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Item type:Publication, Access status: without files , Improving the robustness of the finite cell method for problems with large deformationsZahlreiche Ingenieuranwendungen mit komplexen Strukturen und mikrostrukturierten Materialien wie Schaumstoffen oder 3D-gedruckten Objekten stellen eine Herausforderung für die traditionelle Finite-Elemente-Methode (FEM) dar. Die Finite-Cell-Methode (FCM) kombiniert den fiktiven Gebietsansatz mit hierarchischen Ansatzfunktionen, um den Netzgenerierungsaufwand zu reduzieren. Eine nicht-negative Moment-Fitting-Quadratur-Regel wird entwickelt, um die Integration von gebrochenen Zellen zu verbessern und die NewtonRaphson-Methode in der nichtlinearen Berechnung zu stabilisieren. Zusätzlich wird eine Eigenwert-Stabilisierungs-Technik entwickelt, um die Konditionszahl der Schnittzellen zu reduzieren. Ein Remeshing-Verfahren wird entwickelt, um die FCM-Robustheit zu erhöhen und größere Lastschritte zu ermöglichen. Numerische Beispiele veranschaulichen die Wirksamkeit dieser Ansätze.Publicationtype: Doctoral Thesis67 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Partitioned fluid-structure interaction simulation of maritime applicationsIn this work, the benefits of employing the partitioned approach in fluid-structure interaction simulations are demonstrated in four different examples: Simulations of a floating offshore wind turbine and a wave energy converter are used to analyze dynamic effects and improve the structural design. A multilayered submersible mixer validates the simulations by assessing local strains, thrust, and torque. Finally, an anisotropic ship propeller made of carbon-reinforced polymer is optimized with an evolutionary algorithm based on coupled simulations concerning efficiency, thrust, and cavitation.Publicationtype: Doctoral Thesis60 482 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Experimental testing and numerical simulation of granules as crash absorber for double hull structuresIn the maritime sector, the collision safety is an essential aspect of ship operation. Aiming at further improvements, the filling of the cavity in a double hull structure with granular material is investigated in this thesis. In order to do so, the potential of different granules is determined experimentally, along with the description of their material parameters as particles and as a bulk material, respectively. Furthermore, a simplified side hull structure is designed for experimental testing, followed by a numerical study of different material models for the granules. This leads to the simulation of a realistic collision scenario, showing the potential in the maritime industry.Publicationtype: Doctoral Thesis264 1081 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , The hierarchical finite cell method for nonlinear problems: moment fitting quadratures, basis function removel, and remeshingIn this thesis, several approaches are discussed in order to further enhance the performance of the finite cell method (FCM). Thereby, novel moment fitting quadrature schemes are introduced that allow to reduce the effort of the numerical integration process significantly. Further, a basis function removal scheme is proposed to improve the conditioning behavior of the resulting equation system. Finally, an innovative remeshing strategy is presented that overcomes the problem of severely distorted elements for simulations with large deformations.Publicationtype: Doctoral Thesis235 676 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , A partitioned solution approach for fluid-structure interaction problems in the arterial systemThe present work is concerned with the partitioned solution of the multifield problem arising from a hierarchical modeling approach to cardiovascular fluid-structure interaction. Different strategies to couple the participating field solvers are investigated in detail. This includes staggered and parallel coupling algorithms as well as different methods for convergence acceleration, spatial interpolation and temporal extrapolation of coupling quantities as well as convergence criteria. In the developed modeling and simulation approach, a fully resolved model of a segment of the arterial network is coupled to reduced order models in order to account for the in uence of the surrounding. The resulting problem is solved using five specialized field solvers, namely a fluid and a structural solver for the three-dimensional fluid-structure interaction problem, a one-dimensional blood flow solver for the surrounding vessel network, a solver for diffrent types of windkessel models used to obtain physiological boundary conditions at the distal ends of the one- and three-dimensional models, and a solver for an elastic foundation that describes the surrounding tissue. The applicability of the solution approach is demonstrated in terms of several exemplary applications including studies of idealized and patient specific end-to-side anastomoses of bypass grafts. They are known to be prone to the development of intimal hyperplasia, i.e. a thickening of the vessel wall that may lead to occlusions in the anastomosis region. There is experimental evidence that hemodynamic quantities such as the wall shear stress promote