@inproceedings{KeskinKoberStelldingeretal., author = {Keskin, Akin and Kober, Markus and Stelldinger, Enrico and K{\"u}hhorn, Arnold and B{\"o}hm, Holger and Hornig, Andreas and Hufenbach, Werner}, title = {On the quantification of errors of a pre-processing effort reducing contact meshing approach : AIAA 2015-0408}, series = {53rd AIAA Aerospace Sciences Meeting 2015, Kissimmee, Florida, USA, 5 - 9 January 2015, held at the AIAA SciTech Forum 2015, vol. 4}, booktitle = {53rd AIAA Aerospace Sciences Meeting 2015, Kissimmee, Florida, USA, 5 - 9 January 2015, held at the AIAA SciTech Forum 2015, vol. 4}, publisher = {Curran}, address = {Red Hook, NY}, isbn = {978-1-5108-0117-2}, doi = {10.2514/6.2015-0408}, pages = {3113 -- 3124}, language = {en} } @inproceedings{KoberKuehhornKeskin, author = {Kober, Markus and K{\"u}hhorn, Arnold and Keskin, Akin}, title = {Implicit Time Integration Schemes for the FEM Simulation of Fast Rotating Structures}, series = {WCCM XII \& APCOM VI (World Congress on Computational Mechanics \& Asia Pacific Congress on Computational Mechanics), Seoul, South Korea, 23.-29. Juli 2016}, booktitle = {WCCM XII \& APCOM VI (World Congress on Computational Mechanics \& Asia Pacific Congress on Computational Mechanics), Seoul, South Korea, 23.-29. Juli 2016}, abstract = {Due to the demand of more and more detailed FEM simulation results of whole aero-engine models, the number of DOF of such models (Fig. 1) increases dramatically. Although the available computational power for the solution of such highly nonlinear, dynamic boundary value problems also increased during the last years, the overall computational time of these high-fidelity models for the simulation of a few milliseconds of the running engine is in the order of several weeks even on high performance clusters with thousands of CPU cores. Typically, for such highly dynamic FEM computations explicit time integration schemes are used in order to solve the equations of motion. To keep such a numerical integration stable, the time step size may not exceed a certain critical time step size. Unfortunately, the critical time step size of an explicit time integration algorithm for the engine models under consideration is in the order of 10-8. This means that the simulation of a running engine over a time span of a few seconds results in a computational time of several years with clusters, which are available nowadays. Therefore, implicit time integration schemes, which are unconditionally stable and allow much bigger time steps, have to be used. Since in an implicit integration, in contrast to an explicit one, an equilibrium iteration is always necessary, the computational costs for an implicit time step are much higher than for an explicit time step. Almost all commercial FE codes use the classical Newmark implicit time integration scheme. It will be shown that this algorithm fails especially for the simulation of fast rotating structures if bigger time steps are used. This is demonstrated at the example of academic examples as well as for more realistic rotor models. To overcome the problems of the Newmark integration, a 3-Point-Newmark- Euler-Backward integration scheme [1,2] has been selected, which consists of alternating Newmark and 3-Point-Euler-Backward steps. The additional interpolation information for the Euler-step is provided by the previous Newmark step. This integration procedure has been implemented and successfully applied to the FEM simulation of fast rotating structures, which will be also demonstrated during the presentation.}, language = {en} } @inproceedings{KoberKuehhornKeskinetal., author = {Kober, Markus and K{\"u}hhorn, Arnold and Keskin, Akin and Singh, Kai Navtej}, title = {Challenges and opportunities of thermomechanical FEM simulations of high fidelity aero-engine models by implicit time-integration}, series = {Proceedings of ISABE 2017, ISABE-2017-22568, Manchester, September 3.-8., 2017}, booktitle = {Proceedings of ISABE 2017, ISABE-2017-22568, Manchester, September 3.-8., 2017}, publisher = {ISABE}, pages = {10}, language = {en} } @misc{KoberKuehhorn, author = {Kober, Markus and K{\"u}hhorn, Arnold}, title = {Stable implicit time-integration of flexible rotating structures—explanation for instabilities and concepts for avoidance}, series = {Applied Mathematical Modelling}, volume = {60}, journal = {Applied Mathematical Modelling}, issn = {0307-904X}, doi = {10.1016/j.apm.2018.03.017}, pages = {235 -- 243}, abstract = {Instabilities occurring during the implicit time-integration are still handicapping a time-efficient solution of large FEM systems of equations. Especially the simulation of flexible rotating structures is barely mastered by implicit FEM codes. For this, the Newmark algorithm and related algorithms are used for many years. Here, we derive the reasons for the mentioned inevitable numerical issues and present concepts that lead to an efficient and stable solution.}, language = {en} } @inproceedings{KoberKuehhornKeskin, author = {Kober, Markus and K{\"u}hhorn, Arnold and Keskin, Akin}, title = {Optimization of Newmark-Euler Time-Integration Parameters for a Stable and Efficient Implicit Simulation of Rotating Elastic Structures}, series = {Proceedings of 15th German LS-DYNA Forum}, booktitle = {Proceedings of 15th German LS-DYNA Forum}, publisher = {DYNAmore GmbH}, address = {Stuttgart}, isbn = {978-3-9816215-5-6}, pages = {235 -- 238}, language = {en} } @inproceedings{KoberKuehhornKeskin, author = {Kober, Markus and K{\"u}hhorn, Arnold and Keskin, Akin}, title = {Instability problems in implicit transient FEM simulations of fast rotating elastic structures - Description of the phenomenon and possible solutions}, series = {Proceedings of NAFEMS DACH Conference 2018, Bamberg, May 14-16, 2018}, booktitle = {Proceedings of NAFEMS DACH