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

Convection discretization schemes for LES

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

Like Tree7Likes

Reply
 
LinkBack Thread Tools Display Modes
Old   November 19, 2007, 18:00
Default Dear Forum, just as an obse
  #21
Member
 
Alessandro Spadoni
Join Date: Mar 2009
Location: Atlanta, GA
Posts: 65
Rep Power: 8
gtg627e is on a distinguished road
Dear Forum,

just as an observation that I forgot to attach to the previous message, Uz velocity is indeed symmetric about the x-y plane. This gives me confidence that cyclic boundary conditions are being enforced correctly.


Thank you again,

Alessandro
gtg627e is offline   Reply With Quote

Old   November 19, 2007, 19:04
Default Sorry, I meant to say: "Uz
  #22
Member
 
Alessandro Spadoni
Join Date: Mar 2009
Location: Atlanta, GA
Posts: 65
Rep Power: 8
gtg627e is on a distinguished road
Sorry, I meant to say:

"Uz velocity is indeed cyclic about the x-y plane"

Alessandro
gtg627e is offline   Reply With Quote

Old   July 11, 2012, 03:18
Default
  #23
Member
 
ehsan
Join Date: Mar 2009
Posts: 92
Rep Power: 8
ehsan is on a distinguished road
Hello

Could I kindly ask you whether applying "LES Model: laminar" in Les Property folder will create implicit LES in OPENFOAM? If no, how we should apply implicit LES in OPENFOAM?

Thanks in advance
ehsan is offline   Reply With Quote

Old   April 6, 2015, 09:04
Default
  #24
New Member
 
Join Date: Nov 2011
Posts: 28
Rep Power: 5
Andy_bm is on a distinguished road
Quote:
Originally Posted by eugene View Post
2. Initial conditions: k should have a small positive value (1e-10)
Can you explain why initial conditions for k should have small value? And how to calculate initial conditions for k?
Andy_bm is offline   Reply With Quote

Old   April 9, 2015, 11:27
Default
  #25
Member
 
Eric R
Join Date: Jul 2012
Location: Mississippi State University
Posts: 73
Rep Power: 5
msuaeronautics is on a distinguished road
Quote:
Originally Posted by Andy_bm View Post
Can you explain why initial conditions for k should have small value? And how to calculate initial conditions for k?
K should always tend to zero at the wall, as it represents your modeled portion of turbulence kinetic energy. In the case of LES, K is the (modeled) turbulence energy of the subgrid scales. Since velocity -> 0 as we get down to the wall, so should the energy.
msuaeronautics is offline   Reply With Quote

Old   April 9, 2015, 13:38
Default
  #26
New Member
 
Join Date: Nov 2011
Posts: 28
Rep Power: 5
Andy_bm is on a distinguished road
Thanks,

What about nusgs near wall? Maybe there is a ratio between nusgs and nu? In oneeqeddy nusgs = c*delta*sqrt(k). If k -> 0, delta -> 0 as y+ ~ 1 => nusgs -> 0.But nusgs may varried depending on k and y+.And I don't understand what value of nusgs should be near wall.
Andy_bm is offline   Reply With Quote

Old   April 9, 2015, 14:09
Default
  #27
Member
 
Eric R
Join Date: Jul 2012
Location: Mississippi State University
Posts: 73
Rep Power: 5
msuaeronautics is on a distinguished road
Quote:
Originally Posted by Andy_bm View Post
Thanks,

What about nusgs near wall? Maybe there is a ratio between nusgs and nu? In oneeqeddy nusgs = c*delta*sqrt(k). If k -> 0, delta -> 0 as y+ ~ 1 => nusgs -> 0.But nusgs may varried depending on k and y+.And I don't understand what value of nusgs should be near wall.
nuSgs -> 0

is the correct BC for the wall (if, for nothing else, the equation you wrote there) and really is the only reasonable one.

How nuSgs varies as we start to move away from the wall (as y+ increases) depends on your method. You could either be using a wall function for nuSgs or you could have a sufficiently fine grid (suspect this would be the case for most people using LES) to properly represent it.

Please let me know if I'm not being clear.
msuaeronautics is offline   Reply With Quote

Old   April 9, 2015, 15:19
Default
  #28
New Member
 
Join Date: Nov 2011
Posts: 28
Rep Power: 5
Andy_bm is on a distinguished road
Now I have BC on wall for nusgs - zeroGradient, so I must set fixedValue = 0 if y+ ~ 1?
Sorry,I don't understand initial condition for k on WALL must be near 0 or initial condition on INLET must ~ 0?
Andy_bm is offline   Reply With Quote

Old   April 9, 2015, 15:34
Default
  #29
Member
 
Eric R
Join Date: Jul 2012
Location: Mississippi State University
Posts: 73
Rep Power: 5
msuaeronautics is on a distinguished road
Quote:
Originally Posted by Andy_bm View Post
Now I have BC on wall for nusgs - zeroGradient, so I must set fixedValue = 0 if y+ ~ 1?
Sorry,I don't understand initial condition for k on WALL must be near 0 or initial condition on INLET must ~ 0?
No need to apologize.
Yes, for nuSgs and y+ ~1, you may use nuSgs_WALL ~ 0.
Your condition for K_WALL should also be near 0.

