[petsc-users] projection methods in TS

[Zhang, Hong]

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.

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<mailto: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<mailto: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<mailto: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<mailto: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


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