[petsc-users] convergence problem- 3D Cahn Hillard

[Jed Brown]

Matthew Knepley <knepley at gmail.com> writes:

> On Tue, Jan 5, 2021 at 9:52 PM Barry Smith <bsmith at petsc.dev> wrote:
>
>>
>> Ah, -snes_fd_color so it was already using finite differencing with
>> coloring to compute the Jacobian which explains why the differences below
>> are exactly zero.
>>
>> Implicit time-step schemes essentially add terms like I/dt  to the
>> Jacobian evaluation (and the function defining the ODE) so for tiny
>> time-steps the nonlinear system gets easier and easier to solve (the
>> nonlinear function becomes linear) But we didn't see that with your earlier
>> run where  dt 3.72529e-13 (which is absurdly small).  for tiny time-steps
>> SNES still made no progress. It is hard to understand how this is possible,
>> regardless of the problem you are solving.
>>
>> I would next run the code with valgrind to insure there are no issues of
>> memory corruption or un-initialized data.
>>
>> How are you computing
>>
>> (dp/dt)*(Pxx+Pyy+Pzz)
>>
>>
>> That is, how are you computing Pxx etc?
>>
>> Are you using finite elements for the U and P model? Exactly what elements?
>>
>
> I agree with Barry. This does not seem to make sense, so I would expect
> some kind of inconsistent discretization, or other
> mathematical problem which makes your system unsolvable.

Try -mat_fd_type ds before ruling out sensitivity to differencing parameter.