuum theories
Authors: Bidier Sami , Ehlers Wolfgang
Published: March 2018

Microcontinuum theories enable the consideration of particle‐based microstructures within a continuum mechanical framework. Several classes of microcontinua, such as the micromorphic, the micropolar, the microstrain or the microstrech formulation, have been successfully applied to engineering applications, although a clear physical determination and interpretation of the kinematical extensions and the resulting higher‐order stresses within the formulation is frequently missing. In this regard, the present contribution focuses on establishing the physical link between discrete contact forces, stresses and deformation of particle‐based microstructures and the characteristic stress states of microcontinuum theories. Representative Elementary Volumes (REVs) are therefore constructed on the mesoscale as ensembles of deformable particles from the mircoscale. Establishing the REV balance relations justifies the common generalisation of the angular momentum balance commonly applied in microcontinuum theories. It furthermore leads to the identification of the continuum stresses based on micro‐quantities and enables the application of homogenisation techniques by exploitation of the equilibrium conditions of a REV. In order to investigate the hereby established link from the micro‐ to the macroscale, granular materials are simulated using the Discrete‐Element Method (DEM). In particular, localisation phenomena in granulates, e. g. in biaxial compression tests or during ground‐failure processes are studied. This implies the formulation of the contact between particles in an appropriate constitutive manner in accordance to the envisaged granular material behaviour, e. g. whether loose material, such as sand, or bonded multi‐component material, such as polyurethan‐sand compounds for metal casting applications are of interest. With the full solution of a particle‐based initial‐boundary‐value problem, the homogenisation formalism is applied and enables the study of the extended continuum field quantities, essentially demonstrating the applicability of microcontinuum theories in the field of granular material. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical study of adhesive and cohesive failure of structured interfaces
Authors: Hirsch Franz , Kästner Markus
Published: March 2018

In this contribution, a numerical study is presented, which addresses the failure behaviour of structured interfaces within a computational homogenisation scheme. Different local phenomena, e.g. the surface roughness and different local failure types like cohesive failure of the polymer and adhesive failure of the local interface and their effects on the overall interface characteristics are investigated. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Mean and full field homogenization of artificial long fiber reinforced thermoset polymers
Authors: Kehrer Loredana , Pinter Pascal , Böhlke Thomas
Published: March 2018

Based on two artificial microstructures representing a long fiber reinforced thermoset material, the effective linear elastic material properties are calculated by both a mean and a full field homogenization method. Concerning the mean field method, the effective elastic material properties are approximated using the homogenization scheme by Mori and Tanaka, formulated explicitly in terms of orientation averages. This allows to use orienation tensors of 2nd and 4th order describing the orientation information on the micro level. The full field method is based on the fast Fourier transformation (FFT), for which the effective material properties are determined by volume averaging. The comparison between both methods show good agreements, the deviations are in the range between 2% and 12%. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Variational sensitivity analysis of physical reaction forces and application to multiscale problems
Authors: Kijanski Wojciech , Barthold Franz‐Joseph
Published: March 2018

Nowadays profitability and efficiency of practical and industrial applications and therefore of material design is becoming a more important issue. Due to different well‐known and established approaches for analysis and simulation of complex heterogeneous materials on multiple scales based on numerical homogenisation techniques, development and production of high performance materials (using 3d printers for example) and as a consequence optimisation and design of materials is reality. The objective is to find optimal structures with optimal material distribution under given constraints related to posed problems and tailoring applications to their special requirements. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The effective shear modulus for an n‐layered composite sphere
Authors: Lenz Peter , Dammann Christian , Mahnken Rolf
Published: March 2018

This work presents the derivation of the effective shear modulus for a heterogeneous material composed of multi‐layered composite spheres embedded in a linear elastic matrix. It is based on the composite spheres model known from the literature. In contrast to previous publications the effective shear modulus is obtained by equating the results of two models: In the first model, a heterogeneous sphere is embedded in an equivalent homogeneous material, whereas in the second model, the heterogeneous sphere is replaced by an equivalent homogeneous sphere. In the context of both, a shear stress approach and a shear deformation approach, this results into an overdetermined system of equations which is solved with the least squares method. In a numerical study our results are compared to effective moduli and bounds from the literature. Furthermore, a convincing agreement with experimental data for glass microspheres embedded in a polyester matrix is demonstrated. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical simulation and validation of a solidification experiment using a continuum mechanical two‐phase/‐scale model
Authors: Moj Lukas , Ricken Tim , Foppe Manuel , Deike Rüdiger
Published: March 2018

