COMSOL Multiphysics™

femlin
nullfun and symmetric can be useful in finding out the result of the automatic
null function or the automatic symmetric mechanisms.
Example
The L-Shaped Membrane with Three Subdomains
Take a look at the geometry of the L-shaped membrane for examples of what you
can do. First create the L-shaped membrane and examine the subdomain labels and
edge segment labels by plotting:
clear fem
sq1 = square2(0,0,1);
sq2 = move(sq1,0,-1);
sq3 = move(sq1,-1,-1);
fem.geom = sq1+sq2+sq3;
fem.mesh = meshinit(fem);
geomplot(fem,'edgelabel','on','sublabel','on')
Say you want to use c = 1, 1/2, and 1/3 and f = x, y, and x 2 +1 in subdomains 1, 2,
and 3, respectively. Use Dirichlet boundary conditions on the outer boundaries:
fem.shape = 2;
fem.equ.c = {1 1/2 1/3};
fem.equ.f = {'x' 'y' 'x^2+1'};
fem.bnd.h = 1;
fem.xmesh = meshextend(fem);
fem.sol = femlin(fem);
postsurf(fem,'u')
You can set fem.bnd.h = 1 because fem.border has not been set (and defaults to
off). When fem.border is set to on you must type fem.bnd.h = {1 1 1 0 1 1
0 1 1 1}. Otherwise you get u=0 also on interior boundaries.
Using anisotropic c =
2 x + y
and f=1 in all subdomains can be done by
x + y 10
typing
fem.equ.c = {{{2 'x+y' 10}}};
fem.equ.f = 1;
fem.xmesh = meshextend(fem);
fem.sol = femlin(fem);
postsurf(fem,'u')
Cautionary
When using the general form it is assumed that the coefficients c, α, β, a, q, h have
been computed using fem=femdiff(fem). In the user interface, this is done
automatically.
Compatibility The property Variables has been renamed to Const in FEMLAB 2.3.
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