[petsc-users] strange TS adaptivity behavior

[Mark Adams]

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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