On Wed, Apr 20, 2022 at 5:13 PM Phlipot, Greg <greg.phli...@jhuapl.edu> wrote:
> Hello, > > When using TS with the option TS_EXACT_FINALTIME_MATCHSTEP to force TS > to stop at the final time, I'm seeing the adaptive step controller > choose smaller time steps than the minimum time step that is set with > TSAdaptGetStepLimits. For my problems, this leads to solutions that > won't converge due to numerical issues. > > It seems like this might be due to the fact that TSAdaptChoose ( > https://petsc.org/main/src/ts/adapt/interface/tsadapt.c.html#TSAdaptChoose > ) uses a tolerance of 10*PETSC_MACHINE_EPSILON to compare whether the > candidate time step is close to the final time. For me, > 10*PETSC_MACHINE_EPSILON is much smaller than my smallest time step so > the adaptation scheme occasionally chooses a time step that results in > a time much closer to the max time than the minimum time step. The > subsequent time step is therefore forced to be smaller than the minimum > time step. > > Does anyone have an idea of a workaround (without changing the source > code) where I would be able to modify this logic to use the minimum > time step instead of 10*PETSC_MACHINE_EPSILON? E.g. with Pre/Post stage > callbacks and manually modifying the time step? > It seems that if you want to disallow small timesteps, but also match accurately a final time, then the best choice is interpolation (TS_EXACTFINALTIME_INTERPOLATE). If you only want to be as accurate as your timestep, shouldn't you just let it step over? Thanks, Matt > Thank you, > Greg > -- 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 https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
