CFD Online Logo CFD Online URL
Home > Forums > FLUENT


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

Like Tree1Likes
  • 1 Post By Gernot Lukesch

LinkBack Thread Tools Display Modes
Old   June 3, 2008, 03:16
Default Backflow
khar nab
Posts: n/a
hello all I want to know how to impose a pressure outlet boundary condition without backflow. Rgds
  Reply With Quote

Old   June 3, 2008, 04:42
Default Re: Backflow
Gernot Lukesch
Posts: n/a
I don´t know if this is possible with a pressure outlet but if you really want to avoid backflow, you can take a negative velocity inlet ( but only if its an incompressible fluid ).
  Reply With Quote

Old   June 3, 2008, 08:24
Default Re: Backflow
Posts: n/a
If backflow occurs only in the intermediate solution, take it easy. Otherwise, pls check the pressure setting of your boundary condtion.
  Reply With Quote

Old   June 3, 2008, 19:47
Default Re: Backflow
Posts: n/a
it seems impossible to eliminate backflows in the current Fluent versions if the fluid is imconpressible. Non-reflecting boundary conditions may be applied to compressible flows.
  Reply With Quote

Old   June 4, 2008, 05:26
Default Re: Backflow
khar nab
Posts: n/a
I have understood that using a pressure outlet boundary condition without backflow is impossible. I have tried to use outflow boundary condition, but the pressure at the outlet was not reasonable. So, who can tell me how to impose pressures with a given flow rate. Thanks
  Reply With Quote

Old   June 4, 2008, 06:33
Default Re: Backflow
Gernot Lukesch
Posts: n/a
No it is not impossible - it depends on your geometry. Just at the beginning of a calculation it is very hard to avoid it but when the flow conditions are stable why not ? But as it was often discussed here if the result of the outflow- area is important and you get a backflow then you have to expand your calculation area to regions behind the outflow so that the `new´ outflow there does not influence your `old´ one you are interested in. hope that helps
Pavel Mihailovski likes this.
  Reply With Quote

Old   June 6, 2008, 06:47
Default Re: Backflow
Posts: n/a

How can you say it's not reasonable? Do you have experimental results for the case?

I don't think you should think about imposing the elimination of the backflow, as it comes as the result of the simulation you'd set. If you are sure that this backflow is unreal, you should look for problems in your simulation, and not just "impose" the solution that you think is real.

My best regards, Victor
  Reply With Quote

Old   June 9, 2008, 08:51
Default Re: Backflow
khar nab
Posts: n/a
The following note is mentioned in the Fluent doc

Even if no backflow is expected in the converged solution, you should always set realistic values to minimize convergence difficulties in the event that backflow does occur during the calculation.

In the literature no back flow has been mentioned in all experimental works concerning the subject I am studying (hydrocyclones). What does "realistic values" mean in the note above. Thanks.
  Reply With Quote

Old   December 5, 2010, 12:14
New Member
Join Date: Apr 2010
Posts: 3
Rep Power: 9
zhiyii is on a distinguished road
i have meet this problem.i set smaller gauge pressure at pressure outlet boundary condition and get rid of backflow.
zhiyii is offline   Reply With Quote

Old   December 5, 2010, 12:57
Darren Leong
Join Date: Dec 2010
Posts: 63
Rep Power: 8
Darren Leong is on a distinguished road
Hi Khar Nab,

Is it possible to post a pic of your model? Can't advise specifically where to look at without seeing your setup.

Just for starters:
1. The backflow may indicate turbulence in that area; you can extend the domain to see if this is the case and whether it dissipates.
2. the CFD solution is strictly based on the condition you've applied. I'll advise care when forcing the flow regime; easiest way is to set the boundary as an outlet instead of opening (ur flow will be uni-directional).
3. Is experimental/empirical data your validating against, flow-field or measured results?
4. What turbulence model are you using? =)
5. Hard bit (optional): Run your simulation in steady-state, and then transient for 1 sec. Post the output plots of variables, including the wall and turbulence plot for comparison. It'll help to see if your solution's stable.


Last edited by Darren Leong; December 5, 2010 at 13:29.
Darren Leong is offline   Reply With Quote

Old   July 17, 2014, 04:08
Default backflow
Join Date: Jan 2014
Location: Turkey
Posts: 37
Rep Power: 5
thermal energy is on a distinguished road
hi all,

ı have an rectangular geometry (lenght:76, width:27 mm). heat flux is applied from one of the vertical walls. ı have velocity inlet BC at lower part and also pressure outlet BC at upper part. velocity magnitute is zero (0) (no forced convection), initial gauge pressure is also zero and temperature is 298 K at velocity inlet BC. also, for pressure outlet BC gauge pressure is 0 and backflow temperature is set 298 K. initial temperature is also 298K.

the problem is that reversed flow occured.
"reversed flow in 34 faces on pressure-outlet 17"

the solution is converged but ı am not sure it is right or not.

any suggestion. thanks for you time.
thermal energy is offline   Reply With Quote

Old   July 17, 2014, 10:20
Senior Member
François Grégoire
Join Date: Jan 2010
Location: Laval University, Canada
Posts: 389
Rep Power: 10
macfly is on a distinguished road
Asking the same question all over the place in multiple threads is very annoying, not helping your cause at all, good luck.
macfly is offline   Reply With Quote

Old   July 18, 2014, 04:47
New Member
Join Date: Jul 2012
Posts: 21
Rep Power: 7
thousand is on a distinguished road
sometimes backflow is unreal in problems. its there is for invalid bc in outlet! for example in outlet pressure gradiant is not zero but used from "pressure outlet" in this problem you can increase geometry lengths(adding virtual length) in outlet but in reporting compute pressure in initial(real) outlet!
thousand is offline   Reply With Quote


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
backflow in a jet simulation Hao CFX 0 October 3, 2008 02:23
how to prevent backflow? May Lim FLUENT 2 June 20, 2007 12:17
Backflow term? rayy FLUENT 0 March 15, 2007 12:29
Why would backflow appear? easycad FLUENT 3 April 19, 2005 23:43
backflow detection Max Mueller Main CFD Forum 0 November 10, 2004 09:45

All times are GMT -4. The time now is 22:09.