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<html>
<head>
<title>
FD1D_HEAT_STEADY - Finite Difference Solution of a 1D Steady State Heat Equation
</title>
</head>
<body bgcolor="#EEEEEE" link="#CC0000" alink="#FF3300" vlink="#000055">
<h1 align = "center">
FD1D_HEAT_STEADY <br> Finite Difference Solution of a 1D Steady State Heat Equation
</h1>
<hr>
<p>
<b>FD1D_HEAT_STEADY</b>
is a C++ program which
applies the finite difference method to estimate the solution of
the steady state heat equation over a one dimensional region, which
can be thought of as a thin metal rod.
</p>
<p>
We will assume the rod extends over the range A <= X <= B.
</p>
<p>
The quantity of interest is the temperature U(X) at each point in the rod.
</p>
<p>
We will assume that the temperature of the rod is fixed at known values
at the two endpoints. Symbolically, we write the <i>boundary conditions</i>:
<pre>
U(A) = UA
U(B) = UB
</pre>
</p>
<p>
Inside the rod, we assume that a version of the steady (time independent)
heat equation applies. This assumes that the situation in the rod has
"settled down", so that the temperature configuration has no further
tendency to change. The equation we will consider is
<pre>
- d/dx ( K(X) * d/dx U(X) ) = F(X)
</pre>
Here, the right hand side term F(X) allows us to consider internal
heat sources in the metal - perhaps a portion of the rod is sitting
above a blow torch. The coefficient K(X) is a measure of heat conductivity.
It measures the rate at which the heat from a local hot spot will spread out.
</p>
<p>
If the heat source function F(X) is zero everywhere, and if K(X) is
constant, then the solution U(X) will be the straight line function
that matches the two endpoint values. Making F(X) positive over a small
interval will "heat up" that portion. You can simulate a rod that is
divided into regions of different materials by setting the function K(X)
to have a given value K1 over some subinteral of [A,B] and value K2 over
the rest of the region.
</p>
<p>
To estimate the value of the solution, we will pick a uniform mesh
of N points X(1) through X(N), from A to B. At the I-th point, we will
compute an estimated temperature U(I). To do this, we will need to
use the boundary conditions and the differential equation.
</p>
<p>
Since X(1) = A and X(N) = B, we can use the boundary conditions to
set U(1) = UA and U(N) = UB.
</p>
<p>
For the points in the interior, we need to approximate the differential
equation in a way that allows us to determine the solution values.
We will do this using a finite difference approximation.
</p>
<p>
Suppose we are working at node X(I), which is associated with U(I).
Then a centered difference approximation to
<pre>
- d/dx ( K(X) * d/dx U(X) )
</pre>
is
<pre>
- ( + K(X(I)+DX/2) * ( U(X(I+1)) - U(X(I)) ) / DX )
- K(X(I)-DX/2) * ( U(X(I)) - U(X(I-1)) ) / DX ) / DX
</pre>
</p>
<p>
If we rearrange the terms in this approximation, and set it equal to F(X(I)),
we get the finite difference approximation to the differential equation at X(I):
<pre>
- K(X(I)-DX/2) * U(X(I-1)
+ ( K(X(I)-DX/2) + K(X(I)+DX(2)) ) * U(X(I))
- K(X(I)+DX(2)) * U(X(I+1)) = DX * DX * F(X(I))
</pre>
</p>
<p>
This means that we have N-2 equations, each of which involves
an unknown U(I) and its left and right neighbors, plus the two boundary
conditions, which give us a total of N equations in N unknowns.
</p>
<p>
We can set up and solve these linear equations using a matrix A for
the coefficients, and a vector RHS for the right hand side terms,
and calling a function to solve the system A*U=RHS will give us the solutioni.
</p>
<p>
Because finite differences are only an approximation to derivatives, this
process only produces estimates of the solution. Usually, we can reduce
this error by decreasing DX.
</p>
<p>
This program assumes that the user will provide a calling program, and
functions to evaluate K(X) and F(X).