A numerical simulation of a solidification experiment is presented. The experimental setup consists of an empty screwed up mould, which is filled up with molten steel and cools down due to atmospheric conditions. During the cooling process, the temperature at the centre as well as the shrinkage at the narrow side, respectively, have been recorded. The simulation has been run via the software ANSYS modified by specific provided programmer interfaces, the User Programmable Features (UPFs). All required temperature dependent material parameters have been provided by ThyssenKrupp Steel Europe. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The effective thermal conductivity of carbon nanotube composites
Authors: Nagaraj Manish , Timothy Jithender J. , Meschke Günther
Published: March 2018

The effective thermal conductivity of Carbon Nanotube (CNT)‐polymer composites has been estimated using analytical and computational models. The analytical approach is based on the Cascade Continuum Micromechanics (CCM) model formulated within the framework of mean‐field homogenization and the computational approach is based on numerical homogenization of the composite microstructure using image based Voxel‐FEM (Finite Element Method). Comparison of the analytical and computational model predictions with experimental data show that the interfacial thermal resistance is overestimated by the analytical model as a consequence of not taking into account the CNT fiber tortuosity (curviness). (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Thermomagnetic convection under alternating magnetic fields
Authors: Szabo Peter S. B. , Lee Yeaw‐Chu , Früh Wolf‐Gerrit
Published: March 2018

Thermomagnetic convection within a square cavity filled with a magnetic fluid was studied under alternating magnetic field. The aim was to explore the response of the magnetic fluid towards alternating magnetic fields. A set of frequencies for the sinusoidal variation of the magnetic field intensity was used to study the heat transfer characteristics across the problem geometry. The effect on heat transfer was quantified by an averaged Nusselt number. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
CFD studies on the surge detection of a radial compressor flow by analyzing the unsteady secondary flow field
Authors: Bamberg Stephan , Schnitzer Jakob , Weber Manuel , Rusche Stefan , Eißler Werner
Published: March 2018

The objective of this investigation is the development of a reliable methodology allowing the accurate prediction of compressor surge. In this context a centrifugal compressor used in a commercial application with five main and five splitter blades, installed in regular passenger vehicles, is numerically analyzed. For this purpose a CFD analysis is conducted. The steady‐state and unsteady secondary flow field is analyzed and studied by a frequency analysis to detect critical oscillations. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Evaluating flame turbulence scale in diffusion combustion of diesel fuels
Authors: Loboda Egor , Matvienko Oleg , Agafontsev Mikhail , Reyno Vladimir
Published: March 2018

This paper presents results of mathematical simulations of dynamic flame behavior that occurs in burning diesel fuel, as well as experimental data on turbulent flame eddies provided by infrared thermography. The comparison of the simulated and experimental data documents a good match between the parameters of flame thermodynamics and the combustible gas flow in a turbulent eddy. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical investigations of bubbles rising in water
Authors: Höffmann Ann Kathrin , Ehrhard Peter
Published: March 2018

The hydrodynamics of rising air bubbles in water filled bubble columns are examined using a transient numerical model following the Euler‐Euler method. In this particular case the bubble‐size distribution is assumed to be mono‐disperse and the influence of this assumption on the specific mass‐transfer area is discussed. Additionally, the influence of the bubble size is discussed with regard to the mass‐transfer area, the air volume‐fraction, and the axial air velocity profile. To validate the numerical model, experimental results from tomographic measurements at the Helmholtz‐Zentrum Dresden‐Rossendorf (HZDR) are used. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On transcritical states in viscous flow passing the edge of a horizontal plate
Authors: Scheichl Bernhard , Bowles Robert I.
Published: March 2018

This contribution puts forward some recent advances in the rigorous (asymptotic) theory of gravity‐ (and capillarity‐)driven shallow flow of a viscous liquid past a horizontal plate, originating in jet impingement oblique to it. Hence, our concern is twofold: with steady developed flow over the distance from the jet centre to the trailing edge of the plate, referred to as a pronounced hydraulic jump blurred by viscous diffusion; with the predominantly inviscid transcritical limit arising near the edge due to scale reduction given an intrinsic expansive singularity taking place there. In the latter situation envisaged briefly, condensing nonlinear inertial effects, weak time dependence, and (very) weak streamline curvature as the essential ingredients into a distinguished limit demonstrates the generation of a weak (transcritical) hydraulic jump by a plate‐mounted obstacle. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical Prediction of Aerodynamic Noise Generated from an Aircraft in Low Mach Number Flight
Authors: Rašuo Boško , Jazarević Vladimir
Published: March 2018

