bluuebunny/crossref_metadata_2025 huggingface dataset:
Towards Phase Field Modeling of Ductile Fracture in Gradient‐Extended Elastic‐Plastic Solids
Authors: Aldakheel Fadi , Mauthe Steffen , Miehe Christian
Published: December 2014

The modeling of failure in ductile metals must account for complex phenomena at a micro‐scale as well as the final rupture at the macro‐scale. Within a top‐down viewpoint, this can be achieved by the combination of a micro‐structure‐informed elastic‐plastic model with a concept for the modeling of macroscopic crack discontinuities. In this context, it is important to account for material length scales and thermo‐mechanical coupling effects due to dissipative heating. This can be achieved by the construction of non‐standard, gradient‐enhanced models of plasticity with a full embedding into continuum thermodynamics [1,2]. The modeling of macroscopic cracks can be achieved in a convenient way by recently developed continuum phase field approaches to fracture based on regularized crack discontinuities. This avoids the use of complex discretization methods for crack discontinuities, and can account for complex crack patterns within a pure continuum formulation. Moreover, the phase field modeling of fracture is related to gradient theories of continuum damage mechanics, and fits nicely the structure of constitutive models for gradient plasticity. The main focus of this work is the extensions to gradient thermoplasticity and phase field formulation of ductile fracture, conceptually in line with the work [3]. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Computational aspects of three‐phase modelling using a suction dependent cap model
Authors: Gamnitzer Peter , Hofstetter Günter
Published: December 2014

This contribution is on computational aspects of the application of a suction dependent modified cap model for geomaterials, which was presented at the 84th GAMM annual meeting. The modified cap model features an elasto‐plastic material response and is formulated in terms of two independent stress variables, generalised effective stress and matric suction. It has a smooth, suction dependent yield surface. The cap model is usually used in the context of coupled three‐phase simulations. Thus, the computational aspects that will be discussed below comprise several levels: i) the stress projection level (at which a robust one equation projection scheme with improved start‐up procedure is applied), ii) the nonlinear system solution level (at which a line search is suggested to increase robustness) and iii) the linear system solution level (for which iterative solvers and preconditioners are discussed which have been found suitable for the problem under consideration). (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the simulation of soils under rapid cyclic loading conditions
Authors: Schenke Maik , Markert Bernd , Ehlers Wolfgang
Published: December 2014

When simulating soils, which are subjected to dynamic loadings conditions, base on a convenient soil model, special attention has to be paid to the approximation of the real problem by a suitable initial‐boundary‐value problem (IBVP). In this regard, the semi‐infinite half‐space is split into a near‐field, which is, in general, the domain of interest, and a far‐field. However, truncating the half‐space at the near field, which is often sufficient in quasi‐static simulations, introduces artificial boundaries at which, in dynamic analysis, the incident waves are reflected back into the domain of interest. Several methods have been proposed in the literature to overcome this problem. The present contribution proceeds from a coupling approach, where the near and the far field are discretised with finite and infinite elements, respectively, and where additionally an energy‐absorbing layer is introduced at the interface. The aim of the present contribution is to perform a parametric study associated with a biphasic soil model based on the Theory of Porous Media (TPM). Therein, numerical simulations are carried out in order to judge the energy‐absorbing capabilities of the present far‐field treatment under different parametric conditions. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Macroscopic and mesoscopic modeling based on the concept of generalized stresses for cutting simulation
Authors: Cheng Chun , Mahnken Rolf , Uhlmann Eckart , Ivanov Ivan Mitkov
Published: December 2014

Based on the concept of generalized stresses proposed by GURTIN [2] and FOREST et al. [1] macro‐ and meso‐scopic modelling are presented. For the macroscopic modelling we develop a multi‐mechanism model for strain rate and temperature dependent asymmetric plastic material behavior accompanied by phase transformation with consideration of the trip‐strain. Furthermore, we extend the multi‐mechanism model with the gradient of phase fraction, which is considered as an extra degree of freedom. For mesoscopic modelling a phase field model is implemented for describing phase transformations. For the scenario of a cutting process we have a martensite‐austenite‐martensite transformation. A generalized principle of virtual power is postulated involving generalized stresses and used to derive the constitutive equations for both approaches. Furthermore, parameters of the multi‐mechanism model related to visco‐plasticity with SD‐effect and the trip‐strain are identified for the material DIN 100Cr6. In the examples a cutting simulation for testing the multi‐mechanism model and a phase‐transformation simulation are shown. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A microscopically motivated continuum model for cellular materials
Authors: Diebels Stefan , Grammes Thilo , Goldschmidt Florian , Jung Anne
Published: December 2014

