Immersed boundary methods such as the finite cell method provide a versatile tool for the analysis of structures, which are difficult to discretize in a boundary conforming manner due to their complex geometries. Using Cartesian grids and a fictitious domain approach, the effort is shifted from meshing toward the quadrature, which needs to be adapted to discontinuous integrands for cut cells, i.e., elements cut by the immersed boundary. Further, the condition number of the stiffness matrix of such discretizations is typically much larger compared to boundary fitted discretizations, making the use of iterative solvers challenging. In order to restore the performance of iterative solvers, i.e., lower the condition number of the stiffness matrix, several stabilization methods are available. In this work, we compare the performance of two stabilization methods. The classical $ \alpha $-stabilization method uses a material with low (but non-zero) stiffness in the fictitious domain. The so called eigenvalue- or $ \epsilon $-stabilization is based on an eigendecomposition of the stiffness matrix and selectively stabilizes modes, which are associated with very low eigenvalues. Generally, both stabilization methods introduce an error, which, however, can be corrected. This work includes an investigation of such correction mechanisms for linear and nonlinear problems.
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Figure 2. Schematic of the eigenvalue stabilization procedure for a single cell (from left to right). First, the eigenvalues are computed. Second, the critical non-zero eigenmodes are extracted. Third, the eigenmodes are orthonormalized, and, finally, the eigenvalues are scaled and the stabilization system is constructed
Figure 8. Combination of the data from Fig. 6 (left) and the same data after one corrector iteration (right). For each $ \alpha $-graph $ \alpha = 10^{-4-2\, k} $ and $ \epsilon = 0 $, and for each $ \epsilon $-graph $ \epsilon = 10^{-2-2\, k} $ and $ \alpha = 0 $, where $ k = 0, 1, 2, 3 $
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Illustration of the concept of a fictitious domain method
Schematic of the eigenvalue stabilization procedure for a single cell (from left to right). First, the eigenvalues are computed. Second, the critical non-zero eigenmodes are extracted. Third, the eigenmodes are orthonormalized, and, finally, the eigenvalues are scaled and the stabilization system is constructed
Plate with hole geometry and boundary conditions
High-order boundary fitted FEM mesh (A) and a Cartesian FCM grid (B)
Volume fraction of the cells within the finite cell mesh
Influence of the stabilization parameters on the error in the energy norm (left) and the number of CG-iterations (right)
Condition number of the cut cells for
Combination of the data from Fig. 6 (left) and the same data after one corrector iteration (right). For each
Progress of the error and deviation in energy norm for
Relative error in energy norm for FCM simulation ansatz order
Convergence in energy norm for
Convergence for different right-hand side modifications for
Convergence for different right-hand side modifications for
Evolution of the CG solver convergence indicator over the CG-iteration (left) and the relative error in energy norm for each Newton-Raphson iteration (right)