bluuebunny/crossref_metadata_2025 huggingface dataset:
DISO: A rethink of Taylor diagram
Authors: Hu Zengyun , Chen Xi , Zhou Qiming , Chen Deliang , Li Jianfeng
Published: December 2018

Climate models use quantitative methods to simulate the interactions of the important drivers of climate system, to reveal the corresponding physical mechanisms, and to project the future climate dynamics among atmosphere, oceans, land surface and ice, such as regional climate models and global climate models. A comprehensive assessment of these climate models is important to identify their different overall performances, such as the accuracy of the simulated temperature and precipitation against the observed field. However, until now, the comprehensive performances of these models have not been quantified by a comprehensive index except the existed single statistical index, such as correlation coefficient (r), absolute error (AE), and the root‐mean‐square error (RMSE). To address this issue, therefore, in this study, a new comprehensive index Distance between Indices of Simulation and Observation (DISO) is developed to describe the overall performances of different models against the observed field quantitatively. This new index DISO is a merge of different statistical metrics including r, AE, and RMSE according to the distance between the simulated model and observed field in a three‐dimension space coordinate system. From the relationship between AE, RMSE, and RMS difference (RMSD) (i.e., standard deviation [SD] of bias time series), the new index also has the information of RMSD which is the statistical index in Taylor diagram. An example is applied objectively to display the applications of DISO and Taylor diagram in identifying the overall performances of different simulated models. Overall, with the strong physical characteristic of the distance in three dimensional space and the strict mathematical proof, the new comprehensive index DISO can convey the performances among different models. It can be applied in the comparison between different model data and in tracking changes in their performances.
Osteochondral regeneration using constructs of mesenchymal stem cells made by bio three‐dimensional printing in mini‐pigs
Authors: Yamasaki Atsushi , Kunitomi Yoshihiro , Murata Daiki , Sunaga Takafumi , Kuramoto Tomohide , Sogawa Takeshi , Misumi Kazuhiro
Published: December 2018

Osteoarthritis is a major joint disease that has been extensively investigated in humans and in model animals. In this study, we examined the regeneration of articular cartilage and subchondral bone using artificial scaffold‐free constructs composed of adipose tissue‐derived mesenchymal stem cells (AT‐MSCs) created using bio three‐dimensional (3D) printing with a needle‐array. Printed constructs were implanted into osteochondral defects created in the right femoral trochlear groove of six mini‐pigs, using femoral defects created in the left femurs as controls. Repair within the defects was evaluated at 3 and 6 months post‐implantation using computed tomography (CT) and magnetic resonance (MR) imaging. The radiolucent volume (RV, mm3) in the defects was calculated using multi‐planar reconstruction of CT images. MR images were evaluated based on a modified 2D‐ MOCART (magnetic resonance observation of cartilage repair tissue) grading system. Gross and microscopic pathology were scored according to the ICRS (International Cartilage Repair Society) scale at 6 months after implantation. The percentage RV at 3 months postoperation was significantly lower in the implanted defects than in the controls, whereas total scores based on the MOCART system were significantly higher in the implanted defects as compared with the controls. Although there were no statistical differences in the gross scores, the average histological scores were significantly higher in the implanted defects than in the controls. To our knowledge, this is the first report to suggest that artificial scaffold‐free 3D‐printed constructs of autologous AT‐MSCs can be aid in the osteochondral regeneration in pigs. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1398–1408, 2019.
Journals removed from DOAJ appearing within SCImago's ranks: A study of excluded journals
Authors: Sun Li
Published: December 2018

Researchers applauded Directory of Open Access Journals (DOAJ)’s new selection criteria that removed many existing journals from its index because they are ‘Not Adhering to Best Practice’. However, many of these journals have acceptable scholarly quality. The present study compares such removed journals to a trustworthy bibliometric database, SCImago Journal & Country Rank (the Scopus Database). It finds many overlaps. Some of the overlapping journals have a high score in the SCImago ranks. This paper calls for a more transparent process of journal reviews by DOAJ or even a re‐examination of the selection criteria and/or the evaluation process.
Uridine‐cytidine kinase 2 upregulation predicts poor prognosis of hepatocellular carcinoma and is associated with cancer aggressiveness
Authors: Huang Shanzhou , Li Jian , Tam Nga Lei , Sun Chengjun , Hou Yuchen , Hughes Bridget , Wang Zekang , Zhou Qi , He Xiaoshun , Wu Linwei
Published: December 2018

