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Hello forum.

I am solving the heat equation i 1d using FEM. It works fine for initial condition. However when I tried to run it for t>0 it does not work properly.

Here is my code, you can run it with this inputs and see

[Uh]=ParaFEM1D(1,0,1,10,7)

function [Uh, xnod]=ParaFEM1D(Tf,a, b, Nt, xnel)

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%% This function shows how to set up a finite element solution

%% of a two-point BVP

%% -(ku')' =f, u(a)=ua, u(b)=ub

%% using Galerkin linear Finite Elements

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%% function sol = solfem (a,b,xnel)

%% k,f,ua,ub are provided as functions in code

%% numerical integration is used in computation of stiffness matrix and rhs

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%% INPUT:

%% <a,b> = endpoints of interval

%% <xnel> = the number or location of elements

%% if xnel is a scalar, then xnel = number of elements,

%% if xnel is a vector, then it contains position of first node of each element

%% OUTPUT:

%% <sol> is the computed numerical solution at nodes

%% <xnod> position of all nodes

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

clf;

set(0,'DefaultLineLineWidth',3);

%%%%%%%%%%%%%%%%%%%%%%%%% set up grid

dt = Tf/Nt;

t = 0:dt:Tf;

if size(xnel) == 1 %% uniform grid

nels = xnel;

hel = (b-a)./nels;

xel = a : hel : b-hel;

elseif size(xnel,1) == 1

nels = size(xnel,2);

xel=xnel;

for i=1:nels-1

hel(i) = xnel(i+1)-xnel(i);

end;

else

error('wrong number of elements/grid');

end;

%% set up uniform order of elements = 1 + degree of polynomial =

%% = number of degrees of freedom

ord = zeros(nels,1) + 2; %% type of elements: linear

maxord = max(ord);

%% number of nodes

nnodes = sum(ord-1)+1;

%% derive global indexing of nodes:

%% nod(i,j) is the global number of j'th node in element i

nod = zeros(nels,maxord); myel = zeros(nnodes,2);

n = 1;

for i = 1:nels

for j = 1:ord(i)

nod(i,j) = n;

if j == 1

myel(n,2) = i;

elseif j == ord(i)

myel(n,1) = i;

else

myel(n,1) = i;

myel(n,2) = i;

end;

if j ~= ord(i)

n = n+1;

end

end;

end;

%% xnod (i=1..nnodes): coordinates of node i

xnod = zeros(nnodes,1);

for i=1:nels-1

h = xel(i+1)-xel(i);

hi = h/(ord(i)-1);

for j=1:ord(i)

xnod (nod(i,j)) = xel(i) + hi*(j-1);

end;

end;

i = nels;

h = b-xel(i);

hi=h/(ord(i)-1);

for j=1:ord(i)

xnod (nod(i,j)) = xel(i) + hi*(j-1);

end;

%%%%%%%%%%%%%%%%%%%%%%%%% set up numerical integration

%% set up quadrature parameters on the reference element (-1,1)

%% set up number of integration points nw, nodes xw, and weights w

if maxord == 1 %% exact for linears

nw = 1;

xw(1) = 0.;

w(1) = 2.;

elseif maxord == 2 %% exact for cubics

nw = 2;

xw(1) = -1/sqrt(3); xw(2) = -xw(1);

w(1) = 1; w(2) = 1;

elseif maxord == 3 %% exact for polynomials of degree 5

nw = 3;

xw(1) = -sqrt(3./5.); xw(2)=0.; xw(3) =- xw(1);

w(1) = 5./9.; w(2)=8./9.; w(3)=w(1);

end;

%%%%%%%%%%%%%%%%%%%%%%%%% matrix and rhs of linear system

b_global = sparse(nnodes,nnodes); m_global = sparse(nnodes,nnodes);

gs_global = sparse(nnodes,nnodes); rhsf = zeros(nnodes,1);

for el = 1:nels

x1 = xnod(nod(el,1)); %% left endpoint

x2 = xnod(nod(el,2)); %% right endpoint

dx = (x2-x1)/2.; %% Jacobian of transformation

%% compute element stiffness matrix and load vector

b_local = zeros(ord(el),ord(el)); %% element stiffness matrix

% gs_local = zeros(ord(el),ord(el));

m_local = zeros(ord(el),ord(el));

% gs_local = zeros(ord(el),ord(el)); %% GS = B+dt*A

f = zeros(ord(el),1); %% element load vector

%nw is the order of Gaussian Integration

for i = 1:nw

x = x1 + (1 + xw(i))*dx; %% x runs in true element,

%% xw runs in reference element

[psi,dpsi] = shape(xw(i),ord); %% calculations on ref.element

% kval = feval(@kfun,x);

