[petsc-users] TS question 1: how to stop explicit methods because they do not use SNES(VI)?

[Ed Bueler]

Barry --

>   Sounds like a bug to me. The methods should be checking if an
> IFunction is being provided and error out in that case.

I don't think it is that simple.  I.e. having an IFunction and an explicit
scheme is not, by itself, a bug.  I think.

If the user has provided IFunction
   F(t,u,u_t) = u_t - f(t,u)
which is the convenient form for e.g. TSARKIMEX,
and RHSFunction
  G(t,u)
then I guess I assumed that explicit methods like TSRK should,for
unconstrained cases, get their RHS by callback like this:
  g(t,u) = - F(t,u,0) + G(t,u)
so that the ODE is in form
  u_t = g(t,u) = f(t,u) + g(t,u)

I think that is the behavior I am seeing (i.e. on my problem, using PETSc
master).

The "nonsense" I am referring to is only from the non-enforcement of the
constraint; it would be o.k. for a pure ODE.

I would love to have some projection mechanism to try, for comparing
explicit methods with projection to the SNESVI way (i.e. the right way),
but that is asking for a lot of PETSc refactoring, I think.  For now I just
want to error-out if the method is not going to call the SNES at each time
step.

Ed



On Mon, Feb 13, 2017 at 11:26 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> > On Feb 13, 2017, at 1:53 PM, Ed Bueler <elbueler at alaska.edu> wrote:
> >
> > Dear Petsc --
> >
> > This is the first of two short TS usage questions.
> >
> > My problem is both stiff (diffusive PDE) and constrained, so I require
> >
> >    -snes_type vinewton{rs|ss}ls
> >
> > *and* I split my ODE system into IFunction and RHSFunction
> >
> >    F(t,u,u_t) = G(t,u)
> >
> > where F(t,u,u_t) = u_t + f(t,u) in my case (i.e. no mass matrix needed),
> and the stiff part goes in f(t,u).
> >
> > With this arrangement TS types beuler, theta, bdf, arkimex all work
> quite well.  However, the program runs and produces nonsense with type rk
> and ssp, that is, explicit methods.
>
>    Sounds like a bug to me. The methods should be checking if an IFunction
> is being provided and error out in that case.
>
>   Barry
>
> >
> > So my question is, how do I ask the TS (at run time) whether the chosen
> TS type will or will not call its SNES at each step?  If SNES is not going
> to be used then I want to SETERRQ and stop.  That is, I want to error-out
> if the *method* is fully explicit.
> >
> > Note the constraints are enforced by the SNESVI, as they should be, not
> ad hoc projection.  Also, as a technical matter, I cannot require my
> iterates to be feasible inside my IFunction evaluation because that would
> break VINEWTONSSLS.
> >
> > Neither TSProblemType (mine is NONLINEAR) nor TSEquationType (mine is
> IMPLICIT I guess) seem to address this?  My problem is indeed nonlinear and
> has stiff parts, but it is not a DAE and it *is* constrained.
> >
> > Thanks!
> >
> > Ed
> >
> > PS  I'd prefer not to enumerate the existing TS types and error on the
> bad ones.  It is not nicely-maintainable.
> >
> >
> >
> > --
> > Ed Bueler
> > Dept of Math and Stat and Geophysical Institute
> > University of Alaska Fairbanks
> > Fairbanks, AK 99775-6660
> > 301C Chapman and 410D Elvey
> > 907 474-7693 and 907 474-7199  (fax 907 474-5394)
>
>


-- 
Ed Bueler
Dept of Math and Stat and Geophysical Institute
University of Alaska Fairbanks
Fairbanks, AK 99775-6660
301C Chapman and 410D Elvey
907 474-7693 and 907 474-7199  (fax 907 474-5394)
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