Patients with advanced hepatocellular carcinoma (HCC) continue to have a dismal prognosis. Potential biomarkers to determine prognosis and select targeted therapies are urgently needed for patients with HCC. This study aimed to elucidate the role of UCK2 in HCC prognosis and tumor progression. We performed a screen of public databases to identify functional genes associated with HCC tumorigenesis, progression, and outcome. We identified uridine‐cytidine kinase 2 (UCK2) as a gene of interest for further study. UCK2 promoting HCC aggressiveness was demonstrated by evaluation of clinical samples, in vitro experiments, in vivo tumorigenicity, and transcript analysis. UCK2 expression was generally elevated in HCC and was significantly correlated with poor survival and inferior clinicopathological characteristics of HCC patients. A multivariate analysis revealed that high UCK2 expression was an independent factor for poor prognosis. In HCC cell lines, UCK2 knockdown suppressed cell migration and invasion and inhibited cell proliferation, while UCK2 overexpression had an opposite effect. Animal model experiments confirmed that knockdown of UCK2 suppressed tumor growth in vivo. The bioinformatics analysis demonstrated that UCK2 might associated with metabolsim, splicesome, and adherens junction. UCK2 is highly associated with HCC malignant behavior and is a potential prognostic predictor for HCC patients in the clinic.
Movement Disorders in Treatable Inborn Errors of Metabolism
Authors: Ebrahimi‐Fakhari Darius , Van Karnebeek Clara , Münchau Alexander
Published: December 2018

There are several hundred single‐gene disorders that we classify as inborn errors of metabolism. Inborn errors of metabolism are often rare and highly heterogeneous multisystem diseases with non‐neurological and neurological manifestations, commonly with onset during childhood. Movement disorders are among the most common neurological problems in inborn errors of metabolism, but, in many cases, remain poorly defined. Although movement disorders are usually not the only and often not the presenting symptom, their recognition can facilitate a diagnosis. Movement disorders contribute substantially to the morbidity in inborn errors of metabolism and can have a significant impact on quality of life. Common metabolic movement disorders include the monoamine neurotransmitter disorders, disorders of amino and organic acid metabolism, metal storage disorders, lysosomal storage disorders, congenital disorders of autophagy, disorders of creatine metabolism, vitamin‐responsive disorders, and disorders of energy metabolism. Importantly, disease‐modifying therapies exist for a number of inborn errors of metabolism, and early recognition and treatment can prevent irreversible CNS damage and reduce morbidity and mortality. A phenomenology‐based approach, based on the predominant movement disorder, can facilitate a differential diagnosis and can guide biochemical, molecular, and imaging testing. The complexity of metabolic movement disorders demands an interdisciplinary approach and close collaboration of pediatric neurologists, neurologists, geneticists, and experts in metabolism. In this review, we develop a general framework for a phenomenology‐based approach to movement disorders in inborn errors of metabolism and discuss an approach to identifying the “top ten” of treatable inborn errors of metabolism that present with movement disorders—diagnoses that should never be missed. © 2018 International Parkinson and Movement Disorder Society
Kaiser Permanente MH clinicians seek improved, timely care and more staff
Authors: Canady Valerie A.
Published: December 2018

Observing a need to hire more clinicians to provide timely, appropriate care to patients with mental illness, many of whom have to wait four, six and even eight weeks for return visits, is the thrust behind a five‐day strike by Kaiser Permanente mental health clinicians across California. The strike, which commenced Dec. 10, entered its fourth day as MHW went to press.
A complementary Sierpinski gasket fractal antenna array for wireless MIMO portable devices
Authors: Kaur Amanpreet , Gupta Sheifali
Published: December 2018

This research work reports the design and development of a novel complementary Sierpinski gasket fractal microstrip antenna array to achieve a good impedance bandwidth and hence support high data rates for 4G/5G MIMO applications. The proposed array is a compact structure (136 mm × 136 mm) designed with complimentary Sierpinski gasket fractal up to two iterations and has a defected ground to improve the antenna's performance. The antenna performance reports an 8.2% bandwidth at the center frequency of 4.94 GHz (4.7434‐5.1514 GHz), a simulated diversity gain of 9.99 allows the proposed array to be well suitable for proposed applications and a directivity of around 5.8 dBi suits good range characteristics. The array designing and optimization is carried out with 3D EM tool CST MWS V'14 using perfect boundary approximations. A simulated low value of envelope correlation coefficient (0.04) indicates that the two antennas in the array provide least interference to each other while radiating. The measured S11 results on a VNA E 5071C show a close match with the simulated results. Thus a compact array size with good bandwidth, radiation characteristics, low ECC and a high diversity gain allow the antenna array to be well suitable for portable MIMO 4G/5G wireless applications specifically for C band and military applications.
A system for profiling the IXPs in a region and monitoring their growth: Spotlight at the internet frontier
Authors: Fanou Rodérick , Sanchez‐Agüero Victor , Valera Francisco , Mwangi Michuki , Coffin Jane
Published: December 2018

