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<html>
<head>
<title>
FEM2D_POISSON - Finite Element Solution on Arbitrary 2D Region
</title>
</head>
<body bgcolor="#EEEEEE" link="#CC0000" alink="#FF3300" vlink="#000055">
<h1 align = "center">
FEM2D_POISSON <br>
Finite Element Solution of Poisson's Equation <br>
on a Triangulated Region
</h1>
<hr>
<p>
<b>FEM2D_POISSON</b>
is a C++ program which
applies the finite element method to solve
a form of Poisson's equation over an arbitrary triangulated region.
</p>
<p>
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.
</p>
<p>
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 <a href = "../../cpp_src/triangulation_refine/triangulation_refine.html">
TRIANGULATION_REFINE</a> refine it to something more reasonable.
</p>
<p>
For the following ridiculously small example:
<pre>
4----5
|\ |\
| \ | \
| \ | \
| \| \
1----2----3
</pre>
the node file would be:
<pre>
0.0 0.0
1.0 0.0
2.0 0.0
0.0 1.0
1.0 1.0
</pre>
and the triangle file would be
<pre>
1 2 4
5 4 2
2 3 5
</pre>
</p>
<p>
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:
<pre>
- DEL H(x,y) DEL U(x,y) + K(x,y) * U(x,y) = F(x,y) in the region
U(x,y) = G(x,y) on the boundary
</pre>
</p>
<p>
To specify the right hand side function F(x,y), the linear
coefficients H(x,y) and K(x,y) and the boundary condition function G(x,y),
the user has to modify a file containing three routines,
<ul>
<li>
<b>void rhs ( int node_num, double node_xy[], double node_rhs[] )</b>
evaluates the right hand side of function F(x,y) at a list of
nodes.
</li>
<li>
<b>void h_coef ( int node_num, double node_xy[], double node_h[] )</b>
evaluates the coefficient function H(x,y) at a list of nodes.
</li>
<li>
<b>void k_coef ( int node_num, double node_xy[], double node_k[] )</b>
evaluates the coefficient function K(x,y) at a list of nodes.
</li>
<li>
<b>void dirichlet_condition ( int node_num, double node_xy[], double node_g[] )</b>
evaluates the Dirichlet boundary condition G(X,Y) at a list of nodes.
</li>
</ul>
</p>
<p>
To run the program, the user compiles the user routines,
links them with <b>FEM2D_POISSON</b>, and runs the executable.
</p>
<p>
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.
</p>
<p>
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.
</p>
<h3 align = "center">
Usage:
</h3>
<p>
The user must create an executable by compiling the user routines
and linking them with the main program, perhaps by commands like:
<pre>
g++ -c fem2d_poisson.C
g++ -c user.C
g++ fem2d_poisson.o user.o
mv a.out fem2d_poisson
</pre>
</p>
<p>
Assuming the executable program is called "fem2d_poisson", then
the program is executed by
<blockquote>
<b>fem2d_poisson</b> <i>prefix</i>
</blockquote>
where prefix is the common filename prefix, so that
<ul>
<li>
<i>prefix</i><b>_nodes.txt</b>, is a file containing the node coordinates;
</li>
<li>
<i>prefix</i><b>_elements.txt</b>, is a file listing the 3 nodes that
make up each element;
</li>
</ul>
</p>
<h3 align = "center">
Licensing:
</h3>
<p>
The computer code and data files described and made available on this web page
are distributed under
<a href = "../../txt/gnu_lgpl.txt">the GNU LGPL license.</a>
</p>
<h3 align = "center">
Languages:
</h3>
<p>
FEM2D_POISSON is available in
<a href = "../../cpp_src/fem2d_poisson/fem2d_poisson.html">a C++ version</a> and
<a href = "../../f_src/fem2d_poisson/fem2d_poisson.html">a FORTRAN90 version</a> and
<a href = "../../m_src/fem2d_poisson/fem2d_poisson.html">a MATLAB version.</a>
</p>
<h3 align = "center">
Related Data and Programs:
</h3>
<p>
<a href = "../../cpp_src/fem2d_poisson_cg/fem2d_poisson_cg.html">
FEM2D_POISSON_CG</a>,
a C++ program which
solves Poisson's equation on a triangulated region,
using the finite element method, sparse storage, and a conjugate gradient solver.
