bluuebunny/crossref_metadata_2025 huggingface dataset:
Extending a finite strain hyperelastic micro‐sphere framework towards phase transformations
Authors: Ostwald Richard , Bartel Thorsten , Menzel Andreas
Published: October 2016

A finite strain micro‐sphere framework for hyperelastic solids elaborated by Carol et al. is extended towards the modelling of phase transformations in order to simulate polycrystalline solids under large deformations such as, e.g., shape memory alloys and shape memory polymers. The implemented phase transformation mechanism is based on statistical physics and is not restricted in terms of the number of solid material phases that can be considered, though we restrict the provided examples to two phases for the sake of conceptual clarity. The specifically chosen non‐quadratic format of the Helmholtz free energy functions considered on the micro‐plane level includes Bain‐type transformation strains for each of the phases considered. Following the Voigt assumption on the micro‐scale, identical total micro‐stretches act in each of the material phases, where a multiplicative decomposition into elastic and transformation‐related contributions is applied. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A multiscale view on shape effects in the computational characterization of magnetorheological elastomers
Authors: Rambausek Matthias , Keip Marc‐André , Miehe Christian
Published: October 2016

This contribution highlights the influence of the shape of specimens of magnetorheological composites on their response under homogeneous magnetic fields. The analysis is performed in terms of two‐scale computational characterization using a finite strain homogenization framework. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Large strain constitutive modelling of cellulose aerogels
Authors: Rege Ameya , Itskov Mikhail
Published: October 2016

In this contribution, we propose a non‐linear constitutive model for cellulose aerogels subject to compression. The model is based on the cellular microstructure of aerogels formed of square shaped unit cells of varying pore sizes with an isotropic spatial distribution. Under uniaxial compression, these cells tend to bend non‐linearly until pore collapse. This cell bending is described by the extended version of the Euler‐Bernoulli beam theory for large deflections. The macroscopic free energy of the whole network is obtained by integration of the microscopic energies of cells over all spatial directions. Cellulose aerogels with different cellulose content are simulated using the proposed model. The model predictions show good agreement with experimental data. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Characterization of short fiber reinforced polymers
Authors: Röhrig Céline , Diebels Stefan
Published: October 2016

This contribution presents a possibility how to characterize composite materials with respect to experimental testing. Furthermore a modeling approach is presented based on the experimental results.

Composite materials, like short fiber reinforced polymers, are of great interest because of their good properties for industrial applications. In this work a polybutylene terephthalate (PBT) reinforced by short glass fibers (GF) is characterized. An overview of the properties of the investigated material which are obtained from results of cyclic uniaxial tensile tests under monotonous and cyclic loading at a constant strain rate is given. The short fiber reinforced PBT shows elasto‐plastic material behavior and damage. The complete strain information in all directions is obtained by an optical area analysis over the whole surface using a digital image correlation (DIC) software. The optical sytem uses four cameras to realize a 360°‐3D‐measurement by a two‐sided stereography [2]. Based on the results of the experimental investigations with uniaxial tensile tests, a one‐dimensional macroscopic model to describe the material behavior is introduced and the results of the simulation are provided. The modeling description takes into consideration the observed behavior like elasto‐plasticity and damage. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Variational Treatment and Stability Analysis of Coupled Electro‐Mechanics
Authors: Vallicotti Daniel , Teichtmeister Stephan , Keip Marc‐André , Miehe Christian
Published: October 2016

Dielectric materials such as electro‐active polymers (EAPs) belong to the class of functional materials which are used in advanced industrial environments as sensors or actuators and in other innovative fields of research. Driven by Coulomb‐type electrostatic forces EAPs are theoretically able to withstand deformations of several hundred percents. However, large actuation fields and different types of instabilities prohibit the ascend of these materials. One distinguishes between global structural instabilities such as buckling and wrinkling of EAP devices, and local material instabilities such as limit‐ and bifurcation‐points in the constitutive response. We outline variational‐based stability criteria in finite electro‐elastostatics and design algorithms for accompanying stability checks in typical finite element computations. These accompanying stability checks are embedded into a computational homogenization framework to predict the macroscopic overall response and onset of local material instability of particle filled composite materials. Application and validation of the suggested method is demonstrated by a representative model problem. © 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Microplane modeling of cyclic behavior of concrete: a gradient plasticity‐damage formulation
Authors: Zreid Imadeddin , Kaliske Michael
Published: October 2016