This work aims at designing and implementing a system able to profile and help manage the set of Internet eXchange Points (IXPs) in an Internet region. As part of the Internet Society's strategy to help monitor and understand the evolution of IXPs in a particular region, a route‐collector data analyzer tool was developed before being deployed and tested in AfriNIC. In fact, traffic localization efforts in the African peering ecosystem would be more sustained, and their efficacy assessed if they were supported by a platform, which evaluates and reports in real time about their impact on the Internet. We, thus, built the “African” Route‐collectors Data Analyzer (ARDA), an open source web platform for analyzing publicly available routing information collected since 2005, by local route‐collectors. ARDA evaluates predefined metrics that picture the status of the interconnection at local, national, and regional levels. It shows that a small proportion of AfriNIC ASes (roughly 17%) are peering in the region. Through them, 58% of all African networks are visible at one IXP or more. These have been static from April to September 2017, and even February 2018, underlining the need for increased efforts to improve local interconnectivity. We show how ARDA can help detect the impact of policies on the growth of local IXPs or continually provide the community with up‐to‐date empirical data on the evolution of the IXP substrate. Given its features, this tool will be a helpful compass for stakeholders in the quest for better traffic localization and new interconnection opportunities in the targeted region.
A stable local radial basis function method for option pricing problem under the Bates model
Authors: Company Rafael , Egorova Vera N. , Jódar Lucas , Soleymani Fazlollah
Published: December 2018

We propose a local mesh‐free method for the Bates–Scott option pricing model, a 2D partial integro‐differential equation (PIDE) arising in computational finance. A Wendland radial basis function (RBF) approach is used for the discretization of the spatial variables along with a linear interpolation technique for the integral operator. The resulting set of ordinary differential equations (ODEs) is tackled via a time integration method. A potential advantage of using RBFs is the small number of discrete equations that need to be solved. Computational experiments are presented to illustrate the performance of the contributed approach.
A New Holistic Approach to the Local Buckling Behaviour of Open‐Profile Composite Laminated Beams
Authors: Schreiber Philip , Mittelstedt Christian
Published: December 2018

The present work deals with the local buckling of thin‐walled composite‐beams with I, Z, C, L and T‐cross‐sections under axial load. In this study the analysis of the buckling load of the complete composite‐beam is performed using the Ritz‐method. The individual webs and flanges of the beam are assembled by suitable continuity conditions in one system. In order to achieve that, new displacement shape functions for web and flange that fulfill all boundary conditions have been developed. The present closed‐form analytical method enables the explicit representation of the buckling load for the entire composite‐beam under axial load. The comparison between the present approach and finite element simulations shows a satisfactory conformity. The present method is highly efficient in terms of computational effort and its practical suitability.
Operator splitting for stiff chemical reaction systems
Authors: Lukassen Axel Ariaan , Kiehl Martin
Published: December 2018

Chemical reaction systems often result in high‐dimensional, stiff differential equations. Solving these differential equations is computational demanding. The computational effort can be reduced by the usage of an operator splitting scheme. The most frequently used splitting schemes are the Lie‐Trotter splitting [1] and the Strang splitting [2]. However, the usage of a splitting scheme results in an additional splitting error. The Lie‐Trotter splitting is a first order scheme, and the Strang splitting is a second order scheme. If the timescale of the fastest chemical reaction is smaller than the splitting time step, the Strang splitting suffers from order reduction. Therefore, the Lie‐Trotter splitting and the Strang splitting are first order schemes for reasonable step sizes. The Richardson extrapolation of the Lie‐Trotter splitting is a second order scheme. We show that the Richardson extrapolation does not suffer from order reduction for a splitting between a fast chemical source term and a slow transport term.
PIV Measurements and Numerical Simulations in a 2D‐Extruder
Authors: Hirschfeld Stefan , Wünsch Olaf
Published: December 2018

This work is concerned with the fluid flow in a special fully and partially filled 2D‐extruder model with a double flighted single‐screw and twin‐screw geometry. A shear‐thinning highly viscous silicone oil is used as model substance. The results of Particle Image Velocimetry (PIV) measurements are compared with numerical simulations.
Predicting Skeletal Muscle Force from Motor‐Unit Activity using a 3D FE Model
Authors: Saini Harnoor , Altan Ekin , Ramasamy Ellankavi , Klotz Thomas , Gizzi Leonardo , Röhrle Oliver
Published: December 2018

Homogenised finite‐element (FE) skeletal muscle models provide a good trade‐off between computational efficiency and physiological realism. This trade‐off is in part due to the lack of explicit modelling of muscle fibres (MFs). As a consequence, the organisation of MFs into motor‐unit territories (MUTs) is particularly challenging. The current paper presents a possible technique to adress this issue by assigning MUTs in a homogenised sense in terms of muscle volume to FE skeletal muscle models. This method allows control of inter‐MUT distance, the amount of intermingling between MUTs and the order of MUT assignment. It was found that both assigning smaller MUTs first and allowing a moderate amount of intermingling significantly influence MUT distributions. This technique allows a simplified and straight‐forward way to introduce MUTs into 3D FE skeletal muscle models.
Time step estimates for explicit dynamics with reciprocal mass matrices
Authors: Tkachuk Anton , Schaeuble Anne‐Kathrin , Bischoff Manfred
Published: December 2018

