
[Sponsors] 
July 21, 2012, 02:33 
LES of Turbulent channel flow

#1 
New Member
Sarlak
Join Date: Mar 2012
Posts: 17
Rep Power: 7 
Hello CFD guys,
I'm trying to generate turbulent channel flow using wall resolved LES simulation on a collocated block structured, multigrid accelerated finite volume code. I defined periodic boundaries on the horizontal plane, symmetry on top and no slip wall (no model) for the bottom and started with a log profile with some random noise (1050% magnitude). The problem is the fluctuations are damped rather quickly. u_tau=1, domain: 0.5pi x 1 x pi with 48*48*48 grids (uniform in horizontal direction and stretched in the vertical with the first grid in yplus=0.25). This could partly make sense as the initial fluctuations are not correlated etc. but there is also a suspicion that since the code is is not pseudospectral (unlike many available codes) and is collocated, it's highly dissipative and also not kinetic energy conserving. What is your opinion? Do you have any idea how I should generate the correct turbulence filed? My code has also the problem that the finer the mesh is, the more dissipative it becomes, isn't it strange? 

July 21, 2012, 03:49 

#2  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 3,481
Rep Power: 40 
Quote:
The first issue is that LES is a 3D simulation, you must solve the entire 3D channel, no simmetry at halfheight ... you are forcing a false solution 

July 21, 2012, 07:14 

#3 
Senior Member
cfdnewbie
Join Date: Mar 2010
Posts: 557
Rep Power: 13 

July 21, 2012, 16:46 

#4 
New Member
Sarlak
Join Date: Mar 2012
Posts: 17
Rep Power: 7 

July 21, 2012, 16:52 

#5  
New Member
Sarlak
Join Date: Mar 2012
Posts: 17
Rep Power: 7 
Quote:
That's a good point and I agree that LES is in fact 3D, but does it mean you are never allowed to use symmetry BC in LES? Cause I've seen quite a few papers (mainly atmospheric boundary layer simulations) that have set the top BC as a symmetric surface (I guess they've presumed that the top of the domain can be treated like a laminar flow) 

July 21, 2012, 18:11 

#6 
Senior Member
Alex
Join Date: Jun 2012
Location: Germany
Posts: 1,528
Rep Power: 25 
There are circumstances when a symmetry boundary condition can be applied in LES. The main prerequisite is that the flow is laminar next to the boundary.
For a turbulent channel flow, this condition is not satisfied. 

July 21, 2012, 19:38 

#7 
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 3,481
Rep Power: 40 
in channel flow with LES the condition of simmetry is simply wrong and drives the flow to dissipate artificially the energy ... such a condition can be used in RANS simulation where the variable which is solved has a steady statistical meaning.
Furthermore, I suggest to run the full 3D case with and without SGS model, on the same grid, to assess the real effect of the model... 

Tags 
channel flow, les, turbulence, wall resolved 
Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Open Channel Flow  ElanMorin  FLUENT  4  February 25, 2015 17:26 
turbulent developed lenght in oppen channel flow  zandi  Main CFD Forum  0  January 9, 2010 15:52 
Initial conditionfor turbulent channel flow in LES  pankaj saha  Main CFD Forum  0  November 30, 2007 13:04 
Turbulent channel flow  roberthino  OpenFOAM Running, Solving & CFD  5  August 15, 2007 08:35 
Pressure drop across a turbulent channel flow  shiv  Main CFD Forum  0  September 21, 2005 00:52 