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May 4, 2011, 03:47 
Setting up a DES

#1 
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Max
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Hi,
Im about to set up a DES from a acceptably converged (after approx 150 It.) SST Solution. Calculation of Total Time and Timesteps is no the problem, but whats a good value to choose for Iterations? Do i have to calculate the 150Loops every Timestep or is it sufficient to choose a lower value (around 40, there's not much RES left...) Regards Max 

May 4, 2011, 07:26 

#2 
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Glenn Horrocks
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Transient simulations usually run best on about 35 coeff loops per iteration. Multiphase or other tricky physics (eg chemistry) might go up to 10. This is all described in the documentation.


May 4, 2011, 07:40 

#3 
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Max
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Allright, thanks. Ill take a deeper look.
Another thing  even if i said i knew how to set up timestepping, i was wrong. DES Blending Function wont go anywhere near 0. Am I right to assume my timestep is to big? I cant find anything in the Documentation (wheres in DESchapter cited appendix 55?) and it seems the things i found were wrong. I have to admit, i just lookes around the net 

May 4, 2011, 14:37 

#4 
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Max
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I now tried to subsequently reduce the timestep, and so im getting larger areas with small blending values. but still i do not reach 0. is this an error or maybe connected to the model? is there a possibility to increase the threshhold so cfx will blend earlier to LES?


May 4, 2011, 19:25 

#5 
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Glenn Horrocks
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The DES model is an advanced turbulence model. Unless you are familiar with LES models (things like dissipation, filtering functions and matching it to turbulent spectrums) then I would not recommend using the DES model.
ANSYS does advanced turbulence training courses which describe the use of these models, or you can do it the old fashion way of reading the literature. The 2equation turbulence models (ke, SST, kw) are much simpler to use. This means you are much more likely to correctly apply it. Why not use one of them? 

May 5, 2011, 05:23 

#6  
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Max
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Quote:
I trie to model Vortex movement and turbulent structure over time, and im writing a paper about it done with LES/DES. So its okay if i have to read some literature, ive got plety of time. So I've done mainly about LES, but most of it was about the math, and not how to find the known parameters in Pre. I will go on try to find some more, seems like it is enough for 3 months of work 

May 5, 2011, 07:30 

#7 
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Glenn Horrocks
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That sounds good. There is lots of background knowledge you need before you start on a LES model. Make sure you understand Kolmogorov length scales, Taylor microscales, turbulence energy cascades and all that sort of stuff as a background. Then do some LES models where you attempt to correctly model turbulence energy cascades. Once you can correctly model a turbulent energy cascade you are ready to proceed.


May 5, 2011, 08:58 

#8 
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Max
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Allright i will check those.
Is there a book or something you'd say is a "mustread"? I've got the Peric, Ferzinger CFDBook, and some other german stuff about LESModelling, it should be enough, but if there's something outstanding, I'll check that too. 

May 5, 2011, 18:40 

#9 
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Glenn Horrocks
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Peric and Ferzinger are good. I would also recommend "Turbulence Modelling for CFD" by Wilcox. It is my reference for turbulence modelling.


May 10, 2011, 12:41 

#10 
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Max
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Ok, I've done some research, there's still more to come, but i have a few question...
If I get it right, CFX Docu says boundary layer grid spacing may not fall under 0.1 boundary layer thickness. Now I also found in "Fröhlich,J: LES Simulation [2006]" (german text translated): Upper Value for GridSpacing delta < 0.1 boundary layer, which is the exact opposite. Whats right? At this time, I'm not sure if there's an adavantage in comparison to URANS, since the Mesh is very fine and some Vortex structures are already resolved. Theres some Vortex shedding at BladePS down to BladeSS of the next blade and at the TE of the blade. Those Vortices are then carried out of the domain. (QCrit) Still I am trying to get the Blending function to 0, bur I'm not finding any information on this function. I'm trying timesteps around 1ms (which is still to small for the problem, but way over Kolmogorov. But since im not doing a DNS, it should work...) and getting RMS Courant Numbers around 150. This is said (again, Documentation and Peric, Ferzinger) to be possible. If I go up with my step, Courant also goes up. How far can i go? Since i got the Mesh to work with, I think i have to do some changes there. This again leads to the question above, if I need to coarsen or fine it. I will continue doing research, but I'd be more than happy for any advice Best Reagrds Max 

May 10, 2011, 18:47 

#11 
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Glenn Horrocks
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Often LES is set up to use the taylor microscale as the time step size, or some multiple of it. This means you will be resolving the large eddies but modelling the small ones  of course this is the whole point of LES.
How does your time step compare to the taylor microscale? 

May 11, 2011, 04:56 

#12 
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Max
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Im not sure if i do the Taylor scale calculation right.
Im getting a taylor scale (a lenght) which is approx 1/4 of my timestep (a time?) 

May 11, 2011, 06:45 

#13 
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Glenn Horrocks
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You cannot compare a time and a distance meaningfully. You need to get an eddy turnover time to get the equivalent time scale.


May 13, 2011, 09:44 

#14 
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Max
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Hmm im not getting a appropriate turnover time. until now, i've only found some correlations and so on...
still im looking for ansys tech report on des, cited as [55]...someone got this? 

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