We present a new finite element method, called $\phi$-FEM, to solve numerically elliptic partial differential equations with natural (Neumann or Robin) boundary conditions using simple computational grids, not fitted to the boundary of the physical domain. The boundary data are taken into account using a level-set function, which is a popular tool to deal with complicated or evolving domains. Our approach belongs to the family of fictitious domain methods (or immersed boundary methods) and is close to recent methods of cutFEM/XFEM type. Contrary to the latter, $\phi$-FEM does not need any non-standard numerical integration on cut mesh elements or on the actual boundary, while assuring the optimal convergence orders with finite elements of any degree and providing reasonably well conditioned discrete problems. In the first version of $\phi$-FEM, only essential (Dirichlet) boundary conditions was considered. Here, to deal with natural boundary conditions, we introduce the gradient of the primary solution as an auxiliary variable. This is done only on the mesh cells cut by the boundary, so that the size of the numerical system is only slightly increased. We prove theoretically the optimal convergence of our scheme and a bound on the discrete problem conditioning, independent of the mesh cuts. The numerical experiments confirm these results.
翻译:我们提出了一个新的限定要素方法,称为$phi$-FEM,用不与物理域的边界界限不相适应的简单计算网格解决自然(Neumann或Robin)边界条件的数值椭圆部分方程式。边界数据是使用一个级别设置功能加以考虑的,这是一个处理复杂或演变域的常用工具。我们的方法属于虚构域方法(或浸入边界方法)的大家庭,接近于最近的切割FEM/XFEM类型方法。与后者相反,美元-FEM不需要在剪切网格或实际边界上采用任何非标准的数字组合,同时用任何程度的有限要素确保最佳汇合顺序,并提供相当良好的离散问题。在第一个版本的 $\phi$-FEM 中,只考虑基本(dirichlet)边界条件。在这里,我们采用主要解决方案的梯度作为辅助变量。这只针对边界切割的网格,因此,在剪切网单元上不需要任何非标准的数字组合,因此,以任何程度的有限要素确保最佳汇合,并且提供了合理的分数实验结果。我们最接近的精确地试验。我们只验证了这些数值的精确的公式。