 FEATool Multiphysics  v1.14 Finite Element Analysis Toolbox
Problem Definition

FEATool uses a struct format with several fields to specify and set up problem definitions. The FEATool GUI will automatically manage these fields, however if the model is exported, modified on the command line, or built from scratch it is important to know how these fields are used, which is explained below.

# Problem Definition Struct

The FEATool problem definition struct consists of the following main fields

Field Description
geom Geometry objects
grid Grid structure
sdim Cell array of space dimension names
phys Physics mode structs
dvar Cell array of dependent variable names
sfun Cell array of shape functions
coef Cell array of coefficient and expression definitions
eqn Equation specification struct
bdr Boundary specification struct
pnt Point source and constraint specification struct
edg Edge constraint specification struct (3D)
sol Solution vector and list of solution times

A cell array of geometry objects used for grid generation is contained in the geom.objects field. The geometry is only used by the automatic grid generation and can be omitted if a grid is constructed by other means (for example with the grid function primitives). For example, the following code defines a geometry with a unit circle

fea.geom.objects = { gobj_circle() };


The grid struct field, which defines the computational grid, is explained in the Grid section.

The sdim field contains a cell array of strings with names for the space dimensions. These can then be used in FEATool functions, and equation/boundary specification and postprocessing. Typically the field will take the following forms

fea.sdim = { 'x' };           % 1D.
fea.sdim = { 'x' 'y' };       % 2D.
fea.sdim = { 'r' 'z' };       % 2D - Axisymmetry.
fea.sdim = { 'x' 'y' 'z' };   % 3D.


but the name strings can be substituted arbitrarily.

The optional phys struct contains predefined physics modes that have been added with the addphys function. After editing and setting the appropriate physics mode coefficients the parsephys command will expand the physics to the fea parent dvar, sfun, coef, eqn, and bdr fields.

The dvar field contains a cell array of strings with names of the dependent (equation) variables which should be solved for. For example

fea.dvar = { 'u' 'v' };


defines two dependent variables, labeled u and v.

Similarly, the sfun field contains a cell array of strings with function names of the finite element shape functions to use for each dependent variable. Shape functions are defined by the functions in the ellib directory. For example

fea.dvar = { 'sflag1' 'sflag2' };


defines 1st order conforming basis functions for the first dependent variable and 2nd order for the second one.

The coef field is a (n_coef, 4) sized cell array of coefficient expressions that can be used in equation definitions and postprocessing. The first column gives the coefficient names, second a short description, third a long description, and the fourth column is a sub cell array for the coefficient expression definitions in each subdomain. (If any of the fields const, expr, and vars exist they will be merged internally with the coef field.) The assignment

fea.coef = { 'rho' '' '' { 1  2            } ;
'miu' '' '' { 3 '4*pi*sin(x)' } };


here defines two coefficients rho with values 1 and 2 in two respective subdomains, and miu with values 3 and the expression 4*pi*sin(x).

The eqn, bdr, and pnt structs are used to specify equations, boundary conditions, and point constraints. The composition of these fields is explained below in the corresponding subsections.

After solving a problem the sol field will contain the solution column vector in sol.u with rows corresponding to the degrees of freedom (ordered according to the order of the dependent variables in fea.dvar and fea.eqn.dofm). For time dependent and eigenvalue problems the columns in u correspond to solutions at different times/eigenvalues, and additionally the output times will be stored as a vector in the sol.t or sol.l field, respectively.

## Equation Struct Specification

The equation struct contains the following fields

Field Description
eqn.m.form Form specifications for temporal terms
eqn.m.coef Coefficient specifications for temporal terms
Shape functions are inherited from eqn.a.sfun

Field Description
eqn.a.form Bilinear form specifications
eqn.a.coef Coefficient specifications for bilinear forms
eqn.a.sfun Shape function specification for bilinear forms

Field Description
eqn.f.form Linear form specifications for right hand side/load vector
eqn.f.coef Coefficient specifications for right hand side/load vector
eqn.f.sfun Shape function specifications for right hand side/load vector

Field Description
eqn.dofm Degree of freedom n numbering map for each cell
eqn.ndof Numbers of degrees of freedom for each dependent variable

The eqn.m field contains specifications for the time dependent term (with time derivative). Similarly the eqn.a field contains specifications for the bilinear forms used in the iteration (stiffness) matrix, and the eqn.f field specifies the linear forms in the right hand side/load vector. The above struct fields contain the form field which specifies the (bi-)linear forms to build and assemble. For bilinear forms the first row corresponds to the trial function space, and the second row the test function space. Linear forms only need to contain one row. In the form specification a 1 indicates a function value, 2 x-derivative, 3 y-derivative, and 4 z-derivative. For example a form specification [2 3;2 3] indicates a bilinear form with two terms, one term with both x-derivatives for the test and trial function spaces, and one with y-derivatives for both spaces (which in this case is a typical two-dimensional diffusion operator). Second order derivatives can also be specified as 22 for the xx-derivative, 23 xy-derivative and so on.

The coef field, is a cell array with coefficient values or expressions used for each term in the form field.

The sfun field is a cell array of shape function names used in the form assembly. This field is usually automatically constructed when calling the parseprob function.

dofm is an array specifying the local to global degree of freedom numbering for each dependent variable (size n_ldof, n_c). The rows correspond to local degrees of freedoms on each cell and the columns give the cell numbers. (For linear conforming shape functions the dof mapping will be identical to the grid.c field.) This field is created when parseprob calls the mapdofbdr function.

ndof is simply an help array for the numbers of degrees of freedom for each dependent variable (equals to max(dofm(:)) and also automatically generated by mapdofbdr).

