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Ms.Sheyda October 11, 2012 14:31

Resolved/modelled scales in LES
 
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 (sub-grids) is modeled, right? But,

1- How do you make sure that the appropriate energy-containing 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!

FMDenaro October 11, 2012 14:41

Quote:

Originally Posted by Ms.Sheyda (Post 386191)
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 (sub-grids) is modeled, right? But,

1- How do you make sure that the appropriate energy-containing 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!

Hi,

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 energy-containing 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

Ms.Sheyda October 12, 2012 19:09

Thanks, I appreciate your help. Have more questions now:

Quote:

you have to put the filter lenght within the inertial range
How could I know what is the inertial range present in my flow case?

Quote:

Nyquist frequency must be greater than the frequencies related to the energy-containing eddies
How we can actually know the frequencies related to the energy-containing eddies?

And, from your LES results how would you evaluate the smallest scales that have actually been resolved?

FMDenaro October 13, 2012 17:36

Quote:

Originally Posted by Ms.Sheyda (Post 386353)
Thanks, I appreciate your help. Have more questions now:


How could I know what is the inertial range present in my flow case?


How we can actually know the frequencies related to the energy-containing eddies?

And, from your LES results how would you evaluate the smallest scales (Kolmogorov) that have actually been resolved?


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)

Ms.Sheyda October 16, 2012 18:53

Quote:

Originally Posted by FMDenaro (Post 386459)
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)

Thanks.. and yes I am a bit lost in some concepts of turbulence.
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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