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% $Id$ 
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\chapter{How to Solve A Diffusion Problem} 
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\label{DIFFUSION CHAP} 
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\begin{figure} 
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\centerline{\includegraphics[width=\figwidth]{DiffusionDomain}} 
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\caption{Temperature Diffusion Problem with Circular Heat Source} 
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\label{DIFFUSION FIG 1} 
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\end{figure} 
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\section{\label{DIFFUSION OUT SEC}Outline} 
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In this chapter we will discuss how to solve the time dependenttemperature diffusion\index{diffusion equation} for 
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a block of material. Within the block there is a heat source which drives the temperature diffusion. 
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On the surface, energy can radiate into the surrounding environment. 
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\fig{DIFFUSION FIG 1} shows the configuration. 
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In the next \Sec{DIFFUSION TEMP SEC} we will present the relevant model. A 
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time integration scheme is introduced to calculate the temperature at given time nodes $t^{(n)}$. 
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We will see that at each time step a Helmholtz equation \index{Helmholtz equation} 
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must be solved. 
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The implementation of a Helmholtz equation solver will be discussed in \Sec{DIFFUSION HELM SEC}. 
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In Section~\ref{DIFFUSION TRANS SEC} the solver of the Helmholtz equation is used to build a 
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solver for the temperature diffusion problem. 
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\section{\label{DIFFUSION TEMP SEC}Temperature Diffusion} 
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The unknown temperature $T$ is a function of its location in the domain and time $t>0$. The governing equation 
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in the interior of the domain is given by 
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\begin{equation} 
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\rho c\hackscore p T\hackscore{,t}  (\kappa T\hackscore{,i})\hackscore{,i} = q 
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\label{DIFFUSION TEMP EQ 1} 
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\end{equation} 
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where $\rho c\hackscore p$ and $\kappa$ are given material constants. In case of a composite 
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material the parameters depend on their location in the domain. $q$ is 
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a heat source (or sink) within the domain. We are using Einstein summation convention \index{summation convention} 
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as introduced in \Chap{FirstSteps}. In our case we assume $q$ to be equal to a constant heat production rate 
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$q^{c}$ on a circle or sphere with center $x^c$ and radius $r$ and $0$ elsewhere: 
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\begin{equation} 
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q(x,t)= 
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\left\{ 
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\begin{array}{lcl} 
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q^c & & \xx^c\ \le r \\ 
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& \mbox{if} \\ 
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0 & & \mbox{else} \\ 
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\end{array} 
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\right. 
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\label{DIFFUSION TEMP EQ 1b} 
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\end{equation} 
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for all $x$ in the domain and all time $t>0$. 
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On the surface of the domain we are 
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specifying a radiation condition 
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which precribes the normal component of the flux $\kappa T\hackscore{,i}$ to be proportional 
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to the difference of the current temperature to the surrounding temperature $T\hackscore{ref}$: 
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\begin{equation} 
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\kappa T\hackscore{,i} n\hackscore i = \eta (T\hackscore{ref}T) 
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\label{DIFFUSION TEMP EQ 2} 
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\end{equation} 
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$\eta$ is a given material coefficient depending on the material of the block and the surrounding medium. 
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As usual $n\hackscore i$ is the $i$th component of the outer normal field \index{outer normal field} 
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at the surface of the domain. 
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To solve the time dependent \eqn{DIFFUSION TEMP EQ 1} the initial temperature at time 
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$t=0$ has to be given. Here we assume that the initial temperature is the surrounding temperature: 
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\begin{equation} 
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T(x,0)=T\hackscore{ref} 
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\label{DIFFUSION TEMP EQ 4} 
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\end{equation} 
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for all $x$ in the domain. It is pointed out that 
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the initial conditions satisfy the 
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boundary condition defined by \eqn{DIFFUSION TEMP EQ 2}. 
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The temperature is calculated at discrete time nodes $t^{(n)}$ where 
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$t^{(0)}=0$ and $t^{(n)}=t^{(n1)}+h$ where $h>0$ is the step size which is assumed to be constant. 
