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XFEM coupling of granular flows interacting with surrounding fluids ; Zilian, Andreas ; in ECCOMAS 2012 - European Congress on Computational Methods in Applied Sciences and Engineering, e-Book Full Papers (2012) In this paper, ideas for the simulation of sliding dry granular materials interacting with surrounding fluids are presented and first results are presented. The compressible granular material is modeled ... [more ▼] In this paper, ideas for the simulation of sliding dry granular materials interacting with surrounding fluids are presented and first results are presented. The compressible granular material is modeled as a medium which can show solid-like and fluid-like characteristics. Therefore a weighted decomposition of stress tensors of a solid-like and a fluid-like phase is applied. The surrounding incompressible fluids are described with a Newtonian constitutive model. Interface dynamics are handled with the level-set method. The model equations are discretized with the space-time finite element method. Discontinuous solution characteristics across interfaces are captured numerically by the extended finite element method (XFEM). For all discontinuities the space of ansatz functions is enriched with Heaviside functions. [less ▲] Detailed reference viewed: 123 (0 UL)Modellierung und numerische Simulation von Hangrutschungen ; Zilian, Andreas ; in Busch, W; Niemeier, W; Sörgel, U (Eds.) GeoMonitoring 2011 - Ein Paradigmenwechsel zur Beherrschung von Georisiken (2011) Detailed reference viewed: 108 (0 UL)Untersuchung dünnwandiger Silostrukturen unter exzentrischer Entleerung ; ; Zilian, Andreas in Baustatik-Baupraxis 11 (2011) Detailed reference viewed: 83 (2 UL)A rheological interface model and its space-time finite element formulation for fluid-structure interaction ; Zilian, Andreas ; in International Journal for Numerical Methods in Engineering (2011), 86(6), 667-687 This contribution discusses extended physical interface models for fluid-structure interaction problems and investigates their phenomenological effects on the behavior of coupled systems by numerical ... [more ▼] This contribution discusses extended physical interface models for fluid-structure interaction problems and investigates their phenomenological effects on the behavior of coupled systems by numerical simulation. Besides the various types of friction at the fluid-structure interface the most interesting phenomena are related to effects due to additional interface stiffness and damping. The paper introduces extended models at the fluid-structure interface on the basis of rheological devices (Hooke, Newton, Kelvin, Maxwell, Zener). The interface is decomposed into a Lagrangian layer for the solid-like part and an Eulerian layer for the fluid-like part. The mechanical model for fluid-structure interaction is based on the equations of rigid body dynamics for the structural part and the incompressible Navier-Stokes equations for viscous flow. The resulting weighted residual form uses the interface velocity and interface tractions in both layers in addition to the field variables for fluid and structure. The weak formulation of the whole coupled system is discretized using space-time finite elements with a discontinuous Galerkin method for time-integration leading to a monolithic algebraic system. The deforming fluid domain is taken into account by deformable space-time finite elements and a pseudo-structure approach for mesh motion. The sensitivity of coupled systems to modification of the interface model and its parameters is investigated by numerical simulation of flow induced vibrations of a spring supported fluid-immersed cylinder. It is shown that the presented rheological interface model allows to influence flow-induced vibrations. © 2010 John Wiley & Sons, Ltd. [less ▲] Detailed reference viewed: 119 (0 UL)Extended Finite Element Method ; Zilian, Andreas ; in International Journal for Numerical Methods in Engineering (2011), 86(4-5), 403 [No abstract available] Detailed reference viewed: 151 (3 UL)Model order reduction of the aeroelastic response of a wind turbine rotor blade ; Zilian, Andreas ; Scientific Conference (2011) The dynamic response of a wind turbine is mainly governed by the aecting ow forces of the wind. Concerning life expectancy, the vibration sensitivity of the rotor blades is of particular importance ... [more ▼] The dynamic response of a wind turbine is mainly governed by the aecting ow forces of the wind. Concerning life expectancy, the vibration sensitivity of the rotor blades is of particular importance. In this contribution the dynamics of a rotating blade is described including the deformation-dependant aerodynamic forces [1]. Then, a reduced-order model for the coupled system of structure and uid ow is derived, which is more ecient concerning the calculation through the use of less degrees of freedom, but maintains the stability characteristics of the non-reduced-order model. A single rotor blade is described as a straight Bernoulli beam with a variable non-symmetric sectional prole. The coordinate system refers to the current position of the blade. Besides the structural stiness, mass inertia and aerodynamic loading according