[petsc-users] Suggestions for code with dof >> 1 ?

[Christophe Ortiz]

On Wed, Oct 16, 2013 at 4:22 PM, Christophe Ortiz <
christophe.ortiz at ciemat.es> wrote:

>
>
> On Wed, Oct 16, 2013 at 4:10 PM, Christophe Ortiz <
> christophe.ortiz at ciemat.es> wrote:
>
>>
>>
>> On Wed, Oct 16, 2013 at 3:36 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>>
>>> Christophe Ortiz <christophe.ortiz at ciemat.es> writes:
>>> > I found where is the problem. I had
>>> > set TSARKIMEXSetFullyImplicit(ts,PETSC_TRUE);
>>> > I found it produces wrong solutions, either with ARKIMEX3, 1BEE or A2.
>>> > Solution becomes negative. Without this option, all ARKIMEX types work
>>> very
>>> > well and very fast.
>>>
>>> I'm glad they work as IMEX methods (please check that the solutions are
>>> correct), but I would like to find out why they are not working in fully
>>> implicit mode.  Is your problem similar to ex25?
>>
>>
>> Yes. The only difference is that I put everything (diffusion and reaction
>> terms) under IFunction and IJacobian.
>>
>> My system is
>>
>> u_t - alpha.u_xx + k.u.v = 0
>> v_t - beta.v_xx + k.u.v = 0
>>
>> with Dirichlet boundary conditions
>> u(x=0,t) = u(x=L,t) = cste
>> v(x=0,t) = v(x=L,t) = cste
>>
>>  (If I remember
>>> correctly, that example uses a DAE formulation for boundary conditions
>>> and thus has trouble with some methods with explicit stages.  There may
>>> be more going on.)
>>>
>>
>> Now you say it, I'm having some troubles. After checking that it worked
>> very well with only diffusion terms with ARKIMEX, I introduced the reaction
>> term. With ARKIMEX 1BEE/A2, it goes very fast up to t=0.27 s, then, steps
>> start being rejected and then it goes very slowly, with small timesteps.
>> See output below.
>> Do you think it is due to the fact that I use IFunction to define
>> boundary conditions, which produces troubles with ARKIMEX methods ?
>> What would be the solution ?
>>
>> Christophe
>>
>>
> I just tried with TSARKIMEXSetFullyImplicit(ts,PETSC_TRUE); It goes much
> better and reaches final time....but solution is clearly not correct. The
> initial gaussian distributions do not change. Like if diffusion was null.
>

Hi,

I've tried something else. Since ARKIMEX is able to converge up to t~0.20s
and gives the correct solution for this time, and then comes into trouble,
I thought that it could come from the SNES.
Then, instead of using SNESNEWTONLS, I tried SNESNEWTONTR. I summarize my
findings:

- With NEWTONTR, ARKIMEX (3, 1BEE or A2) converges.

-With NEWTONTR, ARKIMEX 3 converges much faster than 1BEE or A2 (much less
timesteps).

- No conflicts between NEWTONTR and ARKIMEXFullyImplicit as with NEWTONLS.
Give the same results.


Then I came back to SNESNEWTONLS

- With NEWTONLS, ARKIMEX does not converges.

- When ARKIMEXFullyImplicit is used with NEWTONLS, it converges, but
solution clearly incorrect. It seems it does nothing, initial distributions
do not change.


And I played a bit with the tolerances:

- No influence of rtol (increasing or decreasing it). It stops at same
point.

- Changing stol from 1e-8 to 1e-7 seems to ease as it goes up to longer
time (6.98 s instead of 0.20) but then stops with an error:

[0]PETSC ERROR: --------------------- Error Message
------------------------------------
[0]PETSC ERROR: Floating point exception!
[0]PETSC ERROR: Vec entry at local location 450 is not-a-number or infinite
at end of function: Parameter number 3!
...
...
[0]PETSC ERROR: VecValidValues() line 30 in src/vec/vec/interface/rvector.c
[0]PETSC ERROR: SNESComputeFunction() line 1998 in src/snes/interface/snes.c
[0]PETSC ERROR: SNESSolve_NEWTONLS() line 162 in src/snes/impls/ls/ls.c
[0]PETSC ERROR: SNESSolve() line 3636 in src/snes/interface/snes.c
[0]PETSC ERROR: TSStep_ARKIMEX() line 765 in src/ts/impls/arkimex/arkimex.c
[0]PETSC ERROR: TSStep() line 2458 in src/ts/interface/ts.c
[0]PETSC ERROR: TSSolve() line 2583 in src/ts/interface/ts.c
[0]PETSC ERROR: main() line 392 in src/ts/examples/tutorials/diffusion.c

Maybe all this will give you a hint of what's going on...
Saludos,
Christophe
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