CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

Setting up a DES

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

Reply
 
LinkBack Thread Tools Display Modes
Old   May 4, 2011, 03:47
Default Setting up a DES
  #1
Member
 
Max
Join Date: Apr 2011
Posts: 46
Rep Power: 7
max3.2 is on a distinguished road
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
max3.2 is offline   Reply With Quote

Old   May 4, 2011, 07:26
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,832
Rep Power: 100
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
Transient simulations usually run best on about 3-5 coeff loops per iteration. Multiphase or other tricky physics (eg chemistry) might go up to 10. This is all described in the documentation.
ghorrocks is offline   Reply With Quote

Old   May 4, 2011, 07:40
Default
  #3
Member
 
Max
Join Date: Apr 2011
Posts: 46
Rep Power: 7
max3.2 is on a distinguished road
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 DES-chapter cited appendix 55?) and it seems the things i found were wrong. I have to admit, i just lookes around the net
max3.2 is offline   Reply With Quote

Old   May 4, 2011, 14:37
Default
  #4
Member
 
Max
Join Date: Apr 2011
Posts: 46
Rep Power: 7
max3.2 is on a distinguished road
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?
max3.2 is offline   Reply With Quote

Old   May 4, 2011, 19:25
Default
  #5
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,832
Rep Power: 100
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
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 2-equation turbulence models (k-e, SST, k-w) are much simpler to use. This means you are much more likely to correctly apply it. Why not use one of them?
ghorrocks is offline   Reply With Quote

Old   May 5, 2011, 05:23
Default
  #6
Member
 
Max
Join Date: Apr 2011
Posts: 46
Rep Power: 7
max3.2 is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post
The 2-equation turbulence models (k-e, SST, k-w) are much simpler to use. This means you are much more likely to correctly apply it. Why not use one of them?
Thats already done.
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
max3.2 is offline   Reply With Quote

Old   May 5, 2011, 07:30
Default
  #7
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,832
Rep Power: 100
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
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.
ghorrocks is offline   Reply With Quote

Old   May 5, 2011, 08:58
Default
  #8
Member
 
Max
Join Date: Apr 2011
Posts: 46
Rep Power: 7
max3.2 is on a distinguished road
Allright i will check those.
Is there a book or something you'd say is a "must-read"? I've got the Peric, Ferzinger CFD-Book, and some other german stuff about LES-Modelling, it should be enough, but if there's something outstanding, I'll check that too.
max3.2 is offline   Reply With Quote

Old   May 5, 2011, 18:40
Default
  #9
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,832
Rep Power: 100
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
Peric and Ferzinger are good. I would also recommend "Turbulence Modelling for CFD" by Wilcox. It is my reference for turbulence modelling.
ghorrocks is offline   Reply With Quote

Old   May 10, 2011, 12:41
Default
  #10
Member
 
Max
Join Date: Apr 2011
Posts: 46
Rep Power: 7
max3.2 is on a distinguished road
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 Grid-Spacing 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 Blade-PS down to Blade-SS of the next blade and at the TE of the blade. Those Vortices are then carried out of the domain. (Q-Crit)

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
max3.2 is offline   Reply With Quote

Old   May 10, 2011, 18:47
Default
  #11
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,832
Rep Power: 100
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
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?
ghorrocks is offline   Reply With Quote

Old   May 11, 2011, 04:56
Default
  #12
Member
 
Max
Join Date: Apr 2011
Posts: 46
Rep Power: 7
max3.2 is on a distinguished road
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?)
max3.2 is offline   Reply With Quote

Old   May 11, 2011, 06:45
Default
  #13
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,832
Rep Power: 100
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
You cannot compare a time and a distance meaningfully. You need to get an eddy turnover time to get the equivalent time scale.
ghorrocks is offline   Reply With Quote

Old   May 13, 2011, 09:44
Default
  #14
Member
 
Max
Join Date: Apr 2011
Posts: 46
Rep Power: 7
max3.2 is on a distinguished road
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?
max3.2 is offline   Reply With Quote

Reply

Thread Tools
Display Modes

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 Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Cells with t below lower limit Purushothama CD-adapco 2 May 31, 2010 21:58
How to use DES well? Daniel Main CFD Forum 0 October 26, 2008 06:59
DES ( SA+LES) vs DES ( kwSST+LES) for bluff body Phil Main CFD Forum 0 January 8, 2008 10:04
Problem with DES model mehul FLUENT 0 April 12, 2006 07:59
Warning 097- AB CD-adapco 6 November 15, 2004 05:41


All times are GMT -4. The time now is 23:59.