In this contribution, a novel local, node‐based time step estimate for explicit dynamics with reciprocal mass matrices is presented. Reciprocal mass matrices are sparse matrices used in explicit dynamics that allow computation of nodal acceleration from the total force vector. They can be built algebraically and variationally and aim for higher critical time steps or/and accuracy than the lumped mass matrix approximation. Since the element eigenvalue inequality by Fried is not valid for reciprocal mass matrices, element‐based estimates may be not conservative and they are consequently inadequate. Therefore, the nodal time step estimate for diagonally lumped mass matrices based on Gershgorin's theorem is further developed for application to reciprocal mass matrices. Additionally, simplifications of the proposed time step estimate that improve computational efficiency, especially for contact problems with the penalty method, are discussed. These simplifications use subadditivity and submultiplicativity properties of expressions used in the estimate. Finally, efficiency of the proposed estimate is illustrated by a numerical example.
Effects of aging on the thermo‐mechanical response of a die casting alloy
Authors: Martinez Page Maria Angeles , Hartmann Stefan
Published: December 2018

Zamak alloys can be found in a wide variety of components of the automotive, building, electronic, and toy industry. They are preferred to other materials for die casting foundry due to their several advantages. For example, they exhibit a low melting point which allows an economic manufacturing process, they allow the production of components with tight tolerances and are characterized by good mechanical properties. Nonetheless, these alloys show a change in their mechanical behavior over time. This phenomenon is known as aging and it is associated to microstructural transformations such as diffusion and precipitation of alloying elements and phase transformations. In this work, the influence of natural aging on the thermo‐mechanical response is investigated with the help of tension, compression and torsion experiments and a model with aging as an internal variable is proposed.
Droplet deformation, oscillation and detachment on a vibrating plate by a controlled airflow
Authors: Barwari Beawer , Burgmann Sebastian , Janoske Uwe
Published: December 2018

In this work, a detailed experimental study on the influence of fluid properties on the droplet behavior subjected to a shear flow and harmonic surface vibration is conducted. The experiments were executed in a rectangular channel and an acrylic channel wall was utilized to analyze the droplet wetting behavior. Both the effect of shearing mechanism induced by airflow and harmonic vibration induced by an electromagnetic shaker for droplet volumes in the range of 5−40 µl are studied here. Furthermore, the effect of fluid viscosities on the motion of droplets is analyzed for a wide range of water and glycerine mixture ratios. Experiments show that the critical bulk velocity at which the droplet movement begins is a function of fluid properties and droplet sizes. Increasing the droplet volume results in a decrease of the critical bulk velocity, while the droplet deformation increases. On the contrary, increasing the glycerine content of the liquid droplets results in an increase in both the critical bulk velocity and droplet deformation. The droplet movement can be categorized into three motion patterns based on experimental observations: (i) the droplet shape is largely retained with a small tail formation at the rear end, (ii) small droplets detach from the tail at rear end of the main droplet, (iii) the droplet deforms over its entire length resulting in forming of a film.
On computational interdisciplinary modeling for reliability assessment of high‐speed train – bridge interaction
Authors: Adam Christoph , Hirzinger Benjamin , Oberguggenberger Michael , Salcher Patrick
Published: December 2018

Bridge and crossing high‐speed train represent an uncertain interacting dynamic system of high complexity with, in general, time‐dependent system matrices. Thus, full reliability assessment of this system can only be conducted efficiently in an interdisciplinary approach, bringing together the disciplines of structural mechanics, engineering mathematics, and high performance computing. This contribution gives a brief summary on computational modeling issues of uncertain bridge‐train interaction, its efficient stochastic simulation, and an outline of pros and cons of different strategies is provided.
Sensitivity Analysis of a Two‐Lens System for Positioning Feedback
Authors: Schindlbeck Christopher , Pape Christian , Reithmeier Eduard
Published: December 2018

Precise alignment of optical components is crucial for the assembly of optical system. Misaligned components (due to positioning tolerances) lead to a distorted wavefront that deviates from a desired wavefront given by an initial nominal design. Inferring the current poses of optical components solely from detector measurements is a difficult but necessary task in order to calculate corrections for feedback control without using external measurement devices. In this paper, we aim at deducing lens poses of a two‐lens system solely from wavefront measurements. The wavefront can be decomposed into a finite sequence of polynomials which are weighted by coefficients. Since pose deviations directly lead to deviations in the wavefront (and therefore in the wavefront coefficients), a sensitivity matrix can be established. The choice of lens poses and wavefront coefficients determines the sensitivity matrix which is often ill‐conditioned and may potentially suffer from rank loss. We conduct a sensitivity analysis and show how the sensitivity matrix can be utilized to generate a linear estimate of the optical component poses. This estimate can then be used to close the feedback loop. Validation is conducted by simulating an optical system consisting of a laser, two bi‐convex lenses, and a wavefront sensor.
Heuristic pressure profiles in hydraulic fracturing
Authors: Schätzer Markus , Fries Thomas‐Peter
Published: December 2018

