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

Question about CFX turbulence intensity

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

Reply
 
LinkBack Thread Tools Display Modes
Old   July 8, 2013, 16:23
Question Question about CFX turbulence intensity
  #1
Senior Member
 
Anna Tian's Avatar
 
Meimei Wang
Join Date: Jul 2012
Posts: 472
Rep Power: 6
Anna Tian is on a distinguished road
Hi

Here is my approach to detect the flow regime for flow in complex geometry:

1) Run the simulation by SST model (or other turbulence model)

2) Look at the turbulence intensity value to know the flow regime


Question:

1) Does this approach make sense?

2) What is the range of the turbulence intensity value corresponding to laminar flow? And transitional flow?

3) Could CFX output turbulence intensity in CFX post?
__________________
Best regards,
Meimei
Anna Tian is offline   Reply With Quote

Old   July 13, 2013, 07:39
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,646
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
You should not need to do this. Reynolds number can be calculated for most geometries - no need to be exact, just close enough - and compared to the known transition points for pipe flow, flat plates or spheres/airfoils and you will know if you are laminar, turbulent or transitional. In the setup stages, that is all you need to know.

Other non dimensional numbers may also be useful, depending on the simulation. FOr instance Rayliegh number for natural convection flows.
ghorrocks is offline   Reply With Quote

Old   May 6, 2015, 15:32
Default
  #3
Member
 
Hooman
Join Date: Feb 2012
Location: USA
Posts: 65
Rep Power: 5
enayath is on a distinguished road
Dear ghorrocks,

I am wondering if you know the turbulence intensity in a natural convection cases(in an enclosed domain) for a turbulent flow and laminar flow? Do you know how the TI should vary in percentage?
for instance more than 20%, should be called fully turbulent flow?

Thank you,
Hooman
enayath is offline   Reply With Quote

Old   May 6, 2015, 19:58
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,646
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Turbulence intensity is 0% for a laminar flow by definition - a laminar flow is not turbulent. So if the turbulence intensity is larger than 0 then the flow is turbulent.

For buoyancy driven flows the Rayleigh number is often a better measure. There is often a Rayleigh number which transitions from laminar to turbulent flow, just the same as in Reynolds Number.

http://en.wikipedia.org/wiki/Rayleigh_number
ghorrocks is offline   Reply With Quote

Old   May 7, 2015, 12:15
Default
  #5
Member
 
Hooman
Join Date: Feb 2012
Location: USA
Posts: 65
Rep Power: 5
enayath is on a distinguished road
Dear ghorrocks,

Thank you for the reply. Basically, I knew and agree that Ra is important. Suppose you do not the Ra corresponds to transitional or turbulent flow. so maybe a good way is to calculate TI and judge based on that value, that flow is laminar, transitional or turbulent(low-medium or high turbulent)...
So, my concern is about that and I am wondering if you know a reference about the Ti variation in a natural convection case.

Regards,
Hooman
enayath is offline   Reply With Quote

Old   May 7, 2015, 18:13
Default
  #6
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,646
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
The simulation approach used for laminar flows is quite different to turbulent flows - you don't use a turbulence model for one. You need to determine in advance whether the flow is laminar or turbulent and select appropriate models. CFX has a turbulence transition model which is the only model in CFX which can handle laminar and turbulent flows and their transition - and it is only applicable under specific circumstances (flow over airfoils and airfoil like objects).
ghorrocks 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
Question about using turbulence model to laminar flow in CFX Anna Tian CFX 9 May 23, 2015 06:17
Turbulence postprocessing Mohsin FLUENT 0 September 19, 2011 21:05
question about turbulence model selection and sensitivity karananand Main CFD Forum 1 February 26, 2010 05:41
How CFX calculate TKE and Epsilon from intensity? David CFX 0 November 24, 2005 05:53
Naive question about turbulence and LES Renato Main CFD Forum 1 October 4, 2005 23:01


All times are GMT -4. The time now is 23:04.