FREE_FEM_POISSON
Finite Element Solution of Poisson's Equation
on a Triangulated Region

FREE_FEM_POISSON is a C++ program which applies the finite element method to solve a form of Poisson's equation over an arbitrary triangulated region.

The computational region is unknown by the program. The user specifies it by preparing a file containing the coordinates of the nodes, and a file containing the indices of nodes that make up triangles that form a triangulation of the region.

Normally, the user does not type in this information by hand, but has a program fill in the nodes, and perhaps another program that constructs the triangulation. However, in the simplest case, the user might construct a very crude triangulation by hand, and have TRIANGULATION_REFINE refine it to something more reasonable.

For the following ridiculously small example:

        4----5
        |\   |\
        | \  | \
        |  \ |  \
        |   \|   \
        1----2----3
      

         0.0 0.0
         1.0 0.0
         2.0 0.0
         0.0 1.0
         1.0 1.0
      

        1 2 4
        5 4 2
        2 3 5
      

The program is set up to handle the linear Poisson equation with a right hand side function, and nonhomogeneous Dirichlet boundary conditions. The state variable U(X,Y) is then constrained by:

        - ( Uxx + Uyy ) + K(x,y) * U(x,y) = F(x,y)  in the region
                 U(x,y) = G(x,y)  on the boundary
      

A fancier version of the program would handle a more interesting nonlinear PDE, and include optional Neumann boundary conditions.

To specify the right hand side function F(x,y), the linear coefficient K(x,y) and the boundary condition function G(x,y), the user has to modify a file containing three routines,

• void rhs ( int node_num, double node_xy[], double node_rhs[] ) evaluates the right hand side of function F(x,y) at a list of nodes.
• void k_coef ( int node_num, double node_xy[], double node_k[] ) evaluates the coefficient function K(x,y) at a list of nodes.
• void dirichlet_condition ( int node_num, double node_xy[], double node_g[] ) evaluates the Dirichlet boundary condition G(X,Y) at a list of nodes.

To run the program, the user compiles the user routines, links them with FREE_FEM_POISSON, and runs the executable.

The program writes out a file containing an Encapsulated PostScript image of the nodes and elements, with numbers. If there are too many nodes, the plot may be too cluttered to read. For lower values, however, it is a valuable map of what is going on in the geometry.

The program is also able to write out a file containing the solution value at every node. This file may be used to create contour plots of the solution.

Licensing:

The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.

Related Data and Programs:

CVT_TRIANGULATION is a FORTRAN90 program which constructs a CVT triangulation for certain regions.

FEM is a data directory which contains a description of the data files that can be used to describe a finite element model.

FEM_50 is a MATLAB program which is a finite element program in just 50 lines of code.

FEM_50_HEAT is a MATLAB program which is a modified version of FEM_50 suitable for solving the heat equation.

FEM_BASIS_T3_DISPLAY is a MATLAB program which displays a basis function associated with a linear triangle ("T3") mesh.

FEM_BASIS_T6_DISPLAY is a MATLAB program which displays a basis function associated with a quadratic triangle ("T6") mesh.

FEM_IO is a C++ library which reads or writes the node, element and data files that define a finite element model.

FEM_SAMPLE is a C++ library which evaluates a finite element function defined on an order 3 or order 6 triangulation.

FEM_TO_TEC is a MATLAB program which can convert an FEM model into a TEC graphics file.

FEM1D is a C++ program which applies the finite element method, with piecewise linear basis functions, to a linear two point boundary value problem;

FEM1D_ADAPTIVE is a C++ program which applies the finite element method to a linear two point boundary value problem in a 1D region, using adaptive refinement to improve the solution.

FEM1D_NONLINEAR is a C++ program which applies the finite element method to a nonlinear two point boundary value problem in a 1D region.

FEM1D_PMETHOD is a C++ program which applies the p-method version of the finite element method to a linear two point boundary value problem in a 1D region.

FEM2D_HEAT is a C++ program which solves the time dependent heat equation on the unit square.

FEM2D_PACK is a C++ library which includes utitlies for 2D finite element calculations.

FEM2D_POISSON is a C++ program which solves Poisson's equation on a square, using the finite element method.

FFP_SPARSE is a MATLAB program which solves the steady Poisson equations in an arbitrary triangulated 2D region, using MATLAB's sparse matrix storage format and solver.

FREE_FEM_HEAT is a C++ program which solves the time dependent heat equation on an arbitrary triangulated region in 2D.

FREE_FEM_NAVIER_STOKES is a C++ program which solves the steady incompressible Navier Stokes equations in an arbitrary triangulated region, using the finite element method.

FREE_FEM_POISSON is available in a C++ version and a FORTRAN90 version and a MATLAB version.

FREE_FEM_STOKES is a C++ program which solves the steady Stokes flow equations on a triangulated 2D region.

HOT_PIPE is a MATLAB program which can be run with FEM_50_HEAT.

HOT_POINT is a MATLAB program which can be run with FEM_50_HEAT.

