CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM Running, Solving & CFD

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

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   June 14, 2010, 11:25
Default spectrum: LES of a turbulent channel flow finally goes to turbulent
  #1
liu
Senior Member
 
Xiaofeng Liu
Join Date: Mar 2009
Location: State College, PA, USA
Posts: 118
Rep Power: 8
liu is on a distinguished road
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 x-axis (Fig. 5.7), how it f and U defined?

Thanks for you help!
Attached Images
File Type: jpg flowfield.JPG (19.4 KB, 212 views)
File Type: jpg Uplus_yplus.JPG (24.3 KB, 274 views)
File Type: jpg Eugene_Fig5_7.JPG (36.0 KB, 370 views)
liu is offline   Reply With Quote

Old   June 16, 2010, 08:14
Default
  #2
Senior Member
 
Eugene de Villiers
Join Date: Mar 2009
Posts: 725
Rep Power: 12
eugene is on a distinguished road
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
eugene is offline   Reply With Quote

Old   June 16, 2010, 10:39
Default spectrum: LES of a turbulent channel flow finally goes to turbulent
  #3
liu
Senior Member
 
Xiaofeng Liu
Join Date: Mar 2009
Location: State College, PA, USA
Posts: 118
Rep Power: 8
liu is on a distinguished road
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.
liu is offline   Reply With Quote

Old   June 17, 2010, 05:52
Default
  #4
Senior Member
 
Eugene de Villiers
Join Date: Mar 2009
Posts: 725
Rep Power: 12
eugene is on a distinguished road
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.)
eugene is offline   Reply With Quote

Old   June 23, 2010, 20:09
Default
  #5
Senior Member
 
lakeat's Avatar
 
Daniel WEI (老魏)
Join Date: Mar 2009
Location: South Bend, IN, USA
Posts: 688
Blog Entries: 9
Rep Power: 12
lakeat is on a distinguished road
Send a message via Skype™ to lakeat
Hi Liu,

Here are some of my ideas,
  1. I was recommended some years ago to look deep into Pope's "Turbulence Flows" to get a basic idea what turbulence spectrum is.
  2. Taylor frozen turbulence field hypothesis is used, since the spatial resolution is far from enough in common channel flow simulation.

Bye,
__________________
~
Daniel WEI
-------------
NatHaz Modeling Laboratory
Department of Civil & Environmental Engineering & Earth Sciences
University of Notre Dame, USA
Email || My Personal CFD Blog
lakeat is offline   Reply With Quote

Old   February 1, 2011, 16:46
Default How does it go to turbulence?
  #6
Member
 
Charlie
Join Date: Dec 2010
Location: 415 Kinross Dr. Newark, DE 19711
Posts: 78
Rep Power: 6
cheng1988sjtu is on a distinguished road
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!
cheng1988sjtu is offline   Reply With Quote

Old   November 1, 2011, 02:00
Default
  #7
aka
New Member
 
Getnet
Join Date: Aug 2011
Location: LSU
Posts: 20
Rep Power: 6
aka is on a distinguished road
Quote:
Originally Posted by liu View Post
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 x-axis (Fig. 5.7), how it f and U defined?

Thanks for you help!
Hi Liu,
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 k-omega model and nut values are reasonable and are about 1000 times larger than nuSgs in some spatial locations ( mainly close to the mid-section of the channel).

I also appreciate if anyone has compared nut and nuSgs for the same channel flow cases.

Any advice please?

Thanks,
aka is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Fluent LES Channel Flow santosh FLUENT 0 January 7, 2010 22:11
Initial conditionfor turbulent channel flow in LES pankaj saha Main CFD Forum 0 November 30, 2007 13:04
Look for applications to simulate a rough wall channel flow with turbulent model xiuying OpenFOAM Running, Solving & CFD 0 September 25, 2007 22:21
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


All times are GMT -4. The time now is 11:55.