A microplane model is developed to simulate the behavior of concrete under cyclic loading conditions. Pure damage mechanics or pure plasticity models yield satisfactory results for concrete under monotonic loading but cannot capture correctly the unloading and reloading response. Therefore, coupling damage and plasticity is necessary for accurate constitutive modeling of concrete. The microplane model offers a straightforward approach to simulate induced anisotropy by formulating the material laws on many randomly oriented planes. Distinguishing between compression and tension response using the proper plastic yield function and damage laws is considered. Furthermore, gradient enhancement is employed to handle the pathological mesh sensitivity related to strain softening. The new formulation is implemented within a 3D finite element code and a numerical example is simulated and compared to experiments in order to evaluate the capabilities of the model. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A multiscale approach to thermoplastic deformation
Authors: Čanađija Marko , Munjas Neven , Brnić Josip
Published: October 2016

The paper applies a multiscale formulation to thermoplasticity problems. The plasticity model is based on the variationally consistent formulation at the micro level. It is suitable for problems involving large strains. The temperature field is calculated at the macro level and is assumed to be homogeneous at the micro level. The proposed formulation is implemented in the Abaqus software by means of user subroutines. User subroutines are introduced both at the macro and at the micro level. The consistent tangent operator is calculated by the numerical differentiation procedure. Implementation details are briefly addressed. Finally, the methodology is verified on a representative example and relevant results are presented and discussed. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Impact of heterogeneously distributed solid particles in the transition zone between porous and free flow domain
Authors: Falkner Nadine , Steeb Holger
Published: October 2016

We present a direct numerical simulation approach for the simulation of shallow water flow using the particle based meshfree Smoothed Particle Hydrodynamics (SPH) method. Simulations of single phase flow are done to characterize the occurring flow parameters on both macro‐scale and pore‐scale. More precisely, we examine initiation of motion and sediment transport as appearing at the interface between a free flow and porous flow domain under parallel flow conditions. Therefore we evaluate three theoretical models presenting analytical solutions for this coupled problem. Moreover, we discuss the influence of heterogeneities at the interface on forces on single grains by implementing and testing various microstructures into our numerical model. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Model reduction for multi‐component porous‐media models of biological materials using POD‐DEIM
Authors: Fink Davina , Ehlers Wolfgang
Published: October 2016

In the context of clinical treatment, reliable models of biological materials can provide further information of the occurring processes. For this purpose, the prediction of various simulation scenarios or real‐time simulations is desirable. A broad variety of biological materials, such as intervertebral discs or skeletal muscles, exhibit a porous microstructure and are conveniently simulated using a macroscopic continuum‐mechanical modelling approach. Therefore, the complex inner structure is regarded in a multi‐phasic manner using the Theory of Porous Media (TPM). The solution of the descriptive set of coupled partial differential equations (PDE) is approximated with the finite‐element (FE) method.

In the context of FE simulations, computing time and numerical effort is an important issue because the number of degrees of freedom (DOF) of such coupled problems can become very large. To reduce the numerical effort, the method of proper orthogonal decomposition (POD) is applied in combination with the discrete‐empirical interpolation method (DEIM). Using the POD method, representative state variables (so‐called snapshots) are stored in pre‐computations using the initial full system and are approximated with a low‐dimensional subspace. Additionally, snapshots of the nonlinear terms of the differential equation are stored to approximate the nonlinearities using the DEIM. Dealing with porous‐media problems in biomechanical applications, the primary variables (such as the solid displacement or the pore pressure) exhibit a different temporal behaviour. In order to take this into account, the snapshots are divided in separated parts for each primary variable. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
3D Fluid‐Structure Interaction Experiment and Benchmark Results
Authors: Hessenthaler Andreas , Friedhoff Stephanie , Röhrle Oliver , Nordsletten David A.
Published: October 2016

Testing and comparison of well‐known and recently developed numerical methods and algorithms are the key for their thorough assessment and continued improvement. We will present a short overview of a new 3D FSI experiment that proposes steady and dynamic test cases (referred to as Phase I and Phase II), complementing well‐known standard benchmark problems. In this context, we discuss a mathematical FSI model, that has previously proofed capable of good predictions, and demonstrate that the periodic steady‐state solution for Phase II is insensitive to the initial deformation of the solid. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Computational characterization of ferroelectric solids with a 3D Preisach model based on an orientation distribution function
Authors: Labusch Matthias , Schröder Jörg
Published: October 2016

Ferroelectric or ferromagnetic materials show an interaction between mechanical deformations and polarization or magnetization. A few multiferroic materials possess both ferroic properties and exhibit a magneto‐electric (ME) coupling. These ME properties can be achieved in two‐phase composites, which combine ferroelectric and ferromagnetic characteristics. To predict a realistic material behavior and a more precise ME coefficient, the application of suitable material models which describe the nonlinear hysteretic behavior is of particular importance. In the present contribution we focus on the characterization of a nonlinear ferroelectric material behavior, in terms of a 3D Preisach model based on an orientation distribution function. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Modeling the constitutive behavior of ferroelectric, ferromagnetic and multiferroic materials by using the condensed method (CM)
Authors: Lange Stephan , Ricoeur Andreas
Published: October 2016