PLOT_POINTS is a FORTRAN90 program which can make a plot of the nodes that define the region.

TABLE is a file format which is used to store the input and output files used by the program.

TABLE_DELAUNAY is a C++ program which constructs a Delaunay triangulation of a region which is described by a set of points.

TABLE_IO is a C++ library which supplies the routines used to read the data files.

TRIANGULATION_ORDER3 is a data directory which contains a description of the format for the two files needed to describe an order 3 triangulation.

TRIANGULATION_ORDER3_CONTOUR is a MATLAB program which can make contour plots of the computed solution.

TRIANGULATION_PLOT is a C++ program which can display an image of the triangulation used by the program.

TRIANGULATION_REFINE is a C++ program which can refine a triangulation.

Reference:

1. Hans Rudolf Schwarz,
   Finite Element Methods,
   Academic Press, 1988,
   ISBN: 0126330107,
   LC: TA347.F5.S3313.
2. Gilbert Strang, George Fix,
   An Analysis of the Finite Element Method,
   Cambridge, 1973,
   ISBN: 096140888X,
   LC: TA335.S77.
3. Olgierd Zienkiewicz,
   The Finite Element Method,
   Sixth Edition,
   Butterworth-Heinemann, 2005,
   ISBN: 0750663200,
   LC: TA640.2.Z54

Source Code:

• free_fem_poisson.C, the source code;
• free_fem_poisson.csh, commands to compile the partial program;

Examples and Tests:

• ELL is an L-shaped region.
• LAKE is a simulated lake with an island.

The List of Routines:

• MAIN is the main routine of FREE_FEM_POISSON.
• ASSEMBLE_POISSON assembles the system for the Poisson equation.
• BANDWIDTH determines the bandwidth of the coefficient matrix.
• BASIS_ONE_T3 evaluates basis functions for a linear triangular element.
• CH_CAP capitalizes a single character.
• CH_EQI is true if two characters are equal, disregarding case.
• CH_TO_DIGIT returns the integer value of a base 10 digit.
• DGB_FA performs a LINPACK-style PLU factorization of a DGB matrix.
• DGB_MXV multiplies a DGB matrix times a vector.
• DGB_PRINT_SOME prints some of a DGB matrix.
• DGB_SL solves a system factored by DGB_FA.
• DIRICHLET_APPLY accounts for Dirichlet boundary conditions.
• DTABLE_DATA_READ reads the data from a real TABLE file.
• DTABLE_HEADER_READ reads the header from a real TABLE file.
• FILE_COLUMN_COUNT counts the number of columns in the first line of a file.
• FILE_NAME_SPECIFICATION determines the names of the input files.
• FILE_ROW_COUNT counts the number of row records in a file.
• I4_MAX returns the maximum of two I4's.
• I4_MIN returns the smaller of two I4's.
• I4_MODP returns the nonnegative remainder of integer division.
• I4_WRAP forces an integer to lie between given limits by wrapping.
• I4COL_COMPARE compares columns I and J of an I4COL.
• I4COL_SORT_A ascending sorts the columns of an I4COL.
• I4COL_SWAP swaps two columns of an I4COL.
• I4MAT_TRANSPOSE_PRINT_SOME prints some of an I4MAt, transposed.
• ITABLE_DATA_READ reads data from an integer TABLE file.
• ITABLE_HEADER_READ reads the header from an integer TABLE file.
• LVEC_PRINT prints a logical vector.
• POINTS_PLOT plots a pointset.
• QUAD_RULE sets the quadrature rule for assembly.
• R8_ABS returns the absolute value of an R8.
• R8_HUGE returns a "huge" R8.
• R8_NINT returns the nearest integer to an R8.
• R8MAT_TRANSPOSE_PRINT_SOME prints some of an R8MAT, transposed.
• R8VEC_AMAX returns the maximum absolute value in an R8VEC.
• R8VEC_PRINT_SOME prints "some" of an R8VEC.
• REFERENCE_TO_PHYSICAL_T3 maps reference points to physical points.
• RESIDUAL_POISSON evaluates the residual for the Poisson equation.
• S_LEN_TRIM returns the length of a string to the last nonblank.
• S_TO_I4 reads an I4 from a string.
• S_TO_I4VEC reads an I4VEC from a string.
• S_TO_R8 reads an R8 from a string.
• S_TO_R8VEC reads an R8VEC from a string.
• S_WORD_COUNT counts the number of "words" in a string.
• SOLUTION_EVALUATE evaluates the solution at a point in a triangle.
• SOLUTION_WRITE writes the solution to a file.
• SORT_HEAP_EXTERNAL externally sorts a list of items into ascending order.
• TIMESTAMP prints the current YMDHMS date as a time stamp.
• TIMESTRING returns the current YMDHMS date as a string.
• TRIANGLE_AREA_2D computes the area of a triangle in 2D.
• TRIANGULATION_ORDER3_BOUNDARY_NODE indicates nodes on the boundary.
• TRIANGULATION_ORDER3_PLOT plots a triangulation of a set of nodes.

You can go up one level to the C++ source codes.