questions about TS functions

[Matthew Knepley]

2008/9/9 tsjb00 <tsjb00 at hotmail.com>:
> Hi! I have some questions about TS functions.
>
> 1. I tried to use TSDefaultComputeJacobian to provide the Jacobian matrix.
> It worked fine with TS_BEULER, but when I tried to use TS_CN, it didn't
> work. The error message indicates that it is not supported for
> Crank-Nicholson. Is it true or I might do something wrong?

Please always send the complete error message. That default just uses finite
differences to compute the Jacobian. However, for nonlinear problems with
changing Jacobians, we manipulate the matrix directly in CN. In order to use
the FD code, we would need to recode this using a MatShell.

> 2. Is there a specific PETSc function to evaluate Jacobian for TS_CN? Any
> example of using TS_CN that I can refer to?

No. You would need to form the Jacobian yourself.

> 3. I tried to solve a simple time evolving diffusion problem with TS, using
> TS_BEULER and non-linear solver. When I tried to use big time step, sever
> over-prediction is observed. I would appreciate any suggestion on the
> criteria of maximum time step allowed. If I really need to use big time
> steps, any tips to set up TS to improve the results?

Its not clear from your description whether this is a stability or
accuracy problem.

    Matt

> Many thanks in advance!
>
> BJ
>
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