The paper describes numerical prediction of aerodynamic noise generated from an Aircraft. Simulation of turbulent flow is done solving the incompressible Navier‐Stokes equation, where turbulence is modeled applying the orthogonal subgrid scale (OSGS) method with dynamical subscales. Because of comparison, the same simulation is done using the LES (Large Eddy simulation). It is shown how simulation of turbulent flow affects the prediction of acoustic sources calculated using Lighthill's analogy. Translation from the time to frequency domain is done through DFT (Direct Fourier Transform), which gives smaller usage of memory. Acoustic sources are used in the inhomogeneous Helmholtz equation to simulate pressure wave propagation in the domain. It is shown that OSGS with dynamical subscales gives better representation of the spectrum. Overall, better prediction of energy transfer across large and small eddies will give better allocation and presentation of acoustics sources. These sources will change wave propagation of the pressure in the acoustic field. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical investigation of vortex roll‐up processes using different wingtip shapes
Authors: Uhl Anna , Dufhaus Sebastian , Hörnschemeyer Ralf , Stumpf Eike
Published: March 2018

Vortex roll‐up processes of different generic wingtip shapes are investigated using the DLR TAU flow solver in combination with a Reynolds Stress Model. In contrast to a non‐rounded layout an elliptical rounding of the wingtip planform leads to multiple vortices at the wingtip and results in an axial velocity deficit in the vortex core further downstream. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Modeling of a Likelihood Function based on a Global Sensitivity Analysis
Authors: Glaser Philipp , Kosmas Petridis , Schick Michael , Heuveline Vincent
Published: March 2018

In future the model quality will have an increasing priority in the industrial context and suitable techniques and methods to quantify them have to be developed. In this paper we want to focus on the challenge of selecting the most informative measurement data for a time‐dependent simulation model to infer parameter distributions. Sobol indices are used to give a estimate about the global sensitivity of those parameters and they are illustrated for the given benchmark problem. In this context the gained knowledge on the parameters is used to define a Likelihood function for the Bayes' rule. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
SmartFarm – Data based optimization for optimal energy management
Authors: Jung Francesca , Büskens Christof , Lachmann Malin
Published: March 2018

In the project SmartFarm we develop a method that automatically decides which renewable energies are profitable to use at the farm for the next hours. To this means optimization methods are used on various levels. Initially, all necessary data is measured on a demonstration object. This data forms the basis for data based learning methods for the modelling of the consumers (e.g. milk cooling), the producers (e.g. solar plant) and the energy storage systems (e.g. battery storage device). In this paper we will focus on the data based models of the producers and energy storage systems. Optimization methods are used to identify optimal parameters for a general ansatz function, which is derived from a Taylor approximation.

We develop all models with real data of the demonstration object and verify them with this data as well. Numerical results and the further course of action will be discussed in the paper. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Modification of TLS algorithm for solving ℱ2 linear data fitting problems
Authors: Hnětynková Iveta , Plešinger Martin , Žáková Jana
Published: March 2018

It has been proved that the classical TLS algorithm fails to construct a TLS solution of linear data fitting problems AX ≈ B that belong to the class ℱ2. It will be shown how to modify this algorithm in order to reach a TLS solution. Such solution is not necessarily the minimum 2‐norm or Frobenius norm one. A few ideas how to decrease its norm are briefly discussed. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Time Domain BEM Based on Generalised Convolution Quadrature
Authors: Diao Hanzhi
Published: March 2018

We present the application of the generalised convolution quadrature (gCQ) technique to the time domain boundary element method (BEM) which solves the retarded potential boundary integral equation (RPBIE). Our result allows to employ the multi‐stage Runge‐Kutta method as the time stepping scheme for the generalised convolution quadrature which is used in the time domain BEM for acoustic problems in either a bounded three‐dimensional domain or in its unbounded exterior. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Efficient Ritz‐Galerkin Discretization of PDEs with Variable Coefficients in arbitrary Dimensions using Sparse Grids
Authors: Hartmann Rainer , Pflaum Christoph
Published: March 2018

