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Discussion: LES SGSmodels and solvers

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Old   September 7, 2016, 06:43
Default Discussion: LES SGSmodels and solvers
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Giulio
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Dear Foamers ,

I am an OF beginner, playing with OF LES skills on a complex geometry with highly separated flow. I have a series of concerns regarding LES, its implementation and its relationship with OF solvers.
  1. pisoFoam would be the most natural solution for solving highly transient flow, but it's very sensitive to time steps and y+. I usually put 2 to 4 loops and then 1 to 2 nonOrthogonalCorrectors (usually 1). Am I overusing it?
  2. pimpleFoam allows a more stable simulation, but the settings of the Pimple loop are not clear to me (it appears I need much more iterations than PISO and still convergence is very slow). Is it necessary to relax p and U like for SIMPLE? I put 20 to 50 outerCorrectors, 2 Correctors, 1 nOrtho + relaxation (0.3-0.7). Is it sensible (still it's really slow in convergence);
  3. simpleFoam (which is the SIMPLEC of Ansys as far as I understand): can it be used for transient flows? According to the literature yes, but still I don't think it's a good idea to use it with LES (am I right?);
  4. Adaptive time Step is it a good idea to use it with LES? Any suggestion on how to implement it? Any trick (because it usually ends up with a very low time step: 1e-6...)
  5. Any other alternative for solvers?

For LES I mean SGS Smagorinsky model. My idea was to then switch to more advanced SGS models:
  1. dynamicSmagorinsky is not implemented (just the kSGS and the Lagrangian, which is very sensitive to the mesh and the maxCo) from v.3 on, right? (I wanted to try it out)
  2. WALE can be a solution to tackle y+ requirements, is it alright? If so what would you suggest for a y+ value? Furthermore, the simulation is much slower than with Smagorinsky (but I've only tried Pimple).
  3. vanDriest damping is an alternative to WALE for tackling behaviour of Smagorinsky near wall. I get good results even with y+=5. Is it the case, that vanDriest helps to get better solutions even for not perfect y+? (or it's beginner's luck?);

I don't mention the Schemes because we'll end up with an infinite post . I just wanted to hear from more expert users if you have suggestions/experience and if you find something is wrong with what I wrote!

This is a cylinder upstream to a thick blunt plate, without any wake modelled (infinite plate). The mesh is huge, therefore I need to cheat (I cannot make it uniform, I have to stretch cells outside of the separation bubble zone I'm interested in)...

Thank you very much,
Sorry if I've said stupidities!!!
Giulio
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