Due to the multifunctional applicability, smart materials are of particular interest in the field of material modeling. Most of the developed models, describing the nonlinear behavior, are implemented within the framework of the Finite Element Method (FEM). However, most investigations are restricted to simple boundary value problems (BVP) under uniaxial loading and their goal is the calculation of hysteresis loops. Regarding this circumstance, the so‐called condensed method (CM) is introduced to investigate the macroscopic polycrystalline ferroelectric material behavior at a global material point without any kind of discretization scheme. In the presented paper, the CM is extended towards ferromagnetic and multiferroic material behavior. Moreover, numerical results for a pure ferromagnetic behavior and a comparison between the magnetoelectric coupling coefficient calculated by the FEM and the CM are presented. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A phase field model for martensitic transformations with a temperature‐dependent separation potential
Authors: Schmidt Simon , Plate Carolin , Müller Regina , Müller Ralf , Meiser Jerome , Urbassek Herbert M
Published: October 2016

Metallic materials often exhibit a complex microstructure with varying material properties in the different phases. Of major importance in mechanical engineering is the evolution of the austenitic and martensitic phases in steel. The martensitic transformation can be induced by heat treatment or by plastic surface deformation at low temperatures. A two dimensional elastic phase field model for martensitic transformations considering several martensitic orientation variants to simulate the phase change at the surface is introduced in [1]. However here, only one martensitic orientation variant is considered for the sake of simplicity. The separation potential is temperature dependent. Therefore, the coefficients of the Landau polynomial are identified by results of molecular dynamics (MD) simulations for pure iron [1]. The resulting separation potential is applied to analyse the mean interface velocity with respect to temperature and load. The interface velocity is computed by use of the dissipative part to the configurational forces balance as suggested in [3]. The model is implemented in the finite element code FEAP using standard 4‐node elements with bi‐linear shape functions. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Modeling of contaminant migration in groundwater: A continuum mechanical approach using in the theory of porous media
Authors: Seyedpour S.Morteza , Ricken Tim
Published: October 2016

Over the past few decades, soil and subsequently groundwater contamination have raised growing concern as factors threatening the life of humans and other organisms as well as agricultural production sustainability. This study focuses on continuum mechanical multi‐phase, multi‐component formulation within the theory of porous media (TPM) to simulate the convective as well as diffusive driven contaminant tranport in groundwater including conversion processes. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A Magneto‐Mechanically Coupled Phase‐Field Model at Large Deformation
Authors: Sridhar Ashish , Keip Marc‐André , Miehe Christian
Published: October 2016

The aggregate magneto‐mechanical behavior of magneto‐rheological elastomers (MREs) stems from the magnetic properties of the ferromagnetic inclusion and the mechanical properties of the matrix material. We propose a large deformation micro‐magnetic theory, to predict the behavior and interaction of ferromagnetic particles inside an elastomeric matrix. A rate‐type variational principle, with the magnetization as the order parameter is proposed. A large deformation Landau‐Lifshitz‐Gilbert equation for the time evolution of the magnetization, is obtained directly from the proposed variational principle. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
External boundary value problems in the quasi static theory of elasticity for triple porosity materials
Authors: Svanadze Merab
Published: October 2016

In this paper the quasi static linear theory of elasticity for materials with triple porosity 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. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
External boundary value problems in the quasi static theory of viscoelasticity for Kelvin‐Voigt materials with double porosity
Authors: Svanadze Maia M.
Published: October 2016

In the present paper the linear quasi static theory of viscoelasticity for Kelvin‐Voigt materials with double porosity is considered. The basic external boundary value problems (BVPs) of steady vibrations in this theory are formulated. The uniqueness and existence theorems for regular (classical) solutions of the BVPs are proved by using of the potential method (boundary integral equations method) and the theory of singular integral equations. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A stereographic projection based phase field model for ferromagnetics
Authors: Zhou Ziqi , Yi Min , Xu Bai‐Xiang
Published: October 2016

By using the stereographic projection to satisfy the constant magnetization magnitude, a constraint‐free phase field model is developed for ferromagnetic materials. Implemented by finite element method, the model is shown to be capable of simulating magnetic domain, vortex, and hysteresis curves. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Aspects of computational homogenization in magneto‐mechanics
Authors: Zabihyan Reza , Mergheim Julia , Javili Ali , Steinmann Paul
Published: October 2016