Sparse grids can be used to discretize elliptic differential equations of second order on a d‐dimensional cube. Using the Ritz‐Galerkin discretization, one obtains a linear equation system with 𝒪 (N (log N)d−1) unknowns. The corresponding discretization error is 𝒪 (N−1 (log N)d−1) in the H1‐norm. A major difficulty in using this sparse grid discretization is the complexity of the related stiffness matrix. To reduce the complexity of the sparse grid discretization matrix, we apply prewavelets and a discretization with semi‐orthogonality. Furthermore, a recursive algorithm is used, which performs a matrix vector multiplication with the stiffness matrix by 𝒪 (N (log N)d−1) operations. Simulation results up to level 10 are presented for a 6‐dimensional Helmholtz problem with variable coefficients. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On a digital approximation for pseudo‐differential operators
Authors: Vasilyev Alexander , Vasilyev Vladimir
Published: March 2018

We introduce a concept of a discrete pseudo‐differential operator using general ideas of the theory and would like to show correlations between continuous and discrete cases. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Parameter Identification in Stochastic Differential Equations to Model the Degradation of Metal Films
Authors: Pedretscher Barbara , Kaltenbacher Barbara , Bluder Olivia
Published: March 2018

In automotive industry, reliability analysis of smart power semiconductor devices is mandatory for safety reasons. To reliably forecast the lifetime of devices, a mathematical description of the material degradation due to applied stress is required. In this work, the degradation process of a metal film of interest under thermo‐mechanical stress is studied. Therefore, a bi‐layer test structure (metal film on substrate) is used. Degradation refers to the formation, growth and coalescence of voids, initializing cracks that propagate through the metal film. Since the film's elastic‐plastic deformation is triggered by random diffusional activities with drift, stochastic differential equations (SDEs) are used to model the evolution of the microstructural heterogeneity in the metallization.

The choice of an efficient numerical method to estimate the unknown parameters of the inverse problem is complex due to ill‐posedness and rare measurements. To address these challenges, an approach by Dunker and Hohage of 2014 is followed, where parameter identification in SDE models is performed on basis of the Kolmogorov forward (Fokker‐Planck) equation [1]. The mentioned approach has to be adapted to our application. Firstly, a stochastically consistent cost functional is defined, taking into account the different type of measurements and secondly, instead of estimating a function, a finite set of model parameters has to be recovered. For efficient computational optimization an adjoint approach is used to calculate the gradient of the cost functional. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Comparison of a Filter‐ and a Model Predictive Control Based Motion Cueing Algorithm
Authors: Ellensohn Felix , Schwienbacher Markus , Venrooij Joost , Rixen Daniel
Published: March 2018

Motion Cueing Algorithms (MCA) include control strategies to take into account the motion‐based driving simulator's restrictions concerning workspace limits and dynamic boundaries. A typical 6‐DoF simulator consists of a motion system which exhibits three translational and rotational degrees of freedom. Its actuators are capable of realizing accelerations, velocities and positions in a limited range. Based on these facts MCAs aim to generating realistic simulations of the driving motion (such as a driving manoeuvre) in order to immerse persons in virtual environments provided by the simulator. Filter‐based, classical MCAs belong to the most applied algorithms and mainly consist of linear transfer functions. Whereas, Model Predictive Control (MPC) algorithms rest upon a reduced model of the technical system's dynamics and, optionally, a model of the human motion perception system. An optimization problem subject to the restrictions of the motion system predicts the control variables over a time horizon. This paper addresses the strengths and weaknesses of the two stated approaches with focus on Motion Cueing errors. These errors describe discrepancies between the motion that is to be simulated (e.g. a driving manoeuvre) and the motion that is eventually provided by the combination of the MCA and the motion simulator. On basis of the results, it gives an outlook why optimization based algorithms have a higher potential to improve driving simulation. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Two Degree‐Of‐Freedom Online Compensation of a Piezoelectric Micro‐Positioning Unit
Authors: Schindlbeck Christopher , Pape Christian , Reithmeier Eduard
Published: March 2018

Due to their almost unlimited resolution and fast dynamics, piezoelectric actuators are a common choice for mechatronic systems targeting positioning tasks with high demands on precision. However, these piezoelectric actuators inherently suffer from nonlinear characteristics (mainly hysteresis and creep effects) which need to be addressed by appropriate control strategies. The operator‐based modified Prandtl‐Ishlinksii (mPI) approach does not only model hysteresis effects with asymmetries and creep effects but also provides an analytical solution for its inverse model. Online feedforward compensation of the aforementioned nonlinear effects can be realized by using the inverse model and additional weight adaptation. In this paper, online compensation via the mPI model is applied to a commercial micro‐positioning unit driven by piezoelectric actuators with more than one degree of freedom (DOF). For validation of the proposed approach, two coupled trajectories in the X‐Y plane are utilized. Subsequent tracking error analysis validates the efficacy of the stated approach. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Edge‐wise funnel synchronization
Authors: Trenn Stephan
Published: March 2018

