Re: [ff3d-users] Newbie question

[Stephane Del Pino]

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Le mercredi 21 Juillet 2004 14:33, Luigi Bardelli a écrit :
> Hi Stephane,
Hi!

>   thank you for the quick reply! I'm not understanding why the various
> expressions do not work when A is a PDE variable like for example
>
> solve(v1,v2) in MESH{
>   pde(v1)
>     v1*dx(dx(v2))=0
>   pde(v2)
>     .....
> }
> it says:
>    syntax error, unexpected '*', expecting '=' or '-' or '+': "*"
> unexpected
>
> Why? Thank you
>    Luigi
Their are two problems here:
1) As I told you, you cannot write
        f*dx(dx(u))
because of the Green formula. You *must* use
        dx(f*dx(u))
if you are more familiar with variational formulations, you can use the syntax
        solve(u) in MESH{
           test(v)
             int(f*dx(u)*dx(v))=0
           // ...
        }
for witch no order is required. (v is the test function). This is documented 
(section 4.5) and since you have 2 versions ;-)

2) You are trying to solve *directly* a non linear problem!
ff3d does only solve linear problems, so you have to write the nonlinear 
algorithm within ff3d's command file. For instance:
        function v0 = initial_value;
        do {
          solve(u) in MESH {
            pde(u)
                dv(v0*dx(u)) = 0
            // ...
          }
        v0 = u;
        }
Of course, you can use more clever algorithms...

I hope this helps,
Stephane.

PS: when solving vectorial problems, the assignment of equations to "pde(.)" 
statements is essential (see section 4.5 also).

PS2: I committed the change concerning the call of dvips.
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