Cellular materials like metal foams exhibit localised deformation expressed in deformation bands under inelastic strain conditions. According to the compressible behaviour of metal foams, there is localised damage in crushing zones with a thickness of several pores. In the present contribution, a new phenomenological modelling approach has been developed, which allows the explicit consideration of micro structural effects. The model accounts for micro structural effects by a rheological model offering the possibility to reflect the macroscopic stress fluctuations. Connecting a micro structural size with the rheological spring model results in a modified elasticity law and allows the description of localisation effects. By introducing a rheological model, the constitutive equations still contain fundamental parameters, but these are motivated by the micro structure, whereas the continuum model is able to account for size effects. The model is experimentally validated and its microstructurally motivated parameters are experimentally identified in quasi‐static compression tests with unloading. Using digital image correlation (DIC), it was possible to connect the micro structural deformation to the global macroscopic stress‐strain behaviour. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A comparison between a recursive method based on an inverse mechanical formulation and shape optimization for solving inverse form finding problems in isotropic elastoplasticity
Authors: Germain Sandrine , Landkammer Philipp , Steinmann Paul
Published: December 2014

In this paper we compare two methods, a recursive method based on an inverse mechanical formulation and a method based on a recursive shape optimization formulation, in order to solve inverse form finding problems in isotropic elastoplasticity. Both methods are succinctly presented and a numerical example is given. It was found that no difference could be found between the node coordinates on the undeformed configurations computed with both methods. However the convergence to the solution is faster with the recursive method based on an inverse mechanical formulation than with the method based on shape optimization. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Phase field modeling of domain evolution in ferroelectric materials in the context of size effects
Authors: Schrade David , Müller Ralf , Gross Dietmar
Published: December 2014

Phase field modeling provides an efficient tool for the study of domain evolution in ferroelectric materials. Such models naturally introduce an inner length scale which represents the width of the interfaces between domains (domain walls). This inner length scale is of the order of a few unit cells, i.e. about 0.8 nm–2 nm. The focus of this contribution is on size effects in a) the switching behavior of ferroelectric thin films and b) the microstructure evolution in ferroelectric nanodots. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Thermomechanical coupling of geometrically non‐coherent interfaces
Authors: Esmaeili Ali , Javili Ali , Steinmann Paul
Published: December 2014

An evacuated tube collector is composed of a vacuum glass tube forged to a metal tube creating an interface which undergoes thermomechanical cyclic loading. To numerically model such a thermomechanical and geometrically non‐coherent interface, a decohesion element with mixed‐mode capability, based on a normal‐shear decomposition of the displacement discontinuity vector is used, exploiting exponential traction‐separation laws. In addition to the classical strategies, interfacial elements are allowed to have an in‐plane stretch resistance by assuming a thermohyperelastic interface Helmholtz energy, a function of the rank‐deficient interface deformation gradient
$\overline{\bf F}$
and temperature
$\overline{\Theta}$
, as an extension of the surface/interface elasticity theory. The nonlinear governing equations are given and solved using the finite element method. The results are illustrated through a series of numerical examples for different material parameters. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Thermo‐mechanical modelling of cellular hybrid refractories
Authors: Jung Anne , Diebels Stefan
Published: December 2014

Refractory materials are subjected to both quasi‐static and dynamic thermal loading (thermal shock) causing damage up to mechanical failure. Typical refractories are magnesia carbon bricks consisting of periclase (MgO) and carbon inclusions. Recently, a significant improvement of the thermo‐mechanical behaviour could be achieved by cellular hybrid composites made of periclase‐filled carbon foams. The present contribution focuses on MgO‐filled carbon foams and the investigation and optimisation of the structure‐property relationship with respect to a reduction of thermally induced stresses and damage. It is a transient as well as static, fully coupled thermo‐mechanical problem. According to the fact that, in general, refractories are brittle materials a linear elastic model, with a damage criterion was used. To optimise the structural morphology of the cellular refractories, the effect of micro structural changes has been determined. For the investigation of the thermal shock! behaviour, the results correlate very well with the experimentally motivated Hasselman relation. There is a significant size effect depending on the pore size. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The Cyclic Thermomechanical Coupled Problem of Thermal Gradients in Friction Railway Disc Brakes
Authors: Lamjahdy Abdelkrim , Brunel Jean‐François , Lamure Benjamin , Dufrenoy Phillip , Weichert Dieter , Markert Bernd
Published: December 2014

The emergency braking distance of a TGV train at a speed of 320 km/h is almost 3000 m. Dry running brakes are reliable due to their predictable response to external stress and are thus used in such applications. The kinetic energy is dissipated proportionately into the brake disc and brake pad. This induced dissipation of energy and the high frequency of brake application cause high temperatures. These immense temperature changes could cause macroscopic cracks leading to failure of discs and accidents [1]. Generally, hot spotting describes the development of thermal localizations and can lead to early damage, early wear, pad performance loss, and squeal noise [2]. The aim of the present study is to improve a disc‐pad transient numerical model by use of a coupled thermomechanical method. It is based on full 3‐d thermomechanical calculations taking disc rotation into account. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A coupled multi‐component approach for bacterial methane oxidation in landfill cover layers
Authors: Sindern Andrea , Ricken Tim , Bluhm Joachim , Widmann Renatus , Denecke Martin , Gehrke Tobias
Published: December 2014

