[petsc-users] Spatial distribution of the residual after an implicit TS timestep

[Mani Chandra]

The residual of the system of equation that I feed into TS with
TSSetIFunction. I have a system of equations and I want to probe where most
of the residual is coming from. The reason is that after certain time of
evolution, the initial residual at the beginning of each time step
increases by orders of magnitude than what it used to be at the beginning
of the time step at early times. For ex, say at t=200, the SNES norm at the
beginning of a TS timestep with the theta method would be something like
1.0812 and at t = 300, it would be 2e5 at the beginning. SNES then has to
work much harder to reach the abs norm levels and so I want to investigate
what is happening.


On Tue, Mar 18, 2014 at 7:48 PM, Jed Brown <jed at jedbrown.org> wrote:

> Mani Chandra <mc0710 at gmail.com> writes:
>
> > Is there anyway I can get the residual at the beginning of the time
> step? I
> > tried TSSetPreStage and TSSetPreStep but they didn't work. Setting
> > TSSetPreStage would crash the program even if there is nothing in the
> > routine that I set and TSSetPreStep would give me the residual at the end
> > of the previous time step. I want to take a look at the residual at the
> > beginning of the new time step before the nonlinear solver starts.
>
> Residual of what?  What are you going to do with it?
>
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