Buoyancy driven flow

chivaee · March 14, 2012, 09:07

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

sgrshukla · March 14, 2012, 14:07

How about first trying with reducing your URF and making your solution more stable.

mettler · March 14, 2012, 14:26

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.

FMDenaro · March 15, 2012, 08:02

maybe, it can be a case of numerical instability due to the high Gr number...

chivaee · March 15, 2012, 10:03

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 ...

chivaee · March 15, 2012, 10:11

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.

chivaee · March 15, 2012, 10:13

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 ...

chivaee · March 15, 2012, 10:15

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 ...

chivaee · March 19, 2012, 11:39

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 ...

FMDenaro · March 19, 2012, 11:56

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

happy · April 19, 2012, 05:46

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

happy · May 15, 2012, 00:56

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

March 14, 2012, 09:07	Buoyancy driven flow	#1
chivaee New Member Hamid Join Date: Mar 2012 Posts: 20 Rep Power: 14	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 19, 2012, 11:39	problem partly solved, not sure if it's true ...	#9
chivaee New Member Hamid Join Date: Mar 2012 Posts: 20 Rep Power: 14	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 ...

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March 14, 2012, 14:07		#2
sgrshukla New Member Join Date: Jun 2009 Posts: 9 Rep Power: 17	How about first trying with reducing your URF and making your solution more stable.

March 14, 2012, 14:26		#3
mettler Senior Member Andrew Join Date: Mar 2009 Location: Washington, DC Posts: 211 Rep Power: 18	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
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,882 Rep Power: 73	maybe, it can be a case of numerical instability due to the high Gr number...

March 15, 2012, 10:13		#7
chivaee New Member Hamid Join Date: Mar 2012 Posts: 20 Rep Power: 14	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 ...