Methane (CH4), which has a 25 times higher global warming potential than carbon dioxide (CO2), can be oxidated by methanotrophic bacteria into carbon dioxide and water. The biological oxidation of methane can be considered in the passive aftercare phase of landfills in order to reduce climate‐damaging methane emissions. Methanotrophic bacteria are situated within the landfill cover layer and convert the harmful methane emissions arising from the degradation of organic waste to the less harmful carbon dioxide. Hence, the passive aftercare of landfills in terms of methane oxidation layers is an efficient method to reduce contributions to the greenhouse effect.

To model the coupled processes during phase transition from methane to carbon dioxide, the well‐known Theory of Porous Media (TPM) combined with the Mixture Theory has been used in order to develop a multi‐component Finite Element calculation concept, see [1, 3]. The thermodynamic consistent model analyzes the relevant gas productions of methane, carbon dioxide and oxygen. The model also accounts for the driving phenomena of production, diffusion and advection. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Viscoelastic modelling and experimental results of a dielectric electro‐active polymer diaphragm actuator
Authors: Gaida Philipp , Weinberg Kerstin , Jahr Andreas
Published: December 2014

Common actuating principles e.g. solenoids or piezo‐elements have mostly exploited their potential as electromechanical transducers. A new technology based on Dielectric Electro‐active Polymers (DEAPs) enables major progress in efficiency and functionality for actuators. In order to use the advantages of this new technology in a feasible application, a silicone based DEAP diaphragm actuator was tested experimentally. Here, the mechanical properties of the fundamental parts of the DEAP diaphragm actuator, namely the stressstrain behaviour of an equibiaxially pre‐stretched hyperelastic polymer and the mechanical response of a DEAP diaphragm actuator, were determined by appropriate test benches.

The investigated DEAP diaphragm actuator consists of a dielectric, which corresponds to a thin elastic silicone film sandwiched between two compliant electrodes. Furthermore, DEAPs are pre‐stretched, actually up to five times their in‐plane dimensions, in order to improve their performance. In that context, the influence of the pre‐stretched state on the actuator performance was investigated. In addition, a viscoelastic finite‐element model of the electromechanical system has been developed and validated with the experimental results. The mechanical part of the DEAP diaphragm actuator was realized with an incompressible isotropic elastic model, whereas the electrical part was approximated with a circular parallel‐plate capacitor. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Algorithm of Exact Methods Obtaining of the Solution and Dynamical Flexibilities of Subsystem of the Mechatronic system
Authors: Buchacz Andrzej
Published: December 2014

In this paper the application of analysis of transverse vibrating subsystem of mechatronic systems by means of the exact were the main purposes of work to solve the task of assignment of frequency‐modal analysis and characteristics of mechatronic system. At first the problem of analysis in the form of the one differential equation of motion of mechanical subsystem or of the set of state equations of considered mechatronic model of object has been formulated and solved. Classic method to solve this problem have been used. The considered transverse vibrating mechanical subsystems of mechatronic system are a continuous beams with constant cross‐section, with free ends and/or clamped on one end. A ring transducer, which is the integral part of mechatronic system, extorted by harmonic force or voltage excitation, is assumed to be perfectly bonded to the beam surface. Parameters of the transducer have important influence on values of natural frequencies and on form of characteristics of considered mechatronic system. The poles of dynamical characteristic calculated by mathematical exact method and the Galerkin's method have approximately different or the same values; it is depended on the combination of boundary conditions of elementary beam. The results of the calculations were not only presented in mathematical form but also as a transients of examined dynamical characteristic which are function of frequency of assumed excitation. Practical implications of this work is to present the introduction to synthesis of considered class of mechanical and/or mechatronic beam‐systems with a constant changeable cross‐section. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Model reduction and dynamic iteration for coupled nonlinear systems
Authors: Kerler Johanna , Stykel Tatjana
Published: December 2014

The dynamical behavior of coupled systems is determined by different interconnected subsystems that are usually governed by entirely different physical laws and often act in different time and space scales. We discuss the simulation of coupled nonlinear systems using dynamic iteration combined with model order reduction. We also study the convergence of this approach and derive error estimates for approximate solutions. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Effective bulk moduli of materials containing stochastically distributed nano‐inhomogeneities with surface stresses
Authors: Nazarenko Lidiia , Bargmann Swantje
Published: December 2014

