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1  % $Id$  % $Id$
2    \section{Elastic Deformation}
3  The \LinearPDE class is used to define a general linear, steady, second order PDE  \label{ELASTIC CHAP}
4  for an unknown function $u$ on a given $\Omega$ defined through a \Domain object.  In this section we want to discuss the deformation of a linear elastic body caused by expansion through a heat distribution. We want
5  In the following $\Gamma$ denotes the boundary of the domain $\Omega$. $n$ denotes  to displacement field $u\hackscore{i}$ which solves the momentum equation
6  the outer normal field on $\Gamma$.  \index{momentum equation}:
7    \begin{eqnarray}\label{BEAM general problem}
8  For a single PDE with a solution with a single component the linear PDE is defined in the   - \sigma\hackscore{ij,j}=0
9  following form:  \end{eqnarray}
10  \begin{equation}\label{LINEARPDE.SINGLE.1}  where the stress $\sigma$ is given by
11  -(A\hackscore{jl} u\hackscore{,l}){,j}+(B\hackscore{j} u)\hackscore{,j}+C\hackscore{l} u\hackscore{,l}+D u =-X\hackscore{j,j}+Y \; .  \begin{eqnarray}\label{BEAM linear elastic}
12  \end{equation}   \sigma\hackscore{ij}= \lambda u\hackscore{k,k} \delta\hackscore{ij} + \mu ( u\hackscore{i,j} + u\hackscore{j,i})
13  $u_{,j}$ denotes the derivative of $u$ with respect to the $j$-th spatial direction. Einstein's summation convention, ie. summation over indexes appearing twice in a term of a sum is performed, is used.   - (\lambda+\frac{2}{3} \mu)  \; \alpha  \;  (T-T\hackscore{ref})\delta\hackscore{ij} \;.
14  The coefficients $A$, $B$, $C$, $D$, $X$ and $Y$ have to be specified through \Data objects in the  \end{eqnarray}
15  \Function on the PDE or objects that can be converted into such \Data objects.  In this formula $\lambda$ and $\mu$ are the Lame coefficients, $\alpha$ is the
16  $A$ is a \RankTwo, $B$, $C$ and $X$ are \RankOne and $D$ and $Y$ are scalar.  temperature expansion coefficient, $T$ is the temperature distribution and $T_{ref}$ a reference temperature. Note that
17  The following natural  \eqn{BEAM general problem} is similar to eqn{WAVE general problem} introduced in section~\Sec{WAVE CHAP} but the
18  boundary conditions are considered \index{boundary condition!natural} on $\Gamma$:  inertia term $\rho u\hackscore{i,tt}$ has been dropped as we assume a static scenario here. Moreover, in
19  \begin{equation}\label{LINEARPDE.SINGLE.2}  comparison to the \eqn{WAVE stress}
20  n\hackscore{j}(A\hackscore{jl} u\hackscore{,l}+B\hackscore{j} u)+d u=n\hackscore{j}X\hackscore{j} + y  \;.  definition of stress $\sigma$ in \eqn{BEAM linear elastic} an extra term is introduced
21  \end{equation}  to bring in stress due to volume changes trough temperature dependent expansion.  