Conference 2018, Bamberg, May 14-16, 2018}, publisher = {NAFEMS Deutschland, {\"O}sterreich, Schweiz GmbH}, address = {Grafing}, isbn = {978-1-910643-14-3}, pages = {48 -- 52}, language = {en} } @misc{KoberKuehhornKeskin, author = {Kober, Markus and K{\"u}hhorn, Arnold and Keskin, Akin}, title = {Instabilit{\"a}tsprobleme bei der impliziten transienten FEM-Simulation schnell rotierender elastischer Strukturen - Beschreibung des Ph{\"a}nomens und L{\"o}sungsm{\"o}glichkeiten}, series = {NAFEMS-Online-Magazin}, volume = {48}, journal = {NAFEMS-Online-Magazin}, number = {4}, issn = {2311-522X}, pages = {36 -- 49}, language = {en} } @misc{NaveedKuehhornKober, author = {Naveed, Zishan and K{\"u}hhorn, Arnold and Kober, Markus}, title = {Comparative Evaluation of Isogeometric Analysis and Classical FEM with Regard to Contact Anaylsis}, series = {12th European LS-DYNA Conference 2019, 14-16 May 2019, Koblenz}, journal = {12th European LS-DYNA Conference 2019, 14-16 May 2019, Koblenz}, pages = {10}, abstract = {Isogeometric analysis represents a newly developed technique that offers the application of Computer Aided Designs (CAD) concept of Non-uniform Rational B-Splines (NURBS) tool to describe the geometry of the computational domain. The simplified transition of CAD models into the computational domain eliminates the problems arising from the geometrical discontinuities induced by the faceted approximation of the mesh. Moreover, numerical analysis directly on NURBS objects significantly reduces the design-to-analysis time compared to traditional FEA approach. In the field of contact mechanics, when finite elements are applied to geometry with curved surfaces, the result is a non-smooth geometrical representation of interface surfaces which may lead to mesh interlocking, high jumps and spurious oscillations in contact forces. To eliminate these issues, various surface smoothening strategies are to be employed in case of FEM. Isogeometric based analysis alleviates these issues without employing any additional smoothening strategy due to inherent higher order continuity of NURBS basis functions and much more accurate results are obtained compared to conventional FE approach. In the current study, LS-DYNA is used to demonstrate the capabilities and advantage of an isogeometric analysis though an example of pendulum under gravitational load. The numerical simulation results are analytically validated and the comparison of NURBS surfaces with faceted surfaces is carried out to investigate the accuracy.}, language = {en} } @misc{NaveedKuehhornKober, author = {Naveed, Zishan and K{\"u}hhorn, Arnold and Kober, Markus}, title = {Contact Behaviour of Isogeometric Analysis for Rotating Structures and its Industrial Application by Coupling to the Classical Finite Element Method}, series = {VII International Conference on Isogeometric Analysis, M{\"u}nchen, 18.-20. September 2019}, journal = {VII International Conference on Isogeometric Analysis, M{\"u}nchen, 18.-20. September 2019}, pages = {1}, abstract = {Especially for rotating structures like bearings non-smooth contact surfaces, as they appear in classical finite element modeling, lead to various problems during the analysis involving mesh interlocking and spurious oscillations in contact forces. In order to eliminate these issues, very fine meshes and additional smoothing strategies are employed in case of the finite element method (FEM). But also Non-Uniform Rational B-splines (NURBS) based isogeometric analysis (IGA) can be very useful for the contact analysis due to the inherent higher order continuity of NURBS basis functions. In this contribution, the contact behavior using classical FEA and IGA approaches is studied by means of an example of a pendulum under gravitational load. In addition, a more practical example of a coupled IGA-FEM problem with a cylindrical roller bearing is also reported in this paper. This research study of contact analysis has been carried out for the above mentioned examples using LS-DYNA and illustrates that contact surfaces of coarsely meshed geometry lock the rotation of the parts in case of classical FEM. On the contrary, when the contact surface is represented by NURBS elements it allows the rotation of the parts and effectively alleviates the contact force oscillation.}, language = {en} } @misc{NoackKuehhornKoberetal., author = {Noack, Martin and K{\"u}hhorn, Arnold and Kober, Markus and Firl, Matthias}, title = {A new stress-based topology optimization approach for finding flexible structures}, series = {Structural and Multidisciplinary Optimization}, volume = {64}, journal = {Structural and Multidisciplinary Optimization}, number = {4}, issn = {1615-147X}, doi = {10.1007/s00158-021-02960-w}, pages = {1997 -- 2007}, abstract = {This paper presents a new FE-based stress-related topology optimization approach for finding bending governed flexible designs. Thereby, the knowledge about an output displacement or force as well as the detailed mounting position is not necessary for the application. The newly developed objective function makes use of the varying stress distribution in the cross section of flexible structures. Hence, each element of the design space must be evaluated with respect to its stress state. Therefore, the method prefers elements experiencing a bending or shear load over elements which are mainly subjected to membrane stresses. In order to determine the stress state of the elements, we use the principal stresses at the Gauss points. For demonstrating the feasibility of the new topology optimization approach, three academic examples are presented and discussed. As a result, the developed sensitivity-based algorithm is able to find usable flexible design concepts with a nearly discrete 0 - 1 density distribution for these examples.}, language = {en} }