bluuebunny/crossref_metadata_2025 huggingface dataset:
Thrombocytosis is associated with complications after microvascular surgery: An NSQIP data analysis
Authors: Tarabishy Sami P. , Inglesby Dani , Tapp Marion , Corral Gabriel Del , Herrera Fernando A.
Published: November 2019

Background

Complications after microvascular surgery, such as partial flap loss, and arterial/venous compromise cannot only increase morbidity for the patient but also tax the healthcare system. Thrombocytosis, both essential and reactive, can predispose patients to thrombosis and hemorrhage and thus should intuitively have an effect on the outcome of microvascular free tissue transfers. We sought to evaluate the effect of preoperative thrombocytosis on outcomes after microvascular free flap surgery.

Methods

A retrospective review of the 2013–2016 American Collges of Surgeons, National Surgical Quality Improvement Program (ACS NSQIP) database identified 4,299 patients who had microsurgical flaps. Of these 3,744 had preoperative platelet levels recorded. Fifty‐four patients had preoperative thrombocytosis, defined as a platelet count >450 K/CUMM, while 3,690 did not. The groups were compared; demographics, comorbidities, and smoking status were recorded. We then examined outcomes between groups including average operative time, length of hospital stay, need for transfusion, deep vein thrombosis (DVT) postoperatively, and need for reoperation.

Results

When comparing the two cohorts, there was no statistical difference in comorbidities. We found a significant difference between the thrombocytosis and control cohort in need for transfusion (29.6 vs. 12.8%, p = .0002), average days till discharge (8.36 vs. 5.75, p = .009), and need for reoperation (27.8 vs. 13.8%, p = .003). There was no difference in DVT occurrence (0 vs. 0.1%, p = .46) or average operation time (535 m vs. 482 min, p = .088).

Conclusion

Patients with thrombocytosis undergoing microvascular free flaps are at increased risk for complications, including the need for a blood transfusion, prolonged hospital stays, and reoperation.
A note on the density expansion and generating function of the beta Burr XII
Authors: Guerra Renata Rojas , Peña‐Ramirez Fernando A. , Peña‐Ramirez Miguel R. , Cordeiro Gauss M.
Published: November 2019

The five‐parameter beta Burr XII (BBXII) was pioneered as an extension of the Burr XII (BXII) distribution. In this work, we obtain a much simpler linear representation for its density to compute its ordinary and incomplete moments, among other related properties. We verify that the BBXI generating function is not convergent for all terms of the series expansion, and then, it is not adequate for all parameter combinations. Therefore, we derive an accurate linear representation for the BBXII density and a new power series for the BXII generating function. We provide Rspienter scripts for these calculations and a numerical study to illustrate the convergence of the density expansion. Many distributions have been proposed in the literature to extend for computing the moments.
Global transcriptome analysis of porcine oocytes in correlation with follicle size
Authors: Gad Ahmed , Nemcova Lucie , Murin Matej , Kinterova Veronika , Kanka Jiri , Laurincik Jozef , Benc Michal , Pendovski Lazo , Prochazka Radek
Published: November 2019

Although our knowledge regarding oocyte quality and development has improved significantly, the molecular mechanisms that regulate and determine oocyte developmental competence are still unclear. Therefore, the objective of this study was to identify and analyze the transcriptome profiles of porcine oocytes derived from large or small follicles using RNA high‐throughput sequencing technology. RNA libraries were constructed from oocytes of large (LO; 3–6 mm) or small (SO; 1.5–1.9 mm) ovarian follicles and then sequenced in an Illumina HiSeq4000. Transcriptome analysis showed a total of 14,557 genes were commonly detected in both oocyte groups. Genes related to the cell cycle, oocyte meiosis, and quality were among the top highly expressed genes in both groups. Differential expression analysis revealed 60 up‐ and 262 downregulated genes in the LO compared with the SO group. BRCA2, GPLD1, ZP3, ND3, and ND4L were among the highly abundant and highly significant differentially expressed genes (DEGs). The ontological classification of DEGs indicated that protein processing in endoplasmic reticulum was the top enriched pathway. In addition, biological processes related to cell growth and signaling, gene expression regulations, cytoskeleton, and extracellular matrix organization were among the highly enriched processes. In conclusion, this study provides new insights into the global transcriptome changes and the abundance of specific transcripts in porcine oocytes in correlation with follicle size.
Water–fat Dixon cardiac magnetic resonance fingerprinting
Authors: Jaubert Olivier , Cruz Gastão , Bustin Aurélien , Schneider Torben , Lavin Begoña , Koken Peter , Hajhosseiny Reza , Doneva Mariya , Rueckert Daniel , Botnar René M. , Prieto Claudia
Published: November 2019

Purpose

Cardiac magnetic resonance fingerprinting (cMRF) has been recently introduced to simultaneously provide T1, T2, and M0 maps. Here, we develop a 3‐point Dixon‐cMRF approach to enable simultaneous water specific T1, T2, and M0 mapping of the heart and fat fraction (FF) estimation in a single breath‐hold scan.

