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<div class="">On Feb 4, 2017, at 7:35 PM, Matthew Knepley <<a href="mailto:knepley@gmail.com" class="">knepley@gmail.com</a>> wrote:</div>
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<div class="gmail_quote">On Sat, Feb 4, 2017 at 7:24 PM, Zhang, Hong <span dir="ltr" class="">
<<a href="mailto:hongzhang@anl.gov" target="_blank" class="">hongzhang@anl.gov</a>></span> wrote:<br class="">
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<div style="word-wrap:break-word" class="">Can you elaborate a bit more on your problem?
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<div class="">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.</div>
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<div class="">Please define index.</div>
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<div><a href="http://www.scholarpedia.org/article/Differential-algebraic_equations#Index" class="">http://www.scholarpedia.org/article/Differential-algebraic_equations#Index</a></div>
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<div>Hong </div>
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<div class=""> Matt</div>
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<div class="">If you happen to have a Hamiltonian system to solve, I have a symplectic solver in my own branch that you can use directly.</div>
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<div class="">Hong (Mr.)</div>
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<div class="">On Feb 4, 2017, at 9:47 AM, Gideon Simpson <<a href="mailto:gideon.simpson@gmail.com" target="_blank" class="">gideon.simpson@gmail.com</a>> wrote:</div>
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<div style="word-wrap:break-word" class="">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)?
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<div class="">On Feb 3, 2017, at 11:47 PM, Matthew Knepley <<a href="mailto:knepley@gmail.com" target="_blank" class="">knepley@gmail.com</a>> wrote:</div>
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<div dir="ltr" class="">That is one answer. Another one is that this particular system is a DAE and we have methods for that.
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<div class=""> Matt</div>
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<div class="gmail_quote">On Fri, Feb 3, 2017 at 8:40 PM, Barry Smith <span dir="ltr" class="">
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TSSetPostStep(); in your function use TSGetSolution() to get the current solution.<br class="">
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Please let us know how it works out<br class="">
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> On Feb 3, 2017, at 7:14 PM, Gideon Simpson <<a href="mailto:gideon.simpson@gmail.com" target="_blank" class="">gideon.simpson@gmail.com</a>> wrote:<br class="">
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> I’m interested in implementing a projection method for an ODE of the form:<br class="">
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> y’ = f(y),<br class="">
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> 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.<br class="">
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> There were two ways I was thinking of doing this, and I was hoping to get some input:<br class="">
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> 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?<br class="">
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> 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?<br class="">
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What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.<br class="">
-- Norbert Wiener</div>
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-- Norbert Wiener</div>
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