22  Notice that the coefficients $A$, $B$ and $X$ are defined in the PDE. The coefficients $d$ and $y$ are    
23  each a \Scalar in the \FunctionOnBoundary.  Constraints \index{constraint} for the solution prescribing the value of the  Our domain is the unit cube
24  solution at certain locations in the domain. They have the form  \begin{eqnarray} \label{BEAM natural}
25  \begin{equation}\label{LINEARPDE.SINGLE.3}  \Omega=\{(x\hackscore{i} | 0 \le x\hackscore{i} \le 1 \}
26  u=r \mbox{ where } q>0  \end{eqnarray}
27  \end{equation}  On the boundary the normal stress component is set to zero
28  $r$ and $q$ are each \Scalar where $q$ is the characteristic function  \begin{eqnarray} \label{BEAM natural}
29  \index{characteristic function} defining where the constraint is applied.  \sigma\hackscore{ij}n\hackscore{j}=0
30  The constraints defined by \eqn{LINEARPDE.SINGLE.3} override any other condition set by \eqn{LINEARPDE.SINGLE.1}  \end{eqnarray}
31  or \eqn{LINEARPDE.SINGLE.2}. The PDE is symmetrical \index{symmetrical} if  and on the face with $x\hackscore{i}=0$ we set the $i$-th component of the displacement to $0$
32  \begin{equation}\label{LINEARPDE.SINGLE.4}  \begin{eqnarray} \label{BEAM constraint}
33  A\hackscore{jl}=A\hackscore{lj} \mbox{ and } B\hackscore{j}=C\hackscore{j}  u\hackscore{i}(x)=0 & \mbox{ where } & x\hackscore{i}=0 \;
34  \end{equation}  \end{eqnarray}
35  For a system of PDEs and a solution with several components the PDE has the form  For the temperature distribution we use
36    \begin{eqnarray} \label{BEAM temperature}
37    T(x)= \frac{\beta}{\|x-x^{c}\|};
38    \end{eqnarray}
39    with a given positive constant $\beta$ and location $x^{c}$ in the domain\footnote{This selection of $T$ corresponds to
40    a temperature distribution in an indefinite domain created by a nodal heat source at $x^{c}$. Later in \Sec{X} we will calculate
41    the $T$ from an time-dependent temperature diffusion problem as discussed in \Sec{DIFFUSION CHAP}.}
43    When we insert~\eqn{BEAM linear elastic} we get a second oder system of linear PDEs for the displacements $u$ which is called
44    the Lame equation\index{Lame equation}. We want to solve
45    this using the \LinearPDE class to this. For a system of PDEs and a solution with several components the \LinearPDE class
46    takes PDEs of the form
47  \begin{equation}\label{LINEARPDE.SYSTEM.1}  \begin{equation}\label{LINEARPDE.SYSTEM.1}
48  -(A\hackscore{ijkl} u\hackscore{k,l}){,j}+(B\hackscore{ijk} u_k)\hackscore{,j}+C\hackscore{ikl} u\hackscore{k,l}+D\hackscore{ik} u_k =-X\hackscore{ij,j}+Y\hackscore{i} \; .  -(A\hackscore{ijkl} u\hackscore{k,l}){,j}+(B\hackscore{ijk} u\hackscore{k})\hackscore{,j}+C\hackscore{ikl} u\hackscore{k,l}+D\hackscore{ik} u\hackscore{k} =-X\hackscore{ij,j}+Y\hackscore{i} \; .
49  \end{equation}  \end{equation}
50  $A$ is a \RankFour, $B$ and $C$ are each a \RankThree, $D$ and $X$ are each a \RankTwo and $Y$ is a \RankOne.  $A$ is a \RankFour, $B$ and $C$ are each a \RankThree, $D$ and $X$ are each a \RankTwo and $Y$ is a \RankOne.
51  The natural boundary conditions \index{boundary condition!natural} take the form:  The natural boundary conditions \index{boundary condition!natural} take the form:
52  \begin{equation}\label{LINEARPDE.SYSTEM.2}  \begin{equation}\label{LINEARPDE.SYSTEM.2}
53  n\hackscore{j}(A\hackscore{ijkl} u\hackscore{k,l}){,j}+(B\hackscore{ijk} u_k)+d\hackscore{ik} u_k=n\hackscore{j}-X\hackscore{ij}+y\hackscore{i}  \;.  n\hackscore{j}(A\hackscore{ijkl} u\hackscore{k,l})\hackscore{,j}+(B\hackscore{ijk} u\hackscore{k})+d\hackscore{ik} u\hackscore{k}=n\hackscore{j}X\hackscore{ij}+y\hackscore{i}  \;.
54  \end{equation}  \end{equation}
55  The coefficient $d$ is a \RankTwo and $y$ is a    The coefficient $d$ is a \RankTwo and $y$ is a  
56  \RankOne both in the \FunctionOnBoundary. Constraints \index{constraint} take the form  \RankOne both in the \FunctionOnBoundary. Constraints \index{constraint} take the form
57  \begin{equation}\label{LINEARPDE.SYSTEM.3}  \begin{equation}\label{LINEARPDE.SYSTEM.3}
58  u\hackscore{i}=r\hackscore{i} \mbox{ where } q\hackscore{i}>0  u\hackscore{i}=r\hackscore{i} \mbox{ where } q\hackscore{i}>0
59  \end{equation}  \end{equation}
60  $r$ and $q$ are each \RankOne. Notice that at some locations not necessarily all components must  $r$ and $q$ are each \RankOne.
