# Convection discretization schemes for LES

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

 November 19, 2007, 18:00 Dear Forum, just as an obse #21 Member   Alessandro Spadoni Join Date: Mar 2009 Location: Atlanta, GA Posts: 65 Rep Power: 8 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

 November 19, 2007, 19:04 Sorry, I meant to say: "Uz #22 Member   Alessandro Spadoni Join Date: Mar 2009 Location: Atlanta, GA Posts: 65 Rep Power: 8 Sorry, I meant to say: "Uz velocity is indeed cyclic about the x-y plane" Alessandro

 July 11, 2012, 03:18 #23 Member   ehsan Join Date: Mar 2009 Posts: 98 Rep Power: 8 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

April 6, 2015, 09:04
#24
New Member

Join Date: Nov 2011
Posts: 28
Rep Power: 6
Quote:
 Originally Posted by eugene 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?

April 9, 2015, 11:27
#25
Member

Eric R
Join Date: Jul 2012
Posts: 83
Rep Power: 5
Quote:
 Originally Posted by Andy_bm 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.

 April 9, 2015, 13:38 #26 New Member   Join Date: Nov 2011 Posts: 28 Rep Power: 6 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.

April 9, 2015, 14:09
#27
Member

Eric R
Join Date: Jul 2012
Posts: 83
Rep Power: 5
Quote:
 Originally Posted by Andy_bm 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.

 April 9, 2015, 15:19 #28 New Member   Join Date: Nov 2011 Posts: 28 Rep Power: 6 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?

April 9, 2015, 15:34
#29
Member

Eric R
Join Date: Jul 2012
Posts: 83
Rep Power: 5
Quote:
 Originally Posted by Andy_bm 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.

 April 9, 2015, 15:58 #30 New Member   Join Date: Nov 2011 Posts: 28 Rep Power: 6 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+-.

April 9, 2015, 16:01
#31
Member

Eric R
Join Date: Jul 2012
Posts: 83
Rep Power: 5
Quote:
 Originally Posted by Andy_bm 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?

 April 9, 2015, 16:23 #32 New Member   Join Date: Nov 2011 Posts: 28 Rep Power: 6 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.

April 9, 2015, 16:34
#33
Member

Eric R
Join Date: Jul 2012
Posts: 83
Rep Power: 5
Quote:
 Originally Posted by Andy_bm 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.

 April 11, 2015, 00:12 #34 New Member   Join Date: Nov 2011 Posts: 28 Rep Power: 6 And what scheme for grad and div(phi,U) did you use?

April 11, 2015, 21:07
#35
Member

Eric R
Join Date: Jul 2012
Posts: 83
Rep Power: 5
Quote:
 Originally Posted by Andy_bm 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;

 May 18, 2015, 11:53 #36 New Member   Join Date: Nov 2011 Posts: 28 Rep Power: 6 What about internalField for k? Did you set this = k_inlet value or 0?

May 18, 2015, 11:55
#37
Member

Eric R
Join Date: Jul 2012
Posts: 83
Rep Power: 5
Quote:
 Originally Posted by Andy_bm What about internalField for k? Did you set this = k_inlet value or 0?
I usually set this equal to k_inlet.

 November 28, 2015, 14:41 #38 Senior Member   Join Date: Jan 2013 Posts: 252 Rep Power: 6 Dear Eugene, Thank you for this suggestions. Do you know any papers or threads in which the LES results from linear and this filteredLinear2V for convection scheme are compared or validated? In the latter, we have two parameters that can be adjusted: "filteredLinear2V 0.2 0". If the first one (0.2 here) is small, this scheme seems equilevant to the linear. The comments from the source file are: Code: ``` // Scaling corefficient for the gradient ratio, // 0 = linear // 1 = fully limited``` For the second parameter, in the source files, we have the following comments: Code: ``` // Maximum allowed overshoot/undershoot relative to the difference // across the face. // On input: // 0 = no overshoot/undershoot // 1 = overshoot/undershoot equal to the difference across the face // Note: After input 1 is added to l_``` I am not sure I completely understand this usage of the second parameter. Could you please give an example? Thanks. The link for the source file is here: Code: `https://github.com/OpenFOAM/OpenFOAM-2.1.x/blob/master/src/finiteVolume/interpolation/surfaceInterpolation/limitedSchemes/filteredLinear2/filteredLinear2V.H` Thanks.

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post christian OpenFOAM Running, Solving & CFD 3 December 17, 2007 09:21 Mohammad Kazemi CFX 16 December 7, 2004 23:38 Maciej Matyka Main CFD Forum 3 October 7, 2004 04:32 alice FLUENT 1 July 28, 2004 15:58 Nuray Kayakol Main CFD Forum 20 September 16, 1999 04:16

All times are GMT -4. The time now is 01:56.