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In the following the upper index ${(n)}$ refers to a value at time $t^{(n)}$. The simplest 
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and most robust scheme to approximate the time derivative of the the temperature is 
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the backward Euler 
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\index{backward Euler} scheme, see~\cite{XXX} for alternatives. The backward Euler 
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scheme is based 
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on the Taylor expansion of $T$ at time $t^{(n)}$: 
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\begin{equation} 
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T^{(n1)}\approx T^{(n)}+T\hackscore{,t}^{(n)}(t^{(n1)}t^{(n)}) 
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=T^{(n1)}  h \cdot T\hackscore{,t}^{(n)} 
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\label{DIFFUSION TEMP EQ 6} 
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\end{equation} 
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This is inserted into \eqn{DIFFUSION TEMP EQ 1}. By separating the terms at 
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$t^{(n)}$ and $t^{(n1)}$ one gets for $n=1,2,3\ldots$ 
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\begin{equation} 
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\frac{\rho c\hackscore p}{h} T^{(n)}  (\kappa T^{(n)}\hackscore{,i})\hackscore{,i} = q + \frac{\rho c\hackscore p}{h} T^{(n1)} 
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\label{DIFFUSION TEMP EQ 7} 
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\end{equation} 
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where $T^{(0)}=T\hackscore{ref}$ is taken form the initial condition given by \eqn{DIFFUSION TEMP EQ 4}. 
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Together with the natural boundary condition 
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\begin{equation} 
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\kappa T\hackscore{,i}^{(n)} n\hackscore i = \eta (T\hackscore{ref}T^{(n)}) 
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\label{DIFFUSION TEMP EQ 2222} 
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\end{equation} 
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taken from \eqn{DIFFUSION TEMP EQ 2} 
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this forms a boundary value problem that has to be solved for each time step. 
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As a first step to implement a solver for the temperature diffusion problem we will 
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first implement a solver for the boundary value problem that has to be solved at each time step. 
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\section{\label{DIFFUSION HELM SEC}Helmholtz Problem} 
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The partial differential equation to be solved for $T^{(n)}$ has the form 
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\begin{equation} 
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\omega u  (\kappa u\hackscore{,i})\hackscore{,i} = f 
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\label{DIFFUSION HELM EQ 1} 
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\end{equation} 
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where $u$ plays the role of $T^{(n)}$ and we set 
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\begin{equation} 
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\omega=\frac{\rho c\hackscore p}{h} \mbox{ and } f=q+\frac{\rho c\hackscore p}{h}T^{(n1)} \;. 
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\label{DIFFUSION HELM EQ 1b} 
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\end{equation} 
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With $g=\eta T\hackscore{ref}$ the radiation condition defined by \eqn{DIFFUSION TEMP EQ 2222} 
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takes the form 
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\begin{equation} 
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\kappa u\hackscore{,i} n\hackscore{i} = g  \eta u\mbox{ on } \Gamma 
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\label{DIFFUSION HELM EQ 2} 
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\end{equation} 
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The partial differential 
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\eqn{DIFFUSION HELM EQ 1} together with boundary conditions of \eqn{DIFFUSION HELM EQ 2} 
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is called the Helmholtz equation \index{Helmholtz equation}. 
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We want to use the \LinearPDE class provided by \escript to define and solve a general linear PDE such as the 
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Helmholtz equation. We have used a special case of the \LinearPDE class, namely the 
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\Poisson class already in \Chap{FirstSteps}. 
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Here we will write our own specialized subclass of the \LinearPDE to define the Helmholtz equation 
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and use the \method{getSolution} method of parent class to actually solve the problem. 
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The form of a single PDE that can be handled by the \LinearPDE class is 
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\begin{equation}\label{EQU.FEM.1} 
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(A\hackscore{jl} u\hackscore{,l})\hackscore{,j}+D u =Y \; . 
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\end{equation} 
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We show here the terms which are relevant for the Helmholtz problem. 
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The general form and systems is discussed in \Sec{SEC LinearPDE}. 
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$A$, $D$ and $Y$ are the known coeffecients of the PDE. \index{partial differential equation!coefficients} 
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Notice that $A$ is a matrix or tensor of order 2 and $D$ and $Y$ are scalar. 
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They may be constant or may depend on their 
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location in the domain but must not depend on the unknown solution $u$. 
