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November 19, 2007, 18:00 
Dear Forum,
just as an obse

#21 
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Alessandro Spadoni
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Dear Forum,
just as an observation that I forgot to attach to the previous message, Uz velocity is indeed symmetric about the xy 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 
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Alessandro Spadoni
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Sorry, I meant to say:
"Uz velocity is indeed cyclic about the xy plane" Alessandro 

July 11, 2012, 03:18 

#23 
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ehsan
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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 

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April 9, 2015, 11:27 

#25 
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Eric Robertson
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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 

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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  
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Eric Robertson
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Quote:
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 

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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  
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Eric Robertson
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Quote:
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 1equation eddy. The condition for inlet can be K_INLET ~ 1e5 or 1e6. In other words, some small value but not zero. It really depends on your case. The only time I've used 1equation eddy I used that condition and got good results. 

April 9, 2015, 15:58 

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Thanks for your answers!
My case is flow past cylinder.I have mesh with y+ 0.11. And I have following problem if I use k_inlet~0.96 (1.5(UI)^2) > Cd ~0.8. if k_inlet~0.010.0001 > Cd~0.3 but in experiment Cd~0.55+. 

April 9, 2015, 16:01 

#31 
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Eric Robertson
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How are you calculating Cd? Are you using an ensemble average of a list of outputs?


April 9, 2015, 16:23 

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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  
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Eric Robertson
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Quote:
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 

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And what scheme for grad and div(phi,U) did you use?


April 11, 2015, 21:07 

#35 
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Eric Robertson
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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 0.10.3 based on trial and error using a simple case; i.e.
Gauss limitedLinear 0.3; 

May 18, 2015, 11:53 

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What about internalField for k? Did you set this = k_inlet value or 0?


May 18, 2015, 11:55 

#37 
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Eric Robertson
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November 28, 2015, 14:41 

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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 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_ The link for the source file is here: Code:
https://github.com/OpenFOAM/OpenFOAM2.1.x/blob/master/src/finiteVolume/interpolation/surfaceInterpolation/limitedSchemes/filteredLinear2/filteredLinear2V.H 

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