Sudden random deviation of results

[Ben Tay]

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.
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