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November 15, 2012, 14:57 
LES using Fluent

#1 
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MAZI
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Hi
i have tried to simulate the case that has been mentioned in the attached paper, a backward step flow, but when i draw the mean xvelocity profile at specific location (x/H=14), i do not get good results. i did this procedure for simulation: 1) run the case with ke model 2) wrote this command: solve/initialize/initinstantaneousvel (i write this command to generate the instantaneous velocity out of the steady state RANS results as Fluent manual mentioned) 3) wrote this command: solve/initialize/initflowstatistics (i write this command to zero out the initial statistics as Fluent manual mentioned) 4) wrote (rpsetvar 'les2d? #t) to enable on 2D LES model 5) turned on Transient 6) select LES model 7) second order model for time step 8) for perturbation i used turbulent intensity and hydrolic diameter 9) i saved data for every 1 time step and also turned on the "Data Sampling for time statistics" 10) the time step used in the paper is 0.005 but i used even smaller one :0.003 11) run the cod for 2000 time steps when i look at mean xvel contour, it has the fully developed format, but i do not know why the results are different from the paper. i used both outflow and pressure outlet boundary condition for outlet and also a channel with larger length (paper is 34H mine 40 H where H=26.7 mm). i used both 34H and 40H for the simulations and i got different results for the vel. profile at x/H=14. i used 259 and 25 nodes in x and y direction respectively as paper mentioned. and Samgosinesky model. do you have any idea? how many nodes should be used in each direction and how can i find that my results are correct? thanks for your time 

November 15, 2012, 16:04 

#2 
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Filippo Maria Denaro
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1) LES must be used for a 3D flow condition
2) You have to sample the fields for the statistics after the numerical transient is finished. Therefore, you need to run in time until an energy equilibrium is reached. 3) Results will strongly depends on the inflow condition. I suggest to use a supplementary part of domain before the step 4) use dynamic modelling 

November 15, 2012, 16:06 

#3 
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cfdnewbie
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I shudder to think what a "2D LES model" might be....


November 15, 2012, 16:35 

#4 
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MAZI
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But there are many papers used 2D LES


November 15, 2012, 16:39 

#5  
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MAZI
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Quote:
Step too? Also how much should be the amount of Yplus? 

November 15, 2012, 16:42 

#6  
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Filippo Maria Denaro
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Quote:
You need to put some points in the BL, that means 3  4 cells within y+<1 

November 15, 2012, 16:42 

#7 
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Also if I want to use 3D model, how much
Should be the length in the zdirection? How many times of H? For example 4H is enough? I have one more question. How the cell size affect the results? 

November 15, 2012, 16:44 

#8 
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November 15, 2012, 16:45 

#9 
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Filippo Maria Denaro
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November 15, 2012, 16:49 

#10 
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November 15, 2012, 16:54 

#11 
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Filippo Maria Denaro
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November 15, 2012, 17:00 

#12  
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MAZI
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Quote:
That you told 

November 15, 2012, 17:11 

#13 
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Filippo Maria Denaro
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many of those papers are based on periodic conditions along the spanwise direction ... see also the book of Sagaut


November 15, 2012, 17:13 

#14 
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November 15, 2012, 17:19 

#15 
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Filippo Maria Denaro
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see also chapter 8 in the Agard database http://www.efluids.com/efluids/databases/agard.html


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