In the present work, the behavior of heterogeneous magnetorheological elastomers undergoing large deformations under the action of magnetic fields is studied. First‐order computational homogenization is used to derive the homogenized stresses and magnetic inductions of the macro‐structure from the response of the underlying micro‐structure. Different types of boundary conditions are applied to solve the micro problem where the constitutive law is assumed to be known. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Partitioned treatment of surface‐coupled problems with application to the fluid‐porous‐media interaction
Authors: Jamei Said , Asgharzadeh Pouyan , Ehlers Wolfgang
Published: October 2016

Employing a decoupled solution strategy for the numerical treatment of the set of governing equations describing a surface‐coupled phenomenon is a common practice. In this regard, many partitioned solution algorithms have been developed, which usually either belong to the family of Schur‐complement methods or to the group of staggered integration schemes. To select a decoupled solution strategy over another is, however, a case‐dependent process that should be done with special care. In particular, the performances of the algorithms from the viewpoints of stability and accuracy of the results on the one hand, and the solution speed on the other hand should be investigated.

In this contribution, two strategies for a partitioned treatment of the surface‐coupled problem of fluid‐porous‐media interaction (FPMI) are considered. These are one parallel solution algorithm, which is based on the method of localised Lagrange multipliers (LLM), and one sequential solution method, which follows the block‐Gauss‐Seidel (BGS) integration strategy. In order to investigate the performances of the proposed schemes, an exemplary initial‐boundary‐value problem is considered and the numerical results obtained by employing the solution algorithms are compared. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Material modelling of a sheet‐layered lamination stack by homogenization
Authors: Baloglu Maximilian Volkan , Willner Kai
Published: October 2016

In electrical machines, sheet‐layered lamination stacks play an important role for the mechanical behavior of the system. Especially the interlayer between individual sheets and their interaction have a severe influence on the structure. In the context of performance and computational effort, it is desirable to avoid a full FE simulation of a lamination stack with every single sheet. Therefore, homogenization techniques are presented to identify a transversely isotropic surrogate material model, while Zero‐Thickness elements are utilized during this process to cover the interplay of single sheets. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A homogenisation strategy for micromorphic continua based on particle mechanics
Authors: Bidier Sami , Ehlers Wolfgang
Published: October 2016

Materials with a granular microstructure frequently fail in narrow zones due to strain localisation. Examplarily, one may look at the shear‐zone development in dry sand during bi‐ and triaxial loading, where grains in the shear‐zone exhibit large displacements and rotations. Furthermore, localisation is also observed in materials, where the microstructure consists of grains and a binding material, such as for example metal‐casting moulds. Here, sand grains are bound together via a polyurethan‐based material and macroscopic material failure originates from the deformation and breakage of the binder material. Within a continuum‐based modelling approach, these microstructural effects can be accounted for by the consideration of an additional microcontinuum at each material point of the macroscopic body. These extended continuum theories, such as the micromorphic continua and its micropolar and microstrain sub‐formulations, assume a characteristic microcontinuum deformation on a lower scale and have been successfully applied in the field of granular media. Exemplarily, in the framework of a micropolar continua, it is possible to contact forces to stresses and couple stresses via an appropriate homogenisation technique. This method includes the introduction of a Representative Elementary Volume (REV) on the mesoscale situated between the particle and the continuum scale. In this contribution, a homogenisation strategy based on a particle‐centre‐based REV definition is presented that is generally valid for micromorphic and micropolar continua. Therefore, a grain‐binder microstructure is investigated, where particle rotations contribute to the micropolar part, while binder deformations yield the additional macromorphic character. Numerical examples are given, where results from discrete‐element simulations are locally averaged and show the individual activation of the microcontinuum characteristics in the localised zones. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The heterogeneous multiscale finite element method FE‐HMM for the homogenization of linear elastic solids
Authors: Eidel Bernhard , Fischer Andreas
Published: October 2016

The objective of the present paper is a formulation of the Heterogeneous Multiscale Finite Element Method (FE‐HMM) for the homogenization of linear elastic solids in a geometrical linear frame, and doing so, for the first time, of a vector‐valued field problem. The macro stiffness is estimated by stiffness sampling on heterogeneous microdomains in terms of a modified quadrature formula, which implies an equivalence of energy densities of the microscale with the macroscale. Existing a‐priori estimates are assessed and used for optimal micro‐macro refinement strategies in the H1‐norm and the L2‐norm. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Mean field homogenization and experimental investigation of short and long fiber reinforced polymers
Authors: Kehrer Loredana , Pinter Pascal , Böhlke Thomas
Published: October 2016