Methods

Dixon‐cMRF is achieved by combining cMRF with several innovations that were previously introduced for other applications, including a 3‐echo GRE acquisition with golden angle radial readout and a high‐dimensional low‐rank tensor constrained reconstruction to recover the highly undersampled time series images for each echo. Water–fat separation of the Dixon‐cMRF time series is performed to allow for water‐ and fat‐specific T1, T2, and M0 estimation, whereas FF estimation is extracted from the M0 maps. Dixon‐cMRF was evaluated in a standardized T1–T2 phantom, in a water–fat phantom, and in healthy subjects in comparison to current clinical standards: MOLLI, SASHA, T2‐GRASE, and 6‐point Dixon proton density FF (PDFF) mapping.

Results

Dixon‐cMRF water T1 and T2 maps showed good agreement with reference T1 and T2 mapping techniques (R2 > 0.99 and maximum normalized RMSE ~5%) in a standardized phantom. Good agreement was also observed between Dixon‐cMRF FF and reference PDFF (R2 > 0.99) and between Dixon‐cMRF water T1 and T2 and water selective T1 and T2 maps (R2 > 0.99) in a water–fat phantom. In vivo Dixon‐cMRF water T1 values were in good agreement with MOLLI and water T2 values were slightly underestimated when compared to T2‐GRASE. Average myocardium septal T1 values were 1129 ± 38 ms, 1026 ± 28 ms, and 1045 ± 32 ms for SASHA, MOLLI, and the proposed water Dixon‐cMRF. Average T2 values were 51.7 ± 2.2 ms and 42.8 ± 2.6 ms for T2‐GRASE and water Dixon‐cMRF, respectively. Dixon‐cMRF FF maps showed good agreement with in vivo PDFF measurements (R2 > 0.98) and average FF in the septum was measured at 1.3%.

Conclusion

The proposed Dixon‐cMRF allows to simultaneously quantify myocardial water T1, water T2, and FF in a single breath‐hold scan, enabling multi‐parametric T1, T2, and fat characterization. Moreover, reduced T1 and T2 quantification bias caused by water–fat partial volume was demonstrated in phantom experiments.
Analytical solution of stochastic differential equation by multilayer perceptron neural network approximation of Fokker–Planck equation
Authors: Namadchian Ali , Ramezani Mehdi
Published: November 2019

The Fokker–Planck equation is a useful tool to analyze the transient probability density function of the states of a stochastic differential equation. In this paper, a multilayer perceptron neural network is utilized to approximate the solution of the Fokker–Planck equation. To use unconstrained optimization in neural network training, a special form of the trial solution is considered to satisfy the initial and boundary conditions. The weights of the neural network are calculated by Levenberg–Marquardt training algorithm with Bayesian regularization. Three practical examples demonstrate the efficiency of the proposed method.
A mixed variational‐based dynamic simulation method for fiber‐reinforced continua in non‐isothermal rotordynamical systems
Authors: Groß Michael , Dietzsch Julian , Röbiger Chris
Published: November 2019

Three‐dimensional woven carbon‐fiber reinforced polymers (3D‐CFRP) are being increasingly used for blades in rotordynamical systems as turbine fans, shafts as well as disc brake rotors. By using dynamic mixed finite‐element methods, these materials can be simulated as anisotropic continua with non‐isothermal behaviour in the matrix as well as the fiber material. Therefore, we present a novel dynamic mixed finite‐element method which is variational‐based and capable to simulate these materials. This dynamic mixed finite‐element method preserves each basic balance law emanating from the variational principle exact in a discrete setting. We demonstrate this physically‐consistent simulation behaviour by using dynamic simulations of a disc brake rotor and a rotating pipe subject to thermal and mechanical loads.
Reconstruction of the genesis of a Renaissance algorithm to calculate sine tables
Authors: Ullrich Peter
Published: November 2019

Recently, a manuscript has been discovered in which Jost Bürgi (1552–1632) presented an algorithm to calculate sine tables of high precision with only a modest amount of numerical effort [1], [2]. Several modern proofs for the correctness of this algorithm have been given, all of which, however, use methods that were not at hand at Bürgi's time. This raises the question how Bürgi had found his algorithm.

Building on [5], an educated guess is presented how Bürgi may have found his algorithm and also collected enough evidence for its correctness. For this, only methods are needed which were known at Bürgi's time, like prosthaphaeresis and the method of false position.
Green's function for a horizontally layered anisotropic soil
Authors: Waubke Holger , Kreuzer Wolfgang , Hrycak Tomasz , Schmutzhard Sebastian
Published: November 2019

The Green's function for an anisotropic horizontally layered soil can be determined in the wave‐number frequency domain. Only the vertical axis remains in the original domain. For a simulation in 2.5D a numerical inverse Fourier integral transformation, about one horizontal axis is needed. This leads to difficulties at the singularities of the function. For the boundary element method in 2.5D the integrals along the boundary are transformed to the wave‐number domain and solved analytically. The result is a filter that attenuates the spectrum of the Green's function and therefore allows for an inverse Fourier transformation using numerical methods. For an efficient inverse transformation the Filon method together with an adaptive algorithm are used.
Model hierarchy of upper‐convected Maxwell models with regard to simulations of melt‐blowing processes
Authors: Wieland Manuel , Arne Walter , Marheineke Nicole , Wegener Raimund
Published: November 2019

