Date:
Wednesday, 7 December, 2022 - 14:00
Speakers: Andreas Dedner, University of Warwick
Hosted at: SISSA, International School of Advanced Studies, Trieste, Italy, room 134.
Abstract:
Finite Element Methods (FEM) are generally constructed based on so called finite element triples (K,B,L). Here K is an element in the grid (e.g., a triangle), B is the basis of some finite dimensional space (e.g., a set of polynomials), and L is a set of functionals with |L| = |B| (e.g., the evaluation of functions on a set of Lagrange points). The set L are called local degrees of freedom (dofs). Often the aim in the construction of finite element spaces V_h is to achieve some level of conformity, i.e., guaranteeing that V_h is a subset of some function space V. Typical spaces are conforming with V=H^1. Achieving conformity with V=H^2 or V=H(div) is a lot more challenging. For example, the lowest order finite element on triangles which leads to V_h being H^2 conforming requires the use of polynomials of order 5 locally (without using a piecewise definition). Also changing K from triangles to quadrilaterals requires defining a complete new set of basis function B and dofs L. Often a suitable choice for L is not the problem but defining a suitable B can be challenging. The Virtual Element Method (VEM) is a recent approach to define a wide range of spaces on general element shapes including general polygons. In this talk we will provide a description of the virtual element spaces which shows that it can be considered to be a direct extension of the FEM constructing approach. The approach uses a fixed B on each element independent of the choice of L thus avoiding the problem described above. We introduce a VEM tuple and describe how that can be used to define the local spaces. We will focus our presentation on spaces which can be used to solve forth order problems but will also demonstrate how the approach can be used to construct other spaces, i.e., divergence free spaces for fluid dynamics problems. We will show how this approach simplifies the implementation of VEM methods within existing FEM codes and discuss a-priori error analysis and numerical experiments for linear forth order problems with varying coefficients.
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