The effective elastic material properties of short fiber reinforced polypropylen are determined by means of the self‐consistent (SC) method and the interaction direct derivative (IDD) method. In order to account for thermoelastic effective material properties, a Hashin‐Shtrikman (HS) based two‐step homogenization method with variable reference stiffness is used. The influence of the reference stiffness, dependent on a scalar parameter is investigated. Information on the microstructure are derived by computed tomography scans (µCT) and considered within the homogenization schemes. Thermomechanical properties of a long fiber reinforced polymer (LFRP) and a short fiber reinforced polymer (SFRP) are obtained by means of dynamic mechanical analysis (DMA). Simulation results for SFRP are compared to experimental results. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Multiscale simulation for description of rubber friction
Authors: Lang Ronny , Falk Korbinian , Kaliske Michael
Published: October 2016

The interaction between tire and road generates the transferable forces, which are necessary for driving dynamics and safety. These forces are based on friction between rubber material and pavement surface and depend on the roughness of the pavement, the slip velocity, the contact pressure and the temperature. Based on the finite element method, the friction coefficient is calculated by numerical simulation. The roughness of the pavement surface is described by the height difference correlation function (HDCF), which allows partitioning into different length scales. This multiscale approach is suitable to understand and to evaluate friction phenomena. These phenomena are hysteresis friction based on dissipation inside the rubber material and adhesion friction, which describes the direct bonding between two materials. Given, that the material parameters of rubber highly depend on temperature and the frictional dissipation leads to a warming of the rubber, the provision for these effects is necessary for a realistic desciption of friction. The method allows an understanding of friction phenomena on the micro‐scale like the real contact area or the microscopic contact pressure. Also, the temperature distribution inside the tire cross‐section can be illustrated. The resulting coefficient of friction is validated by experimental data based on linear friction tests and compared to analytical solutions. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The Logarithmic finite element method in a multigrid setting
Authors: Schröppel Christian , Wackerfuß Jens
Published: October 2016

The Logarithmic finite element (“LogFE”) method is a novel finite element approach for solving boundary‐value problems proposed in [1]. In contrast to the standard Ritz‐Galerkin formulation, the shape functions are given on the logarithmic space of the deformation function, which is obtained by the exponentiation of the linear combination of the shape functions given by the degrees of freedom.

Unlike many existing multigrid formulations, the LogFE method allows for a very smooth interpolation between nodal values on the coarse grid. It can thus avoid problems with regard to locking and convergence that appear in multigrid applications using only linear interpolation, especially for larger corsening factors.

We illustrate the use of the LogFE method as a coarse grid algorithm, in conjunction with an atomistic finite element method on the fine grid, for calculating the mechanical response of super carbon nanotubes. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Analysis of multiscale methods for time‐harmonic Maxwell's equations
Authors: Henning Patrick , Ohlberger Mario , Verfürth Barbara
Published: October 2016

We shortly revisit the Heterogeneous Multiscale Method (HMM) for the time‐harmonic Maxwell's equations in locally periodic media as introduced in [1]. The optimal a priori bounds in the H(curl) and H−1 norm predicted theoretically are justified by a numerical example in this contribution. The setting consists of a periodic inverse permeability, inspired by [2], and as reference solution a computation on a fine (well‐resolved) grid is used. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the initial phase of the triple deck stage in marginally separated flows
Authors: Braun Stefan , Scheichl Stefan
Published: October 2016

The method of matched asymptotic expansions is used to investigate marginally separated boundary layer flows (laminar or alternatively transitional separation bubbles) at high Reynolds numbers. Typical examples include, among others, the flow past slender airfoils at small to moderate angels of attack and channel flows with suction. As is well‐known, classical (hierarchical) boundary layer computations usually break down under the action of an adverse pressure gradient on the flow, a scenario associated with the appearance of the Goldstein separation singularity. If, however, the parameter controlling the strength of the pressure gradient (the angle of attack or the relative suction rate in the examples mentioned above) is adjusted accordingly, the application of a local viscous‐inviscid interaction strategy is capable of describing localized boundary layer separation. Moreover, taking into account unsteady effects and flow control devices allows the investigation of the conditions leading to forced or self‐sustained vortex generation and the subsequent evolution process culminating in bubble bursting. Within the asymptotic formulation of this stage bubble bursting is associated with the formation of finite time singularities in the solution of the underlying equations and a corresponding break down. The distinct blow‐up structure gives rise to a fully non‐linear triple deck interaction stage featuring shorter spatio‐temporal scales characteristic of the successive vortex evolution process. The paper will focus on the numerical treatment of the initial phase of the latter stage. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Hartmann duct flow at moderate magnetic Reynolds numbers
Authors: Bandaru Vinodh , Boeck Thomas , Schumacher Jörg
Published: October 2016