In hydraulic fracturing, the pressure exerted by the fracking fluid onto the surrounding solid, is typically obtained from solving the Reynolds equation. This is not always robust and leads to a complex behaviour at the crack front (toughness or viscous regimes). In the presented work, the Reynolds equation is replaced by a simplified fluid model, where pre‐defined pressure distributions are assumed which lead to a simpler and robust coupled problem. The mapping of dimensionless prescribed pressure distributions onto the physical crack surface of the mesh is discussed based on a dimensionless distance function. This function is obtained by solving the Laplace‐Beltrami operator on the explicit representation of the manifold, i.e., the surface mesh, where distances are evaluated along streamlines which represent the fluid flow of the fracking fluid within the fracture.
Application and adaption of Dynamic Mode Decomposition to experimental and numerical data of turbomachinery flow
Authors: Walter Christian‐Henrik , Krieg Marinho
Published: December 2018

Unsteady turbomachinery flows are challenging in terms of resolving turbulent characteristics and instability mechanisms like rotating stall, rotor‐stator interaction and von Karmán vortex streets. Furthermore, in part load conditions these flow phenoma can induce vibrations leading to resonance problems and high cycle fatigue. Flowfield measurements of a centrifugal fan were taken by high‐speed particle image velocimetry (PIV). CFD results, calculated as unsteady Reynolds averaged Navier Stokes (URANS) and based on Menter's shear‐stress transport (SST) model, are the source of numerical data. In former examinations discrepancies between experiment and simulation in time‐averaged velocity profil data for part load conditions have been stated. The source of theses discrepancies is still unknown. Hence, a new approach is needed for further examination of measured and calculated flowfields. The Dynamic Mode Decomosition is employed in order to compare experimental and numerical datasets. Information about coherent structures and the dynamical behaviour are contained in the eigenvalues and their associated eigenvectors offered by DMD. While the eigenvectors are describing the coherent structures, the associated eigenvalues describe the dynamic behaviour of the structure. The eigenvectors are denoted as DMD‐Modes. The results acquired by DMD based on experimental and numerical data are compared to investigate congruent behaviour and therefore if it is possible to obtain reliable information about dynamic behaviour of a system even with URANS simulations. Comparisons between singular value and eigenvalues are presented and discussed.
Efficient basis updating for parametric nonlinear model order reduction
Authors: Meyer Christian H. , Lerch Christopher , Karamooz Mahdiabadi Morteza , Rixen Daniel
Published: December 2018

Nonlinear model reduction is used to speed up the numerical solution of finite element models for vibration analysis of structures undergoing large deflections. This speed up is highly desired in design and optimization applications where parametric models are considered and the outcoming high‐dimensional differential equation must be solved multiple times. A first step is to approximate the solution vector i.e. the displacements of the nodes by a linear combination of some basis vectors. One common choice for these basis vectors is a combination of vibration modes and static modal derivatives. However, these vectors depend on parameter values of the parameterized system. This contribution shows how these basis vectors can be updated in an efficient manner. The vibration modes are updated by an inverse free preconditioned Krylov subspace method while the static derivatives are updated by a preconditioned conjugate gradient solver. A case study with a parametric beam gives a first insight into the performance of the proposed method.
Variational Integrators and Optimal Control for a Hybrid Pendulum‐on‐Cart‐System
Authors: Kern Dominik , Groß Michael
Published: December 2018

Hybrid systems combine continuous‐time and discrete‐time dynamics. They emerge in mechanical systems, inter alia, when collisions are modelled by impulse‐like contact forces. During collisions of two or more bodies, the positions of the latter evolve continuously, but their velocities experience jumps. On the one hand, this non‐smoothness significantly complicates the optimal control problem; on the other, it opens up opportunities for new numerical methods. This contribution uses a nested optimization in order to control the swinging up of a hybrid pendulum‐on‐cart‐system. Both, pendulum and cart, move in finite ranges which are bounded by limiters (elastic collisions). The outer layer optimizes the occurence and the momenta of the collisions, while the inner layer optimizes the smooth motions from start to collision, from collision to collision, and finally to the terminal state. The discretization and the optimal control of continuous‐time dynamics are based on the Discrete‐Mechanics‐and‐Optimal‐Control (DMOC) approach.
One‐dimensional turbulence investigation of incompressible and low Mach number variable density pipe‐flow
Authors: Medina Méndez Juan A. , Schmidt Heiko
Published: December 2018

Cylindrical pipe‐flow is evaluated by means of the One‐Dimensional Turbulence (ODT) model applying both an incompressible and a low Mach number variable density framework. In the variable density formulation, the velocity field is decomposed into density‐related (non‐divergence‐free) and density‐unrelated (divergence‐free) contributions, as detailed in Medina et al. [Combust. Flame 190 (2018) 388‐401]. Both contributions to the velocity field are subject to the effects of diffusion and turbulent transport, as it is normally the case for the incompressible velocity field in traditional ODT formulations. The turbulent transport is implemented by means of stochastic eddy events. These are influenced by the available kinetic energy given by the divergence‐free velocity contribution. An eddy may arise from the different regions of shear caused by varying velocity gradients in the 1‐D domain, but also as a consequence of the temperature‐varying dynamic viscosity. The cylindrical low Mach number variable density formulation is tested in a very low heat transfer regime and compared to the passive scalar results of the incompressible pipe‐flow formulation. ODT model parameters are assumed equal for both formulations. ODT results are compared to DNS results from Satake and Kunugi [Int. J. Numer. Methods Heat Fluid Flow 12 (2002) 958‐1008]. The passive scalar and variable density results show very good agreement with each other as well as with DNS data, respectively, thus suggesting new possible applications for ODT.
A task space admittance control algorithm for safe human robot interaction
Authors: Kaserer Dominik , Gattringer Hubert , Müller Andreas
Published: December 2018

