Littérature scientifique sur le sujet « Unfitted mesh methods »

Abstract Two unfitted-mesh methods for a linear incompressible fluid/thin-walled structure interaction problem are introduced and analyzed. The spatial discretization is based on different variants of Nitsche’s method with cut elements. The degree of fluid–solid splitting (semi-implicit or explicit) is given by the order in which the space and time discretizations are performed. The a priori stability and error analysis shows that strong coupling is avoided without compromising stability and accuracy. Numerical experiments with a benchmark illustrate the accuracy of the different methods proposed.

We propose a new geometrically unfitted finite element method based on discontinuous Trefftz ansatz spaces. Trefftz methods allow for a reduction in the number of degrees of freedom in discontinuous Galerkin methods, thereby, the costs for solving arising linear systems significantly. This work shows that they are also an excellent way to reduce the number of degrees of freedom in an unfitted setting. We present a unified analysis of a class of geometrically unfitted discontinuous Galerkin methods with different stabilisation mechanisms to deal with small cuts between the geometry and the mesh. We cover stability and derive a-priori error bounds, including errors arising from geometry approximation for the class of discretisations for a model Poisson problem in a unified manner. The analysis covers Trefftz and full polynomial ansatz spaces, alike. Numerical examples validate the theoretical findings and demonstrate the potential of the approach.

AbstractThe paper studies a higher order unfitted finite element method for the Stokes system posed on a surface in ℝ3. The method employs parametric Pk-Pk−1 finite element pairs on tetrahedral bulk mesh to discretize the Stokes system on embedded surface. Stability and optimal order convergence results are proved. The proofs include a complete quantification of geometric errors stemming from approximate parametric representation of the surface. Numerical experiments include formal convergence studies and an example of the Kelvin--Helmholtz instability problem on the unit sphere.

We propose numerical schemes that enable the application of particle methods for advection problems in general bounded domains. These schemes combine particle fields with Cartesian tensor product splines and a fictitious domain approach. Their implementation only requires a fitted mesh of the domain's boundary, and not the domain itself, where an unfitted Cartesian grid is used. We establish the stability and consistency of these schemes in $W^{s,p}$-norms, $s\in\mathbb{R}$, $1\leq p\leq\infty$.

In this paper, we propose and analyze the least squares finite element methods for the linear elasticity interface problem in the stress-displacement system on unfitted meshes. We consider the cases that the interface is $C^2$ or polygonal, and the exact solution $(\bsigma, \bu)$ belongs to $H^s(\div; \Omega_0 \cup \Omega_1) \times H^{1+s}(\Omega_0 \cup \Omega_1)$ with $s > 1/2$. Two types of least squares functionals are defined to seek the numerical solution. The first is defined by simply applying the $L^2$ norm least squares principle, and requires the condition $s \geq 1$. The second is defined with a discrete minus norm, which is related to the inner product in $H^{-1/2}(\Gamma)$. The use of this discrete minus norm results in a method of optimal convergence rates and allows the exact solution has the regularity of any $s > 1/2$. The stability near the interface for both methods is guaranteed by the ghost penalty bilinear forms and we can derive the robust condition number estimates. The convergence rates under $L^2$ norm and the energy norm are derived for both methods. We illustrate the accuracy and the robustness of the proposed methods by a series of numerical experiments for test problems in two and three dimensions.

One of the main difficulties that has to be faced with fictitious domain approximation of incompressible flows with immersed interfaces is related to the potential lack of mass conservation across the interface. In this paper, we propose and analyze a low order fictitious domain stabilized finite element method which mitigates this issue with the addition of a single velocity constraint. We provide a complete a priori numerical analysis of the method under minimal regularity assumptions. A comprehensive numerical study illustrates the capabilities of the proposed method, including comparisons with alternative fitted and unfitted mesh methods.

