[petsc-users] projection methods in TS

[Matthew Knepley]

On Sat, Feb 4, 2017 at 7:24 PM, Zhang, Hong <hongzhang at anl.gov> wrote:

> Can you elaborate a bit more on your problem?
>
> If your problem is an index-1 DAE, there is no need to use a projection
> method, and it is perfectly fine to set it up as a DAE in PETSc. For
> high-index DAEs, you may have to use TSSetPostStep() to implement your own
> projection algorithm.
>

Please define index.

   Matt


> If you happen to have a Hamiltonian system to solve, I have a symplectic
> solver in my own branch that you can use directly.
>
> Hong (Mr.)
>
> On Feb 4, 2017, at 9:47 AM, Gideon Simpson <gideon.simpson at gmail.com>
> wrote:
>
> Would setting it up as a DAE in petsc be algorithmically euivalent to a
> projected method (i.e., step of standard RK followed by nonlinear
> projection)?
>
> -gideon
>
> On Feb 3, 2017, at 11:47 PM, Matthew Knepley <knepley at gmail.com> wrote:
>
> That is one answer. Another one is that this particular system is a DAE
> and we have methods for that.
>
>    Matt
>
> On Fri, Feb 3, 2017 at 8:40 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>>
>> TSSetPostStep(); in your function use TSGetSolution() to get the current
>> solution.
>>
>>   Please let us know how it works out
>>
>>    Barry
>>
>>
>>
>> > On Feb 3, 2017, at 7:14 PM, Gideon Simpson <gideon.simpson at gmail.com>
>> wrote:
>> >
>> > I’m interested in implementing a projection method for an ODE of the
>> form:
>> >
>> > y’ = f(y),
>> >
>> > such that g(y) = 0 for all time (i.e., g is conserved).  Note that in a
>> projection method, a standard time step is made to produce y* from y_{n},
>> and then this is corrected to obtain y_{n+1} satisfying g(y) = 0.
>> >
>> > There were two ways I was thinking of doing this, and I was hoping to
>> get some input:
>> >
>> > Idea 1: Manually loop through using taking a time step and then
>> implementing the projection routine.  I see that there is a TSStep command,
>> but this doesn’t  seem to be much documentation on how to use it in this
>> scenario.  Does anyone have any guidance?
>> >
>> > Idea 2: Is there some analog to TSMonitor that allows me to modify the
>> solution after each time step, instead of just allowing for some
>> computation of a statistic?
>> >
>> >
>>
>>
>
>
> --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
>
>
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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