% fval = feval(@rhsfun,0,x);

fval = feval(@exfun,0,x);

f = f + fval * psi * w(i)*dx;

b_local = b_local + (psi*psi')* w(i)*dx; %mass vector

m_local = m_local + w(i)*(dt*(dpsi*dpsi')/dx/dx) * dx;%stiffness matrix

% gs_local = gs_local + w(i)*(dt*(dpsi*dpsi')/dx/dx) * dx + (psi*psi')* w(i)*dx ;

end

%% add the computed element stiffness matrix and load vector to

%% the global matrix and vector

rhsf(nod(el,:)) = rhsf(nod(el,:)) + f(:); % F = load vector

b_global(nod(el,:),nod(el,:)) = b_global(nod(el,:),nod(el,:)) + b_local; % B = mass matrix

m_global(nod(el,:),nod(el,:)) = m_global(nod(el,:),nod(el,:)) + m_local; % A = stiffness matrix

% gs_global(nod(el,:),nod(el,:)) = gs_global(nod(el,:),nod(el,:)) + gs_local;

end

%% impose Dirichlet boundary conditions

xa = exfun(0,a); xb = exfun(0,b) ;

Uh = zeros(Nt,nnodes);

for i=1:nnodes

% rhsf(i) = rhsf(i) - m_global(i,1)*xa - m_global(i,nnodes)*xb ;

rhsf(i) = rhsf(i) - (dt*m_global(i,1)-b_global(i,1))*xa - ...

(dt*m_global(i,nnodes)-b_global(i,nnodes))*xb ;

m_global(i,1) =0; m_global(1,i)=0;

b_global(i,1) =0; b_global(1,i)=0;

m_global(i,nnodes)=0; m_global(nnodes,i)=0;

b_global(i,nnodes)=0; b_global(nnodes,i)=0;

end

m_global(1,1) = 1; b_global(1,1) = 1; rhsf(1,1) = xa;

m_global(nnodes,nnodes)=1;b_global(nnodes,nnodes)=1; rhsf(nnodes,1) = xb;

for n=1:Nt

fprintf('itetation %d of %d\n',n,Nt);

% k = t(n+1) - t(n); % time step;

if n == 1

Uh(n,:) = b_global\rhsf;

else

% Uh(ntime,:) = Uh(ntime-1,:)*(gs_global\(dt*b_global));

Uh(n,:) = (b_global+dt*m_global)\(b_global*Uh(n,:)'+dt*rhsf);

end

figure(1)

exfun(dt*(n-1),xnod)'

plot(xnod,exfun(dt*(n-1),xnod),'k',xnod,Uh(n,:),'r--');

pause(0.5),legend('Exact solution','Numerical solution (linear FE)');

end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%% end of algorithm

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

function [y,dy] = shape(x,n)

%% shape function on reference element (-1,1)

%% n = 2: linear

%% n = 3: quadratic (must be coded)

if n == 2

y (1,:) = .5.*(1-x);

y (2,:) = .5.*(1+x);

dy (1,:) = -.5;

dy (2,:) = .5;

end

function y = exfun(t, x)

y= sin(pi*x).*exp(-pi^2*t) + sin(2*pi*x).*exp(-4*pi^2*t);

% dy = pi*cos(pi*x).*exp(-pi^2*t) + 2*pi*cos(2*pi*x).*exp(-4*pi^2*t);

Please help

On 12/10/2012 3:49 PM, RICARDO wrote:

> Hello forum.

> I am solving the heat equation i 1d using FEM. It works fine for

>initial condition. However when I tried to run it for t>0 it does not work properly.

>

how about first fixing these issues which Matlab tells you about?

25: The value assigned to variable 't' might be unused.

31: Variable 'hel' might be set by a nonscalar operator.

31: Variable 'hel' might be set by a nonscalar operator.

40: The value assigned to variable 'hel' might be unused.

40: The variable 'hel' appears to change size on every loop

iteration. Consider preallocating for speed.

120: The value assigned to variable 'gs_global' might be unused.

and few more. You can see these from clicking on tools->code analyzer.

I am not saying these will fix or not fix, your logic errors, but

it won't hurt to fix these first.

I think you are asking here for someone to debug your FEM program?

I hope someone will have to do that. FEM is interesting subject.

If I were you, I'd make a print out of the code, and sit

and go over it carefully to make sure it is doing what

it is supposed to do. debugging is an important skill that needs

to be learned in order to become a good programmer.

--Nasser

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