[petsc-users] TSSetPostStep for projected methods

[Gideon Simpson]

I’ve begun working on implementing a projected time stepping method for solving y’ = f(y), where g(y(t)) is an invariant of the flow.  Per a previous email exchange, it was suggested that I use TSSetPostStep to perform the projection routine, calling TSGetSolution obtain the current iterate, which will be corrected with the projection.  However, I noticed two things:

1.  The documentation for TSGetSolution says "This vector not changed until the solution at the next timestep has been calculated.”  Does that mean that if I make a change to the solution in a PostStep function, it won’t be captured until the next step?  What happens at the final time step?

2.  The projection is nonlinear, requiring the use of a SNES.  I had originally thought that I would create/configure/destroy the SNES within the main routine of the code, and pass this to the PostStep through a user data structure.  However, the TSSetPostStep function seems to only take functions which are exclusively functions of the TS.  Likewise, I would need to create a residual vector, r, for the SNESSetFunction.  Is there a way to pass these data structures, so that they can be reused. or would they have to be created/destroyed within the PostStep function at each iterate?  How costly would that be?


-gideon

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