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-   -   local time step (http://www.cfd-online.com/Forums/cfx/79838-local-time-step.html)

syoon September 6, 2010 06:47

local time step
 
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 September 6, 2010 07:37

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

Quote:

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


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