[petsc-users] Turning off TSADAPT still adapts time step

[Mark Lohry]

Trying a simple stiff test case (y'=y^2*(1-y)) using TSBEULER, to get a 
feel for implicit stepping with JFNK.

I'm giving it a pretty large timestep, and the default appears to start 
adapting the time step around the stiff part of the solution. So I tried 
turning it off:

   TSAdapt adapt;
   TSGetAdapt(its,&adapt);
   TSAdaptSetType(adapt,TSADAPTNONE);

and even fixing the limits:

   TSAdaptSetStepLimits(adapt,40.0,40.0);

And it still ignores it and adapts anyway. At t=800 (where the solution 
gets very stiff, so one would normally see adaptive stepping there if it 
was desired) it's dropping dt from 40 to 10.

  Printing (t,y) here from the ts monitor:

600.000000,0.002790;
640.000000,0.003198;
680.000000,0.003761;
720.000000,0.004606;
760.000000,0.006072;
800.000000,0.010155;
810.000000,0.011452;
820.000000,0.013161;
830.000000,0.015538;
840.000000,0.019126;
850.000000,0.025426;
860.000000,0.043639;
900.000000,0.975519;
940.000000,0.999402;
980.000000,0.999985;
1020.000000,1.000000;
1060.000000,1.000000;
1100.000000,1.000000;
1140.000000,1.000000;
1180.000000,1.000000;

Any idea? Adaptive stepping is great, but I'd still like to be able to 
fix the time step.