Recently, it was suggested in [Shim & Trenn 2015] to use the idea of funnel control in the context of synchronization of multi‐agent systems. In that approach each agent is able to measure the difference of its own state and the average state of its neighbours and this synchronization error is used in a typical funnel gain feedback law, see e.g. [Ilchmann & Ryan 2008]. Instead of considering one error signal for each node of the coupling graph (corresponding to an agent) it is also possible to consider one error signal for each edge of the graph. In contrast to the node‐wise approach this edgewise funnel synchronization approach results (at least in simulations) in a predictable consensus trajectory. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the transformation formulas of the Hankel‐norm approximation
Authors: Benner Peter , Werner Steffen W. R.
Published: March 2018

The Hankel‐norm approximation of a linear time‐invariant system is a method of model reduction which yields the best approximation in the Hankel semi‐norm. For the application of the method to systems with a non‐singular descriptor matrix E we generalize the transformation formulas. The resulting formulas provide additional degrees of freedom which can be used to avoid undesired numerical operations or a resulting ill‐conditioned system. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Construction of a Lyapunov functional for a class of controlled population balance models
Authors: Zuyev Alexander , Kienle Achim , Benner Peter
Published: March 2018

In this paper, we consider a class of controlled population balance equations describing granulation processes in chemical engineering. Such a control system admits an equilibrium which is not asymptotically stable in general. In order to stabilize this equilibrium, we consider the perturbed system and introduce a Lyapunov functional candidate as a weighted L2‐norm. It is shown that the weight function for this construction may be defined in terms of solutions to a certain differential inequality. We present a solution of this differential inequality in a particular case and discuss possible extensions of this approach for multidimensional hyperbolic systems. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A new method for network redesign via rank one updates
Authors: Gernandt Hannes , Krauße Dominik , Sommer Ralf , Trunk Carsten
Published: March 2018

We present a method to place the eigenvalues of an electrical network towards a prescribed set of complex numbers by inserting an additional capacitance into the network. For the proof, we use recent results on rank one perturbations of regular matrix pencils. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A new bound for the distance to singularity of a regular matrix pencil
Authors: Berger Thomas , Gernandt Hannes , Trunk Carsten , Winkler Henrik , Wojtylak Michał
Published: March 2018

For regular matrix pencils 𝒜(s) = sE − A the distance to the nearest singular pencil in the Frobenius norm of the coefficients is called the distance to singularity. We derive a new lower bound for this distance by using the spectral theory of tridiagonal Toeplitz matrices. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
“Pre‐emptive strike”—the case for early treatment of hepatic sinusoidal obstruction syndrome with defibrotide
Authors: Rajagopal Revathi , Phillips Marianne B. , Gottardo Nicholas G.
Published: March 2018

The initial signs of hepatic sinusoidal obstruction syndrome (HSOS) can be challenging to recognize in children, especially outside the hematopoietic stem cell transplantation setting. To assist clinicians to promptly identify HSOS, the European Society for Blood and Marrow Transplantation has proposed pediatric HSOS diagnostic criteria which emphasize unexplained consumptive and transfusion‐refractory thrombocytopenia. To highlight the importance of these “bellwether” early signs of HSOS and the efficacy of pre‐emptive treatment with defibrotide, we describe the case of a child with a right 11th rib primitive neuroectodermal tumor who developed HSOS following focal radiotherapy and actinomycin‐D treatment.
The promise of piezoelectric polymers
Authors: Usher Timothy D , Cousins Kimberley R , Zhang Renwu , Ducharme Stephen
Published: March 2018

Recent advances provide new opportunities in the field of polymer piezoelectric materials. Piezoelectric materials provide unique insights to the fundamental understanding of the solid state. In addition, piezoelectric materials have a wide range of applications, representing billions of dollars of commercial applications. However, inorganic piezoelectric materials have limitations that polymer ferroelectric materials can overcome, if certain challenges can be addressed. This mini‐review is a practical summary of the current research and future directions in the investigation and application of piezoelectric materials with an emphasis on polymeric piezoelectric materials. We will assume that the reader is well versed in the subject of polymers, but not as familiar with piezoelectric materials. © 2018 Society of Chemical Industry