[petsc-users] Programmatically get TS Error estimates without adaptivity

[Mark Lohry]

Hi Emil, maybe I was unclear, let me try to clarify, I only want the local
truncation error at a given step.

Most integrators outside of GLEE also support ts_adapt basic (I'm
specifically testing arkimex and bdf), and they compute the WLTE weighted
error term, e.g. for arkimex:

TSAdapt basic arkimex 0:3 step   0 accepted t=0          + 1.896e-02
dt=1.967e-02  wlte=0.653  wltea=   -1 wlter=   -1

or BDF2:

TSAdapt basic bdf 0:2 step   0 rejected t=0          + 3.673e-02
dt=3.673e-03  wlte= 36.6  wltea=   -1 wlter=   -1

I'd like to have access to that wlte value. I can call TSEvaluateWLTE which
works for BDF2 but crashes on arkimex for not being implemented. Is there a
different avenue to getting this value through arkimex?






On Mon, Sep 16, 2019 at 10:56 AM Constantinescu, Emil M. <emconsta at anl.gov>
wrote:

> Mark, most integrators estimate the error occurring every step for
> adapting the step size. These are called local errors. GLEE are
> specialized integrator that estimate the actual error in time; this is
> called the global error. GLEE returns TimeError because it computes it
> internally, however none of the other solvers does it.
>
> To assess the accuracy for a specific problem you have two options, and
> both involve you estimating the error yourself:
>
> 1. Use an integrator with very small step, store or save the solution
> (call it reference) and do the same for the others and then compute the
> error with respect to the reference. A strategy similar to this is
> implemented in src/ts/examples/tutorials/ex31.c
>
> 2. If you have an exact or manufactured solution, you can compare
> against that.
>
> Emil
>
> On 9/16/19 9:07 AM, Mark Lohry via petsc-users wrote:
> > I'm trying to assess time accuracy for a couple different integrators
> > and timesteps in a setup where I need constant time steps.
> >
> > TSGetTimeError seems to only work for GLEE methods (it returns a 0
> > vector otherwise); is there an equivalent for others?
> >
> > I can run -ts_adapt_type basic, and just set min=max=constant, and it
> > clearly computes some integrator errors for printf, but is there a way
> > I can get that programmatically?
> >
> > Thanks,
> > Mark
> >
> >
> >
>
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