[petsc-users] TimeStepper norm problems. EMIL Please read this

[Jed Brown]

Lisandro Dalcin <dalcinl at gmail.com> writes:

> On 21 March 2015 at 17:32, Emil Constantinescu <emconsta at mcs.anl.gov> wrote:
>> When -ts_theta_adapt is used, then it detects the instability as an error
>> and reduces the step by a lot! wlte=1.24e+03 which means that the reduction
>> should be severe but the controller tries 0.1*dt and that seems to pass but
>> it "jig-saws" (take a look at the next attempted step), which means that it
>> is likely unstable.
>
> I think -ts_theta_adapt is seriously broken, I cannot make sense of
> the way the error estimator is computed.

This was Shri's implementation.  TSEvaluateStep_Theta looks wrong to me,
both using U as an input and assuming that Xdot is set.  I don't even
know what the intended math is for a first-order embedded estimate in
midpoint (theta=0.5).
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