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

[Jed Brown]

Ed Bueler <elbueler at alaska.edu> writes:
> Your question makes me think about why I am splitting the way I am.  For
> sure *yes*, in my case they are separate terms and *no* I am not just
> subtracting Lu from both sides.
>
> The ice sheet thickness problem (u(t,x,y) >= 0 is thickness) is like
> degenerate p-bratu, right?  My form is like
>
> u_t - div (u^k |grad u|^{p-2} grad u) = g(t,u)

Are you also interested in the case with sliding?

> where the right side is only the elevation-dependent surface mass balance
> in my context.  The right-hand stuff is (potentially) coupling to other
> models which will be time-split, and provided by other codes, so the
> "ultimate" coupled model will inevitably be imex in some vague sense.  The
> stiff stuff on the left should be handled implicitly both for imex and
> implicit, of course.  And I want to see the solver robustness effects of my
> imex splitting of the problem.  I also want to compare actual explicit runs
> despite their absurd short time step.
>
> In any case, whatever my motivation, I am currently providing
>
> RHSFunction = g(t,u)
> IFunction = u_t - div (u^k |grad u|^{p-2} grad u)
>
> Currently ARKIMEX, THETA, BDF all seem to work well, but explicit breaks.
> As I understand it, Barry's PR will "fix" this by erroring-out when I try
> to run it explicitly.  This is fine until ya'll work out a better API.

I would just have a PetscOptionsEList() for whether to evaluate the
diffusive term on the left or right.  Shouldn't be more than a few lines
of code.

>> What "fragile code"?
>
> I mean I fear the prospect of a PETSc API which requires so conditionals
> determining which "side"' various terms appear on, depending on user code
> testing what method is running etc..  No such thing for now; my current
> code is not fragile.  (Is an API based on "side" of an equation inherently
> suspect?)

I think it's a bit peculiar and would prefer a named basket of terms if
Barry insists on adding this interface.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 832 bytes
Desc: not available
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20170214/2ba5fbf9/attachment.pgp>