</p>
<p>
<a href = "../../cpp_src/fem2d_poisson_ell/fem2d_poisson_ell.html">
FEM2D_POISSON_ELL</a>,
a C++ library which
defines the geometry of an L-shaped region, as well as boundary
conditions for a given Poisson problem, and is called by FEM2D_POISSON
as part of a solution procedure.
</p>
<p>
<a href = "../../cpp_src/fem2d_poisson_lake/fem2d_poisson_lake.html">
FEM2D_POISSON_LAKE</a>,
a C++ library which
defines the geometry of a lake-shaped region, as well as boundary
conditions for a given Poisson problem, and is called by FEM2D_POISSON
as part of a solution procedure.
</p>
<p>
<a href = "../../cpp_src/fem2d_poisson_sparse/fem2d_poisson_sparse.html">
FEM2D_POISSON_SPARSE</a>,
a C++ program which
solves the steady (time independent) Poisson equation on an arbitrary
2D triangulated region using a version of GMRES for a sparse solver.
</p>
<h3 align = "center">
Reference:
</h3>
<p>
<ol>
<li>
Hans Rudolf Schwarz,<br>
Finite Element Methods,<br>
Academic Press, 1988,<br>
ISBN: 0126330107,<br>
LC: TA347.F5.S3313.
</li>
<li>
Gilbert Strang, George Fix,<br>
An Analysis of the Finite Element Method,<br>
Cambridge, 1973,<br>
ISBN: 096140888X,<br>
LC: TA335.S77.
</li>
<li>
Olgierd Zienkiewicz,<br>
The Finite Element Method,<br>
Sixth Edition,<br>
Butterworth-Heinemann, 2005,<br>
ISBN: 0750663200,<br>
LC: TA640.2.Z54
</li>
</ol>
</p>
<h3 align = "center">
Source Code:
</h3>
<p>
<ul>
<li>
<a href = "fem2d_poisson.cpp">fem2d_poisson.cpp</a>,
the source code;
</li>
<li>
<a href = "fem2d_poisson.sh">fem2d_poisson.sh</a>,
commands to compile the partial program;
</li>
</ul>
</p>
<h3 align = "center">
List of Routines:
</h3>
<p>
<ul>
<li>
<b>MAIN</b> is the main program for FEM2D_POISSON.
</li>
<li>
<b>ASSEMBLE_POISSON</b> assembles the system for the Poisson equation.
</li>
<li>
<b>BANDWIDTH</b> determines the bandwidth of the coefficient matrix.
</li>
<li>
<b>BASIS_ONE_T3</b> evaluates basis functions for a linear triangular element.
</li>
<li>
<b>CH_CAP</b> capitalizes a single character.
</li>
<li>
<b>CH_EQI</b> is true if two characters are equal, disregarding case.
</li>
<li>
<b>CH_TO_DIGIT</b> returns the integer value of a base 10 digit.
</li>
<li>
<b>DGB_FA</b> performs a LINPACK-style PLU factorization of a DGB matrix.
</li>
<li>
<b>DGB_MXV</b> multiplies a DGB matrix times a vector.
</li>
<li>
<b>DGB_PRINT_SOME</b> prints some of a DGB matrix.
</li>
<li>
<b>DGB_SL</b> solves a system factored by DGB_FA.
</li>
<li>
<b>DIRICHLET_APPLY</b> accounts for Dirichlet boundary conditions.
</li>
<li>
<b>FILE_COLUMN_COUNT</b> counts the columns in the first line of a file.
