[petsc-users] Question on TS

[amlan barua]

Hi,
Thanks to Barry and Jed. I might come back later with few other questions.
Amlan


On Wed, Dec 26, 2012 at 11:02 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> On Dec 26, 2012, at 10:24 AM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>
> > On Wed, Dec 26, 2012 at 1:00 AM, amlan barua <abarua at iit.edu> wrote:
> > Hi,
> > Greetings to the team! I am currently using PETSc for my research. Here
> is a brief description of my problem and my query
> > a) I have a set a points distributed on a 3 dimensional lattice.
> > b) Corresponding to each point in this set, 7 odes are defined.
> > c) Of these 7 odes, 6 are uncoupled but one is coupled to nearest
> neighbors.
> >
> > I suggest not optimizing for "missing" coupling to start with. We can do
> the optimization in the solver, perhaps by splitting the DMDA into the
> local and coupled parts.
>
>    I agree with Jed here. Coincidently I am working on a similar problem
> but with thousands of ODEs (mostly decoupled).  You can use
> DMDASetBlockFills(), the ofill parameter to indicate exactly what fields
> are coupled to neighbors and which are not, this reduces the unneeded zero
> Jacobian entries (you can also use the dfill parameter to reduce unneeded
> zero entries in the 7 by 7 block). Eventually we'll use the same
> information to reduce the ghost point communication also.
>
>    Barry
>
> >
> > d) To integrate the odes I am using PETSc's DMDA and TS. But my
> application needs implicit as well as locally high order solver. I am
> looking for an implicit RK4 type method. Does PETSc have an IRK4 support or
> equivalent?
> >
> > If you are happy with a diagonally implicit method, you can use
> TSARKIMEX (these integrators can be IMEX, but can also do any diagonally
> implicit method).
> >
> > If you want a fully implicit RK (like Gauss, Radau IIA, etc) then all
> stages are coupled together. Those methods are not currently implemented in
> PETSc, though you could implement it either as a new TS implementation
> (good for code reuse; you can do this outside of PETSc, but the code you
> write is like library code) or manually using SNES (not reusable).
> >
> > e) Suppose I want to build my own implicit time stepper. Should I
> imitate ex2.c of SNES solver?
> > Thanks
> > Amlan
> > IISER Pune, India
> >
>
>
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