In the present contribution, a mathematical model for the investigation of the effective properties of a material with randomly distributed nano‐particles is proposed. The surface effect is introduced via Gurtin‐Murdoch equations describing properties of the matrix/nano‐particle interface. They are added to the system of stochastic differential equations formulated within the framework of linear elasticity. The homogenization problem is reduced to finding a statistically averaged solution of the system of stochastic differential equations. These equations are based on the fundamental equations of linear elasticity, which are coupled with surface/interface elasticity accounting for the presence of surface tension. Using Green's function this system is transformed to a system of statistically non‐linear integral equations. It is solved by the method of conditional moments. Closed‐form expressions are derived for the effective moduli of a composite consisting of a matrix with randomly distributed spherical inhomogeneities. The radius of the nano‐particles is included in the expression for the bulk moduli. As numerical examples, nano‐porous aluminum and nano‐porous gold are investigated assuming that only the influence of the interface effects on the effective bulk modulus is of interest. The dependence of the normalized bulk moduli of nano‐porous aluminum on the pore volume fraction (for certain radii of nano‐pores) are compared to and discussed in the context of other theoretical predictions. The effective Young's modulus of nano‐porous gold as a function of pore radius (for fixed void volume fraction) and the normalized Young's modulus vs. the pore volume fraction for different pore radii are analyzed. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
An asymptotic approximation for a three dimensional linear elasticity problem with Robin boundary condition in a thick cylinder
Authors: Bare Zoufine , Orlik Julia
Published: December 2014

This contribution shows the asymptotic approximation of the displacements and shear stresses of a three dimensional (3D) linear elasticity boundary value problem in a thick beam with Robin boundary condition at an end. The analytic asymptotic approximation is compared to the numerical 3D Finite element solution. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Incremental Scheme to Homogenize Anisotropic Elastic Properties of Multi‐Phase Composites
Authors: Müller Viktor , Böhlke Thomas
Published: December 2014

An incremental homogenization scheme for the prediction of elastic properties of composites is reviewed. Similar to the differential scheme, the inclusions are included step‐by‐step. This approach accounts for high volume fractions of inclusion of different shape and elastic properties. A numerical example for a composite consisting of a polymeric matrix, glass fibers and voids is shown. The fiber distribution is chosen equivalently to a distribution in an injection molded short‐fiber reinforced composite. The volume fraction of the voids is varied. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A modified approximation of the exponential Cauchy‐Born rule for arbitrary shell‐like nanostructures
Authors: Findeisen Claudio , Wackerfuß Jens
Published: December 2014

The mechanical behaviour of shell‐like nanostructures, such as carbon nanotubes (CNT) can be described within the continuum mechanics by a generalisation of the Cauchy‐Born (CB) rule, introduced by Arroyo and Belytschko [1]. However, the finite element approximation presented in the literature requires the reference configuration to be planar. In case the configuration of interest is initially not planar, a preprocessing step is required that isometrically maps the planar reference configuration into the initial configuration. In order to calculate more complex structures, in this paper an extended finite element approximation is presented which is applicable to shell‐like atomic structures, whose reference configuration does not need to be planar and consequently the afore‐mentioned steps are no longer necessary. The accuracy of the proposed model is verified by two numerical examples. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On a potential‐velocity formulation of incompressible Navier‐Stokes equations
Authors: Marner Florian , Scholle Markus
Published: December 2014

Computational fluid dynamics has emerged as an essential investigative tool in nearly every field of technology. Despite a well‐developed mathematical theory and the existence of readily available commercial software codes, computing solutions to the governing equations of fluid motion remains challenging, especially due to the non‐linearity involved. Additionally, in the case of free surface film flows the dynamic boundary condition at the free surface complicates the mathematical treatment notably.

Recently, by introduction of an auxiliary potential field, a first integral of the two‐dimensional incompressible Navier‐Stokes equations has been constructed leading to a set of equations, the differential order of which is lower than that of the original equations [1]. A useful application to free surface simulation was found in [2]. Moreover the new formulation is naturally extendible to three dimensions via tensor calculus, involving a non‐unique symmetric tensor potential. The corresponding degrees of freedom can be used in order to achieve a numerically convenient representation. Finally an efficient staggered‐grid finite difference scheme is applied to a Stokes flow problem in a 3D lid‐driven cavity to demonstrate the capabilities of the new approach. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Non‐steady interaction of fluids with free surfaces and rigid bodies
Authors: Keisenberg Tobias , Ostermeyer Georg‐Peter
Published: December 2014

Interaction between fluids and rigid bodies is a phenomenon, which occurs for example in floating bearings or turbines. Mostly the focus is on domains with rigid boundaries on every side or defined influx or effusion of fluid over the boundaries.

The interaction between fluids with free surfaces and a rigid body were mostly studied under the aspect of stability for steady‐state conditions, e. g. for fluid‐filled centrifuges. A characteristic property is the instability over non‐empty intervalls of angular velocities, so the analysis of non‐steady behaviour is essential to investigate the stability of the drive through these instable domains.

