[FLASH-USERS] split solver, timestepping/refinement and the time accuracy order

[Andrea Gatto]

Dear Klaus,

thank you for your answer.
Yes using an unsplit solver would definitively solve the problem (because
there isn't such problem anymore), but unfortunately I have to use the
split one.
Basically this timestep reduction is performed due to an enormous amount
of energy injected (Supernova) in the first call of the source terms. At
this point the only safe thing to do is to allow this injection of energy
only in the second call of the source terms.
Many thanks.

Best,

Andrea

> On Tue, 23 Apr 2013, Andrea Gatto wrote:
>
>> for stability reasons I find myself forced to reduce the timestep after
>> the first call (in the first half of the Strang splitting loop) of the
>> source terms when using a split solver in Driver_evolveFlash.F90 .
>> This means that the second call of the solver (in the second half of the
>> loop) is performed on a different (smaller) timestep.
>
> Hello Andrea,
>
> I am wondering why you want to use the split solver?  It seems that
> your questions would not even arise if you used the Unsplit variant of
> Driver_evolveFlash.F90 .
>
>> Could this cause the Strang splitting scheme accuracy order to be
>> reduced
>> from 2 to 1?
>
> Yes, I think so.
>
>> Are there any other problems with this timestep change?
>> Could this problem happen also when I force a refinement in a certain
>> region during the first call of the source terms?
>
> I think changing the Grid in the middle of one "split timestep" (which
> should just be two steps of the same time interval) is asking for trouble;
> again, why not go with the simpler Unsplit scheme?
>
> Klaus
>