61  have a constraint. The system of PDEs is symmetrical \index{symmetrical} if  We can easily identify the coefficients in~\eqn{LINEARPDE.SYSTEM.1}:
62  \begin{eqnarray}\label{LINEARPDE.SYSTEM.4}  \begin{eqnarray}\label{LINEARPDE ELASTIC COEFFICIENTS}
63  A\hackscore{ijkl}=A\hackscore{klij} \\  A\hackscore{ijkl}=\lambda \delta\hackscore{ij} \delta\hackscore{kl} + \mu (
64    +\delta\hackscore{ik} \delta\hackscore{jl}
65    \delta\hackscore{il} \delta\hackscore{jk}) \\
66    X\hackscore{ij}=(\lambda+\frac{2}{3} \mu) \;  \alpha \; (T-T\hackscore{ref})\delta\hackscore{ij} \\
67    \end{eqnarray}
68    The characteristic function $q$ defining the locations and components where constraints are set is given by:
69    \begin{equation}\label{BEAM MASK}
70    q\hackscore{i}(x)=\left\{
71    \begin{array}{cl}
72    1  & x\hackscore{i}=0  \\
73    0  & \mbox{otherwise}   \\
74    \end{array}
75    \right.
76    \end{equation}
77    Under the assumption that $\lambda$, $\mu$, $\beta$ and $T\hackscore{ref}$
78    are constant setting $Y\hackscore{i}=\lambda+\frac{2}{3} \mu) \; \alpha \; T\hackscore{i}$ seems to be also possible. However,
79    this choice would lead to a different natural boundary condition which does not set the normal stress component as defined
80    in~\eqn{BEAM linear elastic} to zero.
82    Analogously to concept of symmetry for a single PDE, we call the PDE defined by~\eqn{LINEARPDE.SYSTEM.1} symmetric if
83    \index{symmetric PDE}
84    \begin{eqnarray}\label{LINEARPDE.SYSTEM.SYMMETRY}
85    A\hackscore{ijkl} =A\hackscore{klij} \\
86  B\hackscore{ijk}=C\hackscore{kij} \\  B\hackscore{ijk}=C\hackscore{kij} \\
87  D\hackscore{ik}=D\hackscore{ki} \\  D\hackscore{ik}=D\hackscore{ki} \\
88  d\hackscore{ik}=d\hackscore{ki} \  d\hackscore{ik}=d\hackscore{ki} \
89  \end{eqnarray}  \end{eqnarray}
90  \LinearPDE also supports solution discontinuities \index{discontinuity} over contact region $\Gamma^{contact}$  Note that different from the scalar case now the coefficients $D$ and $d$ have to be inspected. It is easy to see that
91  in the domain $\Omega$. To specify the conditions across the discontinuity we are using the  the Lame equation in fact is symmetric.
 generalised flux $J$ which is in the case of a systems of PDEs and several components of the solution  
 defined as  
 For the case of single solution component and single PDE $J$ is defined  
 In the context of discontinuities \index{discontinuity} $n$ denotes the normal on the  
 discontinuity pointing from side 0 towards side 1. For a system of PDEs  
 the contact condition takes the form  
 n\hackscore{j} J^{0}\hackscore{ij}=n\hackscore{j} J^{1}\hackscore{ij}=y^{contact}\hackscore{i} - d^{contact}\hackscore{ik} [u]\hackscore{k} \; .  
 where $J^{0}$ and $J^{1}$ are the fluxes on side $0$ and side $1$ of the  
 discontinuity $\Gamma^{contact}$, respectively. $[u]$, which is the difference  
 of the solution at side 1 and at side 0, denotes the jump of $u$ across $\Gamma^{contact}$.  
 The coefficient $d^{contact}$ is a \RankTwo and $y^{contact}$ is a    
 \RankOne both in the \FunctionOnContactZero or \FunctionOnContactOne.  
 In case of a single PDE and a single component solution the contact condition takes the form  
 n\hackscore{j} J^{0}\hackscore{j}=n\hackscore{j} J^{1}\hackscore{j}=y^{contact} - d^{contact}[u]  
 In this case the the coefficient $d^{contact}$ and $y^{contact}$ are eaach \Scalar  
 both in the \FunctionOnContactZero or \FunctionOnContactOne.  
 We have used a special case of the \LinearPDE class, namely the  
 \Poisson class already in \Chap{FirstSteps}.  