I am assuming you are using 1-equation eddy. The condition for inlet can be K_INLET ~ 1e-5 or 1e-6. In other words, some small value but not zero. It really depends on your case. The only time I've used 1-equation eddy I used that condition and got good results.
msuaeronautics is offline   Reply With Quote

Old   April 9, 2015, 15:58
Default
  #30
New Member
 
Join Date: Nov 2011
Posts: 28
Rep Power: 5
Andy_bm is on a distinguished road
Thanks for your answers!

My case is flow past cylinder.I have mesh with y+ 0.1-1. And I have following problem if I use k_inlet~0.96 (1.5(UI)^2) -> Cd ~0.8. if k_inlet~0.01-0.0001 -> Cd~0.3 but in experiment Cd~0.55+-.
Andy_bm is offline   Reply With Quote

Old   April 9, 2015, 16:01
Default
  #31
Member
 
Eric R
Join Date: Jul 2012
Location: Mississippi State University
Posts: 73
Rep Power: 5
msuaeronautics is on a distinguished road
Quote:
Originally Posted by Andy_bm View Post
Thanks for your answers!

My case is flow past cylinder.I have mesh with y+ 0.1-1. And I have following problem if I use k_inlet~0.96 (1.5(UI)^2) -> Cd ~0.8. if k_inlet~0.01-0.0001 -> Cd~0.3 but in experiment Cd~0.55+-.
How are you calculating Cd? Are you using an ensemble average of a list of outputs?
msuaeronautics is offline   Reply With Quote

Old   April 9, 2015, 16:23
Default
  #32
New Member
 
Join Date: Nov 2011
Posts: 28
Rep Power: 5
Andy_bm is on a distinguished road
I use libforces.lib and then plot result in labplot.For k~0.96 pulsation of coeffs occur near 0.8, for small k near 0.3.End time is sufficiently large.
Andy_bm is offline   Reply With Quote

Old   April 9, 2015, 16:34
Default
  #33
Member
 
Eric R
Join Date: Jul 2012
Location: Mississippi State University
Posts: 73
Rep Power: 5
msuaeronautics is on a distinguished road
Quote:
Originally Posted by Andy_bm View Post
I use libforces.lib and then plot result in labplot.For k~0.96 pulsation of coeffs occur near 0.8, for small k near 0.3.End time is sufficiently large.
That makes sense.

I have done study on sphere using DDES. I take the time average of the flow and compute the drag from the sum of pressure and viscous forces acting on the body from the mean solution. Maybe you could try that also for comparison. Drag gets a little tricky when using libforces, especially for an unsteady simulation.
msuaeronautics is offline   Reply With Quote

Old   April 11, 2015, 00:12
Default
  #34
New Member
 
Join Date: Nov 2011
Posts: 28
Rep Power: 5
Andy_bm is on a distinguished road
And what scheme for grad and div(phi,U) did you use?
Andy_bm is offline   Reply With Quote

Old   April 11, 2015, 21:07
Default
  #35
Member
 
Eric R
Join Date: Jul 2012
Location: Mississippi State University
Posts: 73
Rep Power: 5
msuaeronautics is on a distinguished road
Quote:
Originally Posted by Andy_bm View Post
And what scheme for grad and div(phi,U) did you use?
Based on my studies, limitedLinear works pretty well for DES. Avoid the upwind/2nd order upwind schemes like the plague. I use a k value of 0.1-0.3 based on trial and error using a simple case; i.e.

Gauss limitedLinear 0.3;
msuaeronautics is offline   Reply With Quote

Old   May 18, 2015, 11:53
Default
  #36
New Member
 
Join Date: Nov 2011
Posts: 28
Rep Power: 5
Andy_bm is on a distinguished road
What about internalField for k? Did you set this = k_inlet value or 0?
Andy_bm is offline   Reply With Quote

Old   May 18, 2015, 11:55
Default
  #37
Member
 
Eric R
Join Date: Jul 2012
Location: Mississippi State University
Posts: 73
Rep Power: 5
msuaeronautics is on a distinguished road
Quote:
Originally Posted by Andy_bm View Post
What about internalField for k? Did you set this = k_inlet value or 0?
I usually set this equal to k_inlet.
msuaeronautics 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
AMG parallelisation and convection schemes christian OpenFOAM Running, Solving & CFD 3 December 17, 2007 09:21
Discretization scheme for Convection Terms Mohammad Kazemi CFX 16 December 7, 2004 23:38
Convection Term Discretization Maciej Matyka Main CFD Forum 3 October 7, 2004 04:32
discretization schemes alice FLUENT 1 July 28, 2004 15:58
differencing schemes for 3-D Convection-diffusion problems Nuray Kayakol Main CFD Forum 20 September 16, 1999 04:16


All times are GMT -4. The time now is 05:49.