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The following natural boundary conditions \index{boundary condition!natural} that 
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are used in the \LinearPDE class have the form 
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\begin{equation}\label{EQU.FEM.2} 
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n\hackscore{j}A\hackscore{jl} u\hackscore{,l}+du=y \;. 
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\end{equation} 
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where, as usual, $n$ denotes the outer normal field on the surface of the domain. Notice that 
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the coefficient $A$ is already used in the PDE in \eqn{EQU.FEM.1}. $d$ and $y$ are given scalar coefficients. 
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By inspecting the Helmholtz equation \index{Helmholtz equation} 
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we can easily assign values to the coefficients in the 
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general PDE of the \LinearPDE class: 
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\begin{equation}\label{DIFFUSION HELM EQ 3} 
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\begin{array}{llllll} 
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A\hackscore{ij}=\kappa \delta\hackscore{ij} & D=\omega & Y=f \\ 
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d=\eta & y= g & \\ 
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\end{array} 
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\end{equation} 
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$\delta\hackscore{ij}$ is the Kronecker symbol \index{Kronecker symbol} defined by $\delta\hackscore{ij}=1$ for 
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$i=j$ and $0$ otherwise. 
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We want to implement a 
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new class which we will call \class{Helmholtz} that provides the same methods as the \LinearPDE class but 
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is described using the coefficients $\kappa$, $\omega$, $f$, $\eta$, 
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$g$ rather than the general form given by \eqn{EQU.FEM.1}. 
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Python's mechanism of subclasses allows us to do this in a very simple way. 
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The \Poisson class of the \linearPDEsPack module, 
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which we have already used in \Chap{FirstSteps}, is in fact a subclass of the more general 
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\LinearPDE class. That means that all methods (such as the \method{getSolution}) 
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from the parent class \LinearPDE are available for any \Poisson object. However, new 
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methods can be added and methods of the parent class can be redefined. In fact, 
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the \Poisson class redefines the \method{setValue} of the \LinearPDE class which is called to assign 
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values to the coefficients of the PDE. This is exactly what we will do when we define 
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our new \class{Helmholtz} class: 
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\begin{python} 
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from esys.linearPDEs import LinearPDE 
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import numarray 
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class Helmholtz(LinearPDE): 
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def setValue(self,kappa=0,omega=1,f=0,eta=0,g=0) 
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ndim=self.getDim() 
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kronecker=numarray.identity(ndim) 
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self._setValue(A=kappa*kronecker,D=omega,Y=f,d=eta,y=g) 
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\end{python} 
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\code{class Helmholtz(linearPDE)} declares the new \class{Helmholtz} class as a subclass 
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of \LinearPDE which we have imported in the first line of the script. 
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We add the method \method{setValue} to the class which overwrites the 
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\method{setValue} method of the \LinearPDE class. The new method which has the 
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parameters of the Helmholtz \eqn{DIFFUSION HELM EQ 1} as arguments 
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maps the parameters of the coefficients of the general PDE defined 
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in \eqn{EQU.FEM.1}. We are actually using the \method{_setValue} of 
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the \LinearPDE class (notice the leeding underscoure). 
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The coefficient \var{A} involves the Kronecker symbol. We use the 
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\numarray function \function{identity} which returns a square matrix with ones on the 
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main diagonal and zeros off the main diagonal. The argument of \function{identity} gives the order of the matrix. 
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The \method{getDim} of the \LinearPDE class object \var{self} to get the spatial dimensions of the domain of the 
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PDE. As we will make use of the \class{Helmholtz} class several times, it is convenient to 
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put its definition into a file which we name \file{mytools.py} available in the \ExampleDirectory. 
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You can use your favourite editor to create and edit the file. 
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An object of the \class{Helmholtz} class is created through the statments: 
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\begin{python} 
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from mytools import Helmholtz 
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mypde = Helmholtz(mydomain) 
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mypde.setValue(kappa=10.,omega=0.1,f=12) 
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u = mypde.getSolution() 
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\end{python} 
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In the first statement we import all definition from the \file{mytools.py} \index{scripts!\file{mytools.py}}. Make sure 
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that \file{mytools.py} is in the directory from where you started Python. 