 Here we will write our own specialized sub-class of the \LinearPDE to define the Helmholtz equation  
 and use the \method{getSolution} method of parent class to actually solve the problem.  
 The form of a single PDE that can be handled by the \LinearPDE class is  
 -(A\hackscore{jl} u\hackscore{,l})\hackscore{,j}+D u =Y \; .  
 We show here the terms which are relevant for the Helmholtz problem.  
 The general form and systems is discussed in \Sec{SEC LinearPDE}.    
 $A$, $D$ and $Y$ are the known coeffecients of the PDE. \index{partial differential equation!coefficients}  
 Notice that $A$ is a matrix or tensor of order 2 and $D$ and $Y$ are scalar.  
 They may be constant or may depend on their  
 location in the domain but must not depend on the unknown solution $u$.  
 The following natural boundary conditions \index{boundary condition!natural} that  
 are used in the \LinearPDE class have the form  
 n\hackscore{j}A\hackscore{jl} u\hackscore{,l}+du=y  \;.  
 where, as usual, $n$ denotes the outer normal field on the surface of the domain. Notice that  
 the coefficient $A$ is already used in the PDE in \eqn{EQU.FEM.1}. $d$ and $y$ are given scalar coefficients.  
 By inspecting the Helmholtz equation \index{Helmholtz equation}  
 we can easily assign values to the coefficients in the  
 general PDE of the \LinearPDE class:  
 \begin{equation}\label{DIFFUSION HELM EQ 3}  
 A\hackscore{ij}=\kappa \delta\hackscore{ij} & D=\omega & Y=f \\  
 d=\eta & y= g &  \\  
 $\delta\hackscore{ij}$ is the Kronecker symbol \index{Kronecker symbol} defined by $\delta\hackscore{ij}=1$ for  
 $i=j$ and $0$ otherwise.  
 We want to implement a  
 new class which we will call \class{Helmholtz} that provides the same methods as the \LinearPDE class but  
 is described using the coefficients $\kappa$, $\omega$, $f$, $\eta$,  
 $g$ rather than the general form given by \eqn{EQU.FEM.1}.  
 Python's mechanism of subclasses allows us to do this in a very simple way.  
 The \Poisson class of the \linearPDEs module,  
 which we have already used in \Chap{FirstSteps}, is in fact a subclass of the more general  
 \LinearPDE class. That means that all methods (such as the \method{getSolution})  
 from the parent class \LinearPDE are available for any \Poisson object. However, new  
 methods can be added and methods of the parent class can be redefined. In fact,  
 the \Poisson class redefines the \method{setValue} of the \LinearPDE class which is called to assign  
 values to the coefficients of the PDE. This is exactly what we will do when we define  
 our new \class{Helmholtz} class:  
 from esys.linearPDEs import LinearPDE  
 import numarray  
 class Helmholtz(LinearPDE):  
    def setValue(self,kappa=0,omega=1,f=0,eta=0,g=0)  
 \code{class Helmholtz(linearPDE)} declares the new \class{Helmholtz} class as a subclass  
 of \LinearPDE which we have imported in the first line of the script.  
 We add the method \method{setValue} to the class which overwrites the  
 \method{setValue} method of the \LinearPDE class. The new method which has the  
 parameters of the Helmholtz \eqn{DIFFUSION HELM EQ 1} as arguments  
 maps the parameters of the coefficients of the general PDE defined  
 in \eqn{EQU.FEM.1}. We are actually using the \method{_LinearPDE__setValue} of  
 the \LinearPDE class.  
 The coefficient \var{A} involves the Kronecker symbol. We use the  
 \numarray function \function{identity} which returns a square matrix with ones on the  
 main diagonal and zeros off the main diagonal. The argument of \function{identity} gives the order of the matrix.  
 The \method{getDim} of the \LinearPDE class object \var{self} to get the spatial dimensions of the domain of the  
 PDE. As we will make use of the \class{Helmholtz} class several times, it is convenient to  
 put its definition into a file which we name \file{mytools.py} available in the \ExampleDirectory.  
 You can use your favourite editor to create and edit the file.    
 An object of the \class{Helmholtz} class is created through the statments:  
 from mytools import Helmholtz  
 mypde = Helmholtz(mydomain)  
 u = mypde.getSolution()  
 In the first statement we import all definition from the \file{mytools.py} \index{scripts!\file{mytools.py}}. Make sure  
 that \file{mytools.py} is in the directory from where you started Python.  