the progression of this secondary disease. Cardiovascular FSI simulation are therefore of great interest and can aid in surgical planning and optimization of anastomoses shapes and graft materials.Publicationtype: Doctoral Thesis420 2187 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , The finite cell method for the computation of cellular materials(2019); Open cell aluminum foams present an attractive class of materials that are applied in many engineering disciplines ranging from automotive, naval and aerospace industry to biomedical applications. In order to design engineering structures composed of foams, a detailed experimental and numerical investigation of their mechanical properties is necessary. The structure of metal foams consists of a complicated stochastic distribution of pores which requires an immense labor effort to generate a suitable finite element discretization. The finite cell method is a combination of the fictitious domain approach and high order finite elements. Due to the fictitious domain approach the finite cell method has shown to drastically simplify the mesh generation process. Motivated by this promising property we employ the finite cell method and enhance it in order to investigate the mechanical behavior of aluminum foams. The following topics are addressed in this thesis: The starting point for any numerical investigation is the discretization of the geometry. A realistic geometric model of metal foams can be provided using voxel models stemming from computed tomography scans. Such models are of course not free from artifacts. In order to achieve automatic mesh generation for voxel-based models, algorithms known from computer graphics will be applied to remove these artifacts. In addition voxel models will be modified numerically for example by the application of a thin coating to the metal foams. The effect of the coating on the homogenized elastic properties will be examined using the window method. In order to accelerate the homogenization procedure Aitken’s 2-method will be applied in this context. To study the buckling and the finite elastoplastic deformations of cell walls the finite cell method will be extended by a hyperelastic based von Mises plasticity model which makes use of the multiplicative split of the deformation gradient into its elastic and plastic contributions. By employing this model different structural levels of the aluminum metal foams will be analyzed under large deformations. A focus here is on the investigation of single pores that serve to find the microscopic material properties of the aluminum by combining experiments and simulations on pores. Finally computations of larger foam samples will be carried out aiming to find a representative volume element and to verify the material parameters obtained by single pore experiments and inverse computations. Two different contact formulations based on the penalty method – one for sticking and one for frictionless contact conditions – will be introduced and investigated by comparing their results to the analytical solution derived by Hertz. The self-contact of metal foams will be modeled using sticking contact formulation and finally applied to a single pore to show the influence of self-contact on the load displacement curve.Publicationtype: Doctoral Thesis428 1180 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Partitioned solution strategies for strongly-coupled fluid-structure interaction problems in maritime applicationsMany engineering applications are governed by coupled multifield phenomena. In this thesis, a partitioned solution approach is followed to solve these kind of problems, which does not only enable the use of different discretization schemes for each of the subproblems but also allows to reuse specialized and efficient solvers, which enhances modularity, software reusability, and performance. A framework for the partitioned analysis of general multifield problems is proposed and implemented in the generic software library comana, which is verified against various benchmark problems and successfully applied to sophisticated fluid-structure interaction problems from the martitime industry.Publicationtype: Doctoral Thesis795 534 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , The hierarchical finite cell method for problems in structural mechanicsPublicationtype: Doctoral Thesis133 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Partitioned solution strategies for electro-thermo-mechanical problems applied to the field-assisted sintering technologyPublicationtype: Doctoral Thesis60 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Numerical homogenization strategies for cellular materials with applications in structural mechanicsIn this thesis numerical homogenization strategies are developed which allow for capturing the complex behavior of cellular materials whereby the spatial discretization is based on higher-order finite element methods. In the examples anisotropy and size effects within two-dimensional foam-like structures are investigated and verified. In three dimensions CT-scans of real-world foams are transformed into numerical models which are successfully verified and validated, whereby a major focus lies on sandwich plates.Publicationtype: Doctoral Thesis439 678