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October 11, 2012, 14:31 
Resolved/modelled scales in LES

#1 
New Member
Sheyda
Join Date: Oct 2012
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Hi everyone.
Well this is my first post here, hoping to get some insight into LES. We know that in LES the large scales are resolved and the effects of smaller ones (subgrids) is modeled, right? But, 1 How do you make sure that the appropriate energycontaining scales have actually been resolved? 2 How could you know what scales are present in a flow at all? 3 How you can assess what scales have been resolved in your LES? Thank you!
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October 11, 2012, 14:41 

#2  
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Filippo Maria Denaro
Join Date: Jul 2010
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Quote:
1 it depends on the extension of the inertial range of the energy spectra, you have to put the filter lenght within the inertial range. In simple words, the filter lenght is related to the computational grid therefore the Nyquist frequency must be greater than the frequencies related to the energycontaining eddies. 2 The estimation is based on the limit provided by the Kolmogorov scale, the smallest one. 3 You should better formulate the question... If I see correctly what you mean, the resolved scales in LES are (formally) all within the filter length. However, the SGS model acts on the resolved scales, mainly those close to the filter lenght 

October 12, 2012, 19:09 

#3  
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Sheyda
Join Date: Oct 2012
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Thanks, I appreciate your help. Have more questions now:
Quote:
Quote:
And, from your LES results how would you evaluate the smallest scales that have actually been resolved?
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_ Someday we'll all look back at this and laugh _ Last edited by Ms.Sheyda; October 16, 2012 at 19:04. 

October 13, 2012, 17:36 

#4  
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Filippo Maria Denaro
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Quote:
I suppose you are quite new in this field... the Kolmogorov scale depends on the physics (energy production, dissipation and viscosity of the fluid) not on the LES results... in LES the Kolmogorov scale is never resolved... what's more, in LES the dissipative region of the energy spectra remains almost always unresolved, apart when you have a very fine grid in regions of the flow of local low Re number (e.g. a resolved boundary layer) 

October 16, 2012, 18:53 

#5  
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Sheyda
Join Date: Oct 2012
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Rep Power: 6 
Quote:
I understand now that kolmogorov scales are smallest scales and could only be resolved in very fine LES. My question is if the Kolmogrov scales are defined by physics of the flow (i.e Re number and energy dissipation), how we can work it out and actually estimate the kolmogrov scales to check how far is our LES resolution from a fully resolved turbulence? We have a equation relating kolmogrov length scale to dissipation rate and Re number. Say in LES of simple channel flow we have Re number but how we work out the energy dissipation from which we could calculate kolmgrov scale?
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