Setting Gravity for Convection

James Lamb · June 21, 2010, 07:27

I am modelling the passive cooling of a heat sink in an enclosure. I have created a volume of air with a stagnation inlet below and a pressure outlet above. When setting gravity in this model it creates a large flow downwards. I expected to see buoyancy forces at work on the hot components, this does not seem to be the case. Am I not leaving the simulation long enough or could I need more inner iterations? At the moment I am running for a minute with 2 inner iterations per second.

Thanks for the help in advance.

James

Vinicius · June 21, 2010, 10:00

Are you using the coupled solver?

James Lamb · June 21, 2010, 10:05

Not at the moment no.

Vinicius · June 21, 2010, 10:11

Try to use the coupled solver and use more inner iterations, maybe you also can check your time-step, you should have a compromise between your time-step size and your number of inner iterations...

James Lamb · June 21, 2010, 10:13

Ok just re-meshing now with a coupled solver. Thank you for your help

Maddin · June 22, 2010, 02:30

Look at your courant number.

WhoAmI · June 25, 2010, 11:49

To see the gravity effects, your reference density should be really really close to the density of the fluid at that temperature. Make sure the difference between reference density and density less than 1e-7.

Maddin · June 25, 2010, 13:20

Jipp or some steady tests before to check the density "effect" - with and withput gravity.
Or what do you think?

WhoAmI · June 25, 2010, 14:27

After initializing the solution, write a filed function to calculate the difference between the density in a cell ($Density) and the reference density value. Create a scene with this field function to see the density difference. Then try to readjust the reference density value (and also the value in field function) to make the difference as small as possible.

June 21, 2010, 07:27	Setting Gravity for Convection	#1
James Lamb New Member James Lamb Join Date: Jun 2010 Posts: 3 Rep Power: 15	I am modelling the passive cooling of a heat sink in an enclosure. I have created a volume of air with a stagnation inlet below and a pressure outlet above. When setting gravity in this model it creates a large flow downwards. I expected to see buoyancy forces at work on the hot components, this does not seem to be the case. Am I not leaving the simulation long enough or could I need more inner iterations? At the moment I am running for a minute with 2 inner iterations per second. Thanks for the help in advance. James

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Cells with t below lower limit	Purushothama	Siemens	2	May 31, 2010 21:58
Gravity setting	giogio	FLUENT	1	March 22, 2008 17:48
Warning 097-	AB	Siemens	6	November 15, 2004 04:41
On gravity modelling...	Drona	CFX	7	November 22, 2001 10:28
Setting Outlets with gravity..	Javier Larrondo	Phoenics	0	January 17, 2001 10:15

June 21, 2010, 10:00		#2
Vinicius Member Vinicius Girardi Join Date: Mar 2009 Location: Sao Paulo, Brazil Posts: 80 Rep Power: 17	Are you using the coupled solver?

June 21, 2010, 10:05		#3
James Lamb New Member James Lamb Join Date: Jun 2010 Posts: 3 Rep Power: 15	Not at the moment no.

June 21, 2010, 10:11		#4
Vinicius Member Vinicius Girardi Join Date: Mar 2009 Location: Sao Paulo, Brazil Posts: 80 Rep Power: 17	Try to use the coupled solver and use more inner iterations, maybe you also can check your time-step, you should have a compromise between your time-step size and your number of inner iterations...

June 21, 2010, 10:13		#5
James Lamb New Member James Lamb Join Date: Jun 2010 Posts: 3 Rep Power: 15	Ok just re-meshing now with a coupled solver. Thank you for your help

June 22, 2010, 02:30		#6
Maddin Senior Member Join Date: Mar 2009 Posts: 260 Rep Power: 18	Look at your courant number.

June 25, 2010, 11:49		#7
WhoAmI New Member Join Date: Jun 2010 Posts: 3 Rep Power: 15	To see the gravity effects, your reference density should be really really close to the density of the fluid at that temperature. Make sure the difference between reference density and density less than 1e-7.

June 25, 2010, 13:20		#8
Maddin Senior Member Join Date: Mar 2009 Posts: 260 Rep Power: 18	Jipp or some steady tests before to check the density "effect" - with and withput gravity. Or what do you think?

June 25, 2010, 14:27		#9
WhoAmI New Member Join Date: Jun 2010 Posts: 3 Rep Power: 15	After initializing the solution, write a filed function to calculate the difference between the density in a cell ($Density) and the reference density value. Create a scene with this field function to see the density difference. Then try to readjust the reference density value (and also the value in field function) to make the difference as small as possible.