to potential ow theory are included. The aerodynamic loading is linearised following the Scanlan model described in [2]. The linear equation of motion is formulated by means of the principle of virtual deformation assuming a constant rotational speed [3]. The quality of the model order reduction approach strongly depends on the chosen reduced basis. In this approach the basis is built by an appropriate selection of eigenmodes, which are dependent on the aerodynamic loading and parametrized on the rotational speed of the blade. The contribution discusses available methods [4] of creating reduced bases eligible to correctly represent the utter characteristics in a spectrum of operating speeds. [less ▲] Detailed reference viewed: 142 (0 UL)XFEM coupling techniques for landslide-fluid interaction ; Zilian, Andreas Scientific Conference (2011) Selected topographies on earth are threatened by sudden landslides on natural or artificial hillsides. Dur- ing landslides natural granular materials exhibit phase transitions from solid-like to fluid ... [more ▼] Selected topographies on earth are threatened by sudden landslides on natural or artificial hillsides. Dur- ing landslides natural granular materials exhibit phase transitions from solid-like to fluid-like behavior [1]. In order to describe such materials interacting with surrounding fluids a material formulation allow- ing a switch from solid- to fluid-like state is developed. The solid-like state of the granular is modeled as compressible elastic material in an eulerian framework, while the surrounding fluid is described by an incompressible newtonian fluid. Discretization of the balance equations is carried out with a stabilized space-time finite element method [2]. The domain of elastic material utilizes mixed-hybrid space-time elements while for the surrounding fluid standard velocity-pressure elements [3] are used. Interaction of both continua is described with an interface-coupled formulation where the level-set technique [4] is employed for the interface motion. The challenge of the introduced model is the coupling of fields with discontinuous state variables as well as fieldwise different state variables. Available coupling techniques in the context of the extended finite element method and based on localized mixed hybrid formulations [5] will be discussed and evaluated numerically by means of selected 2D examples. [less ▲] Detailed reference viewed: 115 (1 UL)Meshfree collocation method for implicit time integration of ODEs ; Zilian, Andreas in International Journal of Computational Methods (2011), 8(1), 119-137 An implicit time integration meshfree collocation method for solving linear and nonlinear ordinary differential equations (ODEs) based on interpolating moving least squares technique, which uses singular ... [more ▼] An implicit time integration meshfree collocation method for solving linear and nonlinear ordinary differential equations (ODEs) based on interpolating moving least squares technique, which uses singular weights for constructing ansatz functions, is presented. On an example of a system of equations for Foucault pendulum, the flexibility of the proposed approach is shown and the accuracy, convergence, and stability properties are investigated. In a nonlinear case, the method gives accurate results, which is demonstrated by the solution of Lorenz equations. The typical trajectory patterns, e.g. butterfly pattern, were observed and the properties of the method are compared to those of a higher-order time integration method. © 2011 World Scientific Publishing Company. [less ▲] Detailed reference viewed: 137 (2 UL)Developments in Mesh-Moving and Mesh-Update Schemes for Space-Time Finite Element Dicretisations of Fluid Flows ; Zilian, Andreas Scientific Conference (2011) Space-time discretisations of physical problems involving moving and deforming bodies, boundaries and interfaces have been shown to offer advantageous properties, while being methodologically uniform and ... [more ▼] Space-time discretisations of physical problems involving moving and deforming bodies, boundaries and interfaces have been shown to offer advantageous properties, while being methodologically uniform and flexible. Well-known phenomena, which are ideally suited to be analysed by space-time methods, are fluid-structure interaction problems in general as well as fluid flows with subdomain phase boundaries or immersed moving objects. In this contribution existing mesh-moving and mesh-update techniques are comparatively discussed within the framework of finite element discretisations of the Navier-Stokes equations in space and time. The investigation is based on a SUPG/PSPG velocity-pressure formulation on the deforming space-time do- main. Developments of mesh reconnection at fluid-solid interfaces due to mesh-moving/reconnecting proce- dures for space-time discretisations of fluid flows are presented. Furthermore mesh-update/reconnecting techniques are examined occurring in engineering problems involving sliding or rotating space-time fluid meshes. The presented techniques are accompanied by numerical examples of translational fluid flow as well as fluid flow with immersed rotating fluid bodies. The quality of the approximative solution on the