A recent development for programming industrial robotic manipulators is the interactive teaching of task motions where a human operator physically guides the end‐effector of a robot. Admittance control schemes are used to this end. Since the operator is in direct contact with the robot, it must be ensured that the velocities of the robot are within a safe range at any time. Thus, in order to guarantee safe operation, position and velocity limits must be respected within the admittance control. Existing admittance control schemes can either take task space limits or joint space limits into account. In this paper an extended admittance control scheme is presented which is capable of simultaneously taking task space limits as well as joint space limits into account. This is achieved by formulating the admittance control law in terms of a path parameter to account for task space limits and a subsequent determination of the path parameter dynamics with respect to the joint space limits. Admittance control requires an estimation of the interaction wrench to be available. Three different methods for obtaining such an estimation are presented: 1) A six‐axis force/torque sensor mounted on the robot end‐effector, 2) The generalized momentum, 3) A decoupled mechanical model of the motors. The performance of the algorithms is demonstrated by means of experimental results.
Bioreactor development for regenerative tissues of the locomotor system
Authors: Stoffel Marcus , Dursun Gözde , Nachtsheim Julia , Fuhrmann‐Nelles Nadine , Markert Bernd
Published: December 2018

In biomedical engineering it is an important challenge to predict the mechanical behaviour of living biological tissues under dynamic loading conditions. As a basis for theoretical models, experimental set‐ups are necessary to trace evolutive processes, such as remodelling and growth, in biological materials. However, in order to mimic in‐vivo situations most realistically, in‐vitro bioreactor systems with physiologically enhanced loading conditions have to be developed. The present study deals with a set of bioreactor systems covering a wide range of applications for investigating remodelling behaviour of biological tissues, such as cartilage replacement materials in knee joints, tendons, and intervertebral discs.
On the modal analysis of flexible multibody systems with singular mass and stiffness matrices
Authors: Hente Christian , Gebhardt Cristian G. , Hofmeister Benedikt , Rolfes Raimund
Published: December 2018

A complex mechanical system can be described as a combination of individual rigid and/or flexible components and a set of kinematical constraints. The restrictions can be either internal, which come on the scene in the case of parameterizing the configuration manifold of a system with a number of parameters larger than its intrinsic dimension, e.g. by employing three directors to describe the Lie Group SO(3) (basically for rigid bodies and beams) or by employing a single unit director to describe the two dimensional sphere with unit radius embedded in the three‐dimensional Euclidean space, i.e. S2 1 , (basically for inextensible shells, in which the component of the Green‐Lagrange strain tensor E33 is set to be zero at every time), or external ones, which come on the scene in the case of constraining bodies by means of joints, connections or supports. The linearized form of the resulting governing equations possesses usually singular mass and stiffness matrices. This fact represents an issue when the calculation of natural frequencies and natural modes for the whole system is needed. The classical modal analysis (solving directly the generalized eigenvalue problem from the stiffness and mass matrices) is no longer applicable. To overcome this difficulty, it is firstly necessary to utilize the linearized constraint equations that complete the missing information at the level of the mass and stiffness matrices and secondly, to redefine the eigenvalue and ‐vector problem within a suitable framework. In the current work, we set the focus on the development framework for the modal analysis able to deal with singular matrices.
On the construction of a triangular Mixed Finite Element based on the principle of Hellinger‐Reissner
Authors: Viebahn Nils , Steeger Karl , Schröder Jörg
Published: December 2018

A low order finite element based on a Hellinger‐Reissner formulation with weak enforcement of the stress symmetry is presented. The novel element gets along without any additional degrees of freedom beside the displacements and stresses. Therein, lowest order Raviart‐Thomas interpolation is utilized for the stresses and a C0 continuous polynomial interpolation of first order is adopted for the displacements. This combination leads to a very efficient and robust element formulation which will be depicted on a couple of numerical examples.
FFT‐based Homogenization in nonlinear Electroelasticity
Authors: Göküzüm Felix Selim , Nguyen Lu Trong Khiem , Keip Marc‐André
Published: December 2018

Electroactive polymers (EAPs) are a group of materials that is able to respond with large strains to applied electric fields, making them candidates for applications such as artificial muscles or smart fins. The present contribution addresses the computational homogenization of electroactive materials based on fast‐Fourier‐transforms. The focus lies on the formulation of a coupled Lippmann‐Schwinger equation with respect to the deformation gradient as well as the electric displacement, where the reference medium introduced in the Lippmann‐Schwinger equation is also fully coupled. As the reference medium is acting as a preconditioner on the system, this has an impact on the convergence rate and stability of the iterative solver. We provide an algorithmically consistent coupled tangent operator as alternative to finite‐difference based approaches.
Investigation of the laminate free‐edge effect by means of the scaled boundary finite element method
Authors: Dölling Sebastian , Hell Sascha , Becker Wilfried
Published: December 2018

