Boundary condition for natural convection

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 July 26, 2012, 01:28 Boundary condition for natural convection #1 Member   ankur Join Date: May 2012 Location: India Posts: 50 Rep Power: 6 Hey I am new to openfoam, I want to simulate some tubes inside a cylinder having steam, air is entering from bottom of the cylinder and leaving from the top due to buoyancy. I am using the zeroGradient BC for the inlet(bottom) and the outlet(top). But the solution is diverging, can anybody help me what boundary condition should i give for pressure and velocity in this case. Thank you

 July 26, 2012, 01:36 #2 Senior Member   Bernhard Join Date: Sep 2009 Location: Delft Posts: 790 Rep Power: 14 What boundary condition did you give? Which solver do you use? What is diverging exactly? Please provide more information, because there are so many option. If I remember correctly, for p_rgh you have a buoyantPressure bc that may suit your needs.

 July 26, 2012, 02:13 #3 Member   ankur Join Date: May 2012 Location: India Posts: 50 Rep Power: 6 I have used buoyantBoussinesqPisoFoam with Quick scheme, actually I have used uniform 0 for pressure at the inlet and the outlet, zeroGradient for velocity at the inlet and the outlet. After that the courant number is going very high, final residual for pressure is also very high after 3rd iteration only, hence the solution is getting diverged.I am having openfoam 1.6 Last edited by Ank; July 26, 2012 at 02:13. Reason: additional information

 July 26, 2012, 16:21 #4 Senior Member     A_R Join Date: Jun 2009 Posts: 118 Rep Power: 9 you trap pressure in your field so it is normal to diverge. and zero gradient for both side is not good idea. can you show your test case clearly?

 July 27, 2012, 00:09 #5 Member   ankur Join Date: May 2012 Location: India Posts: 50 Rep Power: 6 hey thanks for the reply people, My case is to do simulation of one cylinder having some 20 tubes inside, so I am simulating a quarter of the cylinder having 5 tubes. Steam is flowing from top to bottom at 413 K inside the tubes, and cylinder is open from both the side so that there can be a natural flow of air due to buoyancy. Thanks..

 July 27, 2012, 03:00 #6 Senior Member     A_R Join Date: Jun 2009 Posts: 118 Rep Power: 9 Dear Ank 1- zeroGradient affects the solution of test cases. is your top and bottom limited? I mean " can you increase the length of the main cylinder?" 2- change one of your pressure to zeroGradient. tell me: does it work properly?

 July 27, 2012, 04:01 #7 Member   ankur Join Date: May 2012 Location: India Posts: 50 Rep Power: 6 Hey Niaz Thank you for the reply, I did what you said to me but it is stil now working, I will try some other BC as well..if you know some other Bc then please let me know

 July 27, 2012, 04:36 #8 Senior Member     A_R Join Date: Jun 2009 Posts: 118 Rep Power: 9 add these lines to your controldict adjustTimeStep yes; maxCo 0.2; you may use a big times step. it helps to control courant number. and tiemstep. try and tell me what is the result.

 July 27, 2012, 05:00 #9 Member   ankur Join Date: May 2012 Location: India Posts: 50 Rep Power: 6 I did it, but courant number has already gone to 4, after 4 iterations...so again there is no result. I dont know what is the problem with the pressure..it is fluctuating now..I will let you know if it gets stable..thanks..

 July 27, 2012, 05:03 #10 Senior Member   Bernhard Join Date: Sep 2009 Location: Delft Posts: 790 Rep Power: 14 Did you try decreasing your initial time-step? Please post a log-file, it will contains a lot of information.

July 27, 2012, 05:16
#11
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ankur
Join Date: May 2012
Location: India
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I am using 1e-06 as initial time step, I am posting my case files, please have a look at it..solution has diverged again.
Attached Files
 casefiles.zip (7.1 KB, 9 views)

July 27, 2012, 06:38
#12
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Primoz Ternik
Join Date: Apr 2009
Location: Maribor, Slovenia
Posts: 65
Rep Power: 9
Quote:
 Originally Posted by Ank Hey I am new to openfoam, I want to simulate some tubes inside a cylinder having steam, air is entering from bottom of the cylinder and leaving from the top due to buoyancy. I am using the zeroGradient BC for the inlet(bottom) and the outlet(top). But the solution is diverging, can anybody help me what boundary condition should i give for pressure and velocity in this case. Thank you
Dear all,

I am having some difficulties with the natural convection as well;details were posted here:

buoyantBoussinesqSimpleFoam

After quite some time spent I have concluded that the problem might be with the boundary conditions for p and p_rgh (zero velocity and zero gradient/constant temperature on solid walls are pretty straightforward).

It is true that I have found some (similar) posts regarding the natural convection problem, but no final (and validated) results are available.

Is there someone that can give me usefull hints how to set the correct pressure boundary conditions (not only from numerical but from physical point of veiw as well)?!

Best,
Primoz

July 27, 2012, 09:46
#13
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A_R
Join Date: Jun 2009
Posts: 118
Rep Power: 9
Quote:
 Originally Posted by Ank I am using 1e-06 as initial time step, I am posting my case files, please have a look at it..solution has diverged again.
Dear Ank
at first, change your pressure BC. it is not correct to use uniform 0 for inlet and outlet. this BC means that no flow can in or out and it forces to circulate in the tube.
secondly, I think courant control is not work in of1.6 which you used

 July 28, 2012, 07:59 #14 Member   ankur Join Date: May 2012 Location: India Posts: 50 Rep Power: 6 Hey Niaz I tried other BCs as well, but didnt work out. Can you suggest me some specific BC for this kind of case, which you have used? Thanks

 July 28, 2012, 14:33 #15 Senior Member     A_R Join Date: Jun 2009 Posts: 118 Rep Power: 9 Dear Ank are you check that your case is not turbulent?

 July 30, 2012, 00:40 #16 Member   ankur Join Date: May 2012 Location: India Posts: 50 Rep Power: 6 Hey Niaz, If the flow is turbulent then what difference does it make to the boundary conditions?

 July 30, 2012, 04:26 #17 Senior Member     A_R Join Date: Jun 2009 Posts: 118 Rep Power: 9 dear ank firstly, you have a problem in convergency of your problem. it can affect the solution.

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