The application of a uniform external magnetic field on the turbulent duct flow of an electrically conducting fluid leads to several interesting changes in the structure and the mean charateristics of the flow. This is fairly well understood from the existing studies of duct flows in the low magnetic Reynolds number (Rm) limit. In this paper, we present the results for magnetohydrodynamic duct flow at moderate Rm obtained from direct numerical simulations (DNS). Several differences are observed to occur in this case as compared to low Rm flows, such as increased Hartmann layer thickness and enhanced large scale turbulence in the core region of the duct cross‐section due to partial expulsion of magnetic flux. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Statistical characteristics and time autocorrelation coefficients of the turbulent swirl flow in pipe
Authors: Čantrak Đorđe , Janković Novica , Ilić Dejan
Published: October 2016

In this paper is presented experimental investigation of the turbulent swirl flow in pipe generated by three axial fans of various geometries. One of these fans (model ZP) generates Rankine swirl, while other two, industrial fans, produce mainly solid body circumferential velocity profile. One‐component laser Doppler anemometry (LDA) is employed in this research. Downstream transformations of the non‐dimensional time‐averaged velocity profiles are analyzed. Distributions of turbulence levels are discussed for all three fans in both measuring sections, as well as statistical moments of higher orders for fluctuations in the axial, radial and circumferential directions. Applied correlation theory revealed turbulence structure and its statistical nature. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Comparison of isotropic and anisotropic subgrid scale models in Large Eddy Simulations of a backward‐facing step and square duct flow
Authors: Temme Nils , Niemann Martin , Fröhlich Jochen
Published: October 2016

The present paper employs the anisotropic explicit algebraic subgrid‐stress model (EASM), proposed by Marstorp et al. J. Fluid. Mech. 639, 403 ‐ 432 (2009), developed in the spirit of an earlier explicit algebraic RANS‐model for statistical closures. The EASM in its elementary, computationally efficient non‐dynamic version is used for Large Eddy Simulation of three‐dimensional flows over a backward‐facing step at a bulk Reynolds number of 4805 and a square duct at 2205. Its performance is assessed by comparison with experimental data and an own Direct Numerical Simulation. Furthermore, a set of eddy‐viscosity models, including the recent σ‐model, is employed for comparison. Various statistical quantities are evaluated to assess the respective performance of the different models showing, that the anisotropic EASM compares favorably to the other models. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
New explicit algebraic stress model for three‐dimensional turbulent flows
Authors: Vigdorovich Igor , Foysi Holger
Published: October 2016

An explicit algebraic turbulent‐stress model is built in the framework of so‐called Rodi's weak‐equilibrium approximation, which, taking into account the known model representations for the pressure‐strain‐rate correlation and turbulence‐dissipation rate, reduces the differential equations for the Reynolds‐tensor components to a system of quasi‐linear algebraic equations for the five independent components of the anisotropy tensor B. We propose an original method for solving this quasi‐linear system. The tensor in question B is sought in the form of an expansion in a tensorial basis formed from the mean strain and rotation rate tensors which contains only five elements. The expansion's coefficients are functions of five simultaneous invariants of these tensors. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A semi‐implicit multi‐fluid SPH algorithm suitable for GPU
Authors: Fürstenau Jan‐Philipp , Avci Bircan , Wriggers Peter
Published: October 2016

Multi‐fluid flows are a common problem in process and naval engineering. In this paper an incompressible Smoothed Particle Hydrodynamics (SPH) algorithm is presented, to solve multi‐fluid flows with high density ratios (≤ 1000). The results of the Rayleigh‐Taylor instability and the rising bubble test case are compared with the results obtained by a weakly compressible SPH algorithm with respect to the results obtained by grid‐based solvers. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Simulation of mass transfer in liquid/liquid slug flow
Authors: Heckmann Christian , Ehrhard Peter
Published: October 2016

The mass transport for a liquid/liquid extraction system is examined using a numerical concept following the idea of the interface‐tracking method. Separate, body‐fitted, static computational domains are arranged around an imported steady‐state interface topology. The domains are coupled at the free interface to capture the behaviour of the conjugated system. The steady‐state hydrodynamics are the basis for the simulation of the transient mass transport, which is calculated as a passive scalar concentration or one‐way coupling. The investigation is restricted to the extraction from the disperse to the continuous phase. Simulation results for an extraction from the disperse to the continuous phase show that most of the mass is transferred through the wall‐film region into the continuous phase. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Tangential Homology for Defect Detection in the Time of Flight Diffraction Method (TOFD)
Authors: Cuenca Jose F. , Iske Armin , Labud Patrick A. , Nemitz Oliver
Published: October 2016

