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spectrum: LES of a turbulent channel flow finally goes to turbulent 

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June 14, 2010, 11:25 
spectrum: LES of a turbulent channel flow finally goes to turbulent

#1 
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Xiaofeng Liu
Join Date: Mar 2009
Location: State College, PA, USA
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Hi all,
My LES of turbulent channel flow Retau=395 finally goes to turbulent (or at least it looks like it). What I am trying to do is to reproduce Eugene's case in his thesis. First, I compared the velocity profile with DNS data. Again, it looks OK. The only thing I am not so happy is channelFoam never really get Retau=395. It stays at about 384. Maybe we should not impose bulk velocity, but the pressure gradient. Second, I tried to compare the spectrum. What I don't understand is how Fig. 5.7 in Eugene's thesis was produced. To be specific, I have the following doubts: 1. What is the meaning of k? It says "total resolved turbulent energy"? Is it the k field in the output? Or just k=0.5*(Ux*Ux+Uy*Uy+Uz*Uz), where Ux, Uy, and Uz are velocity components? 2. Is this a spatial spectrum or temporal spectrum? I suppose this is the spatial spectrum. What he did is to average the U over z direction in a xz plane (y+=8) parallel to the wall. Then U_avg(x_i) is only a function of x_i, where x_i is the center of the cells (Say we have 45 cells in x direction, then x_i = 1 to 45). Then we do spectrum on this U_avg(x_i). Am I right? Anyway, I didn't get the same plots as in Fig 5.7. I know something is wrong along the way. 3. Also the wave number on xaxis (Fig. 5.7), how it f and U defined? Thanks for you help! 

June 16, 2010, 08:14 

#2 
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Eugene de Villiers
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Hi,
1. It is neither k = 0.5 (magSqr(U  Umean)) 2. It is a temporal spectrum. 3. f is frequency [Hz], U is the phase velocity = Umean at the measurement location Hope this helps, Eugene 

June 16, 2010, 10:39 
spectrum: LES of a turbulent channel flow finally goes to turbulent

#3 
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Xiaofeng Liu
Join Date: Mar 2009
Location: State College, PA, USA
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Thanks.
I will try it. How long did you take the sample for spectrum? Did you normalize E(k) with something? The order of magnitude for k is much smaller than 1 when calculated. 

June 17, 2010, 05:52 

#4 
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Eugene de Villiers
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I really cannot recall how long the sampling ran for. It would have to be long enough to capture the lowest frequencies, so somewhere of the order of 1 second simulated time.
What order of magnitude do you get for E? Unfortunately, I cant help you with the definition, but it is unlikely that it is anything but turbulent energy since I would have provided some kind of equation if that were the case.(Note that the "k" in "E(k)" refers to the wavenumber, not the turbulent energy.) 

June 23, 2010, 20:09 

#5 
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Daniel WEI (老魏)
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Location: South Bend, IN, USA
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Hi Liu,
Here are some of my ideas,
Bye,
__________________
~ Daniel WEI  NatHaz Modeling Laboratory Department of Civil & Environmental Engineering & Earth Sciences University of Notre Dame, USA Email  My Personal CFD Blog 

February 1, 2011, 16:46 
How does it go to turbulence?

#6 
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Charlie
Join Date: Dec 2010
Location: 415 Kinross Dr. Newark, DE 19711
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Hi Liu,
How long (simulate flow time) have you run the case to get a turbulence? Did you change the code? or the initial condition? k for example? Right now, I'm basically doing the same thing, i.e. reproduce the log law and turbulence intensities. However, I just can't get the turbulence. Maybe you can see the link to my thread: perturbation in channelFoam? Basically, the pressure gradient is dropping, though much slower when run it to 100 seconds. so I just wonder how you set up the case for it. If it's no bother for you, can you show me your case? via email or posting a reply as you like. Thank you! 

November 1, 2011, 02:00 

#7  
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Getnet
Join Date: Aug 2011
Location: LSU
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Quote:
What modification did u do to get turbulent flow in your channel? I am also getting the same problem in my channel simulations. My nuSgs is close to nu (molecular viscosity) in most of the flow domain. However, I run the same case using komega model and nut values are reasonable and are about 1000 times larger than nuSgs in some spatial locations ( mainly close to the midsection of the channel). I also appreciate if anyone has compared nut and nuSgs for the same channel flow cases. Any advice please? Thanks, 

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