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 liu June 14, 2010 11:25

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

3 Attachment(s)
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!

 eugene June 16, 2010 08:14

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

 liu June 16, 2010 10:39

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

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.

 eugene June 17, 2010 05:52

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.)

 lakeat June 23, 2010 20:09

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,

 cheng1988sjtu February 1, 2011 16:46

How does it go to turbulence?

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.

http://www.cfd-online.com/Forums/ope...tml#post293152

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!

 aka November 1, 2011 02:00

Quote:
 Originally Posted by liu (Post 262930) 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.