Ultrasonic non‐destructive testing (NDT) techniques are widely used in industry to evaluate properties of a component, material or system without causing any damage. The time of flight diffraction method (TOFD) is regarded as a valuable procedure for the detection of welding defects. We propose an innovative concept for NDT relying on tangential homology. The novel technique allows us to analyze topological properties of manifolds from discrete data. This in turn provides an automated classification of TOFD defects which does not require any interactions with a human operator. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optimal adaptation of acoustic black holes by evolutionary optimization algorithms
Authors: Rothe Sebastian , Ghaffari Mejlej Vahid , Langer Sabine C. , Vietor Thomas
Published: October 2016

The interest of engineers is focusing increasingly on a reduced sound radiation of constructions. In particular, structures with a large surface, such as cabin linings of airplanes as well as a roof or a bottom plate of cars, tend to be good acoustic radiators and lead to an annoyance of passengers. As countermeasures, often complex and time consuming design changes or expensive active measures are used. In many cases, a more elegant and cheaper option is to improve the acoustic properties by using passive measures. Acoustic black holes are an innovative passive method which combine two advantages: A material reduction by improving acoustic properties is performed.

The main idea is to guide and to focus acoustically critical bending waves by a specific wall thickness diminution. Through targeted local damping placement in the middle of an acoustic black hole, a structure can be globally damped in a very efficient way. The efficiency depends on the position and size of the acoustic black holes [1]. Finding the optimal size and position on the structure is therefore an important challenge.

This paper introduces a new strategy to find an optimal position of acoustic black holes to reduce the sound radiation of plane structures by using evolutionary optimization algorithms. Numerical calculations are exemplarily shown on a rectangular plate. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Elastic Wave Propagation in Soft Microstructured Composites Undergoing Finite Deformations
Authors: Galich Pavel , Slesarenko Viacheslav , Rudykh Stephan
Published: October 2016

We consider the propagation of elastic waves in soft composite materials undergoing large deformations. The analysis is performed in terms of small amplitude motions superimposed on a deformed state. By consideration of 2D periodic laminates and 3D fiber composites, we find that an applied deformation influences the elastic waves through the change in the microstructure, and through the change in the local material properties. These effects can be significantly amplified by the deformation induced elastic instability phenomenon leading to microstructure transformations. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Simulation and analysis of turbulent MHD channel flow with a streamwise magnetic field
Authors: Krasnov Dmitry , Boeck Thomas
Published: October 2016

We perform large‐eddy simulations of turbulent MHD channel flow with a streamwise magnetic field using a pseudo spectral method. The streamwise magnetic field leads to turbulent drag reduction due to the selective Joule damping of certain flow structures. Near the walls, the turbulent mean velocity profile retains the logarithmic layer but the von Karman constant decreases with increasing magnetic field strength. In the outer region, the flow is characterized by persistent streaky structures of large streamwise extent, which lead to a rather flat mean velocity profile. In addition, the streamwise velocity fluctuations develop a pronounced second peak upon increasing the magnetic induction as well as a second logarithmic layer that increases in steepness. We find that Prandtl's classical mixing‐length model with a variable Kármán constant can describe the modified logarithmic layer reasonably accurately in a wide range of Reynolds and Hartmann numbers. However, the flow modification near the center of the channel is not properly captured by this approach. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical investigation of the behavior of thin flexible membranes in interaction with a fluid domain
Authors: Makaremi Masouleh M. , Wozniak G.
Published: October 2016

With the increasing concern over massive damage ensuing from flood, the application of flood protection systems has aroused growing interest. As part of an interdisciplinary project regarding the development of a temporary installable textile protective system against flood, we study, how the thin flexible membrane behaves in interaction with a fluid domain numerically and experimentally. The numerical research was carried out by means of the FV solver available in Star CCM+ based on both morphing motion model and on the basis of overset mesh and morphing motion model, with the help of a FEM solver available in Abaqus and finally through FSI Star CCM+ and Abaqus Co‐Simulation (interconnection between FV solver for fluid domain analysis and FEM solver for solid domain analysis). A key challenge facing us was the implementation of proper damping models to address the problem with oscillation of the membrane. We have examined, how the change in water level affects the behavior of the thin flexible membrane. A comparable physical validation with respect to experimental model has been achieved through numerical analysis. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Drag reduction via spanwise transversal traveling waves of smooth and structured surfaces
Authors: Meysonnat Pascal S. , Schröder Wolfgang
Published: October 2016

The turbulent wall‐shear stress can be influenced through passive and active means. The effect of combining active and passive methods is briefly analyzed using transversal traveling waves for the active method and riblets at an optimum spacing as a passive method. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Active Flow Control Implemented in a Multi‐Stage High‐Speed Axial Compressor
Authors: Schwerdt Lutz , Siemann Jan , Seume Joerg R.
Published: October 2016