</p>
<h3 align = "center">
Usage:
</h3>
<p>
<blockquote>
<b>fd1d_steady_heat</b> ( <i>n, a, b, ua, ub, k, f, x, u</i> )
</blockquote>
where
<ul>
<li>
<i>n</i>, the number of spatial points.
</li>
<li>
<i>a, b</i>, the left and right endpoints.
</li>
<li>
<i>ua, ub</i>, the temperature values at the left and right endpoints.
</li>
<li>
<i>k</i>, the name of the function which evaluates K(X);
</li>
<li>
<i>f</i>, the name of the function which evaluates the right hand side F(X).
</li>
<li>
<i>x</i>, (output) the X coordinates of nodes;
</li>
<li>
<i>u</i>, (output), the computed value of the temperature at the nodes.
</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>
<b>FD1D_HEAT_STEADY</b> is available in
<a href = "../../c_src/fd1d_heat_steady/fd1d_heat_steady.html">a C version</a> and
<a href = "../../cpp_src/fd1d_heat_steady/fd1d_heat_steady.html">a C++ version</a> and
<a href = "../../f77_src/fd1d_heat_steady/fd1d_heat_steady.html">a FORTRAN77 version</a> and
<a href = "../../f_src/fd1d_heat_steady/fd1d_heat_steady.html">a FORTRAN90 version</a> and
<a href = "../../m_src/fd1d_heat_steady/fd1d_heat_steady.html">a MATLAB version</a>
</p>
<h3 align = "center">
Related Data and Programs:
</h3>
<p>
<a href = "../../data/fd1d/fd1d.html">
FD1D</a>,
a data directory which
contains examples of 1D FD files, two text files that can be used to describe
many finite difference models with one space variable, and either no time dependence
or a snapshot at a given time;
</p>
<p>
<a href = "../../cpp_src/fd1d_burgers_lax/fd1d_burgers_lax.html">
FD1D_BURGERS_LAX</a>,
a C++ program which
applies the finite difference method and the Lax-Wendroff method
to solve the non-viscous time-dependent Burgers equation
in one spatial dimension.
</p>
<p>
<a href = "../../cpp_src/fd1d_burgers_leap/fd1d_burgers_leap.html">
FD1D_BURGERS_LEAP</a>,
a C++ program which
applies the finite difference method and the leapfrog approach
to solve the non-viscous time-dependent Burgers equation in one spatial dimension.
</p>
<p>
<a href = "../../cpp_src/fd1d_bvp/fd1d_bvp.html">
FD1D_BVP</a>,
a C++ program which
applies the finite difference method
to a two point boundary value problem in one spatial dimension.
</p>
<p>
<a href = "../../m_src/fd1d_display/fd1d_display.html">
FD1D_DISPLAY</a>,
a MATLAB program which
reads a pair of files defining a 1D finite difference model, and plots the data.
</p>
<p>
<a href = "../../cpp_src/fd1d_heat_explicit/fd1d_heat_explicit.html">
FD1D_HEAT_EXPLICIT</a>,
a C++ program which
uses the finite difference method and explicit time stepping
to solve the time dependent heat equation in 1D.
</p>
<p>
<a href = "../../cpp_src/fd1d_heat_implicit/fd1d_heat_implicit.html">
FD1D_HEAT_IMPLICIT</a>,
a C++ program which
uses the finite difference method and implicit time stepping
to solve the time dependent heat equation in 1D.
</p>
<p>
<a href = "../../cpp_src/fd1d_wave/fd1d_wave.html">
FD1D_WAVE</a>,
a C++ program which
applies the finite difference method to solve the time-dependent
wave equation utt = c * uxx in one spatial dimension.
</p>
<p>
<a href = "../../cpp_src/fd2d_heat_steady/fd2d_heat_steady.html">
FD2D_HEAT_STEADY</a>,
a C++ program which
uses the finite difference method (FDM) to solve the
steady (time independent) heat equation in 2D.
</p>
<p>
<a href = "../../cpp_src/fem1d_heat_steady/fem1d_heat_steady.html">
FEM1D_HEAT_STEADY</a>,
a C++ program which
uses the finite element method to solve the steady (time independent)
heat equation in 1D.
