# Buoyancy driven flow

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 March 14, 2012, 09:07 Buoyancy driven flow #1 New Member   Sarlak Join Date: Mar 2012 Posts: 18 Rep Power: 8 Hi everybody, I'm going to solve a simple flow over flat plate and I'm going to have heat equation and buoyancy effects in my flow. My domain size is 1500(height)x6000x6000 [m] and my first grid center is located 0.03 away from the ground. I have periodic lateral BC, Symmetry top BC and no-slip wall BC. I can simulate the flow when I have a neutral case but with temperature it diverges very fast. Do you have any idea what could be wrong? Note that with this size Grashof number becomes enormous, but I don't know if that's the problem. I appreciate all your suggestions

 March 14, 2012, 14:07 #2 New Member   Join Date: Jun 2009 Posts: 9 Rep Power: 10 How about first trying with reducing your URF and making your solution more stable.

 March 14, 2012, 14:26 #3 Senior Member   Andrew Join Date: Mar 2009 Location: Washington, DC Posts: 195 Rep Power: 11 it sounds like a grid problem. Especially with buoyancy driven flow. Most of the interaction will be close to the wall, so you need a good grid in that region. I am assuming your plate is vertical and not horizontal.

 March 15, 2012, 08:02 #4 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 3,865 Rep Power: 42 maybe, it can be a case of numerical instability due to the high Gr number...

March 15, 2012, 10:03
#5
New Member

Sarlak
Join Date: Mar 2012
Posts: 18
Rep Power: 8
Quote:
 Originally Posted by sgrshukla How about first trying with reducing your URF and making your solution more stable.
I see your point, but I've tried with URFs of down to 0.15 with no success ...

March 15, 2012, 10:11
#6
New Member

Sarlak
Join Date: Mar 2012
Posts: 18
Rep Power: 8
Quote:
 Originally Posted by mettler it sounds like a grid problem. Especially with buoyancy driven flow. Most of the interaction will be close to the wall, so you need a good grid in that region. I am assuming your plate is vertical and not horizontal.
Hmmmm it could be. I've used a simple tanh function to stretch in the wall normal direction and uniform in the other two directions ... the plate is horizontal, in fact its an attempt to model atmospheric boundary layer simulation with 1.5 km height ... my issue is that the stretching rate is more than 30% but it looks quite smooth.

Now, even the temperature equation works fine but only if I disregard the gravitation.

 March 15, 2012, 10:13 #7 New Member   Sarlak Join Date: Mar 2012 Posts: 18 Rep Power: 8 I might also take a look at the block communications (it's a multi-block structured code I'm using) to see if something goes strange ...

March 15, 2012, 10:15
#8
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Sarlak
Join Date: Mar 2012
Posts: 18
Rep Power: 8
Quote:
 Originally Posted by FMDenaro maybe, it can be a case of numerical instability due to the high Gr number...
mmm, yeah, meybe, since it scales up as L^3 (L being characteristic length scale) ... but isn't it what normally happens? I mean I'm just simulating a small part of the real atmosphere with temperature stratification ...

 March 19, 2012, 11:39 problem partly solved, not sure if it's true ... #9 New Member   Sarlak Join Date: Mar 2012 Posts: 18 Rep Power: 8 Hi again, regarding the issue with buoyancy driven flow, I tried to decouple temperature and momentum equations completely i.e. after the solution for pressure and velocities converge, I solve the heat equation and most importantly "add the buoyancy contributions". Seems it solved the issue ... I know it's a general practice to treat the heat equation like this (quasi-steady) but not sure if it's true when you have buoyancy? since buoyant force affects the flow field directly and might make it inaccurate ...

March 19, 2012, 11:56
#10
Senior Member

Filippo Maria Denaro
Join Date: Jul 2010
Posts: 3,865
Rep Power: 42
Quote:
 Originally Posted by chivaee Hi again, regarding the issue with buoyancy driven flow, I tried to decouple temperature and momentum equations completely i.e. after the solution for pressure and velocities converge, I solve the heat equation and most importantly "add the buoyancy contributions". Seems it solved the issue ... I know it's a general practice to treat the heat equation like this (quasi-steady) but not sure if it's true when you have buoyancy? since buoyant force affects the flow field directly and might make it inaccurate ...
are you searching for the steady solution? then your method can be quite good, otherwise you need more accuracy in time, for example doing a predictor-corrector integration

April 19, 2012, 04:46
show me by CFX
#11
Senior Member

Safia
Join Date: Oct 2010
Location: Australia
Posts: 161
Rep Power: 9
Quote:
 Originally Posted by chivaee Hi again, regarding the issue with buoyancy driven flow, I tried to decouple temperature and momentum equations completely i.e. after the solution for pressure and velocities converge, I solve the heat equation and most importantly "add the buoyancy contributions". Seems it solved the issue ... I know it's a general practice to treat the heat equation like this (quasi-steady) but not sure if it's true when you have buoyancy? since buoyant force affects the flow field directly and might make it inaccurate ...
could you please show me how you could do it by CFX?
My problem is the same as yours. I have fluid flow with heat transfer and the buoyancy takes place through my model.
Regards

May 14, 2012, 23:56
#12
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Safia
Join Date: Oct 2010
Location: Australia
Posts: 161
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Quote:
 Originally Posted by happy could you please show me how you could do it by CFX? My problem is the same as yours. I have fluid flow with heat transfer and the buoyancy takes place through my model. Regards
finally, I got it!!
thanks for your good thread.However, I'm still sad because I asked to show me how until I do it after 2 months!!..some guys do not care about the others. you will feel better whan youbecome helpful member

 Tags buoyancy flow, divergence, grashof number

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