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November 28, 2014, 11:30 
Resolved Reynolds Stresses in LES...??!

#1 
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Teagyoung Kim
Join Date: Nov 2014
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Running 3D LES on FLUENT.
Clicked Data Sampling for Time Statistics options. Now I see Resolved UV,UW,VW Reynolds Stresses on export quantities. Q1. Why suddenly showing up after Time Statistics options?? Q2. Can I use this as the input for Acoustic Analogies?! Is it mean valued throughout the simulation?! Q3. Do I just square the resolved x velocity to get Resolved UU Reynolds Stress? Is this correct? Anyone experienced in dealing with Lighthill's Stress Tensor?? Last edited by ktgfeel; November 28, 2014 at 11:32. Reason: confusing title 

November 29, 2014, 02:46 

#2  
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Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
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The Reynolds stresses are timeaveraged quantities. Once you activate time statistics, many timeaveraged mean and rms quantities suddenly become available (such as mean velocity, rms velocities).
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Sorry I am not familiar with Lighthill's Stress Tensor. I am not sure why in the GUI the reynolds stresses are labeled with "Resolved". In the field variables they are not named that way but are simnply named UU UV and UW Reynolds stresses. Information on the meaning of "resolved" is somewhat obscure. 

December 22, 2014, 00:12 

#3 
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Teagyoung Kim
Join Date: Nov 2014
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Thank you very much for your reply. I know understand more. but still haven't reached my goal.
Fluent doesn't have any option on Time Staticstics to have set the length of time to calculate. It only has option to set sampling intervals. However, what I am trying to get is somewhat like Instantaneous Reynolds Stress. I have moved on to LES calculating method because having Reynolds Stress Model converge was almost impossible. ( Reynolds Stress Model,i.e. RSM, does calculate Reynolds stresses right away). Q1. If I just multiply LES calculated(not rms) velocities, what would it mean?? Would it be something like instantaneous reynolds stresses?? Thank you again for your reply and careful explanation. 

December 22, 2014, 00:13 

#4  
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Teagyoung Kim
Join Date: Nov 2014
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Quote:
Fluent doesn't have any option on Time Staticstics to have set the length of time to calculate. It only has option to set sampling intervals. However, what I am trying to get is somewhat like Instantaneous Reynolds Stress. I have moved on to LES calculating method because having Reynolds Stress Model converge was almost impossible. ( Reynolds Stress Model,i.e. RSM, does calculate Reynolds stresses right away). Q1. If I just multiply LES calculated(not rms) velocities, what would it mean?? Would it be something like instantaneous reynolds stresses?? Thank you again for your reply and careful explanation. 

March 1, 2016, 07:58 

#5 
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Andrea
Join Date: Feb 2012
Location: Leeds, UK
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Hi ktgfeel,
I know this is quite an old thread, I was just wondering if you or anybody else reading eventually got how the Resolved Reyonlds Stresses are calculated in Fluent when doing LES and collecting statistics. My idea is that, for instance, the resolved uv stress should be defined as: Resolved uv Reynolds stress = {u''v''} where {} indicates ensemble average over time and u''=[u]{[u]} v''=[v]{[v]} where [u] and [v] are the filtered (i.e. resolved) instantaneous velocity components, and {} again indicates time averaging. Cheers Andrea 

March 1, 2016, 10:46 

#6 
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Andrea
Join Date: Feb 2012
Location: Leeds, UK
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I have created custom field functions for the variables defined in my previous post and I am matching the Resolved Reynolds Stressed reported by Fluent under Unsteady Statistics.
I guess this answers LuckyTran's question about the label "resolved", since the Reynolds stressed are calculated based on the resolved velocity field and the contribution coming from the subgrid scales is neglected. 

March 2, 2016, 18:18 

#7  
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Lucky Tran
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Location: Orlando, FL USA
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Tags 
acoustics, fluent, les, reynolds stress tensor, stress calculation 
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