# Finite Element Interface¶

## Introduction¶

Many application codes use unstructured finite element meshes. This section describes an interface for finite element problems, called the FEI, which is supported in hypre.

FEI refers to a specific interface for black-box finite element solvers, originally developed in Sandia National Lab, see [ClEA1999]. It differs from the rest of the conceptual interfaces in hypre in two important aspects: it is written in C++, and it does not separate the construction of the linear system matrix from the solution process. A complete description of Sandia’s FEI implementation can be obtained by contacting Alan Williams at Sandia (william@sandia.gov). A simplified version of the FEI has been implemented at LLNL and is included in hypre. More details about this implementation can be found in the header files of the FEI_mv/fei-base and FEI_mv/fei-hypre directories.

## A Brief Description of the Finite Element Interface¶

Typically, finite element codes contain data structures storing element connectivities, element stiffness matrices, element loads, boundary conditions, nodal coordinates, etc. One of the purposes of the FEI is to assemble the global linear system in parallel based on such local element data. We illustrate this in the rest of the section and refer to example 10 (in the examples directory) for more implementation details.

In hypre, one creates an instance of the FEI as follows:

LLNL_FEI_Impl *feiPtr = new LLNL_FEI_Impl(mpiComm);


Here mpiComm is an MPI communicator (e.g. MPI\_COMM\_WORLD). If Sandia’s FEI package is to be used, one needs to define a hypre solver object first:

LinearSystemCore   *solver = HYPRE_base_create(mpiComm);
FEI_Implementation *feiPtr = FEI_Implementation(solver,mpiComm,rank);


where rank is the number of the master processor (used only to identify which processor will produce the screen outputs). The LinearSystemCore class is the part of the FEI that interfaces with the linear solver library. It will be discussed later in Sections FEI Solvers and Using HYPRE in External FEI Implementations.

Local finite element information is passed to the FEI using several methods of the feiPtr object. The first entity to be submitted is the field information. A field has an identifier called fieldID and a rank or fieldSize (number of degree of freedom). For example, a discretization of the Navier Stokes equations in 3D can consist of velocity vector having $$3$$ degrees of freedom in every node (vertex) of the mesh and a scalar pressure variable, which is constant over each element. If these are the only variables, and if we assign fieldID $$7$$ and $$8$$ to them, respectively, then the finite element field information can be set up by

nFields   = 2;                 /* number of unknown fields */
fieldID   = new int[nFields];  /* field identifiers */
fieldSize = new int[nFields];  /* vector dimension of each field */

/* velocity (a 3D vector) */
fieldID[0]   = 7;
fieldSize[0] = 3;

/* pressure (a scalar function) */
fieldID[1]   = 8;
fieldSize[1] = 1;

feiPtr -> initFields(nFields, fieldSize, fieldID);


Once the field information has been established, we are ready to initialize an element block. An element block is characterized by the block identifier, the number of elements, the number of nodes per element, the nodal fields and the element fields (fields that have been defined previously). Suppose we use $$1000$$ hexahedral elements in the element block $$0$$, the setup consists of

elemBlkID  = 0;     /* identifier for a block of elements */
nElems     = 1000;  /* number of elements in the block */
elemNNodes = 8;     /* number of nodes per element */

/* nodal-based field for the velocity */
nodeNFields     = 1;
nodeFieldIDs    = new[nodeNFields];
nodeFieldIDs[0] = fieldID[0];

/* element-based field for the pressure */
elemNFields     = 1;
elemFieldIDs    = new[elemNFields];
elemFieldIDs[0] = fieldID[1];

feiPtr -> initElemBlock(elemBlkID, nElems, elemNNodes, nodeNFields,
nodeFieldIDs, elemNFields, elemFieldIDs, 0);


The last argument above specifies how the dependent variables are arranged in the element matrices. A value of $$0$$ indicates that each variable is to be arranged in a separate block (as opposed to interleaving).

In a parallel environment, each processor has one or more element blocks. Unless the element blocks are all disjoint, some of them share a common set of nodes on the subdomain boundaries. To facilitate setting up interprocessor communications, shared nodes between subdomains on different processors are to be identified and sent to the FEI. Hence, each node in the whole domain is assigned a unique global identifier. The shared node list on each processor contains a subset of the global node list corresponding to the local nodes that are shared with the other processors. The syntax for setting up the shared nodes is

feiPtr -> initSharedNodes(nShared, sharedIDs, sharedLengs, sharedProcs);


This completes the initialization phase, and a completion signal is sent to the FEI via

feiPtr -> initComplete();


Next, we begin the load phase. The first entity for loading is the nodal boundary conditions. Here we need to specify the number of boundary equations and the boundary values given by alpha, beta, and gamma. Depending on whether the boundary conditions are Dirichlet, Neumann, or mixed, the three values should be passed into the FEI accordingly.

feiPtr -> loadNodeBCs(nBCs, BCEqn, fieldID, alpha, beta, gamma);


The element stiffness matrices are to be loaded in the next step. We need to specify the element number $$i$$, the element block to which element $$i$$ belongs, the element connectivity information, the element load, and the element matrix format. The element connectivity specifies a set of $$8$$ node global IDs (for hexahedral elements), and the element load is the load or force for each degree of freedom. The element format specifies how the equations are arranged (similar to the interleaving scheme mentioned above). The calling sequence for loading element stiffness matrices is

for (i = 0; i < nElems; i++)
feiPtr -> sumInElem(elemBlkID, elemID, elemConn[i], elemStiff[i],

feiPtr -> loadComplete();