In melt‐blowing processes, polymeric jets are extruded into turbulent high‐speed airflow to produce fibers of micro‐ and nano‐scale. String models supplemented with viscoelastic material laws build a suitable basis for the modeling and simulation of fibrous jets in such processes. We present a model‐hierarchy of upper‐convected Maxwell models for the fibers and therein incorporate the turbulent velocity fluctuations of the underlying airflow by a reconstruction technique. Within the model hierarchy, simulation results are analyzed and evaluated with respect to accuracy and efficiency.
Time‐adaptive computations for the mortar finite element method
Authors: Grafenhorst Matthias , Hartmann Stefan
Published: November 2019

In recent years, mortar finite element methods have been successfully applied as space discretization scheme to a wide range of contact problems. The finite deformation contact formulation taken up is based on a mortar approach using so‐called dual Lagrange multipliers. If the constitutive models of the material bodies are of rate‐type, the entire system of equations represents a non‐smooth system of differential‐algebraic equations. This system will be investigated in connection with higher‐order time integration methods using diagonally implicit Runge‐Kutta methods.
Application of the fast nonlinear analysis method on a clamped beam with a cubic spring
Authors: Akar Özge , Willner Kai
Published: November 2019

In civil engineering reduction methods are used for the mechanical design of buildings in order to avoid unnecessary complexity and computational effort especially in the seismic design of structures. The determination of vibration modes by earthquake excitation is carried out by using Ritz vectors instead of eigenvectors. Thus, only the modes of interest are obtained. Furthermore, the Fast Nonlinear Analysis (FNA) method was formulated to easily insert nonlinearities like concentrated damping in a construction model. The main idea is to take the nonlinear forces to the right hand side of the equation of motion. In the context of this work, the FNA method is applied on a finite element model of a clamped beam with a cubic spring that works at the free end of the beam. First, a harmonic excitement is applied and the vibration modes of the system are obtained by using Ritz vectors. Then, a piecewise exact integration method is implemented to iteratively determine the results for displacement, velocity and acceleration of the discretized beam.
A locally conservative reduced flux reconstruction for elliptic problems
Authors: Rave Stephan , Schindler Felix
Published: November 2019

In the context of model order reduction of parametric elliptic problems, we present a methodology to reconstruct a conforming flux from a given a reduced solution, that is locally conservative with respect to the underlying finite element grid. All components of the procedure depend separably on the parameter and allow for further use in offline/online decomposed computations, for instance in the context of a posterior error estimation or flow problems.
Towards physiological conditions in total knee arthroplasty by a bionics‐inspired tibial implant design
Authors: Eidel Bernhard , Gote Ajinkya , Fritzen Claus-Peter , Ohrndorf Arne , Christ Hans-Jürgen
Published: November 2019

In total knee arthroplasty (TKA) force is transmitted into the tibia by a combined plate‐stem device along with a fully or partially cemented fixation. The present work analyzes this force transmission in finite element simulations with the main aim to avoid reported post‐surgical bone density loss as a consequence of reduced tibial bone loading. In the numerical analysis different implant materials and implant‐to‐stem interface conditions are considered, from a stiff, fully cemented fixation to sliding contact conditions with a low friction coefficient. The impact of these variations on bone loading changes are measured by (i) decomposing the total force into parts mediated by the plate and by the stem and by (ii) post‐surgery strain energy density (SED) deviations. Based on a bionics‐inspired perspective on how nature in pre‐operative conditions carries out force transfer from the knee joint into the tibia, a modified implant‐bone interface is suggested that alters force transmission towards physiological conditions while preserving the geometries of the standard plate‐stem endoprosthesis design. As a result, stress shielding is avoided almost completely, pre‐surgery bone loading is preserved such that bone resorption is not likely to occur.
Implementation of generalized mechanical interfaces
Authors: Spannraft Lucie , Mergheim Julia , Steinmann Paul
Published: November 2019

For an accurate approximation of the mechanical behavior of interfaces undergoing tension, shear and in‐plane stretch deformation, the definition of an appropriate interface constitutive model is crucial. Distinct interface models, such as cohesive zone models are well investigated. However, only little research considering energetic interface models, especially inelastic models, and generalized mechanical interfaces is conducted. Although multiple possibilities to apply cohesive zone models in commercial software exist, to the authors' knowledge, the material properties provided are generally limited by basic cohesive laws and any definition of energetic interface models lacks. In order to benefit from the advantages of commercial FE software, a generalized mechanical interface model is implemented in ABAQUS. The employed constitutive model combines a cohesive zone model with an energetic interface model.
A bottom‐up continuum approach of crystal plasticity for the analysis of fcc microwires under torsion
Authors: Zoller Kolja , Schulz Katrin
Published: November 2019

The microstructural evolution of face‐centered cubic microwires is studied using a physical motivated, homogenized continuum model of crystal plasticity. The dislocation configuration in the three‐dimensional space is thereby described via a Continuum Dislocation Dynamics (CDD) theory including a dislocation source term. The resulting spatial distribution of dislocation densities and strain components are shown for a relaxation problem with torsional loading.
A Beam Propagation Method for Distorted Gaussian Beams
Authors: Pflaum Christoph
Published: November 2019

