Dear Freinds,

In recent days I am studing semi-implicit methods for calculating creep flows. In these methods, what are the time step restriction?

Thanks.

Actually there are no theoratical formulas to determine the time step restriction for the nonlinear differential equations. They are only available for linear PDEs. The rule of thumeb is to make the discretization as implicit as possible. Don't just throw all the nonlinear terms in the source term, you can linearize it and put the higher order truncation terms in the source term. The other qualitative rule is to reduce the time step whenever you reduce the spatial grid size, especially in the dominant flow direction.

1. diffusion term. 2. chemical source terms. 3. convective terms. For a unsteady computation, one better use implicit method of some kind for 1 and 2.