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

Laminar separation bubble

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

Reply
 
LinkBack Thread Tools Display Modes
Old   August 3, 2004, 07:49
Default Laminar separation bubble
  #1
Axilleas Tsompanos
Guest
 
Posts: n/a
Hello everyone,

Can Fluent capture the laminar separation bubble? I have tried but haven't made it.

Thanks a lot.
  Reply With Quote

Old   August 3, 2004, 11:17
Default Re: Laminar separation bubble
  #2
Jonas Larsson
Guest
 
Posts: n/a
If you mean the type of laminar separation bubble that undergoes transition and then re-attaches (common on for example turbine blades) the answer is probably no. Fluent does not include any turbulence models that are capable of modling this type of transition and re-attachement occuring after a laminar separation. When you turn on your turbulence model in Fluent you will most likely get turbulent bounday layers everywhere. Hence, you will not see any laminar separation bubbles. What you could do is to define a small laminar region with a UDF. But this requires that you know where transition/re-attachement occures approximately in order to explicitly turn on your turbulence model at that location.

This doesn't mean that Fluent is a bad CFD code - the vast majority of commercial codes are unable to predict this kind of tricky transitional flows.
  Reply With Quote

Old   August 4, 2004, 04:16
Default Re: Laminar separation bubble
  #3
Axilleas Tsompanos
Guest
 
Posts: n/a
Dear Mr. Larsson

Thanks a lot for your reply. It was very helpful indeed. Do you know of any turbulence models that can capture this transition?

Regards
  Reply With Quote

Old   August 4, 2004, 14:29
Default Re: Laminar separation bubble
  #4
Alex Chirokov
Guest
 
Posts: n/a
Turbulent models that are based on modified laminar N-S equations, work in time averaged sence, and can not capture transition. Only DNS can capture transition phenomena, or some particle based methods like SPH.
  Reply With Quote

Old   August 6, 2004, 00:35
Default Re: Laminar separation bubble
  #5
Dan Brzozowski
Guest
 
Posts: n/a
Turbulence models provide are closure for the Reynolds Averaged Navier Stokes (RANS) Equations. These equations are averaged *only* over the timescale of turbulent fluctuations, *not* all time. Using the unsteady solver, you can capture a great deal of unsteady phenomena --- that is, as long as the phenomena occur on a timescale much larger than that of the turbulent fluctuations.

Try using the "Enhanced Wall Functions", which should do a good job modelling laminar boundary layers. Also make sure you have a nice fine mesh near the wall.

Good Luck

-Dan
  Reply With Quote

Old   August 6, 2004, 12:24
Default Re: Laminar separation bubble
  #6
Alex Chirokov
Guest
 
Posts: n/a
You are absolutely right it is well known closure problem. The bottom line is that you have to use something to get a closed system. Thus behavior of your system is usually greatly depend on what closing assumptions you used (read: depend on your turbulence model).

There is no question that you can capture unsteady phenomena with turbulent models but this unsteadiness is not related to transitions at all! Because you already assumed that the flow is turbulent when use introduced closing assumptions! You can use very elaborate closing assumptions that work in low Re # as well as in hi Re #, but you can't simulate the transition.

Again there is not way in fluent (or any other commercial CFD) to predict transitions from laminar to turbulent, and to simulate these effects.

Enhanced Wall Functions will help to simulate turbulent boundary layer (you will need very fine mesh for this, not transitions.
  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
Laminar bubble separation Gilles CFX 1 July 24, 2008 21:57
Separation bubble, hypersonic compression corner ben akih CFX 3 December 10, 2006 17:33
separation bubble ..... zqnwpu Main CFD Forum 1 November 23, 2004 14:45
Please help with separation bubble Nael FLUENT 5 March 22, 2002 03:50
Laminar bubble separation Daniel Bruno FLUENT 0 May 10, 2001 23:11


All times are GMT -4. The time now is 23:49.