moving and deforming mesh as well as its conservation properties are investigated. [less ▲] Detailed reference viewed: 89 (2 UL)Finite element method for strongly-coupled systems of fluid-structure interaction with application to granular flow in silos ; Zilian, Andreas ; in Proceedings of the 4th International Conference on Computational Methods for Coupled Problems in Science and Engineering, COUPLED PROBLEMS 2011 (2011) A monolithic approach to fluid-structure interactions based on the space-time finite element method (STFEM) is presented. The method is applied to the investigation of stress states in silos filled with ... [more ▼] A monolithic approach to fluid-structure interactions based on the space-time finite element method (STFEM) is presented. The method is applied to the investigation of stress states in silos filled with granular material during discharge. The thin-walled siloshell is modeled in a continuum approach as elastic solid material, whereas the flowing granular material is described by an enhanced viscoplastic non-Newtonian fluid model. The weak forms of the governing equations are discretized by STFEM for both solid and fluid domain. To adapt the matching mesh nodes of the fluid domain to the structural deformations, a mesh-moving scheme using a neo-Hookean pseudo-solid is applied. The finite element approximation of non-smooth solution characteristics is enhanced by the extended finite element method (XFEM). The proposed methodology is applied to the 4D (space-time) investigation of deformation-dependent loading conditions during silo discharge. [less ▲] Detailed reference viewed: 121 (1 UL)Hybridized enriched space-time finite element method for analysis of thin-walled structures immersed in generalized Newtonian fluids Zilian, Andreas ; in Computers and Structures (2010), 88(21-22), 1265-1277 The paper addresses the numerical treatment of a specific class of fluid-structure interaction problems: flow-immersed thin structures undergoing considerable motion and deformation. The simultaneous ... [more ▼] The paper addresses the numerical treatment of a specific class of fluid-structure interaction problems: flow-immersed thin structures undergoing considerable motion and deformation. The simultaneous solution procedure uses a mixed-hybrid velocity-based formulation of both fluid and structure discretized by a stabilized time-discontinuous space-time finite element method. The continuity at the interface is ensured by a localized mixed-hybrid interface method avoiding Lagrange multipliers and penalty approaches. The XFEM is utilized for enrichment of the approximation space of the fluid variables in order to represent non-smooth (discontinuous) solution features resulting from immersing a thin structure in a fluid. © 2010 Elsevier Ltd. All rights reserved. [less ▲] Detailed reference viewed: 138 (2 UL)Finite element methods for strongly-coupled systems of fluid-structure interaction with application to granular flow in silos ; Zilian, Andreas ; in PAMM (2010), 10(1), 381--382 A monolithic approach to fluid-structure interactions based on the space-time finite element method is presented to investigate stress states in silos filled with granular material during discharge. The ... [more ▼] A monolithic approach to fluid-structure interactions based on the space-time finite element method is presented to investigate stress states in silos filled with granular material during discharge. The thin-walled silo-shell is discretized by continuum based, mixed-hybrid finite elements, whereas the flowing granular material is described by an enhanced viscoplastic non-Newtonian fluid model. To adapt the mesh nodes of the fluid domain to the structural deformations, a mesh-moving scheme using a pseudo-solid is applied. The level-set-method involving XFEM is used, including a 4D split algorithm for the space-time finite elements, in order to describe free surfaces. The method is applied to 3D silo discharges. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) [less ▲] Detailed reference viewed: 182 (7 UL)On time integration in the XFEM ; Zilian, Andreas in International Journal for Numerical Methods in Engineering (2009), 79(1), 69-93 The extended finite element method (XFEM) is often used in applications that involve moving interfaces. Examples are the propagation of cracks or the movement of interfaces in two-phase problems. This ... [more ▼] The extended finite element method (XFEM) is often used in applications that involve moving interfaces. Examples are the propagation of cracks or the movement of interfaces in two-phase problems. This work focuses on time integration in the XFEM. The performance of the discontinuous Galerkin method in time (space-time finite elements (FEs)) and time-stepping schemes are analyzed by convergence studies for different model problems. It is shown that space-time FE achieve optimal convergence rates. Special care is required for time stepping in the XFEM due to the time dependence of the enrichment functions. In each time step, the enrichment functions have to be evaluated at different time levels. This has important consequences in the quadrature used for the integration of the weak form. A time-stepping scheme that leads to optimal or only