Distorted Gaussian beams arise in several applications in optics. One of them is a laser amplifier. In a laser amplifier a Gaussian beam is distorted by thermal lensing effects, polarization effects and gain guiding. This leads to a decrease of the beam quality at the output of a laser amplifier. An important question is how to calculate this decrease of the beam quality and how to compute the amplified beam. There exist several difficulties of existing simulation techniques for the computation of optical beams. These are the high computational amount in order to resolve the phase of the beam and modelling non‐absorbing boundary conditions. We present a new beam propagation method, which circumvents these difficulties. The idea is to decompose the beam as a product of a Gaussian Beam TEM00 and an unknown distortion function Ξ. This leads to an interesting partial differential equation for Ξ, which contains a beam spreading convection term. This PDE is solved numerically by finite elements and a Crank‐Nicolson space stepping discretization. The resulting linear equation system is solved by GMRES. The q‐parameter of the Gaussian beam is calculated in advance by an ABCD matrix method. This leads to a highly efficient simulation technique. It can be applied to simulate the amplification of Gaussian beams in laser amplifiers.
A model order reduction method for finite strain FFT solvers using a compressed sensing technique
Authors: Gierden Christian , Kochmann Julian , Manjunatha Kiran , Waimann Johanna , Wulfinghoff Stephan , Svendsen Bob , Reese Stefanie
Published: November 2019

We present a model order reduction (MOR) method for finite strain FFT solvers to reduce the computational costs of the FFT simulation scheme of a two‐scale FE‐FFT simulation. The underlying method is based on a reduced set of frequencies which leads to a reduced fixed‐point scheme. The reduced set of frequencies is determined offline, based on the Fourier grid and predominantly consists of low frequencies. After performing the entire simulation with this reduced set of frequencies, the compressed sensing technique is used to reconstruct highly resolved micromechanical fields in a post‐processing step. Compared to the unreduced solution scheme, a significant speed‐up in the CPU time at a negligibly small loss of accuracy in the overall solution is observed. As a numerical example an elastic composite in a finite strain setting is investigated.
A framework and finite element formulation for incompatibility tensor‐based gradient plasticity
Authors: Kaiser Tobias , Menzel Andreas
Published: November 2019

The present contribution deals with a gradient plasticity formulation that accounts for energetic contributions of plastic incompatibilities that are interpreted in terms of dislocation densities. At the outset of the theory, an extended form of the dissipation inequality in terms of a nonlocality residual is assumed in accordance with [1]. Taking into account dislocation density tensors as additional arguments of the free energy density function, this form naturally gives rise to the balance equation of a generalised (quasi) nonlocal stress field that is energetically conjugated to the plastic deformation. Against this background one furthermore observes that the energetic dual to the dislocation density tensor is closely related to a back stress‐type part of the generalised stress field. For the solution of the resulting system of coupled partial differential equations, a multi‐field finite element approach is taken, with the plastic consistency conditions being accounted for on global level by means of Fischer‐Burmeister complementarity functions.
Drift‐diffusion problems with Gauss–Fermi statistics and field‐dependent mobility for organic semiconductor devices
Authors: Glitzky Annegret , Liero Matthias
Published: November 2019

We consider a drift‐diffusion model for organic semiconductor devices including Gauss–Fermi statistics and application‐specific mobility functions. The charge transport in organic materials is realized by hopping of carriers between adjacent energetic sites and is described by complicated mobility laws with a strong nonlinear dependence on temperature, carrier densities and the electric field strength. We report on the existence of weak solutions to the stationary problem as well as global weak solutions to the instationary problem. Moreover, L∞ bounds for the solutions are established.
Topological solvability analysis and its application to liquid flow networks coupled to Functional Mock‐up Units
Authors: Baum Ann-Kristin , Kolmbauer Michael , Offner Günter , Pöchtrager Bernhard
Published: November 2019

We present a unified topological analysis for physical networks coupled to black‐box models. While the governing equations of the physical network are derived by representing the network as a linear graph, the equations of the black‐box model are characterized as an abstract class of Differential‐Algebraic equations (DAEs) of a specific form (Functional Mock‐up Unit). Combining those two representations, existence and uniqueness results can be formulated based on the connectivity of the physical network and a concrete classification of the black‐box models. This is the natural extension of the approach derived in [3].
Time discretization of nonlinear hyperbolic systems on networks
Authors: Altmann Robert , Zimmer Christoph
Published: November 2019

In view of gas networks, the simulation of hyperbolic systems on networks has recently caused large interest. We consider the case with a nonlinear damping term and a small parameter ϵ such that – in the linear case – the system turns parabolic for ϵ =0. Based on this property and an expansion in ϵ, we derive a novel time integration scheme.
Prandtl‐Reuss model without safe‐load conditions
Authors: Kisiel Konrad
Published: November 2019