In the present study, a coupled stress and energy criterion within the framework of finite fracture mechanics is used to predict interlaminar crack onset in composite laminates induced by the free‐edge effect. In order to obtain the required quantities for the evaluation of the coupled criterion the scaled boundary finite element is implemented. This semi‐analytical approach reduces the numerical effort significantly in comparison to standard finite element method. To assess the reliability of the obtained results a comparison to a finite element model is presented.
On a relaxation‐based and time‐incremental approach to damage modeling
Authors: Schwarz Stephan , Hackl Klaus , Junker Philipp
Published: December 2018

Damage models, which are characterized by softening effects, suffer from ill‐posed boundary value problems that result in mesh‐dependent finite‐element results. Regularization strategies counteract these problems by taking into account the non‐local behavior such as realized by the gradient‐enhanced formulation presented in [1]. Two variational equations are resulting at the expense of numerical effort but, however, mesh‐independent finite‐element results are provided. In contrast, we present a novel approach to the regularization of damage processes based on relaxed energies of two‐state models that are formulated in a time‐incremental way. The energies within each time step remain convex but the overall behavior is non‐coercive in order to maintain the characteristic stress‐behavior of damage models. Therefore, mesh‐independent results with less numerical effort are achieved.
A modified plate model for an efficient analytical treatment of stress concentrations at notches
Authors: Felger Julian , Becker Wilfried
Published: December 2018

In the present work, a modified plate model is proposed lying in‐between the established Kirchhoff‐Love and first‐order shear deformation plate theories. The resulting system of partial differential equations can be solved using a complex potential approach which allows for tackling complex geometries such as sharp or rounded notches. Using the proposed model, the singular stress field in the vicinity of a sharp notch can be obtained explicitly and the influence of the notch opening‐angle on the singular behaviour is investigated. It is shown that resulting bending moments and transverse shear forces match with asymptotic solutions according to first‐order shear deformation plate theory and are in very good agreement with numerical results.
A fluid model for the simulation of fluid‐structure interaction in the Scaled Boundary Finite Element Method for prismatic structures
Authors: Wasmer Paul , Krome Fabian , Bulling Jannis , Prager Jens
Published: December 2018

The Scaled Boundary Finite Element Method is known as an efficient method for the simulation of ultrasonic wave propagation. As to investigate acoustic wave behavior in case of fluid‐structure interaction, a fluid model is implemented in the SBFEM for prismatic structures. To omit coupling terms a displacement‐based formulation is used. Spurious modes, which occur in the solution, are suppressed using a penalty parameter. To verify this formulation dispersion curves obtained with Comsol Multiphysics are compared to results of SBFEM. The results of both methods are in very good agreement.
Numerical Simulation of Wind Turbine Considering the Soil‐Structure‐Interaction
Authors: Schauer Marco
Published: December 2018

Whenever vibrations are emitted to soil, they induce waves traveling through the ground. This work is focused on the Soil‐Structure‐Interaction effects on the dynamic behavior of operating wind turbines. Two different mechanical problems have to be addressed and to be solved simultaneously. There is the near‐field, containing the structure of the wind turbine and its foundation as well as parts of the soil modeled by the Finite Element Method (FEM), and the far‐field, the surrounding infinite half‐space discretized by the Scaled Boundary Finite Element Method (SBFEM) [1]. Both domains are discretized separately in order to reduce the effort of the preprocessing. This leads to non‐matching meshes in general. To overcome this issue state projection methods are utilized to exchange state variables at the common interface while solving the coupled FEM‐SBFEM approach.
A quantitative comparison of two‐ and three‐dimensional modeling approaches for magnetorheological elastomers
Authors: Metsch Philipp , Kalina Karl Alexander , Brummund Jörg , Kästner Markus
Published: December 2018

In this contribution, a qualitative and quantitative comparison of two‐ and three‐dimensional finite element simulations for magneto‐rheological elastomers is presented. Based on a continuum formulation, a microscopic modeling approach is applied. To understand basic deformation mechanisms, the differences between simplified two‐dimensional and realistic three‐dimensional simulations are initially shown for the example of chain‐like structures with different arrangements of the particles. Subsequently, a numerical homogenization is used to connect microscopic and macroscopic quantities: different two‐ and three‐dimensional microstructures are compared with regard to their effective magneto‐mechanical behavior.
Convergence properties of the enriched Scaled Boundary Finite Element Method in fracture mechanics
Authors: Bremm S. , Hell S. , Becker W.
Published: December 2018