A first approach to this topic is the qualitative investigation of a fluid domain with the shallow water theory. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Hybrid Solutions for Stationary and Instationary Three‐Dimensional Compressible Navier‐Stokes Layer
Authors: Nastase Adriana
Published: December 2014

New, original hybrid solutions for the three‐dimensional instationary compressible full Navier‐Stokes layer are here proposed. These numerical solutions have also important analytical properties, due to the analytical hybridization and are split, due to the use of the logarithmic density function (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Introduction of an Adaptive Smoothed Particles Hydrodynamics Formulation
Authors: Schnabel Dirk , Müller Alexandra , Eberhard Peter
Published: December 2014

In this work an adaptive Smoothed Particles Hydrodynamics (SPH) formulation is presented which is capable of adjusting the discretization level of a simulation at runtime. In order to increase and decrease the local resolution, the adaptive refinement and coarsening approach introduced in literature was extended to merge a userdefined number of particles at each simulation step. Depending on the simulation different coarsening strategies may be advantageous. Considering for example an SPH formulation for elastic solids, the particles discretizing the continuum experience considerably less disordering than in fluid simulations allowing for an advanced coarsening algorithm which takes each particles refinement history under consideration and preferably merges particles originating from one and the same particle, whereas in SPH for fluids such a strategy would not be reasonable since fluid particles usually experience great relative displacement. Hence different coarsening strategies may be recommended for different simulation scenarios. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical simulation of solutal Rayleigh‐Bénard‐Marangoni convection in a layered two‐phase system
Authors: Köllner Thomas , Boeck Thomas
Published: December 2014

We present a two‐dimensional simulation of solutal Rayleigh‐Bénard‐Maragoni convection in a layered system. In the initial state, the solute concentration is homogeneous in each layer but not in partition equilibrium. Diffusive transfer of solute leads to convective instability. Marangoni convection dominates initially as it operates on a smaller length scale. Rayleigh convection appears later as an instability of the mixed unstably stratified fluid near the interface. Compared to pure Marangoni convection the dynamics is more disordered due to additional flow in the bulk. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Influence of turbulence on the performance of a laboratory scale HAWT
Authors: Al‐Abadi Ali , Ertunc Özgür , Seok Julie , Härtl Michael , Delgado Antonio
Published: December 2014

The oncoming wind to horizontal axis wind turbines (HAWT) may change its speed and direction stochastically in time. Hence, turbine blades are exposed to flows both with fluctuating angle of attack and fluctuating yaw angles. The modern wind turbines are reacting to those changes by pitch angle and torque control not only to exploit as much power as possible but also stabilize energy production and prevent any damage of the turbine. However, time scales of wind fluctuations and sudden changes of wind properties can be very short and with very high in amplitude. In the present study we focus on the influence of turbulence on the performance of a HAWT. Main motivation of the investigations is to figure out best strategies for the aerodynamic design of the blades operating under turbulent conditions. A laboratory scale HAWT and a performance measurement set‐up are employed to measure the influence of the oncoming wind. The tests are conducted in the closed loop wind tunnel of our institute. The test section of the tunnel is 1.87 m in width, 1.4 m in height and 2 m in length. The rotor blades are specially designed and optimized for this wind tunnel and the generator used. The turbulence is generated by two static squared mesh grids; fine and coarse one. Hence, two mainly different turbulence scales are obtained. In addition, the distance between the wind‐turbine and the grid is adjusted to have additional sub‐turbulence scales for each grid. The turbulence is nearly isotropic and decays in the flow direction. The developments of Taylor's micro scale (λg) and integral scale (Lg) of the turbulence in the flow direction at various incoming wind velocities (8−16 m/s) are measured. Hence, the facility allows to expose the wind‐turbine to turbulence with various energy and length scale content. Those measurements are conducted with hot‐wire anemometry in the absence of the wind‐turbine. Upstream and downstream turbulence intensities (TI) distributions are measured to give insight on the surrounding free stream and turbine wake interaction and how can different turbulence eddies scales contribute in the influence of the performance of the turbine. Performance measurements are conducted with and without turbulence and the results are compared. The study shows that the higher the turbulence, the more the power extracted by the turbine. This is due to the higher interaction of large eddies with the turbine wake and with the boundary layer, which helps to keeping it attached. Furthermore, higher TI's help in suppressing the tip vortex, thus, reduce turbine tip losses. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Hybrid LES‐RANS Modeling of Complex Turbulent Flows
Authors: Breuer Michael , Schmidt Stephan
Published: December 2014

The paper describes a state‐of‐the‐art hybrid LES‐URANS method for the simulation of complex internal and external turbulent flows. Relying on a unified LES‐URANS approach with a soft interface the methodology is designed for wall‐bounded non‐equilibrium flows. The unsteady Reynolds‐averaged Navier‐Stokes (URANS) mode within the hybrid approach is taken into account by an explicit algebraic Reynolds stress model (EARSM), which guarantees an appropriate representation of the anisotropic near‐wall turbulence. All non‐closed terms in the transport equation of the turbulent kinetic energy are modeled by enhanced formulations using the EARSM (production and diffusion term) and the splitting of the dissipation rate into a homogeneous and an inhomogeneous contribution. The former is expressed analytically by a Taylor series expansion of the homogeneous lateral Taylor microscale in the vicinity of the wall guaranteeing the correct asymptotic behavior. The latter is incorporated into the diffusion term. The interface location between the large‐eddy simulation (LES) mode and the URANS mode is determined automatically on‐the‐fly based on the modeled turbulent kinetic energy and the distance to the wall. For transitional (external) flows an additional dynamic transition criterion is applied which determines the laminar and the turbulent flow regimes and is combined with the existing interface criterion. An internal flow over a periodic arrangement of hills and an external flow past a SD7003 airfoil with a laminar separation bubble are taken into account for a detailed evaluation of the method. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Turbulent Entrainment in the Atmospheric Boundary Layer
Authors: Mellado Juan Pedro
Published: December 2014

Turbulent entrainment – the process by which turbulence inside of the atmospheric boundary layer entrains air from the free troposphere above it – has been investigated using direct numerical simulations in two configurations, one without a cloud and one with a cloud. With the first configuration, we have learned that the entrainment zone in a convective boundary layer growing into a linearly stratified troposphere is better described in terms of a two‐layer structure, with different characteristic scales associated with each of the two sub‐layers. With the second configuration, we have explained how wind shear across the entrainment zone capping a stratocumulus cloud can render evaporative cooling as important as radiative cooling in driving convective motions. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A mixing‐length model for magnetohydrodynamic flows in channels and ducts with wall‐parallel magnetic field
Authors: Boeck Thomas , Krasnov Dmitry
Published: December 2014

A spanwise magnetic field leads to turbulent drag reduction in channel flow of a conducting liquid due to the selective Joule damping of certain flow structures. This effect can be captured by a simple modification of Prandtl's classical mixing‐length idea. The mixing length over which a turbulent fluctuation loses its momentum is not only constrained geometrically but also by magnetic damping. We therefore introduce a magnetic damping length that is proportional to friction velocity and the Joule damping time. The limitation of mixing length is implemented by using the harmonic mean between wall distance and this damping length. By combining this ansatz with the van‐Driest model for turbulent stresses in channel flow we obtain a satisfactory prediction for the mean velocity distribution in magnetohydrodynamic channel flow with spanwise field for different Reynolds and Hartmann numbers. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Friction law for turbulent boundary layers on a flat plate with suction
Authors: Vigdorovich Igor
Published: December 2014

A universal friction law for turbulent boundary layers on a flat plate with suction is established. Experimental skin‐friction distributions obtained for various suction factors and Reynolds numbers are described in similarity variables by a single universal curve. The law is valid for the entire range of suction velocities from zero one till the limiting values corresponding to asymptotic‐suction boundary layers. The analysis is not based on any particular turbulence model. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical Simulation of Bubbles in Shear Flow
Authors: Anderl Daniela , Bauer Martin , Rauh Cornelia , Rüde Ulrich , Delgado Antonio
Published: December 2014

To gain a basic understanding of foam flow, as it can be found e.g. in transport of aerated food, simulation tools can help to provide better insight. Shearing of the bubbles appears in different flow geometries and is for a bubble assembly not captured analytically. Also experimentally, those flow fields are hard to observe so that simulations are the method of choice.

Our method to simulate foams uses a volume of fluid approach that is based on the free surface algorithm by Körner et al. [1]. Different from classical multiphase methods, only the liquid phase is simulated and special boundary conditions at the liquid‐gas interface account for the gas phase. With this approach high density ratios, e.g. in water‐air systems, are easier to realize than in other methods. High density ratios are even necessary to physically justify the model, where the dynamics of the lighter phase are partially neglected. This method is integrated in the Lattice Boltzmann software framework waLBerla [3] (widely applicable Lattice Boltzmann solver from Erlangen†) that can be used on massively parallel computers and thus allows to simulate even large bubble assemblies.