Purpose The purpose of this paper is as follows: to significantly reduce the computation time (by a factor of 1,000 and more) compared to known numerical techniques for real-world problems with complex interfaces; and to simplify the solution by using trivial unfitted Cartesian meshes (no need in complicated mesh generators for complex geometry). Design/methodology/approach This study extends the recently developed optimal local truncation error method (OLTEM) for the Poisson equation with constant coefficients to a much more general case of discontinuous coefficients that can be applied to domains with different material properties (e.g. different inclusions, multi-material structural components, etc.). This study develops OLTEM using compact 9-point and 25-point stencils that are similar to those for linear and quadratic finite elements. In contrast to finite elements and other known numerical techniques for interface problems with conformed and unfitted meshes, OLTEM with 9-point and 25-point stencils and unfitted Cartesian meshes provides the 3-rd and 11-th order of accuracy for irregular interfaces, respectively; i.e. a huge increase in accuracy by eight orders for the new 'quadratic' elements compared to known techniques at similar computational costs. There are no unknowns on interfaces between different materials; the structure of the global discrete system is the same for homogeneous and heterogeneous materials (the difference in the values of the stencil coefficients). The calculation of the unknown stencil coefficients is based on the minimization of the local truncation error of the stencil equations and yields the optimal order of accuracy of OLTEM at a given stencil width. The numerical results with irregular interfaces show that at the same number of degrees of freedom, OLTEM with the 9-points stencils is even more accurate than the 4-th order finite elements; OLTEM with the 25-points stencils is much more accurate than the 7-th order finite elements with much wider stencils and conformed meshes. Findings The significant increase in accuracy for OLTEM by one order for 'linear' elements and by 8 orders for 'quadratic' elements compared to that for known techniques. This will lead to a huge reduction in the computation time for the problems with complex irregular interfaces. The use of trivial unfitted Cartesian meshes significantly simplifies the solution and reduces the time for the data preparation (no need in complicated mesh generators for complex geometry). Originality/value It has been never seen in the literature such a huge increase in accuracy for the proposed technique compared to existing methods. Due to a high accuracy, the proposed technique will allow the direct solution of multiscale problems without the scale separation.

IntroductionSource analysis of Electroencephalography (EEG) data requires the computation of the scalp potential induced by current sources in the brain. This so-called EEG forward problem is based on an accurate estimation of the volume conduction effects in the human head, represented by a partial differential equation which can be solved using the finite element method (FEM). FEM offers flexibility when modeling anisotropic tissue conductivities but requires a volumetric discretization, a mesh, of the head domain. Structured hexahedral meshes are easy to create in an automatic fashion, while tetrahedral meshes are better suited to model curved geometries. Tetrahedral meshes, thus, offer better accuracy but are more difficult to create.MethodsWe introduce CutFEM for EEG forward simulations to integrate the strengths of hexahedra and tetrahedra. It belongs to the family of unfitted finite element methods, decoupling mesh and geometry representation. Following a description of the method, we will employ CutFEM in both controlled spherical scenarios and the reconstruction of somatosensory-evoked potentials.ResultsCutFEM outperforms competing FEM approaches with regard to numerical accuracy, memory consumption, and computational speed while being able to mesh arbitrarily touching compartments.DiscussionCutFEM balances numerical accuracy, computational efficiency, and a smooth approximation of complex geometries that has previously not been available in FEM-based EEG forward modeling.

Abstract Polygonal finite elements offer an increased freedom in terms of mesh generation at the price of more complex, often rational, shape functions. Thus, the numerical integration of rational interpolants over polygonal domains is one of the challenges that needs to be solved. If, additionally, strong discontinuities are present in the integrand, e.g., when employing fictitious domain methods, special integration procedures must be developed. Therefore, we propose to extend the conventional quadtree-decomposition-based integration approach by image compression techniques. In this context, our focus is on unfitted polygonal elements using Wachspress shape functions. In order to assess the performance of the novel integration scheme, we investigate the integration error and the compression rate being related to the reduction in integration points. To this end, the area and the stiffness matrix of a single element are computed using different formulations of the shape functions, i.e., global and local, and partitioning schemes. Finally, the performance of the proposed integration scheme is evaluated by investigating two problems of linear elasticity.

Cette thèse est dédiée à la simulation numérique des systèmes mécaniques impliquant l'interaction entre une structure mince déformable et un fluide incompressible interne ou qui l'entoure.Dans la première partie, nous introduisons deux nouvelles classes de schémas de couplage explicites en utilisant des maillages compatibles. Les méthodes proposées combinent une certaine consistance Robin dans le système avec (i) un schéma à pas fractionnaire pour le fluide ou (ii) une discrétisation temporelle d'ordre deux pour le fluide et le solide. Les propriétés de stabilité des méthodes sont analysées dans un cadre linéaire représentatif. Cette partie inclut aussi une étude numérique exhaustive dans laquelle plusieurs schémas de couplage (dont certains proposés ici) sont comparés et validés avec des résultats expérimentaux. Dans la seconde partie, nous considérons des maillages non compatibles. La discrétisation spatiale est basée, dans ce cas là, sur des variantes de la méthode de Nitsche avec éléments coupés. Nous présentons deux nouveaux types de schémas de découplage qui exploitent la susmentionée condition de Robin en utilisant des maillages incompatibles. Le caractère semi-implicite ou explicite du couplage en temps dépend de l'ordre dans lequel les discrétisations spatiales et temporelles sont effectuées. Dans le cas d'un couplage avec des structures immergées, la vitesse et la pression discrètes permettent des discontinuités faibles et fortes à travers l'interface, respectivement. Des estimations de stabilité et d'erreur sont fournies dans un cadre linéaire. Une série de tests numériques illustre la performance des différentes méthodes proposées
This thesis is devoted to the numerical approximation of mechanical systems involving the interaction of a deformable thin-walled structure with an internal or surrounding incompressible fluid flow. In the first part, we introduce two new classes of explicit coupling schemes using fitted meshes. The methods proposed combine a certain Robin-consistency in the system with (i) a projection-based time-marching in the fluid or (ii) second-order time-stepping in both the fluid and the solid. The stability properties of the methods are analyzed within representative linear settings. This part includes also a comprehensive numerical study in which state-of-the-art coupling schemes (including some of the methods proposed herein) are compared and validated against the results of an experimental benchmark. In the second part, we consider unfitted mesh formulations. The spatial discretization in this case is based on variants of Nitsche’s method with cut elements. We present two new classes of splitting schemes which exploit the aforementioned interface Robin-consistency in the unfitted framework. The semi-implicit or explicit nature of the splitting in time is dictated by the order in which the spatial and time discretizations are performed. In the case of the coupling with immersed structures, weak and strong discontinuities across the interface are allowed for the velocity and pressure, respectively. Stability and error estimates are provided within a linear setting. A series of numerical tests illustrates the performance of the different methods proposed

