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

[Erik Schnetter]

I think I see know what is going wrong. The commented-out code works
fine the first time, but it overshoots slightly. Carpet then
apparently does not exit the simulation for some reason. The next
time, the commented out code reverses direction, since it tries to get
back to the final time.

I am fine with the patch. Overshooting over the final time is not a
bad thing, and is to be expected when one uses variable time steps.

On a side node, I have used RK4 with adaptive time stepping in the
past (outside Cactus), and I also naively tried to reduce the time
step size to exactly hit certain target times for output. This turned
out to cost a lot of performance, since it increased the number of
time steps and confused the automatic step size calculations. I have
since come to the conclusion that having output (or having simulations
finish) at irregular times is acceptable.

Do others want to comment on this?

-erik

On Thu, Mar 3, 2011 at 10:15 AM, David Radice <david.radice at aei.mpg.de> wrote:
> Erik,
>
> thanks for the reply.
>
> On Thu, 2011-03-03 at 09:55 -0500, Erik Schnetter wrote:
>> 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.
>
> I am using Carpet, but with unigrid. The timestepping is done with MoL,
> RK3.
>
> This issue disappears either by using a terminate condition based on the
> number of iterations or by applying that patch. (I checked that
> courant_wave_speed > 0).
>
> David
>
> --
> David Radice
> Max Planck Institute for Gravitational Physics
> Albert Einstein Institute
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>
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>
>



-- 
Erik Schnetter <schnetter at cct.lsu.edu>   http://www.cct.lsu.edu/~eschnett/