 \var{mydomain} is the \Domain of the PDE which we have undefined here. In the third statment values are  
 assigned to the PDE parameters. As no values for arguments \var{eta} and \var{g} are  
 specified, the default values $0$ are used. \footnote{It would be better to use the default value  
 \var{escript.Data()} rather then $0$ as then the coefficient would be defined as being not present and  
 would not be processed when the PDE is evaluated}. In the fourth statement the solution of the  
 PDE is returned.  
 To test our \class{Helmholtz} class on a rectangular domain  
 of length $l\hackscore{0}=5$ and height $l\hackscore{1}=1$, we choose a simple test solution. Actually, we  
 we take $u=x\hackscore{0}$ and then calculate the right hand side terms $f$ and $g$ such that  
 the test solution becomes the solution of the problem. If we assume $\kappa$ as being constant,  
 an easy calculation shows that we have to choose $f=\omega \cdot x\hackscore{0}$. On the boundary we get  
 $\kappa n\hackscore{i} u\hackscore{,i}=\kappa n\hackscore{0}$.    
 So we have to set $g=\kappa n\hackscore{0}+\eta x\hackscore{0}$. The following script \file{helmholtztest.py}  
 \index{scripts!\file{helmholtztest.py}} which is available in the \ExampleDirectory  
 implements this test problem using the \finley PDE solver:  
 from mytools import Helmholtz  
 from esys.escript import Lsup  
 from esys.finley import Rectangle  
 #... set some parameters ...  
 #... generate domain ...  
 mydomain = Rectangle(l0=5.,l1=1.,n0=50, n1=10)  
 #... open PDE and set coefficients ...  
 #... calculate error of the PDE solution ...  
 print "error is ",Lsup(u-x[0])  
 The script is similar to the script \file{mypoisson.py} dicussed in \Chap{FirstSteps}.  
 \code{mydomain.getNormal()} returns the outer normal field on the surface of the domain. The function \function{Lsup}  
 is imported by the \code{from esys.escript import Lsup} statement and returns the maximum absulute value of its argument.  
 The error shown by the print statement should be in the order of $10^{-7}$. As piecewise bi-linear interpolation is  
 used to approximate the solution and our solution is a linear function of the spatial coordinates one might  
 expect that the error would be zero or in the order of machine precision (typically $\approx 10^{-15}$).  
 However most PDE packages use an iterative solver which is terminated  
 when a given tolerance has been reached. The default tolerance is $10^{-8}$. This value can be altered by using the  
 \method{setTolerance} of the \LinearPDE class.  
 \subsection{The Transition Problem}  
 Now we are ready to solve the original time dependent problem. The main  
 part of the script is the loop over time $t$ which takes the following form:  
 while t<t_end:  
 \var{kappa}, \var{rhocp}, \var{eta} and \var{Tref} are input parameters of the model. \var{q} is the heat source  
 in the domain and \var{h} is the time step size. Notice that the \class{Hemholtz} class object \var{mypde}  
 is created before the loop over time is entered while in each time step only the coefficients  
 are reset in each time step. This way some information about the representation of the PDE can be reused  
 \footnote{The efficience can be improved further by setting the coefficients in the operator  
 \var{kappa}, \var{omega} and \var{eta} before entering the \code{while}-loop and only updating the coefficients  
 in the right hand side \var{f} and \var{g}. This needs a more careful implementation of the \method{setValue}  
 method but gives the advantage that the \LinearPDE class can save rebuilding the PDE operator.}. The variable \var{T}  
 holds the current temperature. It is used to calculate the right hand side coefficient \var{f} in the  
 Helmholtz equation in \eqn{DIFFUSION HELM EQ 1}. The statement \code{T=mypde.getSolution()} overwrites \var{T} with the  
 temperature of the new time step $\var{t}+\var{h}$. To get this iterative process going we need to specify the  
 initial temperature distribution, which equal to $T\hackscore{ref}$.  
 The heat source \var{q} which is defined in \eqn{DIFFUSION TEMP EQ 1b} is \var{qc}  
 in an area defined as a circle of radius \var{r} and center \var{xc} and zero outside this circle.  
 \var{q0} is a fixed constant. The following script defines \var{q} as desired:    
 from esys.escript import length  
 \var{x} is a \Data class object of  
 the \escript module defining locations in the \Domain \var{mydomain}.  
