# Backflow

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

 June 3, 2008, 03:16 Backflow #1 khar nab Guest   Posts: n/a hello all I want to know how to impose a pressure outlet boundary condition without backflow. Rgds

 June 3, 2008, 04:42 Re: Backflow #2 Gernot Lukesch Guest   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 ).

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

 June 3, 2008, 19:47 Re: Backflow #4 David Guest   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.

 June 4, 2008, 05:26 Re: Backflow #5 khar nab Guest   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

 June 4, 2008, 06:33 Re: Backflow #6 Gernot Lukesch Guest   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

 June 6, 2008, 06:47 Re: Backflow #7 Victor Guest   Posts: n/a Hello, 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

 June 9, 2008, 08:51 Re: Backflow #8 khar nab Guest   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.

 December 5, 2010, 12:14 #9 New Member   cz Join Date: Apr 2010 Posts: 3 Rep Power: 8 i have meet this problem.i set smaller gauge pressure at pressure outlet boundary condition and get rid of backflow.

 December 5, 2010, 12:57 #10 Member   Darren Leong Join Date: Dec 2010 Posts: 63 Rep Power: 7 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. -dazza Last edited by Darren Leong; December 5, 2010 at 13:29.

 July 17, 2014, 04:08 backflow #11 Member   Join Date: Jan 2014 Location: Turkey Posts: 36 Rep Power: 4 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.

 July 17, 2014, 10:20 #12 Senior Member   François Grégoire Join Date: Jan 2010 Location: Laval University, Canada Posts: 377 Rep Power: 9 Asking the same question all over the place in multiple threads is very annoying, not helping your cause at all, good luck.

 July 18, 2014, 04:47 #13 New Member   khalil Join Date: Jul 2012 Posts: 21 Rep Power: 5 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!

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Hao CFX 0 October 3, 2008 02:23 May Lim FLUENT 2 June 20, 2007 12:17 rayy FLUENT 0 March 15, 2007 12:29 easycad FLUENT 3 April 19, 2005 23:43 Max Mueller Main CFD Forum 0 November 10, 2004 09:45

All times are GMT -4. The time now is 16:43.