# LES time step

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 January 28, 2008, 14:19 LES time step #1 Chris Guest   Posts: n/a Hi If you are running a LES simulation with CFX which is a fully implicit code then how would you choose your time step. Thank you Chris

 January 28, 2008, 17:59 Re: LES time step #2 Glenn Horrocks Guest   Posts: n/a Hi, Either do a mesh independence check (this can be tricky with LES), estimate a timestep based on the flow timescales or just try the recommended 3-5 iterations per timestep and see if the final results are OK. Glenn Horrocks

 January 28, 2008, 18:08 Re: LES time step #3 Chris Guest   Posts: n/a What do you mean by flow time scales. By iterations do you mean coefficient loops? I can only check final results once i have run my simulation for a week. This is not a good option. Is there something else that you can recommend. Chris

 January 29, 2008, 19:31 Re: LES time step #4 Glenn Horrocks Guest   Posts: n/a Hi, Flow time scales - LES simulations typically resolve the turbulent eddies down to the inertial sub-range. Therefore you need to resolve to this frequency and filter out the inertial sub-range. Iterations/coefficient loops - yes, I mean the same thing. Glenn Horrocks

 January 31, 2008, 12:37 Re: LES time step #5 Magnoli Guest   Posts: n/a Hi, A common guideline for LES is to keep the CFL number below 1.0. You can find even more rigorous guidelines recommending CFL ~ 0.5. The CFX manual also recommends this range. In several papers, researchers claim to have used CFL between 0.4 and 0.8. If you cannot afford such small time steps, maybe you could consider switching to SAS, but then you have to carefully evaluate if your transient results quantitatively correspond to reality, because there may be a tendency to damp the turbulent effects. Regards, Magnoli.

 January 31, 2008, 20:36 Re: LES time step #6 CFD newbie Guest   Posts: n/a Thanks for your answer Magnoli. We can fix the max courant number to whatever value we like under simulation type form. However this still does not give us our time step. How can we calculate time step? In my current simulation i have fixed the max courant number to 0.5 and am using adaptive time step, but still i had to put an initial value for time step. How do we find an initial time step then, if we use adaptive time stepping. CFD newbie

 January 31, 2008, 20:43 Re: LES time step #7 CFD newbie Guest   Posts: n/a If person who initially posted this message have understood how to calculate LES time step and could share his understanding with me, it will be very helpful. CFD newbie