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

help :turbulence 2D or 3D?

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

Reply
 
LinkBack Thread Tools Display Modes
Old   November 16, 2009, 05:04
Default help :turbulence 2D or 3D?
  #1
New Member
 
Join Date: Nov 2009
Posts: 6
Rep Power: 7
fofo is on a distinguished road
Why can we use a 2D model for turbulence? because I have learnt taht turbulence is always 3 or 4 dimensionals. Is it ok ti use a 2D model for a turbulent flow in a pipe with orifice meter?

Thanks
fofo is offline   Reply With Quote

Old   November 16, 2009, 05:32
Default
  #2
Senior Member
 
Herman
Join Date: Nov 2009
Posts: 122
Rep Power: 7
enry is on a distinguished road
Hi.
Yes, of course, turbulence is ALWAYS 3D ! But, in order to reduce computational cost , is often advised to use 2d model, because there are many problems where turbulence don't execute a fundamental role, so 2d and 3d results may be the same.
enry is offline   Reply With Quote

Old   November 16, 2009, 10:48
Default
  #3
Senior Member
 
Max
Join Date: Mar 2009
Posts: 133
Rep Power: 8
coglione is on a distinguished road
hello fofo,

regarding turbulence modelng you have to use your experience a/o intuition.
Using one of the classical RMS models (k-eps, k-omega), you imply that turbulent eddies are isotropic in space and can be approximated by their average in time.
If you expect anisotropic turbulence in space but eddies can be still modeled equally distributed (random with a characteristic mean value) in time you may use RSM.
If you want to capture turbulence in all its beauty go for LES or DNS but think twice before that.

cheers
coglione is offline   Reply With Quote

Old   November 17, 2009, 04:38
Default
  #4
Senior Member
 
sbaffini's Avatar
 
Paolo Lampitella
Join Date: Mar 2009
Location: Italy
Posts: 507
Blog Entries: 14
Rep Power: 17
sbaffini will become famous soon enough
When you talk about 2D turbulence models you're actually talking about RANS/URANS simulations. In this case the solution is affected by a statistical operator (e.g., the Reynolds average in RANS) hence any direction of statistical homogeneity can be collapsed (in space and/or time). Thid does not mean that the model used in 2D can correctly account of all the cases of statistical homogeneity.

For example, it is well known that in the case of the backward facing step, for some Re numbers, 3D large structures appears in the flow but the spanwise direction is still a direction of statistical homogeneity (in the ideal case of an infinitely long spanwise extension) hence there is no reason to go 3D in RANS for this case. They are all up to the model, which have to represent the correct "average effect" of those structures in 2D. The same is true for any simmetry, there is no reason to not apply it with RANS wherever there is the opportunity.

The URANS case is somehow different as large, non turbulence related, 3D structures can exists and they have to be properly simulated (hence 2D/simmetries are not generally suitable in this case)
sbaffini is offline   Reply With Quote

Old   November 17, 2009, 05:04
Default
  #5
Senior Member
 
Herman
Join Date: Nov 2009
Posts: 122
Rep Power: 7
enry is on a distinguished road
However, I suggest you to keep always in your mind that turbulence : is ALWAYS UNSTEADY - THREE DIMENSIONAL - and "HAS A MEMORY OF THE PAST". ALL the URANS (Unsteady Reynolds Average Navier-Stokes) and RANS ((steady) Reynolds Average Navier-Stokes) are based on non physics approximations ( intrinsic hypothesis and specific hypotesis, that introduce the concept of eddy viscosity ), that allow us to reduce computational cost.

Keep also in your mind that: RANS solutions are equal to Time-averaged experimental data IF and ONLY IF model for <u'u'> is exact ( impossible!) .

A more physic method is LES (Large Eddy Simulation), but is not so used because of the high computational cost.

LES --> DNS while RANS-/-> DNS !!!!!!!

So after your simulation is recommended to compare your results with experimental data.

enry 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 on Turbulence Intensity Eric FLUENT 1 March 7, 2012 05:30
Turbulence length scale and integral length scale rizhang CFX 1 September 10, 2009 06:38
Discussion: Reason of Turbulence!! Wen Long Main CFD Forum 3 May 15, 2009 09:52
Code release: Flow Transition and Turbulence Chaoqun Liu Main CFD Forum 0 September 26, 2008 17:15


All times are GMT -4. The time now is 17:26.