[Users] Negative timesteps with timestep_method="courant_speed" and terminate="time"

[Erik Schnetter]

The commented out code ensures that you never move much beyond the
final time, reducing the step size if necessary. However, this doesn't
quit hit the final time, but moves a tiny bit beyond it. I don't see
how this could cause a problem.

Are you using mesh refinement in these simulations? Carpet's time
stepping algorithm doesn't support changing the time step size. (It is
fine if you use unigrid.) If you do, this may explain your symptoms,
because the different time levels on the different refinement levels
would become seriously inconsistent.

-erik

On Thu, Mar 3, 2011 at 4:01 AM, David Radice <david.radice at aei.mpg.de> wrote:
> Hello,
>
> I have experienced a curious problem when trying to use
>
>        Time::timestep_method = "courant_speed"
>
> with
>
>        Cactus::terminate = "time".
>
> What happens is that, after the final time is reached, the timestep
> becomes negative, afterwards the code quickly crushes (my equations are
> not time-reversible and I guess that dt >= 0 is also assumed in many
> other parts of the code)...
>
> The attached patch for the Time thorn fixes the problem, even tough I am
> not exactly sure on why the commented-out part of the code leads to the
> problem: it would seem to me that cctkGH->cctk_time should always be
> less then cctk_final_time when this routine is executed...
>
> Do you have any idea of what is going on?
>
> David
>
>
> --
> David Radice
> Max Planck Institute for Gravitational Physics
> Albert Einstein Institute
> Am Muehlenberg 1
> D-14476 Potsdam
> Germany
>
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> E-Mail : david.radice at aei.mpg.de
>
>
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-- 
Erik Schnetter <schnetter at cct.lsu.edu>   http://www.cct.lsu.edu/~eschnett/