The standard Finite Element Method (FEM) yields optimal rates of convergence for smooth domains. The presence of cracks, corners or notches within the considered domain leads to low rates of convergence which are independent of the polynomial degree of the employed trial functions. In order to regain full rates of convergence, an appropriate adaptation and a skilful choice of test and trial space of the FEM is necessary. The Scaled Boundary FEM (SBFEM) has turned out to be an appropriate method for the treatment of 2D crack problems, in which the singularity is entirely located within the considered domain. However, in 3D problems, singularities are still present at the meeting points of the crack front and the discretized boundary. Then, the SBFEM suffers from the same drawbacks as the standard FEM. In order to dispel this inconvenience, an extension of the trial space with asymptotic crack tip displacement solutions is considered resulting in improved rates of convergence.
Finite volume based multigrid preconditioners for discontinuous Galerkin methods
Authors: Versbach Lea Miko , Birken Philipp , Gassner Gregor J.
Published: December 2018

Our aim is to construct efficient preconditioners for high order discontinuous Galerkin (DG) methods. We consider the DG spectral element method with Gauss‐Lobatto‐Legendre nodes (DGSEM‐GL) for the 1D linear advection equation. It has been shown in [4] that DGSEM‐GL has the summation‐by‐parts (SBP) property and an equivalent finite volume (FV) discretization is presented in [3]. Thus we present a multigrid (MG) preconditioner based on a simplified FV discretization.
A Simplification of the RitzGenEO Recycling Strategy for Adaptive Multi‐Preconditioned FETI Applied to Multiple Right‐Hand Sides
Authors: Leistner Michael C. , Gosselet Pierre , Rixen Daniel J.
Published: December 2018

In this article, a method to accelerate the solution of multiple right‐hand side problems when using the adaptive multi‐preconditioned finite element tearing and interconnecting algorithm is presented. This is done by deflating the conjugate gradient algorithm by means of a coarse space, which is built by a simplification of the recently published RitzGenEO method. While the proposed method no longer requires to solve local eigenproblems, it is able to maintain a significant part of the reduction in computational cost. Numerical results show a comparison of the methods and appropriate applications for either method are discussed.
In‐situ and ex‐situ micro mechanical testing of open‐cell metal foams
Authors: Jung Anne , Luksch Jutta , Bleistein Thomas , Adrien Jerome , Maire Eric
Published: December 2018

Metal foams are cellular materials with structural features resembling to lightweight load‐bearing materials such as bones. Their high stiffness‐to‐weight‐ratio coupled with their long flat stress‐strain response make them ideal candidates as energy absorbers. Their macroscopic properties are strongly influenced by both the struts and their structure at the microscopic length‐scale based on a strong structure‐property relationship. Whereas macroscopic mechanical characterisation is widespread, micromechanical characterisation and assessment of parameters on single struts is very limited. Micromechanical characterisation of individual struts is very challenging but an emerging field of research. The present contribution deals with the mechanical characterisation of open‐cell foams on the meso and micro scale. In‐situ and ex‐situ micro tensile tests were conducted on individual struts. There is a large scattering in the micro material parameters deduced from individual struts. X‐ray computed tomography (CT) scans during micro tensile testing and ex‐situ micro tensile tests performed on struts, where the local microstructure (blowholes, pores, cracks and intermetallic inclusions) was previously determined in CT scans, were performed. The scattering in the material parameters is largely connected to the occurring defects in the microstructure of individual struts.
Investigating incompressible temporally developing turbulent boundary layers using One‐Dimensional Turbulence
Authors: Rakhi  , Schmidt Heiko
Published: December 2018

We investigate incompressible temporally developing turbulent boundary layers for the first time using the One‐Dimensional Turbulence (ODT) model. ODT is spatially fully resolved along the 1D domain, only turbulent advection is represented by stochastic mapping events. We compare the mean and root mean square velocity profiles with the Direct Numerical Simulations (DNS) results for bulk Reynolds numbers, Reb = 500 and 1000 (Reb = Ubd/ν, where ν is the kinematic viscosity, Ub is the velocity provided at the bottom wall and d is the trip wire diameter for experiments). The study demonstrates that ODT has the capability to systematically investigate developing boundary layers.
Towards a hybrid turbulent mixing model based on hierarchical parcel‐swapping and one‐dimensional turbulence
Authors: Pupo Sá da Costa Pedro , Schmidt Heiko
Published: December 2018

One‐dimensional turbulence (ODT) based models are able to generate good results for canonical flow problems compared to Direct Numerical Simulations. ODT has been successfully implemented as a sub‐grid scale model for LES problems (ODTLES) delivering promising results. However, due to the computational cost of the ODT model in this scope, there is a need for an even faster model for real world parameter regimes. The Hierarchical Parcel Swapping (HiPS) algorithm used in combination with the ODT model to simulate a channel flow recently provided results with an acceptable quality in a fraction of the time of an ODT simulation. HiPS uses a binary tree to define the fluid domain, and stochastic eddy events are sampled in this tree. A local instantaneous and complete mixing of the cells is performed when an eddy event happens at the Kolmogorov scale. The biggest limitation of this model is that the viscosity of the solution is implicitly defined by the number of cells in a mesh. Further development of HiPS is planned in order to create a computationally cheaper alternative for the ODT model to be used in ODTLES flow simulations.