slightly sub-optimal convergence rates is systematically constructed in this work. © 2009 John Wiley & Sons, Ltd. [less ▲] Detailed reference viewed: 114 (3 UL)A localized mixed-hybrid method for imposing interfacial constraints in the extended finite element method (XFEM) Zilian, Andreas ; in International Journal for Numerical Methods in Engineering (2009), 79(6), 733-752 The paper proposes an approach for the imposition of constraints along moving or fixed immersed interfaces in the context of the extended finite element method. An enriched approximation space enables ... [more ▼] The paper proposes an approach for the imposition of constraints along moving or fixed immersed interfaces in the context of the extended finite element method. An enriched approximation space enables consistent representation of strong and weak discontinuities in the solution fields along arbitrarily-shaped material interfaces using an unfitted background mesh. The use of Lagrange multipliers or penalty methods is circumvented by a localized mixed hybrid formulation of the model equations. In a defined region in the vicinity of the interface, the original problem is re-stated in its auxiliary formulation. The availability of the auxiliary variable enables the consideration of a variety of interface constraints in the weak form. The contribution discusses the weak imposition of Dirichlet- and Neumann-type interface conditions as well as continuity requirements not fulfilled a priori by the enriched approximation. The properties of the proposed approach applied to two-dimensional linear scalar- and vector-valued elliptic problems are investigated by studying the convergence behavior. © 2009 John Wiley & Sons,Ltd. [less ▲] Detailed reference viewed: 125 (0 UL)Projection-based reduction of fluid-structure interaction systems using monolithic space-time modes Zilian, Andreas ; ; in Computer Methods in Applied Mechanics and Engineering (2009), 198(47-48), 3795-3805 The focus of this work is the development of reduced models for engineering applications in complex bidirectional fluid-structure interaction. In the simultaneous solution procedure, velocity variables ... [more ▼] The focus of this work is the development of reduced models for engineering applications in complex bidirectional fluid-structure interaction. In the simultaneous solution procedure, velocity variables are used for both fluid and solid, and the whole set of model equations is discretized by a stabilized time-discontinuous space-time finite element method. Flexible structures are modeled using a three-dimensional continuum approach in a total Lagrangian setting considering large displacements and rotations. In the flow domain the incompressible Navier-Stokes equations describe the Newtonian fluid. A continuous finite element mesh is applied to the entire spatial domain, and the discretized model equations are assembled in a single set of algebraic equations, considering the two-field problem as a whole. The continuous fluid-structure mesh with identical orders of approximation for both solid and fluid in space and time automatically yields conservation of mass, momentum and energy at the fluid-structure interface. A mesh-moving scheme is used to adapt the nodal coordinates of the fluid space-time finite element mesh to the structural deformation. The computational approach for strongly coupled fluid-structure interaction is used to create suitable reduced models of generic nonlinear problems. Reduction is performed with monolithic projection-based space-time modes, ensuring strong coupling of fluid and structure in the reduced model. The contribution discusses results using proper orthogonal decomposition (POD) for determination of monolithic space-time modes in the reduction of fluid-structure systems. © 2009 Elsevier B.V. All rights reserved. [less ▲] Detailed reference viewed: 166 (7 UL)Space-Time Meshfree Collocation Method for PDEs ; Zilian, Andreas in PAMM (2009), 9(1), 651--652 An innovative Space-Time Meshfree Collocation Method (STMCM) for solving systems of nonlinear ordinary and partial differential equations by a consistent discretization in both space and time is proposed ... [more ▼] An innovative Space-Time Meshfree Collocation Method (STMCM) for solving systems of nonlinear ordinary and partial differential equations by a consistent discretization in both space and time is proposed as an alternative to established mesh-based methods. The STMCM belongs to the class of truly meshfree methods, i.e. the methods which do not have any underlying mesh, but work on a set of nodes only, without an a priori node-to-node connectivity. A regularization technique to overcome the singularity-by-construction and to compute all necessary derivatives of the kernel functions is presented. The method combines the simplicity and straightforwardness of the strong-form computational techniques with the advantages of meshfree methods over the classical ones, especially for coupled engineering problems involving moving interfaces. The key features of the proposed approach are: (i) no need to generate a mesh, (ii) simplified imposition of boundary conditions, (iii) no need to evaluate integral forms of governing equations, (iv) applicability to complex irregularly-shaped domains. The proposed STMCM is applied to linear and nonlinear ordinary and partial differential equations of different types and its accuracy and convergence properties are studied. