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#21 |
Member
Alessandro Spadoni
Join Date: Mar 2009
Location: Atlanta, GA
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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 x-y plane. This gives me confidence that cyclic boundary conditions are being enforced correctly. Thank you again, Alessandro |
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#22 |
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Alessandro Spadoni
Join Date: Mar 2009
Location: Atlanta, GA
Posts: 65
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Sorry, I meant to say:
"Uz velocity is indeed cyclic about the x-y plane" Alessandro |
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#23 |
Senior Member
Ehsan
Join Date: Mar 2009
Posts: 112
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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 |
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#24 |
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Join Date: Nov 2011
Posts: 32
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#25 |
Member
Eric Robertson
Join Date: Jul 2012
Posts: 95
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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.
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#26 |
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Join Date: Nov 2011
Posts: 32
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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. |
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#27 | |
Member
Eric Robertson
Join Date: Jul 2012
Posts: 95
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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. |
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#28 |
Member
Join Date: Nov 2011
Posts: 32
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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? |
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#29 | |
Member
Eric Robertson
Join Date: Jul 2012
Posts: 95
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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 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. |
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#30 |
Member
Join Date: Nov 2011
Posts: 32
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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+-. |
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#31 |
Member
Eric Robertson
Join Date: Jul 2012
Posts: 95
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How are you calculating Cd? Are you using an ensemble average of a list of outputs?
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#32 |
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Join Date: Nov 2011
Posts: 32
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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.
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#33 | |
Member
Eric Robertson
Join Date: Jul 2012
Posts: 95
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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. |
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#34 |
Member
Join Date: Nov 2011
Posts: 32
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And what scheme for grad and div(phi,U) did you use?
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#35 |
Member
Eric Robertson
Join Date: Jul 2012
Posts: 95
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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 0.1-0.3 based on trial and error using a simple case; i.e.
Gauss limitedLinear 0.3; |
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#36 |
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Join Date: Nov 2011
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What about internalField for k? Did you set this = k_inlet value or 0?
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#37 |
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Eric Robertson
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#38 |
Senior Member
Join Date: Jan 2013
Posts: 372
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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/OpenFOAM-2.1.x/blob/master/src/finiteVolume/interpolation/surfaceInterpolation/limitedSchemes/filteredLinear2/filteredLinear2V.H |
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#39 | |
Senior Member
Timofey Mukha
Join Date: Mar 2012
Location: Stockholm, Sweden
Posts: 119
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Quote:
This is an old thread but I would like to revive it. I assume much more people have done LES on OF by now. Any developments on convective schemes? As for filteredLinear, we have 5 to choose from (including the V variants) and also a bunch of parameters. Any guidelines/suggestions? |
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