As first validation, single bubbles are sheared with different capillary numbers and the simulation results are compared to literature [2] and show good agreement. The next step is shearing a bubble assembly which is arranged like a dense sphere packing. In order to investigate the geometrical configuration of the assembly and its impact on the behavior during a shear deformation, the bubble assembly is rotated with different angles with respect to the shear direction. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Rising bubble simulations using lattice Boltzmann method coupled with level set interface capturing
Authors: Safi Mohammad Amin , Turek Stefan
Published: December 2014

A multiphase Lattice Boltzmann (LB) scheme coupled with a level set interface capturing module is used for the simulation of multiphase flows, and in particular, rising bubbles under moderate and high density and viscosity ratios. We employ a consistent, robust discretization of the pressure forces which enables us to preserve stability and accuracy. The accuracy of the present scheme is shown to be competitive with that of the benchmark data from the finite element solutions in [3]. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Explosive‐crystallization waves in thin amorphous layers on heat conducting substrates with thermal contact resistance
Authors: Buchner Christoph , Schneider Wilhelm
Published: December 2014

A model for explosive crystallization in a thin amorphous layer on a heat conducting substrate is presented. Rate equations are used to describe the kinetics of the homogeneous amorphous‐crystalline transition. Heat conduction into the substrate and thermal contact resistance at the interface between layer and substrate are taken into account.

The whole process is examined as a wave of invariant shape in a moving frame of reference. A coupled system of an integro‐differential equation and ordinary differential equations is obtained and solved numerically. The propagation velocity of the wave is obtained as an eigenvalue of the system of equations. Some representative solutions are shown. Crystallization‐wave velocities are compared with experimental values for explosive crystallization in germanium. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Electromagnetic flow control in liquid metals using Lorentz force techniques
Authors: Dubovikova Nataliia , Karcher Christian , Kolesnikov Yuri
Published: December 2014

Flow measurement and flow control in opaque and aggressive liquids such as metal melts are challenging tasks in industrial fluid mechanics. Optical measurement techniques as well as submerging probes or control units cannot be applied in case of highly severe environment; hence, contactless electromagnetic methods are of interest for practical flow control because of relatively high electrical conductivity of investigated materials. In this paper we present working principle and experimental results of two such contactless techniques. First, time‐of‐flight Lorentz force velocimetry (LFV), which allows to determine the flow rate of liquid metal without information about any fluid properties or magnetic field magnitude by cross‐correlation of two Lorentz force signals. Secondly, Lorentz torque velocimetry (LTV) ‐ a technique, which uses an electromagnetic pump with a torque sensor connected to the pump's shaft. In the methods so‐called Lorentz force [1] is measured, which has electromagnetic nature and is proportional to velocity and flow rate of conductive liquid. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The relaxed micromorphic continuum model
Authors: Ghiba Ionel‐Dumitrel , Neff Patrizio , Madeo Angela
Published: December 2014

We formulate a relaxed linear elastic micromorphic continuum model with symmetric Cauchy force‐stresses and curvature contribution depending only on the micro‐dislocation tensor. We prove that the new relaxed micromorphic model supports well‐posedness results. Relations with other theories are also pointed out. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Rank‐one convexity and polyconvexity of Hencky‐type energies
Authors: Neff Patrizio , Ghiba Ionel‐Dumitrel , Lankeit Johannes , Martin Robert
Published: December 2014

We investigate a family of isotropic volumetric‐isochorically decoupled strain energies based on the Hencky‐logarithmic (true, natural) strain tensor log U. The main result of this note is that for n = 2 the considered energies are rank‐one convex for suitable values of two material parameters. We also conjecture that there are values of the material parameters such that the corresponding energies are polyconvex. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Uniqueness Aspects of Semi‐Discretized Navier‐Stokes Equations
Authors: Zanger Florian
Published: December 2014

We consider the Navier‐Stokes equations on a time interval [0, T) in the weak formulation given by

Here we assume
ν > 0, G ⊂ ℝ3 open, bounded with
C2,1
boundary,
f ∈ L2 (0, T, L2), and
u0 ∈ V, where V
is the closure of
C∞0,σ in (H10, ‖∇ ·‖) and (·, ·)
denotes the scalar product in L2. The above equations for
u ∈ L2(0, T, V ∩ H2) with ∂tu ∈ L2 (0, T, L2 )
can be semi‐discretized with respect to time in different ways. The existence of solutions to the resulting schemes can be shown with the Galerkin method provided the viscous term is not treated fully explicitly. In the case of linear schemes, uniqueness can easily be proven with the Lax‐Milgram lemma. However, schemes that are implicit in both factors of the convective term seem to cause difficulties with respect to uniqueness, even for arbitrary small step sizes. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Stochastic Approaches of Nonlinear Model‐Based Simulations for Vibratory Compaction Process
Authors: Debeleac Carmen , Nastac Silviu
Published: December 2014