 The \function{length()} imported from the \escript module returns the  
 Euclidean norm:  
 \begin{equation}\label{DIFFUSION HELM EQ 3aba}  
 } = \function{esys.escript.length}(y)  
 So \code{length(x-xc)} calculates the distances    
 of the location \var{x} to the center of the circle \var{xc} where the heat source is acting.  
 Note that the coordinates of \var{xc} are defined as a list of floating point numbers. It is independently  
 converted into a \Data class object before being subtracted from \var{x}. The method \method{whereNegative} of  
 a \Data class object, in this case the result of the expression  
 \code{length(x-xc)-r}, returns a \Data class which is equal to one where the object is negative and  
 zero elsewhere. After multiplication with \var{qc} we get a function with the desired property.  
 Now we can put the components together to create the script \file{diffusion.py} which is available in the \ExampleDirectory:  
 from mytools import Helmholtz  
 from esys.escript import Lsup  
 from esys.finley import Rectangle  
 #... set some parameters ...  
 # ... time, time step size and counter ...  
 #... generate domain ...  
 mydomain = Rectangle(l0=0.05,l1=0.01,n0=250, n1=50)  
 #... open PDE ...  
 # ... set heat source: ....  
 # ... set initial temperature ....  
 # ... start iteration:  
 while t<tend:  
       print "time step :",t  
 The script will create the files \file{T.1.dx},  
  \file{T.2.dx}, $\ldots$, \file{T.50.dx} in the directory where the script has been started. The files give the  
 temperature distributions at time steps $1$, $2$, $\ldots$, $50$ in the \OpenDX file format.  
 \caption{Results of the Temperture Diffusion Problem for Time Steps $1$ $16$, $32$ and $48$.}  
 \label{DIFFUSION FIG 2}  
 An easy way to visualize the results is the command  
 dx -edit diffusion.net &  
 where \file{diffusion.net} is an \OpenDX script available in the \ExampleDirectory.  
 Use the \texttt{Sequencer} to move forward and and backwards in time.  
 \fig{DIFFUSION FIG 2} shows the result for some selected time steps.  
 \section{Bending Beam under Uniform Load}  
 \label{BEAM CHAP}  
 \sectionauthor{Jannine Eisenmann}{}  
 Given is a two-dimension bending beam which is fixed in all directions  
 at the left end and free at the other, see \fig{BEAM FIG 1}. Furthermore the beam is loaded  
 with a uniform load $p$.  
 % \centerline{\includegraphics[width=\figwidth]{DiffusionRes1}}  
 \caption{Loaded Beam.}  
 \label{BEAM FIG 1}  
 For emphasizing the displacement this picture is drawn (uebertrieben),  
 cause since we use the linear theory otherwise no displacements would  
 be visible.  
 There are two ways of solving this problem: on the one hand one can  
 use the differential equation for the deformed neutral fibre of the  
 beam. This classical differential equation is based on several simplifying  
 assumptions concerning the statics and kinematics of the problem.  
 However the results are known to be highly accurate at least for slender  
 beams with length to hight ratios $> 10$. Alternatively, in connection  
 with finite element based differential equation toolkits one may simply  
 consider the beam as a special case of a 2D or 3D elastic continuum  
 problem and solve the stress equilibrium equations combined with Hooke's  
 law for the specific boundary conditions of a beam. Both cases assume  
 isotropic and linear elastic material properties.  
 The beam equation is easily solved analytically. The analytical solutions  
 can be used for benchmarkung finite element solutions. In section  
 1.2 we formluate a finite element code for general isotropic elasticity  
 problems including thin and deep beams under arbitrary loading conditiond  
 in 2D or 3D.  
 As the stress equilibrium equation is a partial differential equation  
 (PDE), we choose to use Finley to solve it, since Finley is a finite  
 element kernel library, that has been incorporated as a differential  
 equation solver into the python based numerical toolkit called escript.  
 We divided the beam into ten square elements of the size 1x1. Each  
 element consists of 8 nodes, which leads to a quadratic interpolation  
 of the model point displacements \\  
 The key ingredients is \textbf{Hooks Law}. We use Hooks Law because  
 we are dealing with \textbf{linear elastic material} \textbf{behaviour}.  
