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[LES] How do Reynolds stresses exist in no model simulations? 

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January 20, 2021, 17:50 
[LES] How do Reynolds stresses exist in no model simulations?

#1 
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Hello,
As I studied the concept of LES and SGS models, the Reynolds stresses are a part of the SGS stresses and are modeled by SGS model. But how do they exist and plotted in simulations where no SGS model used?(implicit LES) Regards 

January 20, 2021, 17:54 

#2  
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Filippo Maria Denaro
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Quote:
In ILES they are simply disregarded in the computation. It is supposed that the local truncation error acts as a model for them. 

January 21, 2021, 00:53 

#3 
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Tahnk you dear Prof.
In the following figure, the dashedx plots (xxx) are the Reynolds stresses of a no model simulation. They also have bigger values than others! You mean that all of them are caused by truncation errors? Then why doesn't this error have a significant effect on the simulations with a SGS model? 

January 21, 2021, 03:46 

#4 
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I should look at the details of the paper but, in general, what you typically see in these sort of plots is related to the solved reynolds stresses, which you can compute from any unsteady computation. Basically, they are computed as if the computation was a DNS. Because DNS and ILES actually solve the same continuous equations.


January 21, 2021, 03:59 

#5  
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What I mean, is that while the Reynolds stresses are appeared and defined only when we use filtered or averaged equations, then I suppose that it does not have a meaning to have them in DNS or ILES. 

January 21, 2021, 04:12 

#6  
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That's it, it makes total sense. How do you think people compare RANS models to DNS results if they can't compute such averages? If you read some pure DNS paper you will find that there are a lot of such averages. EDIT: of course, these terms are not, explicitly, part of the equations you are solving in DNS or ILES but, besides comparisons with models, they also have a relevant fluid dynamic role, that's another reason people want to look at them. 

January 21, 2021, 04:37 

#7  
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Filippo Maria Denaro
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Paolo addressed the correct answer, what you see are the RMS evaluated from the timeaveraged LES velocity field. Of course, they are not exactly the same fluctuations one would get from the DNS field. 

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