We consider the Prandtl‐Reuss dynamical model of elasto‐perfect plasticity. In the case of non‐homogeneous mixed boundary conditions we were able to omit the standard assumption of safe‐load conditions by introducing much easier one (admissibility of Neumann boundary data).
A higher‐order shear deformation approach to the mixed‐mode buckling problem in composite laminated beams
Authors: Mittelstedt Siham , Mittelstedt Christian
Published: November 2019

This paper presents a semi‐analytical method for the mixed‐mode buckling problem of composite beams, i.e. the interaction between the local buckling modes of the segments of shear deformable symmetrically laminated composite I‐beams. The buckling analysis employs Reddy's third‐order shear deformation theory and the Ritz‐method in conjunction with adequate series expansions for the buckling degrees of freedom wherein the webs and flanges are considered separately and are later on unified by employing Lagrangian multipliers. The results are compared with the buckling loads as obtained by finite element computations, and a very satisfying agreement is found.
Model Order Reduction of a Modular Scale Model of a High Rise Building
Authors: Walker Nadine , Eberhard Peter
Published: November 2019

This contribution deals with the model order reduction of a modular structure. In the following the model will be shortly introduced and it is dealt with the reduction of the assembled system as well as with the application of substructuring.
Noise reduction for DIC measurements
Authors: Reis Martin , Diebels Stefan , Jung Anne
Published: November 2019

Digital image correlation (DIC) is well known as a contact‐less, full‐field strain and displacement measuring method. The use of the method is widely spread in experimental mechanics. Nevertheless, there are error sources such as light settings, speckle pattern and noisy images, which strongly affect measurement quality, but often stays uninvestigated. Therefore, this work outlines a simple routine to identify errors and enhance the accuracy of DIC measurements. A calibration sample adapted to the specifications of the setup using the beam theory to link machine displacement, DIC measurements and forces. All evaluations are done in the commercial software ISTRA4D. The noise in the measurements is reduced to an error of under two per cent. The routine allows estimating the quality of the images before performing the experiments. Furthermore, the routine is transferable to all experimental setups. The calibration of the setup allows getting reliable experimental results with a known error. With this estimated stochastic error, the advantages of DIC can be used to evaluate mechanical experiments such as three‐point bending tests on single struts of an open cell metal foam.
Interfacial energies depending on curvature: the drastic loss of elastic coupling on small scales and its impact on multiphase mechanics
Authors: Kurzeja Patrick
Published: November 2019

Interfacial energies are often assumed to be well approximated by a constant surface tension. This assumption however fails at very small radii, e.g., for fluid bubbles or lipid layers. Below a cut‐off radius, the interface elasticity can even turn negative and interface resonance vanishes, diminishing the resonance spectrum and dispersion.
Sensor positioning in the context of wave‐based damage identification in dams
Authors: Reichert Ina , Alalade Muyiwa , Lahmer Tom
Published: November 2019

Dams and their adjourning reservoirs play an important role in the drinking water supply as well as in the flood defense. Their world‐wide use and possible massive devastation due to dam failures make the monitoring of the structural health of these huge structures imperative. Therefore, it is necessary to identify faults/damages in the dams' structure non‐invasively in order to proceed with the current use. Additionally, monitoring of these huge structures is very complex, time‐consuming and costly. To circumvent the aforementioned challenges, an inverse analysis, here an enhanced, regularized, cyclic full‐waveform inversion is proposed based on wavelengths between seismic and ultrasonic waves. Parameters of the dam material, such as stiffness, density and permeability, are estimated in synthetic experiments. Having this, an optimal design of experiment approach is used to identify the most informative sensor positions regarding the reconstruction quality. An optimized sensor layout may further improve the efficiency of the solution of the inverse problem.
An anisotropic creep model for continuously and discontinuously fiber reinforced materials
Authors: Hohe Jörg , Fliegener Sascha
Published: November 2019

The present study is concerned with the formulation, implementation and validation of anisotropic creep laws for neat thermoplastic matrix materials, discontinuously long fiber reinforced thermoplastic (LFT) composite materials with process dependent fiber preference directions as well as unidirectionally fiber reinforced thermoplastic tapes. Based on a simple three term Kelvin‐Voigt approach, an anisotropic model is formulated and implemented into a finite element program. The model is validated against an experimental data base of creep experiments on continuously and discontinuously fiber reinforced materials.
Gradient Recovery for the BEM‐based FEM and VEM
Authors: Seibel Daniel , Weißer Steffen
Published: November 2019

In this article, we propose new gradient recovery schemes for the BEM‐based Finite Element Method (BEM‐based FEM) and Virtual Element Method (VEM). Supporting general polytopal meshes, the BEM‐based FEM and VEM are highly flexible and efficient tools for the numerical solution of boundary value problems in two and three dimensions. We construct the recovered gradient from the gradient of the finite element approximation via local averaging. For the BEM‐based FEM, we show that, under certain requirements on the mesh, superconvergence of the recovered gradient is achieved, which means that it converges to the true gradient at a higher rate than the untreated gradient. Moreover, we propose a simple and very efficient a posteriori error estimator, which measures the difference between the unprocessed and recovered gradient as an error indicator. Since the BEM‐based FEM and VEM are specifically suited for adaptive refinement, the resulting adaptive algorithms perform very well in numerical examples.
Structural optimization of electrically charged anodes
Authors: Homberger Stefanie , Werner Marek , Weinberg Kerstin
Published: November 2019

