[petsc-users] strange TS adaptivity behavior

[Mark Adams]

On Mon, Apr 6, 2020 at 3:47 PM Jed Brown <jed at jedbrown.org> wrote:

> This typically happens when your model is discontinuous or you activate
> some fast transitien.  You have a really aggressive lower bound on your
> time step so its hard to tell here.
>

The time step goes down to like 1e-3.
I am suspecting that my source term needs to be smoother and am working on
that now.
It is disconcerting that the truncation error goes up as the time stp is
reduced, but in this example it does start to go down again.
Thanks,
Mark


>
> Mark Adams <mfadams at lbl.gov> writes:
>
> > I have a problem that is fairly smooth and when it decreases the time
> step
> > it just keeps decreasing almost forever. I wanted to see if anyone has
> any
> > clue what is going on here.
> >
> > This test reaches a quasi equilibrium and the adaptivity uses fairly
> large
> > time steps with dU/dt - F(u,t) = 0. I set a max time step of 10. A source
> > starts at one point (dU/dt - F(u,t) = S(t)). It cools pretty fast.
> >
> > THe truncation error increase a lot when the source starts but it does
> not
> > decrease the time step immediately because I had a mat time step set:
> >  ....
> >   Nonlinear solve converged due to CONVERGED_FNORM_RELATIVE iterations 4
> > [0] TSAdaptChoose_Basic(): Estimated scaled local *truncation error
> > 0.000214757*, accepting step of size 10.
> >       TSAdapt basic arkimex 0:1bee step *386* accepted t=2292.68    +
> > 1.000e+01 dt=1.000e+01  wlte=0.000215  wltea=   -1 wlter=   -1
> > ...
> > [0] TSAdaptChoose_Basic(): Estimated scaled local *truncation error
> > 0.500399,* accepting step of size 10.
> >       TSAdapt basic arkimex 0:1bee step *387* accepted t=2302.68    +
> > 1.000e+01 dt=1.000e+01  wlte=  0.5  wltea=   -1 wlter=   -1
> > 388) species-0: charge density= -1.6422881272166e+01 z-momentum=
> >  7.2180929658011e-03 energy=  9.7881753794625e+05
> >
> > It goes for another few time steps before it rejects the time step for
> the
> > first time, and then it just keeps reducing and reducing:
> >
> > [0] TSAdaptChoose_Basic(): Estimated scaled local truncation error
> > 0.939966, accepting step of size 10.
> >       TSAdapt basic arkimex 0:1bee step *395* accepted t=2382.68    +
> > 1.000e+01 dt=9.283e+00  wlte= 0.94  wltea=   -1 wlter=   -1
> > ....
> > [0] TSAdaptChoose_Basic(): Estimated scaled local truncation error
> 1.33533,
> > *rejecting* step of size 9.28296
> >       TSAdapt basic arkimex 0:1bee step *396* rejected t=2392.68    +
> > 9.283e+00 dt=7.230e+00  wlte= 1.34  wltea=   -1 wlter=   -1
> >
> > [0] TSAdaptChoose_Basic(): Estimated scaled local truncation error
> 1.44054,
> > *rejecting* step of size 7.22994
> >       TSAdapt basic arkimex 0:1bee step *396* rejected t=2392.68    +
> > 7.230e+00 dt=2.711e+00  wlte= 1.44  wltea=   -1 wlter=   -1
> >
> > [0] TSAdaptChoose_Basic(): Estimated scaled local truncation error
> 1.63529,
> > *rejecting* step of size 2.71072
> >       TSAdapt basic arkimex 0:1bee step *396* rejected t=2392.68    +
> > 2.711e+00 dt=9.539e-01  wlte= 1.64  wltea=   -1 wlter=   -1
> >
> > [0] TSAdaptChoose_Basic(): Estimated scaled local truncation error
> 1.60415,
> > *rejecting* step of size 0.953894
> >       TSAdapt basic arkimex 0:1bee step *396* rejected t=2392.68    +
> > 9.539e-01 dt=5.000e-01  wlte=  1.6  wltea=   -1 wlter=   -1
> >
> > [0] TSAdaptChoose_Basic(): Estimated scaled local truncation error
> > 1.49714, *accepting
> > because step size 0.5 is at minimum*
> >       TSAdapt basic arkimex 0:1bee step *396* accepted t=2392.68    +
> > 5.000e-01 dt=5.000e-01  wlte=  1.5  wltea=   -1 wlter=   -1
> >
> > Anyone have any idea what is going on here?
> >
> > Thanks,
> > Mark
>
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