When to use local timescale or physical timescale

Daniel Frederik · June 25, 2014, 08:41

Hi everybody,

Simulating a radial gas turbine a came to this case which is very hard to make converge.
Just to get a feeling of what the different timesteps will do i simulated with different ones. In the picture there are the convergence criterions plottet with different timesteps. The last one, the local timestep, looks weird. I saw that before in another case. Why does it not converge donewards. It looks like it would converge to a "convergence criteria which doesn't make sense.

Any Ideas?

ghorrocks · June 25, 2014, 08:45

Your simulation is clearly very numerically unstable. Local time stepping is not helping you much - so do not use it. Physical time stepping will probably work better for you.

That is why all these options are available - different simulations need different approaches.

Daniel Frederik · June 25, 2014, 08:47

Thanks for your rapid answer Glenn,
Could it also be that it is phisically unstable instead of numerical? Because with different mass Flows and Speeds of revolution i don't have that sort of problem.

I mean, i am doing a steady state analysis, but what if the flow is unsteady. Thats what i mean with phisicaly unstable.

ghorrocks · June 25, 2014, 08:55

Yes, physical instability will result in numerical instability as well.

So if the flow is truly transient then a steady state run will be unstable. This is all explained in the FAQ this thread has linked to.

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June 25, 2014, 08:45		#22
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	Your simulation is clearly very numerically unstable. Local time stepping is not helping you much - so do not use it. Physical time stepping will probably work better for you. That is why all these options are available - different simulations need different approaches.

June 25, 2014, 08:47		#23
Daniel Frederik Member Daniel Frederik Join Date: Apr 2014 Location: Switzerland Posts: 38 Rep Power: 12	Thanks for your rapid answer Glenn, Could it also be that it is phisically unstable instead of numerical? Because with different mass Flows and Speeds of revolution i don't have that sort of problem. I mean, i am doing a steady state analysis, but what if the flow is unsteady. Thats what i mean with phisicaly unstable.

June 25, 2014, 08:55		#24
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	Yes, physical instability will result in numerical instability as well. So if the flow is truly transient then a steady state run will be unstable. This is all explained in the FAQ this thread has linked to.