The latest battery developments focus on the design of the anode and try to ensure a safe charging of the electrical device. Here an anode is studied which is made of graphite or silicon and subjected to several physical effects like thermal expansion, electrical current conductivity, and mechanical deformation. Typical design objectives like compliance or conductivity are competing against each other. On the one hand, the compliance is adapted as a design objective and is subjected to a minimization condition by applying external mechanical loads. On the other hand, the conductivity is aimed to be maximized. As a consequence, there is no global optimum structure. Therefore, an univariate optimization of certain objectives is investigated at first. Next, for a bivariate optimization Pareto optimality is used and finite element based numerical simulations are performed. The resulting structures will be used for a simulation of the charging process involving multi‐field physics.
Sparse discretization of sparse control problems
Authors: Herberg Evelyn , Hinze Michael , Schumacher Henrik
Published: November 2019

We consider optimal control problems that inherit a sparsity structure, especially we look at problems governed by measure controls. Our goal is to achieve maximal sparsity on the discrete level. We use variational discretization of the control problems utilizing a Petrov‐Galerkin approximation of the state, which induces controls that are composed of Dirac measures. In the parabolic case this allows us to achieve sparsity on the discrete level in space and time. Numerical experiments show the differences of this approach to a full discretization approach.
On a simple nonlinear system with circulatory forces
Authors: Barakat Ahmed A. , Hagedorn Peter
Published: November 2019

Self‐excited vibrations in mechanical system have been always gathering attention since being problematic for various applications, including brake squeal, aerodynamic flutter, machining chatter, and galloping transmission lines, or in some cases beneficial as in some musical instruments (ex. violin) or resonant MEMS. These vibrations can have different origins, such as ‘negative damping’ (such as in the classical Van der Pol oscillator) and/or circulatory forces (i.e. ‘follower forces’, non‐conservative positional forces). Both the negative damping and the circulatory forces can have different physical origins. It is well known that frictional forces generated between bodies in sliding contact can generate circulatory terms as well as damping and other types of forces. In particular, if one of two bodies in frictional contact moves with a constant speed or angular velocity, the other one being capable to oscillate, this implies an energy source for the oscillating part of the system. This has been identified as the cause for many self‐excited vibrations. In this presentation a very simple nonlinear two‐degree‐of‐freedom system of this general type is examined in some detail. It has recently been proposed in the literature as a paradigm for frictionally generated circulatory forces, for which the equations of motion can be derived from first principles. Other related systems have been presented in the literature before, but this newer system seems to be much simpler than the earlier ones and presents a great wealth of dynamic behavior. The nonlinear equations of motion of this 2‐DoF system always have the trivial solution and, depending on the system's parameters, also non‐trivial stationary solutions. The stability of the different stationary solutions is discussed in some detail. It was interesting to note that the increase of the friction parameter drives the system towards instability. This does however not happen, if the stiffness of both springs are identical, which practically can't be guaranteed.
Thin liquid film formation on hemispherical and conical substrate
Authors: Scholle Markus , Marner Florian , Gaskell Philip H.
Published: November 2019

The deposition and coating of thin films onto curved rigid substrate, involving displacement of air by a liquid, has numerous applications within the technology sectors but faces two major challenges: (i) control of the local film thickness; (ii) ensuring that the coating remains stable. The work reported here investigates the full coverage of three‐dimensional curved geometries, of hemispherical and conical shape, by a continuously fed, gravity‐driven, thin liquid layer. The modelling approach adopted utilises a first integral formulation [1,2] of the Navier‐Stokes equations leading to a variational formulation in the case of steady flow and an advantageous re‐formulation of the dynamic boundary condition at the free surface [3]. Asymptotic analysis, underpinned by the long‐wave approximation, enables analytic solutions for the local film thickness to be obtained.
Towards multifield discrete element modeling of concrete structures
Authors: Flack Christian , Dinkler Dieter
Published: November 2019

Concrete structures are built extensively all over the world. The prediction of the lifetime of a structure is very important in terms of costs and usability. A numerical modeling concept is presented that can describe concrete on the mesoscale. The focus of the model development is on the coupling of mechanics, heat conduction and chemical reactions. The Discrete Element Method (DEM) is used to describe the aggregates with spherical bonded particles. A voronoi tessellation provides access to the free space between the aggregates and leads to a pore network model (PNM) approach accounting for transport phenomena. Chemical reactions are handled within a processzone which couples the DEM and PNM. In the scope of this framework the sulfate attack on concrete is modeled and computed. The local deterioration in the concrete matrix is directly captured and the impact on the macroscopic behavior is presented.
Numerical approaches for the simulation of the yeast distribution in a fermentation tank
Authors: Klembt Daniel , Meironke Heiko
Published: November 2019

