# 2nd resolution Wave_t_ex

Christophe Geuzaine Christophe.Geuzaine at ulg.ac.be
Tue Jun 5 16:49:24 CEST 2001

Lin Ji wrote:
>
> Hi, Christophe,
>     The 2nd resolution now worked for the wave equation example you send us.
> But, it still has problem with our wave equation. The solution value I get is
> all zeros. As far as I can remember, I changed the source function in

Yes (indeed, the solution is not zero, but scaled from the correct
solution by a very big factor). I explained the problem in my last
e-mail:

> A _big_ warning about the second resolution scheme: if you plan to use
> it, you have to explicitly decouple the constraint (here weakly imposed
> on \partial_n u) into space and time parts. The reason is the following.
> The system of equations gets only assembled once at the first time step,
> with the constraint taken as it is for this time step (in our case, this
> means we consider dfdt[] at time t==t0). During the computation, the
> only treatment that is applied is the modulation of the constraint by
> the function given as the argument of the 'Update[]' command (here:
> TimeFct[]). What you really impose is thus 'dfdt[t==t0]*TimeFct[]'. With
> your definition (TimeFct[]==dfdt[]), this is obviously not correct
> (which causes the huge difference in the values obtained by the first
> and second resolution). You should thus express TimeFct[] and dfdt[] in
> consequence.

--
Christophe Geuzaine

Tel: 32 (0) 4 366 37 10    http://geuz.org
Fax: 32 (0) 4 366 29 10    mailto:Christophe.Geuzaine at ulg.ac.be