
[Sponsors] 
November 15, 2012, 14:57 
LES using Fluent

#1 
Senior Member
MAZI
Join Date: Oct 2009
Posts: 100
Rep Power: 10 
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 
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 3,673
Rep Power: 41 
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 
Senior Member
cfdnewbie
Join Date: Mar 2010
Posts: 557
Rep Power: 13 
I shudder to think what a "2D LES model" might be....


November 15, 2012, 16:35 

#4 
Senior Member
MAZI
Join Date: Oct 2009
Posts: 100
Rep Power: 10 
But there are many papers used 2D LES


November 15, 2012, 16:39 

#5  
Senior Member
MAZI
Join Date: Oct 2009
Posts: 100
Rep Power: 10 
Quote:
Step too? Also how much should be the amount of Yplus? 

November 15, 2012, 16:42 

#6  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 3,673
Rep Power: 41 
Quote:
You need to put some points in the BL, that means 3  4 cells within y+<1 

November 15, 2012, 16:42 

#7 
Senior Member
MAZI
Join Date: Oct 2009
Posts: 100
Rep Power: 10 
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 
Senior Member
MAZI
Join Date: Oct 2009
Posts: 100
Rep Power: 10 

November 15, 2012, 16:45 

#9 
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 3,673
Rep Power: 41 

November 15, 2012, 16:49 

#10 
Senior Member
MAZI
Join Date: Oct 2009
Posts: 100
Rep Power: 10 

November 15, 2012, 16:54 

#11 
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 3,673
Rep Power: 41 

November 15, 2012, 17:00 

#12  
Senior Member
MAZI
Join Date: Oct 2009
Posts: 100
Rep Power: 10 
Quote:
That you told 

November 15, 2012, 17:11 

#13 
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 3,673
Rep Power: 41 
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 
Senior Member
MAZI
Join Date: Oct 2009
Posts: 100
Rep Power: 10 

November 15, 2012, 17:19 

#15 
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 3,673
Rep Power: 41 
see also chapter 8 in the Agard database http://www.efluids.com/efluids/databases/agard.html


Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Postprocessing of LES data in FLUENT  anee  FLUENT  0  September 27, 2011 01:10 
LES and near wall treatment approach in Fluent  khosro1355  Main CFD Forum  1  July 10, 2009 10:49 
LES in fluent  Sham  FLUENT  0  August 1, 2007 21:31 
LES using FLUENT  sarah_ron  FLUENT  7  October 22, 2004 17:36 
LES with Fluent  sat  FLUENT  0  August 2, 2004 18:32 