The decrease of fuel consumption is a main objective in the development of modern aircraft engines and heavy‐duty gas turbines. Especially at off‐design conditions, one promising approach to suppress flow losses and to increase the efficiency of the compressor is Active Flow Control (AFC) by aspiration or injection. Aerodynamically, the compressor flow of a gas turbine responds more sensitively to volatile flow conditions than the turbine flow because of the positive pressure gradient in a compressor achieved by flow deceleration. In a decelerating flow, particularly at off‐design operating conditions, the compressor flow tends to separate from the blade surfaces. This flow separation causes unstable operating conditions inside the flow path resulting in low overall engine efficiency. Thus it is obvious that counter‐measures against increased flow losses at off‐design operation should be concentrated on the compressor. Considering industrial objectives, both the performance increase and the operating range enhancement are subjects of current compressor research, as is a reduction of vanes or even entire stages. While a reduced vane count reduces cost, even greater benefits can be gained if entire stages could be eliminated and thus the number of rotor discs reduced, which further reduces cost along with reducing the length of the rotor which could also improve rotor dynamics. For all of these purposes, different AFC approaches were implemented in a four stage axial compressor (4AV) and were experimentally examined. This paper presents an overview of the past and ongoing AFC research at the Institute of Turbomachinery and Fluid Dynamics (TFD). (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Experimental investigation of a flexible high‐water protection system in a down‐scale water channel flow
Authors: Schade Klaus‐Peter , Sommer Oliver , Wozniak Günter
Published: October 2016

As part of an interdisciplinary major project regarding the development of a flexible and efficiently installable high‐water protective system we investigated the behaviour of different system parts of flexible channel walls (thin, plastic slides/membranes) and types of self‐installing water barricades subjected to different water channel flows experimentally and numerically. In the experiments we studied the strain of different flexible channel walls and the efficiency of various water barricades in a down‐scale model by optical deformation, flow velocity and time measurements, respectively. We varied the water height as well as the representative flow velocity of the water channel flow and we also examined the influence of water waves onto these high‐water protective system parts. Selected results will be shown and described concerning the stress on the used model‐slide. In addition we applied similarity theory not only for general data presentation but also to generate findings regarding a future practical implementation in technical applications. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Wall‐cooling effect on linear stability of compressible streamwise corner‐flow
Authors: Staudenmeyer Jennifer , Rist Ulrich
Published: October 2016

The influence of constant wall temperatures on the compressible flow along a right‐angled corner and its stability behaviour is investigated by temporal local linear stability analysis. Wall‐cooling up to 90 % of the adiabatic wall‐temperature for a subsonic flow (Ma = 0.95) is considered. The maximum cross‐flow velocity along the corner bi‐sector for the base flow with adiabatic wall boundary condition is higher than for the base flow with constant adabatic wall‐temperature. For increasing wall temperature, the amplification rate of the viscous modes decreases to a lesser extent than that of the corner mode, therefore the critical Reynolds‐number is further on defined by the first viscous mode. The spatial behaviour is displayed using N‐factors computed from Gaster‐transformed modes. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Enhanced and reduced natural convection using magnetic fluid in a square cavity
Authors: Szabo Peter S. B. , Früh Wolf‐Gerrit
Published: October 2016

Natural convection using a magnetic fluid was studied in a square cavity under the influence of a permanent magnet. The aim was to explore the degree by which heat transfer may be controlled, enhanced or reduced, by investigating a set of different distances of a permanent magnet to the cavity. These distances of the magnet were set such that the cavity was in some cases fully dominated by buoyancy or by the magnetic body force and in other cases partly dominated by either of both body forces in different parts of the fluid. The effect on heat transfer was characterised by an averaged Nusselt number, Rayleigh and magnetic Rayleigh number. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
An entropic gradient structure for quasi‐steady‐state approximations of chemical reactions
Authors: Disser Karoline
Published: October 2016

Passing to the limit of an infinite reaction rate in a slow‐fast system of chemical reactions provides a quasi‐steady state approximation (QSSA) of these systems. In case of reactions with detailed balance condition, this approximation includes a dimension reduction to a smaller state space. We show that the limit dynamics carry an entropic gradient structure on this smaller space. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Rank‐one convexity implies polyconvexity for isotropic, objective and isochoric elastic energies in the two‐dimensional case
Authors: Martin Robert J. , Neff Patrizio , Ghiba Ionel‐Dumitrel
Published: October 2016

We show that in the two‐dimensional case, every objective, isotropic and isochoric energy function which is rank‐one convex on GL+(2) is already polyconvex on GL+(2). Thus we negatively answer Morrey's conjecture in the subclass of isochoric nonlinear energies, since polyconvexity implies quasiconvexity. Our methods are based on different representation formulae for objective and isotropic functions in general as well as for isochoric functions in particular. We also state criteria for these convexity conditions in terms of the deviatoric part of the logarithmic strain tensor. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)