</p>
<h3 align = "center">
Reference:
</h3>
<p>
<ol>
<li>
George Lindfield, John Penny,<br>
Numerical Methods Using MATLAB,<br>
Second Edition,<br>
Prentice Hall, 1999,<br>
ISBN: 0-13-012641-1,<br>
LC: QA297.P45.
</li>
</ol>
</p>
<h3 align = "center">
Source Code:
</h3>
<p>
<ul>
<li>
<a href = "fd1d_heat_steady.cpp">fd1d_heat_steady.cpp</a>,
the library.
</li>
<li>
<a href = "fd1d_heat_steady.hpp">fd1d_heat_steady.hpp</a>,
the include file.
</li>
<li>
<a href = "fd1d_heat_steady.sh">fd1d_heat_steady.sh</a>,
commands to compile the library.
</li>
</ul>
</p>
<h3 align = "center">
Examples and Tests:
</h3>
<p>
<ul>
<li>
<a href = "problem1.cpp">problem1.cpp</a>,
uses K(X) = 1, F(X) = 0, so the solution should be the straight
line that connects the boundary values.
</li>
<li>
<a href = "problem1.sh">problem1.sh</a>,
commands to compile the problem and run it with the library.
</li>
<li>
<a href = "problem1_nodes.txt">problem1_nodes.txt</a>,
the coordinates of the nodes.
</li>
<li>
<a href = "problem1_values.txt">problem1_values.txt</a>,
the computed temperatures at the nodes.
</li>
</ul>
</p>
<p>
<ul>
<li>
<a href = "problem2.cpp">problem2.cpp</a>,
uses K(X) which is set to different constants over three subregions,
and F(X) = 0.0, so the solution will be a piecewise linear function
that connects the boundary values.
</li>
<li>
<a href = "problem2.sh">problem2.sh</a>,
commands to compile the problem and run it with the library.
</li>
<li>
<a href = "problem2_nodes.txt">problem2_nodes.txt</a>,
the coordinates of the nodes.
</li>
<li>
<a href = "problem2_values.txt">problem2_values.txt</a>,
the computed temperatures at the nodes.
</li>
</ul>
</p>
<p>
<ul>
<li>
<a href = "problem3.cpp">problem3.cpp</a>,
uses K(X) = 1, F(X) defines a heat source, so the solution can
rise above the boundary values.
</li>
<li>
<a href = "problem3.sh">problem3.sh</a>,
commands to compile the problem and run it with the library.
</li>
<li>
<a href = "problem3_nodes.txt">problem3_nodes.txt</a>,
the coordinates of the nodes.
</li>
<li>
<a href = "problem3_values.txt">problem3_values.txt</a>,
the computed temperatures at the nodes.
</li>
</ul>
</p>
<p>
<ul>
<li>
<a href = "problem4.cpp">problem4.cpp</a>,
uses K(X) = 1, F(X) defines a heat source and a heat sink, so the
solution can go above and below the boundary values.
</li>
<li>
<a href = "problem4.sh">problem4.sh</a>,
commands to compile the problem and run it with the library.
</li>
<li>
<a href = "problem4_nodes.txt">problem4_nodes.txt</a>,
the coordinates of the nodes.
</li>
<li>
<a href = "problem4_values.txt">problem4_values.txt</a>,
the computed temperatures at the nodes.
</li>
</ul>
</p>
<h3 align = "center">
List of Routines:
</h3>
<p>
<ul>
<li>
<b>FD1D_HEAT_STEADY</b> solves the steady 1D heat equation.
</li>
<li>
<b>R83NP_FS</b> factors and solves an R83NP system.
</li>
<li>
<b>R8MAT_WRITE</b> writes an R8MAT file.
</li>
<li>
<b>R8VEC_EVEN</b> returns an R8VEC of values evenly spaced between ALO and AHI.
</li>
<li>
<b>TIMESTAMP</b> prints the current YMDHMS date as a time stamp.
</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 19 May 2011.
</i>
<!-- John Burkardt -->
</body>
</html>