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

[Jed Brown]

Christophe Ortiz <christophe.ortiz at ciemat.es> writes:

>> Does a shorter time step fix the oscillations?
>
>
> Hi Jed,
>
> Thanks for the prompt reply.
>
> No, it did not in this case. However, I noticed for "smoother" cases that
> max timestep matters. I try to fix it to finaltime/100 to avoid too large
> timesteps. I also use -ts_adapt_basic_clip 0.1,1.1 to avoid large timesteps.

This just slows the rate of increasing time step.

>>  Is this with
>> -ts_arkimex_type 1bee or something else?
>>
>
> It occurs with 1bee, a2 or arkimex 3.

Stick with 1bee until we understand better.

>> Can you try -ts_arkimex_fully_implicit and add -snes_mf_operator if
>> necessary to get SNES to converge?
>
>
>
> I tried -ts_arkimex_fully_implicit and it gave a wrong result. As if there
> was no diffusion. Seems there is an artefact with fullyimplicit option.

That does not sound right.  Can you send a reproducible test case (best
as a patch to PETSc)?

> With -snes_mf_operator I got an error message:
> [0]PETSC ERROR: No support for this operation for this object type!
> [0]PETSC ERROR: Mat type mffd!

ALWAYS, ALWAYS, ALWAYS send the ENTIRE error message!

(Sorry about shouting, but I have to type this many times per day.
Truncating error messages kills kittens!)

>>  (I'm assuming you have used an IMEX
>> formulation here, but perhaps you already use fully implicit?)
>>
>
> I put everything under IFunction and IJacobian. This part is not clear to
> me actually. I understand that in IMEX methods, the LHS is solved
> implicitely and the RHS explicitely. What happens when I use an IMEX method
> with no RHS, everything in the LHS ? Is there any explicit stage ?

No, if the RHS is empty, it should reduce to an implicit method.

> Actually, I could solve the problem by adding mesh points. Since I start
> from steep gaussian distributions with large peak values, maybe there was a
> problem with large gradients.
>
> BTW, is it possible to have adaptive mesh in 1D with PETSc ? I am thinking
> of steep profiles that evolve and that require a fine mesh at the beginning.

You can do moving meshes (r-adaptivity) by writing coordinates as a
coupled system.  Or you can create a new grid and interpolate.  You can
use DMRefine and DMCoarsen.
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