## Boundary Struct Specification

The boundary struct contains the following fields

Field Description
bdr.d Dirichlet boundary coefficients
bdr.n Neumann (flux) boundary coefficients
bdr.bdrm Boundary degree of freedom numbering maps

Dirichlet boundary conditions are used to prescribe and fix a specific value for the dependent variables, while in contrast Neumann conditions represent a inward or outward directed flux (which are functions of the gradients of the dependent variables). Mixed Robin conditions are currently not available in FEATool. If a Dirichlet condition is prescribed on a boundary, the corresponding Neumann flux entry will be ignored. Alternatively, if a Dirichlet boundary coefficient entry is empty the Neumann flux contribution will be computed and added (however, all default Neumann contributions are zero corresponding to homogeneous do-nothing Neumann boundary conditions).

Dirichlet boundary coefficient values or string expressions for each boundary segment are specified in a cell array in the bdr.d field. Two syntaxes are supported, standard syntax where d is a cell array of size n_dvar x n_bdr, or extended syntax (supported in versions 1.5 or later) with d a 1 x n_dvar cell array where each entry itself is a n_bc_groups x n_bdr cell array containing the coefficients. In the extended syntax the number of boundary condition groups n_bc_groups is due to the chosen finite element shape function (specifically the number of nLDof rows which is 1+n_sdim for Hermite elements and 1 for all others). The extended syntax is used to prescribe separate conditions on the different local degrees of freedom (that is the basis function values and derivatives for Hermite elements). The Dirichlet boundary conditions are prescribed by the function bdrsetd.

If a Dirichlet boundary coefficient entry is empty, then Neumann (flux) conditions will be prescribed instead for the corresponding boundary segment. Similar to Dirichlet conditions, Neumann conditions are specified in the n field in either standard or extended syntax (bdrneu). An empty cell indicates homogeneous Neumann condition to be used (zero flux) for that boundary segment.

The bdrm arrays specifies the boundary condition mapping for each dependent variable. In a bdrm array (size 5+n_sdim, n_bdof) the first row gives the cell number, followed by edge/face, boundary, global and local degree of freedom numbers, and local coordinates on edges/faces.

The d and n fields must be prescribed by the user or derived from the physics modes with a call to parsephys, the bdrm field is automatically created when parseprob calls the mapdofbdr function.

For example, assume a two dimensional time dependent problem with one dependent variable or unknown u, then the following standard boundary syntax

fea.bdr.d = {  1  'sin(pi*t*x)+u'  []       []       };
fea.bdr.n = { []         0          2  'nx*ux+ny*uy' };


will prescribe u constant u = 1 on the first boundary, a string expression u(x,t) = sin(pi*t*x)+u on the second boundary, a constant flux Neumann condition 2 on the third boundary, and finally the flux expression nx*ux+ny*uy on the fourth and last boundary. Note that here in the string expressions it is perfectly valid to use constants, MATLAB functions, time t, function values and derivatives of the dependent variables, and also the normals, here nx and ny.

Assuming using a finite element basis functions with only one boundary condition group the equivalent extended syntax looks like the following

fea.bdr.d = { {  1  'sin(pi*t*x)+u'  []        []       } };
fea.bdr.n = { { []         0          2   'nx*ux+ny*uy' } };


If the default solvers solvestat and solvetime detects nonlinear Neumann boundary conditions they will attempt to linearize them by moving terms involving linear forms from the explicit right hand side to the implicit matrix (a boundary contribution to eqn.a instead of eqn.f).

## Point Struct Specification

The point struct contains the following fields

Field Description
pnt.index Index to grid point (in grid.p)
pnt.type Specifies either sources or constraint
pnt.dvar Integer or string corresponding a dependent variable
pnt.expr scalar or string expression for constraint

Point sources and Dirichlet point constraints are specified in a pnt struct. The type field specifies either a point 'source' or 'constraint' for each point with a corresponding string value. Point sources are applied to the right hand side load vector f while constraints are applied to both f and the global matrix A with the information in the finite element problem struct (pntsetf and pntset). The sources or constraints will be applied to the degree of freedom closest to the grid point specified in the index field. The dvar field contains an integer or string expression pointing to a dependent variable in dvar. The sources or constraints are specified in the expr field either as a string expression or scalar so that pnt.dvar(grid.p(:,pnt.index)) = pnt.expr. (Note that a point constraint will override the usual Dirichlet boundary conditions at the given point).

## Edge Struct Specification

The edge struct is only applicable to 3D problems and contains the following fields to specify Dirichlet edge constraints

Field Description
edg.index Index to edge
edg.type Must be constraint type
edg.dvar Integer or string corresponding a dependent variable
edg.expr scalar or string expression for constraint

Dirichlet edge constraints are specified in a edg struct. The type field must be set to the string 'constraint'. Edge constraints are applied to both the load vector f and the global matrix A with the information in the finite element problem struct (edgset). The index field specifies the edge to apply the constraint to (as reconstructed by gridbdre and can be visualized with plotedg). The dvar field contains an integer or string expression pointing to a dependent variable in dvar. The constraints are specified in the expr field either as a string expression or numeric scalar.