In recent years, in addition to the experimental investigation of a flow phenomena, numerical investigation has become increasingly established. Especially in investigations where conventional flow measurement techniques such as PIV or LDA fail, numerical computation can offer new possibilities. Models for the simulation of the natural convection flow in a fermentation tank and in second step a superposition of the natural convection by gas bubbles have already been presented in earlier publications. Based on these results, this paper examines the simulation of yeast using the Euler‐Lagrange model. The aim is to extend the existing simulation, which is based on the Eulerian‐Eulerian time‐averaged model and to validate it with measurements from real experiments with the Ultrasonic Doppler Velocimetry (UDV). Starting point is the Dense Discrete Phase Model (DDPM), which is based on the Kinetic Theory of Granular Flow (KTGF). Especially the physical interaction models, for example the lift force, virtual mass force and collision models, play an important role. These fundamental studies must be conducted because the behaviour of the real yeast is really complex. During real fermentation, the yeast increases over time and also combines with other yeast particles. As a result, the proportion of yeast in the process increases and the equivalent particle size also increases due to the combination of the yeast. The goal of this paper is a numerical simulation of the flow processes in the fermentation tank, based on the Eulerian‐Eulerian model with the additional DDPM as realistic as possible, taking into account the natural convection flow and the actual bubble behaviour as well as the yeast distribution.
An ISS characterization for discontinuous discrete‐time systems
Authors: Geiselhart R. , Noroozi N.
Published: November 2019

Input‐to‐state stability (ISS) is an important concept to study robustness properties of nonlinear control systems. The ISS property can be, in particular, characterized by ISS Lyapunov functions. For discrete‐time nonlinear control systems two different forms of ISS Lyapunov functions (implication‐form and dissipation‐form) are known to be equivalent if the dynamics are continuous. However, for discontinuous dynamics the equivalence is no longer satisfied. In this work, we discuss this phenomenon and, eventually, give a complete characterization of ISS in terms of ISS Lyapunov functions.
A penalization scheme for the numerical solution of optimal control problems with control complementarity constraints
Authors: Clason Christian , Deng Yu , Mehlitz Patrick , Prüfert Uwe
Published: November 2019

We suggest a simple penalty method for the numerical solution of optimal control problems with control complementarity constraints which is based on the famous Fischer–Burmeister function. The distinct advantage of this approach is the resulting smoothness of the penalty term and the unconstrainedness of the associated penalized surrogate problems. A numerical example from elliptic PDE control is presented as well.
On boundary conditions and constraints for representative volume elements of a two‐scale shell formulation
Authors: Zoller Jan , Gruttmann Friedrich
Published: November 2019

In this contribution a coupled two‐scale shell model with application of the FE2‐method based on the algorithm in [1] is presented. The representative volume element (RVE) is meshed with solid elements. In the RVE boundary effects can be reduced or even avoided using periodic boundary conditions. However, this requires proper constraints to prevent rigid body movements. For this purpose, the theory in [2] for beams, which considers the shape of normal stresses through the cross section, is transferred to the shell problem. Furthermore, the RVE reacts to shear deformations not only with shear stresses but also with bending stresses. This leads to a vanishing shear stiffness with an increase of the freely selectable in‐plane size. This dependence of the shear stiffness is eliminated by a further constraint on the normal stresses. Both constraints are incorporated using the method of Lagrange multipliers.
An Efficient Treatment of the Laplacian in a Gradient‐Enhanced Damage Model
Authors: Schwarz Stephan , Hackl Klaus , Junker Philipp
Published: November 2019

As is well‐known, softening effects that are characteristic for damage models are accompanied by ill‐posed boundary value problems. This arises from non‐convex and non‐coercive energies and results in mesh‐dependent finite element results. For that reason, regularization strategies, which somehow take into account the non‐local behavior, have to be applied in order to prevent ill‐posedness and to achieve mesh‐independence. Hereto, most commonly gradient‐enhanced formulations [1, 2] are considered, but also integral‐type [3,4] and viscous [5,6] regularization are well‐known.

Gradient‐enhanced damage models such as [1,2], to what group our new model [7] basically belongs, come along with a field function acting on the non‐local level. Two variational equations are resulting and, however, usually the number of nodal unknowns is increased and consequently the numerical effort is increased as well.

In contrast, we present an improved algorithm for brittle damage [7] combining finite element and meshless methods resulting in a quick update of the field function. Thereby, an efficient evaluation of the Laplace operator is applied as introduced in [8]. In the end, the numerical effort of the novel approach is almost comparable with an elastic problem while maintaining well‐posedness and therefore mesh‐independent results.
Artificial neural networks in structural dynamics
Authors: Stoffel Marcus , Bamer Franz , Markert Bernd
Published: November 2019

The aim of the present study is to investigate how structural models with inelastic material behaviour can be enhanced by means of artificial intelligence. To this end, artificial neural networks are proposed to replace continuum mechanical models in structural dynamics. This concerns predictions of shock wave‐loaded plates and finite element simulations with neural network enhanced material modelling. By comparison to experiments, the accuracy and the computational effort of the new approach is investigated.
The non‐real spectrum of a singular indefinite Sturm–Liouville operator with regular left endpoint
Authors: Behrndt Jussi , Schmitz Philipp , Trunk Carsten
Published: November 2019