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\var{mydomain} is the \Domain of the PDE which we have undefined here. In the third statment values are 
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assigned to the PDE parameters. As no values for arguments \var{eta} and \var{g} are 
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specified, the default values $0$ are used. \footnote{It would be better to use the default value 
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\var{escript.Data()} rather then $0$ as then the coefficient would be defined as being not present and 
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would not be processed when the PDE is evaluated}. In the fourth statement the solution of the 
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PDE is returned. 
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To test our \class{Helmholtz} class on a rectangular domain 
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of length $l\hackscore{0}=5$ and height $l\hackscore{1}=1$, we choose a simple test solution. Actually, we 
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we take $u=x\hackscore{0}$ and then calculate the right hand side terms $f$ and $g$ such that 
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the test solution becomes the solution of the problem. If we assume $\kappa$ as being constant, 
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an easy calculation shows that we have to choose $f=\omega \cdot x\hackscore{0}$. On the boundary we get 
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$\kappa n\hackscore{i} u\hackscore{,i}=\kappa n\hackscore{0}$. 
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So we have to set $g=\kappa n\hackscore{0}+\eta x\hackscore{0}$. The following script \file{helmholtztest.py} 
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\index{scripts!\file{helmholtztest.py}} which is available in the \ExampleDirectory 
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implements this test problem using the \finley PDE solver: 
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\begin{python} 
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from mytools import Helmholtz 
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from esys.escript import Lsup 
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from esys.finley import Rectangle 
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#... set some parameters ... 
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kappa=1. 
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omega=0.1 
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eta=10. 
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#... generate domain ... 
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mydomain = Rectangle(l0=5.,l1=1.,n0=50, n1=10) 
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#... open PDE and set coefficients ... 
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mypde=Helmholtz(mydomain) 
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n=mydomain.getNormal() 
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x=mydomain.getX() 
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mypde.setValue(kappa,omega,omega*x[0],eta,kappa*n[0]+eta*x[0]) 
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#... calculate error of the PDE solution ... 
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u=mypde.getSolution() 
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print "error is ",Lsup(ux[0]) 
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\end{python} 
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The script is similar to the script \file{mypoisson.py} dicussed in \Chap{FirstSteps}. 
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\code{mydomain.getNormal()} returns the outer normal field on the surface of the domain. The function \function{Lsup} 
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is imported by the \code{from esys.escript import Lsup} statement and returns the maximum absulute value of its argument. 
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The error shown by the print statement should be in the order of $10^{7}$. As piecewise bilinear interpolation is 
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used to approximate the solution and our solution is a linear function of the spatial coordinates one might 
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expect that the error would be zero or in the order of machine precision (typically $\approx 10^{15}$). 
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However most PDE packages use an iterative solver which is terminated 
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when a given tolerance has been reached. The default tolerance is $10^{8}$. This value can be altered by using the 
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\method{setTolerance} of the \LinearPDE class. 
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\section{The Transition Problem} 
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\label{DIFFUSION TRANS SEC} 
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Now we are ready to solve the original time dependent problem. The main 
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part of the script is the loop over time $t$ which takes the following form: 
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\begin{python} 
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t=0 
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T=Tref 
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mypde=Helmholtz(mydomain) 
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while t<t_end: 
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mypde.setValue(kappa,rhocp/h,q+rhocp/h*T,eta,eta*Tref) 
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T=mypde.getSolution() 
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t+=h 
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\end{python} 
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\var{kappa}, \var{rhocp}, \var{eta} and \var{Tref} are input parameters of the model. \var{q} is the heat source 
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in the domain and \var{h} is the time step size. Notice that the \class{Hemholtz} class object \var{mypde} 
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is created before the loop over time is entered while in each time step only the coefficients 
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are reset in each time step. This way some information about the representation of the PDE can be reused 
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\footnote{The efficience can be improved further by setting the coefficients in the operator 
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\var{kappa}, \var{omega} and \var{eta} before entering the \code{while}loop and only updating the coefficients 
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in the right hand side \var{f} and \var{g}. This needs a more careful implementation of the \method{setValue} 
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method but gives the advantage that the \LinearPDE class can save rebuilding the PDE operator.}. The variable \var{T} 
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holds the current temperature. It is used to calculate the right hand side coefficient \var{f} in the 
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Helmholtz equation in \eqn{DIFFUSION HELM EQ 1}. The statement \code{T=mypde.getSolution()} overwrites \var{T} with the 
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temperature of the new time step $\var{t}+\var{h}$. To get this iterative process going we need to specify the 
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initial temperature distribution, which equal to $T\hackscore{ref}$. 