 We have \\  
 $\sigma_{ik}=2G\left(\varepsilon_{ik}+\frac{\nu}{1-2\nu}\cdot e\cdot\delta_{ik}\right)$\hfill{}(1)\\  
 where the engineering strain$\varepsilon_{ik}$is defined as:  
 \item e= Volume strain = $\varepsilon_{kk}$  
 \item $\delta_{ik}$= Kronecker symbol  
 Inserting equation (2) in (1) (and with further mathematical conversions)  
 leads to the following partial differential equation :\\  
 For tension equilibrium we require:\\  
 which leads to the following expression:\\  
 with the natural boundary condition:  
 $p_{i}$ representing a uniform load on top of the beam.  
 A Dirichlet Boundary condition is assumed on the left end of the beam  
 for which no displacements can occure.\\  
   width=0.60\linewidth,bb = 0 0 200 100, draft, type=eps]{/home/jeannine/sandbox/report/draws/dir_cond_beam.eps}\\  
 This is described in the code with the setting a mask for the left  
 end and setting values to that mask:  
 q = xNodes{[}0{]}.whereZero(){*}{[}1.0,1.0{]}  
 r = Vector({[}0.0, 0.0{]}, where = nodes)  
 The Finley template PDE reads:\\  
 with the natural boundary condition:  
 Yields by comparing the coefficients :  
 \item $A_{ijkl}$= $\left[\mu\left(\delta_{ik}\delta_{ij}+\delta_{jl}\delta_{il}\right)+\lambda\left(\delta_{ij}\delta_{kl}\right)\right]$  
 \item $y_{i}$= $-p_{i}$  
 \item $u_{k}$= displacement $u$  
 Where 0 in the last line is taken as a scalar, vector or tensor, depending  
 on what the belonging coefficient is.  
 These equations are the base for the \textbf{Finley Script}:  
 from ESyS import {*}  
 import Finley  
 \#mu lamda  
 def mu (E, nu): \#= shear modul G  
      return E/(2{*}(1+nu))  
 def lamda (E, nu):  
      return (nu{*}E)/((1-2{*}nu){*}(1+nu))  
 def main()  
      \#material, beam PROPERTIES  
      L = 10.0         \#length of beam {[}m{]}  
      h = 1            \#height of beam {[}m{]}  
      p = 1            \#outer uniform load {[}kN/m{]}  
      E0 = 210000      \#Young's modulus {[}kN/m\textasciicircum{}2{]}  
      nu = 0.4         \#Poisson ratio {[}-{]}  
      print \char`\"{}L=\char`\"{}, L  
      print \char`\"{}h=\char`\"{}, h  
      print \char`\"{}p=\char`\"{}, p  
      print \char`\"{}E=\char`\"{}, E0  
      print \char`\"{}nu=Poisson =\char`\"{}, nu  
      print \char`\"{}mu=\char`\"{}, mu (E0,nu)  
      print \char`\"{}lamda=\char`\"{}, lamda (E0,nu)  
      \#SET MESH for FE  
      mesh= Finley.Rectangle(n0=10 ,n1=1 ,order=2, l0=L, l1=h)  
      nodes = mesh.Nodes()  
      xNodes = nodes.getX()  
      elements = mesh.Elements()  
      faceElements = mesh.FaceElements()  
      xFaceElements = faceElements.getX()  
      \#DISPLACEMENT boundary  
      q = xNodes{[}0{]}.whereZero(){*}{[}1.,1.0{]}    \#setting the                                           mask for r  
      r = Vector({[}0.0, 0.0{]}, where = nodes) \#fixed end  
      \#STRESS boundary  
      ymask = xFaceElements{[}1{]}.whereEqualTo(h)  
      y = Vector({[}0, -p{]}, where = faceElements)  
      y = y{*}ymask  
      \#Finley coeff.  
      A = Tensor4(0, where = elements)  
      for i in range (2) :  
         for j in range (2) :  
            A{[}i,i,j,j{]}+= lamda (E0,nu)  
            A{[}j,i,j,i{]}+= mu (E0,nu)  
            A{[}j,i,i,j{]}+= mu (E0,nu)  
      M,b = mesh.assemble(A=A, B=B, q=q,          
      u= M.iterative(b, tolerance=1e-8,iter\_max=50000,  
      print \char`\"{}u{[}0{]}=\char`\"{},u{[}0{]}  
      print \char`\"{}u{[}1{]}=\char`\"{},u{[}1{]}  
 The finer the mesh the more exact is the solution. E.g. a 20x2 mesh  
 is more exact than a 10x1 mesh.  

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