We provide bounds on the non‐real spectra of indefinite Sturm–Liouville differential operators of the form (Af)(x) = sgn(x)(−f″(x) + q(x)f(x)) on the interval [a, ∞), −∞ < a < 0, with real potential q ∈ L1(a, ∞). The bounds depend only on the L1‐norm of the negative part of q and the boundary condition at the regular endpoint a.
Transformations and Enhancement Approaches for the Large Deformation EAS Method
Authors: Pfefferkorn Robin , Betsch Peter
Published: November 2019

The computer simulation of large deformation solid mechanics requires highly efficient finite elements. One widely used method enabling the construction of such elements is the enhanced assumed strain (EAS) method (cf. [1]). Within that framework, many transformations for the enhanced as well as the compatible deformation gradient have been used in previous publications (see e.g. [2]). We propose a new frame‐invariant alternative which fulfills the patch test and exhibits superior bending performance. Finally, novel mixed finite element methods based on the combination of mixed finite elements for polyconvexity [3] with the EAS method are presented. It is shown that this combination provides a promising framework for novel finite element methods.
Computationally effective spot‐weld fatigue life estimation for vehicle components
Authors: Zigo Milan , Arslan Eray , Mack Werner , Kepplinger Gerhard
Published: November 2019

To further speed up the frequency‐domain fatigue life estimation procedure for vehicle components with large numbers of spot‐welds, several measures are proposed. A two‐scale FE‐approach based on a pre‐processed library for various spot‐weld‐ / sheet‐geometries, applying the cutting plane method, and a priori stress estimations reduce the necessary CPU‐time considerably and allow for an efficient consideration of different load histories.
The Cauchy‐Riemann operator on smooth Fréchet‐valued functions with exponential growth on rotated strips
Authors: Kruse Karsten
Published: November 2019

We study the Cauchy‐Riemann operator

on spaces

of smooth functions on an open set

with values in a complex Fréchet space E which grow exponentially (γ = 1) on strips rotated by an angle θ. We state sufficient conditions for the surjectivity of

.
Numerical and experimental investigation on the flow behavior of liquids in narrow gaps
Authors: Luberto Luca , de Payrebrune Kristin M.
Published: November 2019

Lapping is one of the manufacturing processes with geometrically undefined cutting edges and is used to produce technical surfaces with high dimensional and shape accuracy. In order to predict the surface generation, a process model is developed that combines models of the material removal mechanism, the kinematics of the lapping particles and the flow of the lapping fluid. Due to many interactions in the process and the inhomogeneous flow of the lapping fluid (loose lapping particles in the fluid) a multiphysical model is needed. In a first step, only the flow behavior of the fluid without and with a simplified lapping particle (cuboid) is numerically and experimentally investigated. Particle Image Velocimetry is used to measure the flow field in the gap. The distance between the two plates, the velocity of the upper plate and the cuboid size can be varied. The experimental setup is simulated using a CFD model. Analogous to the experiment, the velocity of the upper plate and the cuboid size are also varied in the numerical study and the results agree well with the measured flow field. The CFD model helps to analyze further parameters, such as the influence of workpiece velocity on the flow. In the future, the model will be used to analyze the interaction between lapping particles and lapping fluid, which is very important for studying the mechanisms of material removal during lapping.
Comparative study of continuously differentiable Galerkin time discretizations for the wave equation
Authors: Bause Markus , Anselmann Mathias
Published: November 2019

In this work we compare numerically two families of space‐time finite element approximation schemes for wave equations. Continuously differentiable in time discrete solutions are provided by the schemes. The first family of schemes is based on a computationally cheap post‐processing of the continuous in space and time finite element approximation of the wave equation. The second family combines the variational approximation in time by finite element techniques with the concepts of collocation methods. Both families show equal order of convergence. The corresponding error constants are reviewed here.
Optimal periodic control of nonlinear chemical reactions with a time‐varying flow rate
Authors: Zuyev Alexander , Seidel-Morgenstern Andreas , Benner Peter
Published: November 2019

This paper deals with a mathematical model of a non‐isothermal chemical reaction described by nonlinear ordinary differential equations with two‐dimensional control. The control variables correspond to the possibility of manipulating the concentration of the input reactant and the total flow rate. We consider the task of maximizing the reactor performance as an isoperimetric optimal control problem with periodic boundary conditions. Necessary optimality conditions are derived by the Pontryagin maximum principle with Lagrange multipliers corresponding to the isoperimetric constraints.
Optimization Problems for Interacting Particle Systems and Corresponding Mean‐field Limits
Authors: Pinnau René , Totzeck Claudia
Published: November 2019

We summarize the relations of optimality systems for an interacting particle dynamic in the microscopic and in the kinetic description. In particular, we answer the question if the passing to the mean‐field limit and deriving the first order optimality system can be interchanged without affecting the results. The answer is affirmative, if one derives the optimality system on the kinetic level in the metric space (𝒫2, 𝒲2). Moreover, we discuss the relation of to the adjoint PDE derived in the L2‐sense. Here, the gradient can be derived as expected from the calculus in Wasserstein space.