[syoon]

Hello.

I'm doing a calculation which simulates a steam turbine diffuser.
I generally use a local time step which I gradually increase from 1 to 50.
This is generally okay for most of my cases.

However, for some outlet boundary conditions, the calculation does not converge and the flow separates significantly.
When I have these cases, I can often (not always!) make the solution converged by increasing the local time step to a very large number (say 200)

I wonder what this means.
Does it mean that my actual time step (delta t) is increased significantly and misses small time scale fluctuations?

Kind Regards,

[ghorrocks]

You should not use local time stepping for the final run to convergence. Always use physical timesteps on the final run to convergence.

Does it mean that my actual time step (delta t) is increased significantly and misses small time scale fluctuations?

I would say that the Reynolds averaging is averaging the small fluctuations out to a steady flow - different way of saying the same thing.

Your experience agrees with the comments on the FAQ about convergence in steady state simulations:
http://www.cfd-online.com/Wiki/Ansys...gence_criteria