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March 14, 2012, 09:07 
Buoyancy driven flow

#1 
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Sarlak
Join Date: Mar 2012
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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 noslip 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 
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Join Date: Jun 2009
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How about first trying with reducing your URF and making your solution more stable.


March 14, 2012, 14:26 

#3 
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Andrew
Join Date: Mar 2009
Location: Washington, DC
Posts: 195
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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 
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Filippo Maria Denaro
Join Date: Jul 2010
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maybe, it can be a case of numerical instability due to the high Gr number...


March 15, 2012, 10:03 

#5 
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Sarlak
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March 15, 2012, 10:11 

#6  
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Sarlak
Join Date: Mar 2012
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Quote:
Now, even the temperature equation works fine but only if I disregard the gravitation. 

March 15, 2012, 10:13 

#7 
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Sarlak
Join Date: Mar 2012
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I might also take a look at the block communications (it's a multiblock 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
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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 
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Sarlak
Join Date: Mar 2012
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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 (quasisteady) 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  
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Filippo Maria Denaro
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Quote:


April 19, 2012, 04:46 
show me by CFX

#11  
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Safia
Join Date: Oct 2010
Location: Australia
Posts: 161
Rep Power: 7 
Quote:
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:
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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