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The heat source \var{q} which is defined in \eqn{DIFFUSION TEMP EQ 1b} is \var{qc} 
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in an area defined as a circle of radius \var{r} and center \var{xc} and zero outside this circle. 
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\var{q0} is a fixed constant. The following script defines \var{q} as desired: 
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\begin{python} 
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from esys.escript import length 
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xc=[0.02,0.002] 
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r=0.001 
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x=mydomain.getX() 
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q=q0*(length(xxc)r).whereNegative() 
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\end{python} 
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\var{x} is a \Data class object of 
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the \escript module defining locations in the \Domain \var{mydomain}. 
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The \function{length()} imported from the \escript module returns the 
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Euclidean norm: 
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\begin{equation}\label{DIFFUSION HELM EQ 3aba} 
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\y\=\sqrt{ 
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y\hackscore{i} 
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y\hackscore{i} 
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} = \function{esys.escript.length}(y) 
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\end{equation} 
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So \code{length(xxc)} calculates the distances 
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of the location \var{x} to the center of the circle \var{xc} where the heat source is acting. 
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Note that the coordinates of \var{xc} are defined as a list of floating point numbers. It is independently 
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converted into a \Data class object before being subtracted from \var{x}. The method \method{whereNegative} of 
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a \Data class object, in this case the result of the expression 
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\code{length(xxc)r}, returns a \Data class which is equal to one where the object is negative and 
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zero elsewhere. After multiplication with \var{qc} we get a function with the desired property. 
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Now we can put the components together to create the script \file{diffusion.py} which is available in the \ExampleDirectory: 
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\index{scripts!\file{diffusion.py}}: 
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\begin{python} 
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from mytools import Helmholtz 
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from esys.escript import Lsup 
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from esys.finley import Rectangle 
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#... set some parameters ... 
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xc=[0.02,0.002] 
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r=0.001 
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qc=50.e6 
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Tref=0. 
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rhocp=2.6e6 
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eta=75. 
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kappa=240. 
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tend=5. 
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# ... time, time step size and counter ... 
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t=0 
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h=0.1 
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i=0 
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#... generate domain ... 
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mydomain = Rectangle(l0=0.05,l1=0.01,n0=250, n1=50) 
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#... open PDE ... 
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mypde=Helmholtz(mydomain) 
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# ... set heat source: .... 
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x=mydomain.getX() 
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q=qc*(length(xxc)r).whereNegative() 
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# ... set initial temperature .... 
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T=Tref 
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# ... start iteration: 
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while t<tend: 
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i+=1 
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t+=h 
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print "time step :",t 
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mypde.setValue(kappa=kappa,omega=rhocp/h,f=q+rhocp/h*T,eta=eta,g=eta*Tref) 
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T=mypde.getSolution() 
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T.saveDX("T%d.dx"%i) 
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\end{python} 
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The script will create the files \file{T.1.dx}, 
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\file{T.2.dx}, $\ldots$, \file{T.50.dx} in the directory where the script has been started. The files give the 
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temperature distributions at time steps $1$, $2$, $\ldots$, $50$ in the \OpenDX file format. 
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\begin{figure} 
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\centerline{\includegraphics[width=\figwidth]{DiffusionRes1}} 
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\centerline{\includegraphics[width=\figwidth]{DiffusionRes16}} 
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\centerline{\includegraphics[width=\figwidth]{DiffusionRes32}} 
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\centerline{\includegraphics[width=\figwidth]{DiffusionRes48}} 
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\caption{Results of the Temperture Diffusion Problem for Time Steps $1$ $16$, $32$ and $48$.} 
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\label{DIFFUSION FIG 2} 
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\end{figure} 
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An easy way to visualize the results is the command 
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\begin{verbatim} 
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dx edit diffusion.net & 
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\end{verbatim} 
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where \file{diffusion.net} is an \OpenDX script available in the \ExampleDirectory. 
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Use the \texttt{Sequencer} to move forward and and backwards in time. 
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\fig{DIFFUSION FIG 2} shows the result for some selected time steps. 