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# A Simple Example

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# A Simple Example   As a preview example to give the flavor of a FlexPDE descriptor file, we will construct a model of heatflow on a square domain.

The heatflow equation is

If K is constant and Source = 4*K, the heat equation will be satisfied by the function

Temp = Const - x^2 - y^2 .

We define a square region of material of conductivity K = 1, with a uniform heat source of 4 heat units per unit area.

We further specify the boundary value

Temp = 1 - x^2 - y^2

Since we know the analytic solution, we can compare the accuracy of the FlexPDE solution.

The text of the descriptor is as follows:

{ *******************************************************************

SIMPLE.PDE

This sample demonstrates the simplest application of FlexPDE to

heatflow problems.

******************************************************************* }

TITLE "Simple Heatflow"

VARIABLES

temp                { Identify "Temp" as the system variable }

DEFINITIONS

k = 1                { declare and define the conductivity }

source = 4                { declare and define the source }

texact = 1-x^2-y^2        { exact solution for reference }

INITIAL VALUES

temp = 0        { unimportant in linear steady-state problems,

but necessary for time-dependent or nonlinear

systems }

EQUATIONS        { define the heatflow equation :}

BOUNDARIES   { define the problem domain: }

REGION 1                { ... only one region }

START(-1,-1)        { specify the starting point }

{ specify Dirichlet boundary at exact solution: }

VALUE(temp)=texact

LINE TO (1,-1)        { walk the boundary }

TO (1,1)

TO (-1,1)

TO CLOSE        { bring boundary back to starting point }

MONITORS

CONTOUR(temp)        { show the Temperature during solution }

PLOTS          { write these plots to disk at completion: }

CONTOUR(temp)        { show the solution }

SURFACE(temp)        { show a surface plot as well }

{ display the solution error :}

CONTOUR(temp-texact) AS "Error"

{ show a vector flow plot: }

VECTOR(-dx(temp),-dy(temp)) AS "Heat Flow"

END                        { end of descriptor file }