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setup problems  LES pipe flow with cyclic BC (1) and direct mapped inlet (2) 

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November 20, 2009, 06:49 

#21  
New Member
Andrea Aprovitola
Join Date: Nov 2009
Posts: 16
Rep Power: 8 
I Florian
I'm working on LES of turbulent channel flow and so I'm interested on the validation on the OpenFOAM results. What kind of time Discretization Schemes did you selected for your problem ? As I'm concerned in the study of both spatial than temporal discretizations OpenFOAM schemes for LES, my doubt is if selecting the flag CrankNicholson in ddt schemes would mean the correct use of such scheme as in the FOAM ProgrammersGuide P43 it is said that: "The Crank Nicholson scheme can be implemented by the mean of implicit and explicit terms: solve ( fvm::ddt(phi) == kappa*0.5*(fvm::laplacian(phi) + fvc::laplacian(phi)) ) ". If is this so, I'have to rewrite the left hand side of the corresponding UEqn.H exploiting both fvm and fvc operator in order to implement the Crank Nicholson scheme. Otherwise I would not have an effective Crank Nicholson time integration schemes and consequently losing the second order time accuracy. cheers Andrea Quote:


March 15, 2013, 10:27 

#22 
Senior Member
Julien
Join Date: Jun 2012
Location: France
Posts: 152
Rep Power: 6 
Hi Florian,
in my opinion, you need to make some kind of average to check if your flow is completely developped. I would make a time averaging, and check that my average flow is not changing ( d./dt = 0 for timeaveraged quantities). You need to define properly your time length of averaging, of course. With taylor's hypothesis, maybe my approach (time averaging) is quite similar to a spatial averaging, isn't it? By the way, I do not understand why usually, users do not map the p field in the inlet, using standard zeroGradient BC ? In the cyclic formulation, everything field is cyclic ! Can you explain why didn't you map your pfield? 

June 13, 2013, 21:25 
Question on LES cases

#23  
Senior Member
Hasan K.J.
Join Date: Dec 2011
Location: Bristol, United Kingdom
Posts: 196
Rep Power: 6 
Hey Florian,
I am trying to set up LES case in openFOAM at the moment i am having trouble with the Courant number since u seem to have had experience validating LES case, i think u are the right person to ask.  you have mentioned since ur courant number is below 0.5 your time step is right ? i have a time step like 1e8 and still my courant number goes out of the roof and my case crashes i have not been able to set up any case of my own but the tutorials run fine any suggestion ?  Have u intilized any cases using inlet from RANS if so how  i also noticed you had used dynSmag ? how did u get this model i thought this is not available in openFOAM ? i am using openFOAM 2.1.x any suggestions ? Thanks a lot for your time Best Regards, Hasan K.J Quote:


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