# 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: 9
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 P-43 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:
 Originally Posted by florian_krause Hi guys and especially LES-pipe-flow-experts ;-) On OF-1.6.x, I am using pisoFOAM with LES turbulence model to simulate a fully turbulent pipe flow with Re=7000 based on the centreline velocity. The pipe is fully 3D and the dimensions are L (pipe length) =5*D (pipe diameter). My grid look attachment grid.jpg . I have 3 grid points in the viscous sublayer, whereas I I calculated the size of the viscous sublayer from the friction velocity calculated by a DNS for the same flow. I performed now two different simulations (first one explained here, second one in the following post) 1.) I used cyclic boundaries on the inlet outlet pair and initialized the flow with a uniform flow field (flow in z-direction) and some reasonable turbulent kinetic energy k (corresponding to 10% turbulent intensity) The problem is now, that my flow and my velocity profiles developes into Poseuille flow profile, but I have small amount of (z-)vorticity and turbulentt kinetic energy. I attached the corresponding plots for Uz, k and z-vorticity plot and contour plot. For the whole computation time, my courant number is below 0.5 so my timestep should be ok. My LES SGS model is the oneEqnEddyViscosity model. Do you guys have any clue how I could improve my setup for my boundary conditions ? My fvSolution and my fvScheme files are copied from the pisoFoam -> les -> pitzDaily case ...see next post....

 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 time-averaged 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 p-field?

June 13, 2013, 21:25
Question on LES cases
#23
Senior Member

Hasan K.J.
Join Date: Dec 2011
Location: Bristol, United Kingdom
Posts: 199
Rep Power: 7
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 1e-8 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:
 Originally Posted by florian_krause Hi guys and especially LES-pipe-flow-experts ;-) On OF-1.6.x, I am using pisoFOAM with LES turbulence model to simulate a fully turbulent pipe flow with Re=7000 based on the centreline velocity. The pipe is fully 3D and the dimensions are L (pipe length) =5*D (pipe diameter). My grid look attachment grid.jpg . I have 3 grid points in the viscous sublayer, whereas I I calculated the size of the viscous sublayer from the friction velocity calculated by a DNS for the same flow. I performed now two different simulations (first one explained here, second one in the following post) 1.) I used cyclic boundaries on the inlet outlet pair and initialized the flow with a uniform flow field (flow in z-direction) and some reasonable turbulent kinetic energy k (corresponding to 10% turbulent intensity) The problem is now, that my flow and my velocity profiles developes into Poseuille flow profile, but I have small amount of (z-)vorticity and turbulentt kinetic energy. I attached the corresponding plots for Uz, k and z-vorticity plot and contour plot. For the whole computation time, my courant number is below 0.5 so my timestep should be ok. My LES SGS model is the oneEqnEddyViscosity model. Do you guys have any clue how I could improve my setup for my boundary conditions ? My fvSolution and my fvScheme files are copied from the pisoFoam -> les -> pitzDaily case ...see next post....

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