Cette thèse porte sur la modélisation, l'analyse numérique et à la simulation de problèmes d'interaction fluide-structure pour des structures minces immergées dans un fluide visqueux incompressible. La motivation sous-jacente de ce travail est la simulation des phénomènes d'interaction fluide-structure impliqués dans la simulation des valves cardiaques. Du point de vue méthodologique, un accent particulier est mis sur des méthodes avec maillage non conformes qui permettent de garantir la précision du résultat en minimisant le coût computationnel. Un aspect essentiel est de garantir la conservation de la masse à travers l'interface fluide-structure. Une extension de la méthode de maillage non conforme Nitsche-XFEM présentée dans Alauzet et al. (2016) à trois dimensions est d'abord proposée, portant à la fois sur des domaines fluides entièrement et partiellement intersectés. Pour y parvenir, un algorithme de tessellation général et robuste a été développé sans recourir à des générateurs de maillage de type boîte noire. De plus, une nouvelle approche pour imposer la continuité dans des domaines partiellement intersectés est introduite. Cependant, dans les situations impliquant des phénomènes de contact avec de multiples interfaces, l'implémentation informatique devient extrêmement complexe, notamment en 3D. Ensuite, une méthode de domaine fictif innovante d'ordre inférieur est introduite, qui atténue les problèmes inhérents de conservation de la masse résultant de l'approximation continue de la pression en incorporant une seule contrainte de vitesse. Une analyse complète des erreurs a priori pour un problème de Stokes avec une contrainte de Dirichlet sur une interface immergée est fournie. Enfin, cette approche de domaine fictif est formulée dans un cadre d'interaction fluide-structure avec des solides minces et appliquée avec succès pour simuler la dynamique de la valve aortique
The present thesis is dedicated to the modeling, numerical analysis, and simu- lation of fluid-structure interaction problems involving thin-walled structures immersed in incompressible viscous fluid. The underlying motivation behind this work is the simulation of the fluid-structure interaction phenomena involved in cardiac valves. From a methodological standpoint, special focus is placed on unfitted mesh methods that guarantee accuracy without compromising computational complexity. An essential aspect is ensuring mass conservation across the fluid-structure interface. An extension of the unfitted mesh Nitsche-XFEM method reported in Alauzet et al. (2016) to three dimensions is first pro- posed, addressing both fully and partially intersected fluid domains. To achieve this, a robust general tessellation algorithm has been developed without relying on black-box mesh generators. Additionally, a novel approach for enforcing continuity in partially intersected domains is introduced. However, in situations involving contact phenomena with multiple interfaces, the computational implementation becomes exceedingly complex, particularly in 3D. Subsequently, an innovative low-order fictitious domain method is introduced, which mitigates inherent mass conservation issues arising from continuous pressure approximation by incorporating a single velocity constraint. A comprehensive a priori error analysis for a Stokes problem with a Dirichlet constraint on an immersed interface is provided. Finally, this fictitious domain approach is formulated within a fluid-structure interaction framework with general thin-walled solids and successfully applied to simulate the dynamics of the aortic valve

This document presents a description of the octree mesh-generation capabilities and of the parallel mesh adaptation kernel. As it is discussed in Section 1.3.2 of part B of the project proposal there are two parallel research lines aimed at developing scalable adaptive mesh refinement (AMR) algorithms and implementations. The first one is based on using octree-based mesh generation and adaptation for the whole simulation in combination with unfitted finite element methods (FEMs) and the use of algebraic constraints to deal with non-conformity of spaces. On the other hand the second strategy is based on the use of an initial octree mesh that, after make it conforming through the addition of templatebased tetrahedral refinements, is adapted anisotropically during the calculation. Regarding the first strategy the following items are included: