LES modeling with fluent
 

Hello all,

I was wondering what is the meaning of "statistically stationery state" and how I can check that while LES model is running. It is one of the steps suggested for LES modelling in tutorials. I've attached screen shots of surface and volume monitoring of velocity which seem stable from the beginning (very small changes in each iteration). I appreciate if anyone could give me a hint on how we could judge that the flow is in statistically stationery state.

Regards
Zeinab

Probably this figure can elucidate. It is the monitor, in time T, of the volume average of the instantaneous kinetic energy for a channel flow.

Up to T=20 you see that the volume average is changing in time. After T=20, it still changes, but only because of "turbulence", it fluctuates around a steady state. That's the meaning of statistically steady state. It's not actually steady. It's the mean of the fluctuation that is steady.

Obviously this is only possible for flows which actually have a statistically steady state. For example, this would be impossible for, say, the flow in a combustion chamber of an ICE or, say, if you want to simulate anything intrinsically not steady in the mean.

Thanks Paolo for your reply.

Is there any way that I can track this stationery state while the program is running? I've read in FLUENT tutorials that monitoring velocity or pressure could help us understand if the flow statistics are stationery. However, when I monitor the velocity, it seems steady from the beginning. Is it a wrong way to check the stationery state? Do I need to stop the simulation, check the flow statistics and re-run the program?

P.S. my problem is flow past a cylinder.

Thank you

If you look at my picture, you see that what i was monitoring was a volume average, over the whole domain, of a custom field function i have defined by myself. Such custom field function is just the local kinetic energy:

0.5 x density x Vmag^2

You should refer to the manual for how to define a custom field function and how to activate volume average monitor.

For the flow over a cylinder, you will definitely reach a statistically steady state.

Obviously this has nothing to do with your LES settings, which might be wrong and never produce "turbulence". You should search for other posts in this forum about the correct LES settings or refer to the Fluent best practices.