Sudden random deviation of results

[Barry Smith]

  Ben,

   First make sure that the residual has actually gotten
to the tolerance you want. PETSc KSP does NOT stop if the
linear system has not converged; you should call KSPGetConvergedReason()
after each solve to make sure it has converged (as a quick check
you can run with -ksp_converged_reason.)

  Next try decreasing the KSP tolerance factor a great deal. Does
1) the solution you get for your physics problem look the same for small
   time (after a few time-steps they will start to drift).
2) the residual blow up at the same time, or later?

   Barry


On Thu, 7 Dec 2006, Ben Tay wrote:

> Hi,
> 
> I have been using a few different Krylov linear solver package such as
> nspcg,sparsekit and now petsc to solve the linear eqns for my NS solver
> momentum and poisson eqn. For moving bodies simulation, while solving the
> poisson eqn, sometimes the result suddenly changes drastically, although the
> iterations seems to have converged e.g. at the previous time step,
> everything is fine, but at the next time step, the velocity suddenly
> explodes from 2-3 to 20-30+.
> 
> The strange thing is that this deviation happens at different time step for
> different linear solver package ie for nspcg, it happens at time=20 but for
> petsc, it happens at time=40. It seems "random" in this sense. Changing the
> type of solver such as gmres or bcgs does not change the time of deviation.
> 
> Does anyone know why this is happening? Is it a characteristic of krylov
> solver to happen once in a while? It does not happen in the stationary body
> simulation though.
>