Example

See Section1.7.

CHAPTER

SEVEN

The esys.finley Module

The esys.finley library allows the creation of domains for solving linear, steady partial differential equations (PDEs) or systems of PDEs using isoparametrical finite elements. It supports unstructured 1D, 2D and 3D meshes. The PDEs themselves are represented by the LinearPDE class of esys.escript. esys.finley is paral- lelized under both OpenMP and MPI. A more restricted form of this library (dudley) is described in Section7.6.

7.1

Formulation

For a single PDE that has a solution with a single component the linear PDE is defined in the following form: Z Ω Ajl· v,ju,l+ Bj· v,ju + Cl· vu,l+ D · vu dΩ + Z Γ d · vu dΓ + Z Γcontact dcontact· [v][u] dΓ = Z Ω Xj· v,j+ Y · v dΩ + Z Γ y · v dΓ + Z Γcontact ycontact· [v] dΓ (7.1)

7.2

Meshes

To understand the usage of esys.finley one needs to have an understanding of how the finite element meshes are defined. Figure7.1shows an example of the subdivision of an ellipse into so-called elements. In this case, triangles have been used but other forms of subdivisions can be constructed, e.g. quadrilaterals or, in the three- dimensional case, into tetrahedra and hexahedra. The idea of the finite element method is to approximate the solution by a function which is a polynomial of a certain order and is continuous across its boundary to neighbour elements. In the example of Figure7.1a linear polynomial is used on each triangle. As one can see, the triangu- lation is quite a poor approximation of the ellipse. It can be improved by introducing a midpoint on each element edge then positioning those nodes located on an edge expected to describe the boundary, onto the boundary. In this case the triangle gets a curved edge which requires a parameterization of the triangle using a quadratic polynomial. For this case, the solution is also approximated by a piecewise quadratic polynomial (which explains the name isoparametrical elements), see Reference [40,5] for more details. esys.finley also supports macro elements. For these elements a piecewise linear approximation is used on an element which is further subdivided (in the case of esys.finley halved). As such, these elements do not provide more than a further mesh refinement but should be used in the case of incompressible flows, see StokesProblemCartesian. For these problems a linear approximation of the pressure across the element is used (use the reduced solution FunctionSpace) while the refined element is used to approximate velocity. So a macro element provides a continuous pressure approximation together with a velocity approximation on a refined mesh. This approach is necessary to make sure that the incompressible flow has a unique solution.

Node with reference number id

Element with reference number

id 17 13 12 7 4 1 0 20 19 15 16 11 9 10 ₈ 5 6 2 3 18 11 10 8 7 9 5 6 4 3 1 0

FIGURE7.1: Subdivision of an Ellipse into triangles order 1 (Tri3)

The union of all elements defines the domain of the PDE. Each element is defined by the nodes used to describe its shape. In Figure 7.1the element, which has type Tri3, with element reference number 19 is defined by the nodes with reference numbers 9, 11 and 0. Notice that the order is counterclockwise. The coefficients of the PDE are evaluated at integration nodes with each individual element. For quadrilateral elements a Gauss quadrature scheme is used. In the case of triangular elements a modified form is applied. The boundary of the domain is also subdivided into elements. In Figure7.1line elements with two nodes are used. The elements are also defined by their describing nodes, e.g. the face element with reference number 20, which has type Line2, is defined by the nodes with the reference numbers 11 and 0. Again the order is crucial, if moving from the first to second node the domain has to lie on the left hand side (in the case of a two-dimensional surface element the domain has to lie on the left hand side when moving counterclockwise). If the gradient on the surface of the domain is to be calculated rich face elements need to be used. Rich elements on a face are identical to interior elements but with a modified order of nodes such that the ’first’ face of the element aligns with the surface of the domain. In Figure7.1elements of the type Tri3Face are used. The face element reference number 20 as a rich face element is defined by the nodes with reference numbers 11, 0 and 9. Notice that the face element 20 is identical to the interior element 19 except that, in this case, the order of the node is different to align the first edge of the triangle (which is the edge starting with the first node) with the boundary of the domain.

Be aware that face elements and elements in the interior of the domain must match, i.e. a face element must be the face of an interior element or, in case of a rich face element, it must be identical to an interior element. If no face elements are specified esys.finley implicitly assumes homogeneous natural boundary conditions, i.e. d =0 and y =0, on the entire boundary of the domain. For inhomogeneous natural boundary conditions, the boundary must be described by face elements.

If discontinuities of the PDE solution are considered, contact elements are introduced to describe the contact region Γcontact_{even if d}contact_{and y}contact_{are zero. Figure7.2shows a simple example of a mesh of rectangular}

elements around a contact region Γcontact. The contact region is described by the elements 4, 3 and 6. Their element type is Line2 Contact. The nodes 9, 12, 6 and 5 define contact element 4, where the coordinates of nodes 12 and 5 and nodes 4 and 6 are identical, with the idea that nodes 12 and 9 are located above and nodes 5 and 6 below the contact region. Again, the order of the nodes within an element is crucial. There is also the option of using rich elements if the gradient is to be calculated on the contact region. Similarly to the rich face elements these are constructed from two interior elements by reordering the nodes such that the ’first’ face of the element above and the ’first’ face of the element below the contact regions line up. The rich version of element 4 is of type Rec4Face Contact and is defined by the nodes 9, 12, 16, 18, 6, 5, 0 and 2. Table 7.1shows the interior element types and the corresponding element types to be used on the face and contacts. Figure7.3, Figure7.4and Figure7.5show the ordering of the nodes within an element.