</li>
<li>
<b>FILE_ROW_COUNT</b> counts the number of row records in a file.
</li>
<li>
<b>I4_MAX</b> returns the maximum of two I4's.
</li>
<li>
<b>I4_MIN</b> returns the smaller of two I4's.
</li>
<li>
<b>I4_MODP</b> returns the nonnegative remainder of integer division.
</li>
<li>
<b>I4_WRAP</b> forces an integer to lie between given limits by wrapping.
</li>
<li>
<b>I4COL_COMPARE</b> compares columns I and J of an I4COL.
</li>
<li>
<b>I4COL_SORT_A</b> ascending sorts the columns of an I4COL.
</li>
<li>
<b>I4COL_SWAP</b> swaps two columns of an I4COL.
</li>
<li>
<b>I4MAT_DATA_READ</b> reads data from an I4MAT file.
</li>
<li>
<b>I4MAT_HEADER_READ</b> reads the header from an I4MAT file.
</li>
<li>
<b>I4MAT_TRANSPOSE_PRINT_SOME</b> prints some of an I4MAt, transposed.
</li>
<li>
<b>LVEC_PRINT</b> prints a logical vector.
</li>
<li>
<b>POINTS_PLOT</b> plots a pointset.
</li>
<li>
<b>QUAD_RULE</b> sets the quadrature rule for assembly.
</li>
<li>
<b>R8_ABS</b> returns the absolute value of an R8.
</li>
<li>
<b>R8_HUGE</b> returns a "huge" R8.
</li>
<li>
<b>R8_NINT</b> returns the nearest integer to an R8.
</li>
<li>
<b>R8MAT_DATA_READ</b> reads the data from an R8MAT file.
</li>
<li>
<b>R8MAT_HEADER_READ</b> reads the header from an R8MAT file.
</li>
<li>
<b>R8MAT_TRANSPOSE_PRINT_SOME</b> prints some of an R8MAT, transposed.
</li>
<li>
<b>R8MAT_WRITE</b> writes an R8MAT file.
</li>
<li>
<b>R8VEC_AMAX</b> returns the maximum absolute value in an R8VEC.
</li>
<li>
<b>R8VEC_PRINT_SOME</b> prints "some" of an R8VEC.
</li>
<li>
<b>REFERENCE_TO_PHYSICAL_T3</b> maps reference points to physical points.
</li>
<li>
<b>RESIDUAL_POISSON</b> evaluates the residual for the Poisson equation.
</li>
<li>
<b>S_LEN_TRIM</b> returns the length of a string to the last nonblank.
</li>
<li>
<b>S_TO_I4</b> reads an I4 from a string.
</li>
<li>
<b>S_TO_I4VEC</b> reads an I4VEC from a string.
</li>
<li>
<b>S_TO_R8</b> reads an R8 from a string.
</li>
<li>
<b>S_TO_R8VEC</b> reads an R8VEC from a string.
</li>
<li>
<b>S_WORD_COUNT</b> counts the number of "words" in a string.
</li>
<li>
<b>SOLUTION_EVALUATE</b> evaluates the solution at a point in a element.
</li>
<li>
<b>SORT_HEAP_EXTERNAL</b> externally sorts a list of items into ascending order.
</li>
<li>
<b>TIMESTAMP</b> prints the current YMDHMS date as a time stamp.
</li>
<li>
<b>TRIANGLE_AREA_2D</b> computes the area of a triangle in 2D.
</li>
<li>
<b>TRIANGULATION_ORDER3_BOUNDARY_NODE</b> indicates nodes on the boundary.
</li>
<li>
<b>TRIANGULATION_ORDER3_PLOT</b> plots a triangulation of a set of nodes.
</li>
</ul>
</p>
<p>
You can go up one level to <a href = "../cpp_src.html">
the C++ source codes</a>.
</p>
<hr>
<i>
Last revised on 06 December 2010.
</i>
<!-- John Burkardt -->
</body>
</html>