The paper presents a basic approach regarding nonlinear models intended for computational dynamics of vibratory compaction process in order to evaluate the consolidation level. Taking into account the complex rheology of terrains and the relative variable dynamic load in time for each point inside the working area, a stochastic approach both of the material characteristics, and of the excitatory charges have been appropriate. Numerical simulations has developed based on nonlinear models containing conservative and complex dissipative rheological elements, which include two main groups of stochastic data as follows: terrain characteristics from preconditioning experimental tests, and vibratory actions from analytical approaches supposing the soil‐drum contact area and the number of dynamic load transitions over this area. Simulation results dignify small variances between the real and computed terrain responses, comparative with the case supposing full determinist parameters. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the preliminary design of axial fans: extending the validity of the flow assumptions over the whole hydraulic characteristic and improvement of modeling
Authors: Semel Matthias , Willinger Bettina , Grilliat Julien , Delgado Antonio
Published: December 2014

In order to carry out a preliminary axial fan design, established assumptions are made onto the flow structure within the fan at the design point. As an example the very common hypothesis of radial equilibrium, states that the radial velocity component downstream the fan is zero, and that the flow in this area is axisymmetric. With the help of this hypothesis, the flow is idealized in order to draw quantitative conclusions. The present communication aims at extending these assumptions on the whole flow regime in order to help fan designers to reduce the try‐and‐error cycle number. An analytical formulation for axial fan velocity distributions at design and off design conditions is proposed for an arbitrary work distribution. It is shown that the total pressure characteristics can be described by means of hyperbola and straight line. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Comparison of AR and ARMA processes in structural health monitoring.
Authors: Alexander Ostermann , Spielberger Georg
Published: December 2014

In this short note we discuss the application of AR and ARMA processes in structural health monitoring with output only data. We highlight the advantages of both types of processes for different purposes. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical Comparison of Optimization Methods for Process Parameters Identification in Forming Technology
Authors: Rozgi` c Marco , Stiemer Marcus
Published: December 2014

The SAND formulation known from structural and topology optimization is applied to a forming problem arising from certain technological forming processes. The SAND approach is compared to a well known and widely used method on a model problem within the scope of linear elasticity. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optimization of sit to stand motions of elderly people for the design and control of physical assistive devices
Authors: Mombaur Katja
Published: December 2014

Sit to stand motions represent a big challenge for many elderly people even if other types of motions such as walking can still be achieved with less problems. It is therefore important to gain a deeper understanding of this type of motions and provide strategies to perform sit to stand transfers even in the presence of different pathologies and joint weaknesses. In this paper we present a numerical study of sit to stand motions based on mechanical multibody models and optimal control techniques. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Simulation‐based optimisation of wastewater flow and composition for on‐site treatment in the food and beverage industry utilising reference nets in combination with genetic algorithms
Authors: Hubert Stefan , Pettigrew Liam , Helmers Thorben , Mießner Ulrich , Groß Frauke , Räbiger Norbert , Delgado Antonio
Published: December 2014

This study introduces the concept of computer modelling and simulation of complex bioprocesses and systems using an approach that combines the reference net formalism with machine learning and optimisation techniques. Reference nets are an extension of high level Petri Nets, which can be used as a central visualisation and modelling tool. The net‐in‐net paradigm used by reference nets makes it possible to model complex processes, such as those found in the food and beverage industry. A plugin/interface system based on the java programming language allows implementation of advanced mathematical modelling techniques at specific points in entire system simulations. Separate optimisation tools can also run and modify existing reference net models for fast solutions to efficiency problems. We present an example system that simulates a specific section of a beer brewery using the reference net formalism, which is optimised using a genetic algorithm. We show in detail how the different software packages can be combined for a simulation based optimisation approach. The optimisation technique specifically addresses the wastewater pollution load in regard to its chemical oxygen demand. A beer brewery was chosen as an example for this study due to the constantly increasing requirements to lower energy and water consumption in this industry. One possibility to lower the energy and water demands is to effectively treat wastewater produced by the brewery, which can introduce cost savings by providing recycled water and biogas. Most approaches to wastewater treatment are end‐of‐pipe solutions that do not consider the brewery as a whole. A brewery contains many processes that can be running concurrently and interacting with one another (e.g. brewing, clean‐in‐place and bottling) with each process producing varying amounts of wastewater with different pollution loads. Optimisation of the scheduling of the different processes with respect to the wastewater production will allow for more effective wastewater treatment, and therefore cost and energy savings. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Improving robustness of the FEAST algorithm and solving eigenvalue problems from graphene nanoribbons
Authors: Galgon Martin , Krämer Lukas , Lang Bruno , Alvermann Andreas , Fehske Holger , Pieper Andreas
Published: December 2014

We consider the FEAST eigensolver, introduced by Polizzi in 2009 [5]. We describe an improvement concerning the reliability of the algorithm and discuss an application in the solution of eigenvalue problems from graphene modeling. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Phase Retrieval from 4N−4 Measurements: A Proof for Injectivity
Authors: Philipp Friedrich
Published: December 2014

We prove by means of elementary methods that phase retrieval of complex polynomials p of degree less than N is possible with 4N − 4 phaseless Fourier measurements of p and p′. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)