LES quality assessement

mirabella · September 4, 2013, 15:12

Hello everybody!

How can I get the resolved turbulent kinetic energy and the subgrid turbulent kinetic energy for an LES simulation with CFD-Post? I want to compare the resolved part with the modeled one, to assess the quality of LES calculated with CFX

nice greetings

ghorrocks · September 4, 2013, 17:53

Which LES model are you using?

To get the resolved turbulent kinetic energy you need to do some mathematics on your results file. You need to split the velocity field into a mean flow and turbulent component. You can do that by time averaging, spatial averaging or ensemble averaging (for a periodic flow like an IC engine). Once you have the average you can evaluate the resolved turbulent component.

mirabella · September 5, 2013, 07:30

Actually, I am using the Smagorinsky model.
Lead these ways of averaging to same results?
For time avergaing, shall I calculate an area averaged turbulent kinetic energy for each transient file, then calculate the mean values over time?
Do you know, how can I get that with CFD-POST?
Thank you!

ghorrocks · September 5, 2013, 08:21

It is a bit tricky with the Smagorinsky model, it is a dissipative model with no explicit sub grid kinetic energy component. If you hunt through the turbulence modelling textbooks you might find something but I cannot recall off the top of my head what it is.

No, the three methods of averging I mentioned do not usually lead to the same results. That would be far too simple

.

No, do not do a area averaged k then time averge that. That would be some weirdo space and time average. The literature does one or the other and that makes sense to me.

To do time averaging you have to activate the time averaging options in the CFX-Pre output file tab. To do spatial averaging you can do that on a results file in post-processing - but it is not simple, you have to define a averaging length scale and there are no simple functions in CFX I know of to do that.

September 4, 2013, 15:12	LES quality assessement	#1
mirabella New Member mira Join Date: Jun 2011 Posts: 6 Rep Power: 14	Hello everybody! How can I get the resolved turbulent kinetic energy and the subgrid turbulent kinetic energy for an LES simulation with CFD-Post? I want to compare the resolved part with the modeled one, to assess the quality of LES calculated with CFX nice greetings

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September 4, 2013, 17:53		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Which LES model are you using? To get the resolved turbulent kinetic energy you need to do some mathematics on your results file. You need to split the velocity field into a mean flow and turbulent component. You can do that by time averaging, spatial averaging or ensemble averaging (for a periodic flow like an IC engine). Once you have the average you can evaluate the resolved turbulent component.

September 5, 2013, 07:30		#3
mirabella New Member mira Join Date: Jun 2011 Posts: 6 Rep Power: 14	Actually, I am using the Smagorinsky model. Lead these ways of averaging to same results? For time avergaing, shall I calculate an area averaged turbulent kinetic energy for each transient file, then calculate the mean values over time? Do you know, how can I get that with CFD-POST? Thank you!

September 5, 2013, 08:21		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	It is a bit tricky with the Smagorinsky model, it is a dissipative model with no explicit sub grid kinetic energy component. If you hunt through the turbulence modelling textbooks you might find something but I cannot recall off the top of my head what it is. No, the three methods of averging I mentioned do not usually lead to the same results. That would be far too simple . No, do not do a area averaged k then time averge that. That would be some weirdo space and time average. The literature does one or the other and that makes sense to me. To do time averaging you have to activate the time averaging options in the CFX-Pre output file tab. To do spatial averaging you can do that on a results file in post-processing - but it is not simple, you have to define a averaging length scale and there are no simple functions in CFX I know of to do that.