CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Mesh requirements for k-w models

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

Like Tree4Likes
  • 4 Post By cfd seeker

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   October 19, 2012, 16:27
Default Mesh requirements for k-w models
  #1
Member
 
Join Date: Mar 2011
Location: Canada
Posts: 35
Rep Power: 14
Jeremie84 is on a distinguished road
Hello,

I am wondering if it is relevant to use the k-w models (standard or SST) with coarse mesh near the wall (y+>50). I read that in the Fluent User's guide and I am a little confused:

"Both k-w models available in ANSYS FLUENT are available as low-Reynolds-number models as well as high-Reynolds-number models. Therefore, the mesh guidance should be the same as for the enhanced wall treatment."

Also, what the advantage of using the "Low-Re corrections" option for the k-w models? Is it relevant only for fine meshes (y+<5)?

Thank you

Jérémie
Jeremie84 is offline   Reply With Quote

Old   October 19, 2012, 16:40
Default
  #2
Senior Member
 
Syavash Asgari
Join Date: Apr 2010
Posts: 473
Rep Power: 17
syavash is on a distinguished road
Quote:
Originally Posted by Jeremie84 View Post
Hello,

I am wondering if it is relevant to use the k-w models (standard or SST) with coarse mesh near the wall (y+>50). I read that in the Fluent User's guide and I am a little confused:

"Both k-w models available in ANSYS FLUENT are available as low-Reynolds-number models as well as high-Reynolds-number models. Therefore, the mesh guidance should be the same as for the enhanced wall treatment."

Also, what the advantage of using the "Low-Re corrections" option for the k-w models? Is it relevant only for fine meshes (y+<5)?

Thank you

Jérémie


If you have a High Reynolds problem, you may implement k-w with y+ between 30 and 300. If your problem regards low Reynolds, then you can check "Transitional flow" option to take into account low Reynolds correction. in this case y+ should be in order of 1.0.

Good luck
syavash is offline   Reply With Quote

Old   October 19, 2012, 17:14
Default
  #3
Member
 
Join Date: Mar 2011
Location: Canada
Posts: 35
Rep Power: 14
Jeremie84 is on a distinguished road
Thank you Syavash for your answer.

I know that there is not an universal rule, but is there a typical value of Re from which we can consider a high-Re problem?
I am studying a VAWT, and I have a range of Re from 100 000 to 500 000.

Thanks again

Jérémie
Jeremie84 is offline   Reply With Quote

Old   October 19, 2012, 21:17
Default
  #4
Senior Member
 
Syavash Asgari
Join Date: Apr 2010
Posts: 473
Rep Power: 17
syavash is on a distinguished road
Quote:
Originally Posted by Jeremie84 View Post
Thank you Syavash for your answer.

I know that there is not an universal rule, but is there a typical value of Re from which we can consider a high-Re problem?
I am studying a VAWT, and I have a range of Re from 100 000 to 500 000.

Thanks again

Jérémie

Actually I'm not sure, I think we should consider transitional length in which flow turns into turbulence. If that would be substantial length in comparison with the characteristic length, we may assume it to be low Reynolds regime.
But I would rather prefer to compare the numerical results with experimental ones to make sure which model I should choose.
syavash is offline   Reply With Quote

Old   October 20, 2012, 04:31
Default
  #5
Senior Member
 
Join Date: Mar 2011
Location: Germany
Posts: 552
Rep Power: 20
cfd seeker is on a distinguished road
Quote:
Originally Posted by Jeremie84 View Post
Hello,

I am wondering if it is relevant to use the k-w models (standard or SST) with coarse mesh near the wall (y+>50). I read that in the Fluent User's guide and I am a little confused:

"Both k-w models available in ANSYS FLUENT are available as low-Reynolds-number models as well as high-Reynolds-number models. Therefore, the mesh guidance should be the same as for the enhanced wall treatment."

Also, what the advantage of using the "Low-Re corrections" option for the k-w models? Is it relevant only for fine meshes (y+<5)?

Thank you

Jérémie
Standard and SST versions of kw model by default use Enhanced Wall Treatment(B.L is resolved all the way upto viscous sublayer) and therefore wall y+<5 are generally recommended but theses models have the flexibility to switch over to Wall functions approach once a course mesh(wall y+ > 30) is used. So you can use it either way.
But if your sure that in your case transition is taking place then don't use "Low Re. No" option because in Fluent 13 they themselves have mentioned that "Low Re. No" does not perform good in such a situation. In Fluent 12 onward they have incorporated two new Turbulence models to properly capture Transition phenomenon, these models are Transition k-kl-w(3 eq. model) and Transition SST kw(4 eq. model). But again these models give you an extra headache because of their stringent mesh requirements i.e
1. Wall y+ <1
2. Sufficient Mesh resolution in streamwise direction
3. Proper estimate of Turbulence Intensity at the Inlet boundaries

Regards
cfd seeker is offline   Reply With Quote

Old   June 13, 2014, 10:49
Default
  #6
Senior Member
 
Join Date: Mar 2010
Posts: 172
Rep Power: 17
Jonathan is on a distinguished road
Hi Seeker,

I am looking at using the Transition SST model to predict transition on a turbine endwall and aerofoil.

I was wondering whether you knew whether it is a good idea to use the Low Re Correction's (i.e. Wilcox's damping functions) as well as the additional equations for transistion, or whether one should leave out the Low Re Corrections when using the additional transition equations.

I havent read anywhere that you shouldn't, but when i did, i didn't quite get intermittancy levels of '1' where i thought i would - i got a max of ~0.5 where i thought the BL should have been fully turbulent!

Any comments or thought based on your experience???

thanks very much
cheers
jonathan

Quote:
Originally Posted by cfd seeker View Post
Standard and SST versions of kw model by default use Enhanced Wall Treatment(B.L is resolved all the way upto viscous sublayer) and therefore wall y+<5 are generally recommended but theses models have the flexibility to switch over to Wall functions approach once a course mesh(wall y+ > 30) is used. So you can use it either way.
But if your sure that in your case transition is taking place then don't use "Low Re. No" option because in Fluent 13 they themselves have mentioned that "Low Re. No" does not perform good in such a situation. In Fluent 12 onward they have incorporated two new Turbulence models to properly capture Transition phenomenon, these models are Transition k-kl-w(3 eq. model) and Transition SST kw(4 eq. model). But again these models give you an extra headache because of their stringent mesh requirements i.e
1. Wall y+ <1
2. Sufficient Mesh resolution in streamwise direction
3. Proper estimate of Turbulence Intensity at the Inlet boundaries

Regards
Jonathan is offline   Reply With Quote

Reply

Tags
near wall mesh, turbulence model

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
[ICEM] Negative volume error in hybrid mesh siw ANSYS Meshing & Geometry 4 September 3, 2014 06:25
Mesh motion with Translation & Rotation Doginal CFX 2 January 12, 2014 07:21
[ICEM] Generating Mesh for STL Car in Windtunnel Simulation tommymoose ANSYS Meshing & Geometry 48 April 15, 2013 05:24
[Gmsh] 2D Mesh Generation Tutorial for GMSH aeroslacker OpenFOAM Meshing & Mesh Conversion 12 January 19, 2012 04:52
Icemcfd 11: Loss of mesh from surface mesh option? Joe CFX 2 March 26, 2007 19:10


All times are GMT -4. The time now is 23:48.