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) [less ▲] Detailed reference viewed: 192 (0 UL)Space-time meshfree collocation method: Methodology and application to initial-boundary value problems ; Zilian, Andreas in International Journal for Numerical Methods in Engineering (2009), 80(3), 355-380 A novel space-time meshfree collocation method (STMCM) for solving systems of non-linear ordinary and partial differential equations by a consistent discretization in both space and time is proposed as an ... [more ▼] A novel space-time meshfree collocation method (STMCM) for solving systems of non-linear ordinary and partial differential equations by a consistent discretization in both space and time is proposed as an alternative to established mesh-based methods. The STMCM belongs to the class of truly meshfree methods, i.e. the methods that do not have any underlying mesh, but work on a set of nodes only without any a priori node-to-node connectivity. Instead, the neighbouring information is established on-the-fly. The STMCM is constructed using the Interpolating Moving Least-squares technique, which allows a simplified implementation of boundary conditions due to fulfillment of the Kronecker delta property by the kernel functions, which is not the case for the major part of other meshfree methods. The method is validated by several examples ranging from interpolation problems to the solution of PDEs, whereas the STMCM solutions are compared with either analytical or reference ones. © 2009 John Wiley & Sons, Ltd. [less ▲] Detailed reference viewed: 177 (5 UL)Numerical model for tsunami generation by subaerial landslides ; Zilian, Andreas ; in PAMM (2008), 8(1), 10519--10520 A discretization method based on stabilized space–time finite elements is presented for the numerical analysis of three–fluid flows of immiscible and incompressible fluids. Signed distance functions are ... [more ▼] A discretization method based on stabilized space–time finite elements is presented for the numerical analysis of three–fluid flows of immiscible and incompressible fluids. Signed distance functions are used to assign the material properties to each spatial point in the domain. The motion and the change in topology of fluid–fluid interfaces are implicitly described by the level–set method. Strong and weak discontinuities in the fields of the physical state variables are captured by locally enriched approximations based on the partition–of–unity concept. An interior penalty method enforces interfacial conservation of mass and momentum. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [less ▲] Detailed reference viewed: 212 (1 UL)Enriched space-time finite elements for fluid-structure interaction ; Zilian, Andreas in European Journal of Computational Mechanics (2008), 17(5-7), 725-736 Detailed reference viewed: 122 (2 UL)The enriched space-time finite element method (EST) for simultaneous solution of fluid-structure interaction Zilian, Andreas ; in International Journal for Numerical Methods in Engineering (2008), 75(3), 305-334 The paper introduces a weighted residual-based approach for the numerical investigation of the interaction of fluid flow and thin flexible structures. The presented method enables one to treat strongly ... [more ▼] The paper introduces a weighted residual-based approach for the numerical investigation of the interaction of fluid flow and thin flexible structures. The presented method enables one to treat strongly coupled systems involving large structural motion and deformation of multiple-flow-immersed solid objects. The fluid flow is described by the incompressible Navier-Stokes equations. The current configuration of the thin structure of linear elastic material with non-linear kinematics is mapped to the flow using the zero iso-contour of an updated level set function. The formulation of fluid, structure and coupling conditions uniformly uses velocities as unknowns. The integration of the weak form is performed on a space-time finite element discretization of the domain. Interfacial constraints of the multi-field problem are ensured by distributed Lagrange multipliers. The proposed formulation and discretization techniques lead to a monolithic algebraic system, well suited for strongly coupled fluid-structure systems. Embedding a thin structure into a flow results in non-smooth fields for the fluid. Based on the concept of the extended finite element method, the space-time approximations of fluid pressure and velocity are properly enriched to capture weakly and strongly discontinuous solutions. This leads to the present enriched space-time (EST) method. Numerical examples of fluid-structure interaction show the eligibility of the developed numerical approach in order to describe the behavior of such coupled systems. The test cases demonstrate the application of the proposed technique to problems where mesh moving strategies often fail. Copyright © 2007 John Wiley & Sons, Ltd. [less ▲] Detailed reference viewed: 185 (6 UL) |
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